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Water Quality Control and Treatment Water Treatment

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Page 1: Water Quality Control and Treatment Water Treatment

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Water Quality Control and Treatment

Water Treatment

ByDr. Khamis AL-Mahallawi

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Water treatment• Overview of the Water Treatment Process• Preliminary Treatment

– Presedimentation– Aeration– Primary Sedimentation– Sedimentation and flotation zones

• Aeration • Adsorption• Ion Exchange• Coagulation and Flocculation• Filtration• Membrane Processes / Electro-dialysis• Nanofiltration and reverse osmosis• Softening • Treatment

– Groundwater types and treatment– Surface water treatment

• Disinfection

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Water Treatment Process abstraction - treatment – transport – storage - distribution

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Water treatment processesPreliminary Treatment

• Preliminary treatment is any physical, chemical or mechanical process used

on water before it undergoes the main treatment process.

• The purpose of preliminary treatment processes is to remove any materials

which will interfere with further treatment.

• Pretreatment may include screening, presedimentation, chemical addition,

flow measurement, and aeration.

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Preliminary Treatment / Screens• The screens are used to remove rocks, sticks, leaves, and

other debris. • Very small screens can be used to screen out algae in the

water. • All objects are removed by physical size separation • Screens on the outside of intakes are often cleaned by

flushing water from the treatment plant backwards• There are two primary types of screens - bar screens and

wire-mesh screens. • A bar screen is used to remove large debris. The spaces

between the bars are two to four inches wide.• A wire-mesh screen is used to remove smaller debris. The

gaps are about half an inch wide. • Water must be flowing slowly in order to pass through a

wire-mesh screen - velocities should be no greater than 3.5 inches per second.

• A wire-mesh screen

A bar screen

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Preliminary Treatment / Presedimentation - Aeration

Presedimentation is to settle out sand, grit, and gravel which will settle rapidly out of the water without the addition of chemicals at the beginning of the treatment process.

Presedimentation depends on gravity and includes no coagulation and flocculation.  Presedimentation will reduce the load on the coagulation/flocculation basin and on the sedimentation chamber, as well as reducing the volume of coagulant chemicals required to treat the water. Presedimentation basins are useful because raw water entering the plant from a reservoir is usually more uniform in quality than water entering the plant without such a holding basin

Here in pretreatment, the purpose of sedimentation is to make the chemical treatment phase of the water treatment process more efficient by removing sediment from the raw water.

In presedimentation basin, activated carbon may be added to the basin for taste, odor, and color problems,  and some chemicals to control the growth of algae. Aeration removes carbon dioxide and hydrogen sulfide from the water.  It also oxidizes the iron and manganese. 

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Preliminary Treatment / Monitoring

• Flow Measurement : to adjust chemical feed rates, calculate detention times, and monitor the amount of water being treated.

• It is also monitored for a variety of characteristics including pH, turbidity, total alkalinity, temperature, and coliform bacteria.

• The pH and total alkalinity of the water will influence the amount of alkali to be added and can also influence the flocculation conditions

• The level of turbidity will influence the amount of polymer (coagulant) added to the water. 

• Temperature is also measured since cold water does not floc as well as warm water and requires the addition of more polymer

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Primary Sedimentation

Notes:• sedimentation may not be necessary in

low turbidity water of less than 10 NTU

• In this case, coagulation and flocculation are used to produce pinpoint (very small) floc which is removed from the water in the filters  

• Sedimentation is a treatment process in which the velocity of the water is lowered below the suspension velocity and the suspended particles settle out of the water due to gravity. 

• The process is also known as settling or clarification• Settled solids are removed as sludge, and floating solids are removed as scum• The efficiency or performance of the process is controlled by: detention time,

temperature, tank design, and condition of the equipment.

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Primary Sedimentation / Location in the Treatment Process

• The most common form of sedimentation follows coagulation and flocculation and precedes filtration. 

• This type of sedimentation requires chemical addition (in the coagulation/flocculation step) and removes the resulting floc from the water. 

• sedimentation following coagulation/flocculation is meant to remove most of the suspended particles in the water before the water reaches the filters,

• Sedimentation at this stage in the treatment process should remove 90% of the suspended particles from the water, including bacteria. 

• The purpose of sedimentation here is to decrease the concentration of suspended particles in the water, reducing the load on the filters.  

• Sedimentation can also occur as part of the pretreatment process, where it is known as presedimentation. 

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Types of sedimentation basins

Rectangular basins: have a variety of advantages - predictability, cost-effectiveness, and low

maintenance.  In addition, rectangular basins are the least likely to short-circuit, especially if the

length is at least twice the width.  A disadvantage of rectangular basins is the large amount of land

area required. 

Double-deck rectangular basins: This type of basin conserves land area - has higher operation and

maintenance costs.

Square or circular sedimentation basins with horizontal flow are known as clarifiers. This type

of basin is likely to have short-circuiting problems.

Solids-contact clarifiers , also known as upflow solids-contact clarifiers or upflow sludge-

blanket clarifiers combine coagulation, flocculation, and sedimentation within a single

basin. found in packaged plants and in cold climates where sedimentation must occur indoors

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Sedimentation and flotation zones • All sedimentation basins have four zones -

the inlet zone, the settling zone, the

sludge zone, and the outlet zone.

• In a clarifier, water typically enters the

basin from the center rather than from one

end and flows out to outlets located around

the edges of the basin.  But the four zones

can still be found within the clarifier

A rectangular sedimentation basin

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Sedimentation and flotation zones/Inlet ZonePurposes of the inlet zone of a sedimentation basin are • to distribute the water and to control the water's velocity as it

enters the basin. 

• inlet devices act to prevent turbulence of the water.

• The incoming flow must be evenly distributed across the width of the basin to prevent short-circuiting.

• Short-circuiting is a problematic circumstance in which water bypasses the normal flow path through the basin and reaches the outlet in less than the normal detention time. 

• If the water velocity is greater than 0.5 ft/sec, then floc in the water will break up due to agitation الهياج)) of the water. 

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Sedimentation and flotation zones/Inlet Zone

Two types of inlets.

1. The stilling (التسكين) wall, also known as

a perforated baffle wall , spans the entire basin from

top to bottom and from side to side.  Water leaves the

inlet and enters the settling zone of the sedimentation

basin by flowing through the holes evenly spaced across

the stilling wall. 

2. The second type of inlet allows water to enter the

basin by first flowing through the holes evenly spaced

across the bottom of the channel and then by flowing

under the baffle in front of the channel. 

The combination of channel and baffle serves to evenly

distribute the incoming water

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  Sedimentation and flotation / Settling Zone• water enters the settling zone where water velocity

is greatly reduced.  • the bulk of floc settling occurs and this zone will

make up the largest volume of the sedimentation basin.

• For optimal performance, the settling zone requires a slow, even flow of water. 

• The settling zone may be simply a large expanse of open water.  But in some cases, tube settlers and lamella plates, such as those shown below, are included in the settling zone.  

• Tube settlers and lamella plates • Water flows up through slanted tubes or along

slanted plates. Floc settles out in the tubes or plates and drifts back down into the lower portions of the sedimentation basin. 

• Clarified water passes through the tubes or between the plates and then flows out of the basin.

Actual Area vs.Effective Area

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 Sedimentation and flotation / Settling ZoneWhy Tube settlers and lamella plates:

To increase the settling efficiency and speed in sedimentation basins.  Each tube or plate functions as a miniature sedimentation basin, greatly increasing the settling area.  Tube settlers and lamella plates are very useful in plants where site area is limited, or to increase the capacity of shallow basins.Adding inclined settling surface technology to an existing clarifier can increase water treatment flow by as much as 75%. 

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 Sedimentation and flotation / Settling Zone

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 Sedimentation and flotation / Settling Zone

Horizontal Parallel Plate Clarifiers

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 Sedimentation and flotation / Settling Zone

Vertical Parallel Plate Clarifiers

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Traditional Circular Clarifiers / Settling Zone

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Sedimentation and flotation / Outlet ZoneOutlet Zone is designed to:• prevent short-circuiting of water in the basin. • ensure that only well-settled water leaves the basin and enters the filter.• control the water level in the basin. • ensure that the water flowing out of the sedimentation basin has the minimum

amount of floc suspended in it.• A typical outlet zone begins with a baffle in front of the effluent. • This baffle prevents floating material from escaping the sedimentation basin

and clogging the filters.• The weirs serve to skim the water evenly off the tank  

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Sedimentation and flotation / Sludge Zone• The sludge zone is found across the bottom of the sedimentation basin. 

• Velocity should be very slow to prevent resuspension of sludge. 

• The tank bottom should slope toward the drains

• Sludge removal by ( automated equipment or manually at least twice per year).

• The best time of cleaning when water demand is low, (April and October).

• Many plants have at least two sedimentation basins so that water can continue to be treated while one basin is being cleaned, maintained, and inspected. 

• If sludge is not removed from enough, the effective volume of the tank will decrease, reducing the efficiency of sedimentation.

• Sludge built up on the bottom of the tank may become septic (anaerobically). 

• Septic sludge may result in taste and odor problems or may float to the top of the water and become scum or resuspended to be carried over to the filters. 

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Aeration

Types of Aerators

• air into the water

• water into the air

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Aeration Efficiency

Surface contact between air and water• Smaller bubble size, greater surface contact with water.• Smaller drop size, greater surface contact with the air.

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Aeration • Aeration is the process of bringing water and air into close contact.

• Aeration is the process to remove dissolved gases, such as carbon dioxide, hydrogen sulfide, and to oxidize dissolved metals such as iron. It can also be used to remove volatile organic chemicals (VOC).

It happened by:• Exposing drops or thin sheets of water to

the air or• introducing small bubbles of air and letting

them rise through the water.the aeration is accomplished the desired

results by: • Sweeping الكنس)) or scrubbing action

caused by the turbulence of water and air mixing together

• Oxidizing certain metals and gases

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CHEMICAL SUBSTANCES AFFECTED BY AERATION

The constituents that are commonly affected by aeration are: • Volatile organic chemicals, such as benzene, found in gasoline,

or trichloroethylene, dichloroethylene, and perchloroethylene, examples of solvents are used in dry cleaning or industrial processes.

• Carbon dioxide • Hydrogen sulfide (rotten-egg odor) • Methane (flammable) • Iron (will stain clothes and fixtures) • Manganese (black stains) • Various chemicals causing taste and odor

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CHEMICAL SUBSTANCES AFFECTED BY AERATION / CARBON DIOXIDE

• Surface waters have a low CO2 content ( 0 to 2 mg/l).

• Deep lake or reservoir can have high CO2 content due to the respiration of microscopic animals and lack of abundant plant growth at the lake bottom.

• aerators remove CO2 by the physical scrubbing or sweeping action caused by

turbulence.

• aeration can reduce the CO2 content to 4.5 mg CO2 /l

• Concentration of CO2 in groundwater are usually higher than in surface water.

• Water from a deep well normally contains less than 50 mg/l, but a shallow well can have a much higher level, up to 50 to 300 mg/l.

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CARBON DIOXIDE REMOVAL• The most appropriate treatment for carbon dioxide may be aeration, addition of

an alkali, or a combination of the two • CO2 gas dissolves easily in water, resulting in carbonic acid: 

• H2O + CO2 <===> H2CO3

• CO2 is neutralized through the addition of an alkali (basic, ionic salt), such as lime (Ca(OH)2) or soda ash (Na2CO3). 

• Lime reacts with carbon dioxide, removing the carbon dioxide from the water as shown below:

• CO2  + Ca(OH)2 <===> CaCO3 + H2O

CO2 above 5 to 15 mg/l in raw water can cause three operating problems: • It increases the acidity of the water, making it corrosive by forming a “weak”

acid, H2CO3. • It tends to keep iron in solution, thus making iron removal more difficult. • It reacts with lime added to soften water, causing an increase in the amount of

lime needed for the softening reaction.

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HYDROGEN SULFIDE• A poisonous gas (Brief exposures--less than 30 minutes in concentrations as low as 0.03

percent by volume in the air) - rotten-egg odor

• H2S occurs mainly in groundwater supplies.

• It may be caused by the action of iron or sulphur reducing bacteria in the well.

• Occasional disinfection of the well can reduce the bacteria producing the H2S

• H2S in a water supply will disagreeably alter the taste of coffee, tea, and ice.

• H2S is corrosive to piping, tanks, water heaters, and copper alloys that it contacts.

Operational problems:

• Disinfection of the water can become less effective because of the chlorine demand exerted by the hydrogen sulfide.

– H2S + Cl2 + O2- → S + H2O + 2Cl-

– H2S + 4Cl2 + 4 H2O → H2SO4 + 8 HCl

• There could be corrosion of the piping systems and the water tanks.

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H2S Removal by Aeration method

• Hydrogen sulfide is physically removed by agitating يج�الته)) the water via bubbling or cascading التعاقب)) and then separating or "stripping التعرية))" the hydrogen sulfide in a container. H2S + O2 = water (H2O) + elemental sulfur

• Aeration is most effective when hydrogen H2S are lower than 2.0 mg/l.

• At higher concentrations, this method may not remove all of the offensive odor unless the air is used to oxidize hydrogen sulfide chemically into solid sulfur, which is then filtered.

• In a typical aeration system, ambient air is introduced into the water using an air compressor or blower.

• Well-designed aeration tanks maintain a pocket of air in the upper third or upper half of the tank.

• If the tank does not maintain an air pocket, sulfur odor may return.

• When sulfur levels exceed 10 mg/l, larger aeration tanks, repressurization systems, chlorination systems, or a combination may be needed.

الخدمات المتعددة
تهيج)
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METHANE• Methane gas can be found in groundwater.

• It may be formed by the decomposition of organic matter.

• It can be found in water from aquifers that are near natural-gas deposits.

• Methane is a colorless gas that is highly flammable and explosive.

• When mixed with water, methane will make the water taste like garlic.

• The gas is only slightly soluble in water and therefore is easily removed by

the aeration of the water.

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IRON AND MANGANESE• Iron and manganese minerals are found in soil and rock. • Iron and manganese can dissolve into groundwater as it percolates through the

soil and rock. • Iron in the ferrous form and manganese in the manganous form are

objectionable. • more than 0.3 mg/l of iron will cause yellow to reddish-brown stains of

plumbing fixtures or almost anything that it contacts. • If the concentration exceeds 1 mg/l, the taste of the water will be metallic and

the water may be turbid. • Manganese even at levels as low as 0.1 mg/l, will cause blackish staining of

fixtures and anything else it contacts. • If the water contains both iron and manganese, staining could vary from dark

brown to black.• Consumer complaints are laundry is stained and that the water is red or dirty. • iron and manganese should not be aerated unless filtration is provided.

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Iron and Manganese Removal

• Iron and manganese in well waters occur as soluble ferrous and manganous bicarbonates.

• In the aeration process, the water is saturated with oxygen to promote the following reactions:

• The oxidation products, ferric hydroxide and manganese dioxide, are insoluble.

• After aeration, they are removed by clarification or filtration.

• Occasionally, strong chemical oxidants such as chlorine (Cl2 or potassium permanganate (KMnO4 (may be used following aeration to ensure complete oxidation.

4Fe(HCO3)2 + O2 + 2H2O = 4Fe(OH)3- + 8CO2

ferrous bicarbonate oxygen water ferric hydroxide carbon dioxide 2Mn(HCO3)2 + O2 = 2MnO2 + 4CO2 - + 2H2O

manganese bicarbonate

oxygen manganese dioxide

carbon dioxide

water

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TASTE AND ODOR & DISSOLVED OXYGEN TASTE AND ODOR • Aeration is effective in removing tastes and odors that are caused by volatile

materials • Volatile materials (e.g Methane and hydrogen sulfide) have low boiling point

and will vaporize very easily. • Many taste and odor problems in surface water could be caused by oils and by-

products that algae produce. • Since oils are much less volatile than gases, aeration is only partially effective. DISSOLVED OXYGEN • Oxygen is injected into water through aeration to remove the flat taste. • The amount of oxygen that the water can hold is dependent on the temp. • The colder the water, the more oxygen the water can hold. • Water that contains excessive amounts of oxygen can become very corrosive.• Excessive oxygen can cause air binding of filters.

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TYPES OF AERATORS

Aerators fall into two general categories.

• introduce air into the water or water into the air.

• The water-to-air method is designed to produce small drops of

water that fall through the air

• The air-to-water method creates small bubbles of air that are

injected into the water stream.

• All aerators are designed to create a greater amount of contact

between the air and water to enhance the transfer of the gases.

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WATER INTO AIR Cascade Aerators • consists of a series of steps that the water

flows over.• aeration is accomplished in the splash

zones. • The aeration action is similar to a

flowing stream. • Splash areas are created by placing

blocks across the incline.• Cascade aerators used to oxidize iron

and to partially reduce dissolved gases. • the oldest and most common type of

aerators.Cone Aerators • are used primarily to oxidize iron and

manganese prior to filtration.• the water pumped to the top of the cones

and then allowed to cascade down through the aerator.

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WATER INTO AIRSlat (الشريحة) and Coke Aerators • similar to the cascade and cone types. • They usually consist of three-to-five stacked trays, which have spaced

wooden slats in them. • The trays are filled with fist-sized (( اليد قبضة ,pieces of coke, rock حجم

ceramic balls, limestone, or other materials. • The primary purpose of the materials is provide additional surface contact

area between the air and water.

Spray Aerators spray aeration is successful in oxidizing iron and manganese and is successful in increasing the dissolved oxygen of the water.

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WATER INTO AIRDraft Aerators: the air is

induced by a blower.Types:• external blowers mounted at

the bottom of the tower to induce air from the bottom of the tower.

• Water is pumped to the top and allowed to cascade down through the rising air.

• The other, an induced-draft aerator, has a top-mounted blower forcing air from bottom vents up through the unit to the top.

• Both types are effective in oxidizing iron and manganese before filtration.

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AIR INTO WATER• These are not common types used in water treatment. • The air is injected into the water through a series of nozzles submerged in

the water.• It is more commonly used in wastewater treatment for the aeration of

activated sludge.

Air-into-water• Diffuser• Draft tube

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AIR INTO WATERPressure Aerators • Uses a pressure vessel.• The water to be treated is sprayed into the

high-pressure air, allowing the water to quickly pick up dissolved oxygen.

• A pressure aerator commonly used in pressure filtration is a porous stone installed in a pipeline before filtration.

• The air is injected into the stone and allowed to stream into the water as a fine bubble, causing the iron to be readily oxidized.

• The higher the pressure, the more readily the transfer of the oxygen to the water.

• more O2 is available, more readily the oxidation of the Fe or Mn.

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AIR STRIPPING• can be quite effective in removing volatile

organic chemicals (VOCs) from water.

• A major concern is that VOCs may be

carcinogens.

• Air stripping capable of removing up to 90

percent of the most highly volatile VOCs.

• water flow over cascade aerators or in

specially designed air-stripping towers.

• water is allowed to flow down over a

support medium or packing contained in the

tower, while air is being pumped into the

bottom of the tower.

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VOC removable by air stripping

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AIR STRIPPING

Diffused aeration air stripper

Low profile sieve tray air stripper

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COMMON OPERATING PROBLEMS• Aeration raises the dissolved oxygen content of the water.

• If too much oxygen is injected into the water, the water becomes supersaturated

• Supersaturation may cause corrosion whenever water and oxygen come into contact with metallic surfaces

• False Clogging Of Filters-Air Binding (the spaces between the filter media particles begin to fill with bubbles).

• Air binding causes the filter to plugged and in need of backwashing.

• Slow removal of the hydrogen sulfide from the towers, algae production, clogged filters, and overuse of energy.

• Hydrogen sulfide is most efficiently removed, not by oxidation, but by the physical scrubbing action of aeration.

• This removal is dependent on the pH of the water. At a pH of 6 or less, the hydrogen sulfide is easily removed.

• high pH, the hydrogen sulfide will ionize, preventing removal by aeration.

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OPERATIONAL TESTING

Three basic control tests are involved in the operation of the aeration process:

• Dissolved oxygen (to estimate whether the process is over or under aerated)

• pH (give an indication of the amount of carbon dioxide removal. pH increases

as the carbon dioxide is removed. pH can also be used to monitor the effective

range for hydrogen sulfide, iron, and manganese removal)

• Temperature (is important as the saturation point of oxygen increases as the

temperature decreases.

• As water temperature drops, the operator must adjust the aeration process to

maintain the correct DO level.

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Dissolved oxygen

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Adsorption

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Adsorption• Adsorption: is the adhesion of atoms, ions, biomolecules or molecules of gas,

liquid, or dissolved solids to surface

• This process creates a film of the adsorbate (the molecules or atoms being accumulated) on the surface of the adsorbent.

• It differs from absorption, in which a fluid permeates or is dissolved by a liquid or solid. 

• The term sorption encompasses both processes, while desorption is the reverse of adsorption. It is a surface phenomenon.

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Types of Adsorption

• Types of Adsorption: Depending on the nature of the forces between adsorbate & adsorbent, Adsorption is of two types.

• 1. Physical Adsorption or physisorption: Forces of attraction between adsorbate and adsorbent are Van der Waal’s forces, they are weak.

• 2. Chemical Adsorption or chemisorption: Forces of attraction between adsorbate and adsorbent are as strong as chemical bonds, they are strong. This type of adsorption cannot be easily reversed.

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Adsorption

• Adsorption, is sorption processes in which certain adsorbates are selectively

transferred from the fluid phase to the surface of insoluble, rigid particles

suspended in a vessel or packed in a column.

• Adsorption systems treat water by adding a substance, such as activated carbon

or alumina, to the water supply.

• Adsorbents attract contaminants by chemical and physical processes that cause

them to ‘stick’ to their surfaces for later disposal.

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Adsorption

• the most commonly-used adsorbent is activated carbon

• Powdered activated carbon is often used when temporary quality problems

arise; it can simply be added to the water and discarded with waste sludge.

• Granular activated carbon is often arranged in a bed through which source

water is slowly passed or percolated.

• Activated alumina treatment  is used to attract and   remove contaminants,

like arsenic and fluoride, which have negatively charged ions.

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Physical adsorption (Physisorption)• In physical adsorption, the forces of attraction between the molecules of the

adsorbate and the adsorbent are of the weak van der Waals' type.

• van der Waals forces is the sum of the attractive or repulsive forces between molecules (or between parts of the same molecule) other than those due to covalent bonds or to the electrostatic interaction of ions with one another or with neutral molecules

• A van der Waals bonding already illustrates the difference in bonding between two molecules and one molecule with a solid.

• A van der Waals bonding between two molecules can be described as the interaction between two point dipoles.

• Since the forces of attraction are weak, the process of physisorption can be easily reversed by heating or decreasing the pressure of the adsorbate (as in the case of gases). The attraction is not fixed to a specific site and the adsorbate is relatively free to move on the surface

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Chemical adsorption (Chemisorption)• In chemisorption, the forces of attraction between the adsorbate and the adsorbent

are very strong;

• the molecules of adsorbate form chemical bonds with the molecules of the adsorbent present in the surface.

• Adsorption is generally accompanied by release of energy, that is, most adsorption processes are exothermic in nature.

• Enthalpy is a measure of the total energy of a thermodynamic system.

• It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.

• Enthalpy of Physisorption, which is the enthalpy change for the adsorption of one mole of an adsorbate on an adsorbent surface, is in the range of 20 kJ/mole to 40kJ/mole

• Enthalpy values for chemisorption, are an order of magnitude high, that is, 200 kJ/mole to 400 kJ/mole

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Comparison between Physisorption and Chemisorption

Physisorption Chemisorption

Forces of attraction are vander Waals’ forces Forces of attraction are chemical bond forces

Low enthalpy of adsorption (20 - 40 k.J/mole)

High enthalpy of adsorption (200 - 400 k.J/mole)

This process is observed under conditions of low temperature

This process takes place at high temperatures

It is not specific It is highly specific

Multi-molecular layers may be formed Generally, monomolecular layer is formed

This process is reversible This process is irreversible

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Adsorption Equillibria

• Adsorption equilibrium is a dynamic concept achieved when the rate at which molecules adsorb onto a surface is equal to the rate at which they desorb.

• Most of the adsorption theories have been developed for gas solid systems because the gaseous state is better understood than the liquid.

• Till now the statistical theories developed for gas – solid systems were applied for liquid solid systems with little confidence for designing of the equipment.

• The most commonly used equilibrium models to understand the adsorption systems was Freundlich and Langmuir isotherm equation

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ADSORPTION EQUILIBRIA / Isotherm• Adsorption is usually described through isotherms

• If the adsorbent and adsorbate are contacted long

enough an equilibrium will be established between

the amount of adsorbate adsorbed and the amount of

adsorbate in solution. The equilibrium relationship

is described by isotherms.

• isotherms is the amount of adsorbate on the adsorbent

as a function of its pressure (if gas) or concentration

(if liquid) at constant temperature.

• The quantity adsorbed is nearly always normalized by

the mass of the adsorbent to allow comparison of

different materials.

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Some general isotherms are shown in the figure below

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Isotherm models• The figures below show that there are four common models for isotherms.

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Freundlich: K Freundlich isotherm coefficient and n • The amount of adsorbate adsorbed is a function of the liquid-

phase concentration and called adsorption equilibrium isotherm.• Different functions can be used to describe the adsorption

equilibrium. • Freundlich isotherm data can be used. • This adsorption isotherm empirical equation is.

• in which:• qmax = loading capacity (g/kg)

• cs = equilibrium concentration (g/m3)

• x = adsorbed amount of compound (g)• m = mass of activated carbon (kg)• K = Freundlich constant ((g/kg).(m3/g)n)• n = Freundlich constant (-)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0Concentration

Mas

s So

rptio

n n = 0.5n = 0.2

n = 2n = 1

n = 5

n = 0.1

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Freundlich: K Freundlich isotherm coefficient and n

• The Freundlich constants K and n are influenced by water temperature, pH, type of carbon and the concentration of other organic compounds.

• Using laboratory experiments the Freundlich constants can be determined for a single substance with a specific type of activated carbon.

• Taking logs and rearranging:

   log q = log K + n log Cs.

• When these graphs are plotted on a logarithmic scale the Freundlich constant K can be determined from the intersection of the graph with the y-axis.

• The slope of the line is equal to the Freundlich constant n.

• The higher the K-value, the better the adsorption.

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Freundlich: K Freundlich isotherm coefficient and n

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Langmuir• In 1916, Irving Langmuir published a new model isotherm for gases

adsorbed to solids. It is a semi-empirical isotherm derived from a proposed kinetic mechanism.

It is based on four assumptions:– The surface of the adsorbent is uniform, that is, all the adsorption sites are

equivalent.

– Adsorbed molecules do not interact.

– All adsorption occurs through the same mechanism.

– At the maximum adsorption, only a monolayer is formed: molecules of adsorbate do not deposit on other, already adsorbed, molecules of adsorbate, only on the free surface of the adsorbent.

• These four assumptions are seldom all true

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Langmuir• Langmuir suggested that adsorption takes place through this mechanism: 

where Ag is a gas molecule and S is an adsorption site.

• Letting xo represent the total concentration of available sites on a given amount of fresh solid substrate, we can define a fractional coverage (θ) as

                                                    (1)where x is the concentration of occupied sites.  • The rate of adsorption va will be proportional to the concentration of gas or liquid

(c) above the surface and the fraction of the surface that is not covered (1- θ), yielding a rate equation

      (2)

where ka is the rate constant for adsorption. 

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Langmuir• The rate of desorption is simply proportional to the fraction of the surface that is

already occupied, so the rate equation is

•                                                 (3)

• and kd is the rate constant for desorption.  Setting equations (2) and (3) equal yields an equilibrium statement that can be written as

                                 (4)

The ratio of rate constants in equation (4) is equal to an equilibrium constant

(K = ka/kd).

• Upon substitution and further rearrangement, the fractional coverage is given by

                                 (5)

 

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Langmuir• Equation (5) is plotted for an arbitrary value of K, (is often called a Langmuir

absorption isotherm) • The surface sites becomes saturated as the concentration rises.• The magnitude of K quantifies the relative affinity that a given solute has for

surface adsorption. • Like all equilibrium constants, K is temperature dependent. 

fractional coverage

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BET Isotherm• Often molecules do form multilayers, that is, some are adsorbed on already

adsorbed molecules and the Langmuir isotherm is not valid.

• In 1938 Stephen Brunauer, Paul Emmett, and Edward Teller developed a

model isotherm that takes that possibility into account. Their theory is

called BET theory, after the initials in their last names.

• They modified Langmuir's mechanism as follows:• A(g) + S   AS⇌• A(g) + AS   A2S⇌• A(g) + A2S   A3S and so on⇌

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Characteristics and general requirements• Adsorbents are used in the form of spherical pellets, rods, moldings صب)), or

monoliths (أعمدة) with hydrodynamic diameters between 0.5 and 10 mm.

• They must have high abrasion resistance للحك high thermal stability ,((مقاومةand small pore diameters, which results in higher exposed surface area and hence high surface capacity for adsorption.

• The adsorbents must also have a distinct pore structure which enables fast transport of the gaseous vapors.

Most industrial adsorbents fall into one of three classes:• Oxygen-containing compounds – Are typically hydrophilic and polar, including

materials such as silica gel and zeolites.

• Carbon-based compounds – Are typically hydrophobic and non-polar, including materials such as activated carbon and graphite.

• Polymer-based compounds - Are polar or non-polar functional groups in a porous polymer matrix.

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Silica gel• a chemically inert, nontoxic, polar and dimensionally stable (< 400 °C)

amorphous form of SiO2. • Prepared by the reaction between sodium silicate and acetic acid, which is

followed by a series of after-treatment processes such as aging, pickling.• After treatment methods results in various pore size distributions.• Silica is used for drying of process air (e.g. oxygen, natural gas) and adsorption

of heavy (polar) hydrocarbons from natural gas• silica gel is a solid • it has an average pore size of 2.4 nanometers and has a strong affinity for water

molecules .

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Zeolites• Natural or synthetic crystalline aluminosilicates.• Havin a repeating pore network and release water at high temperature.• Zeolites are polar in nature. They are manufactured by hydrothermal synthesis of sodium

aluminosilicate or another silica source in an autoclave followed by ion exchange with certain cations (Na+, Li+, Ca2+, K+, NH4+).

• The channel diameter of zeolite cages usually ranges from 2 to 9 Å. • The ion exchange process is followed by drying of the crystals, which can be pelletized

with a binder to form macroporous pellets.• Zeolites are applied in drying of process air, CO2 removal from natural gas, CO removal

from reforming gas, air separation…• Non-polar (siliceous) zeolites are synthesized from aluminum-free silica sources or by

dealumination of aluminum-containing zeolites. • The dealumination process is done by treating the zeolite with steam at elevated

temperatures, typically greater than 500 °C. • This high temperature heat treatment breaks the aluminum-oxygen bonds and the

aluminum atom is expelled from the zeolite framework.

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Activated carbon• non-polar and cheap, highly porous, amorphous solid consisting of

microcrystallites with a graphite lattice

• Usually prepared in small pellets or a powder.

• Its main drawbacks (it is reacts with oxygen at moderate temp. (over 300 °C).

• manufactured from carbonaceous material, including coal, peat, wood, or nutshells (e.g., coconut).

• The manufacturing process consists of two phases, carbonization and activation.

• The carbonization process includes drying and then heating to separate by-products, including tars and other hydrocarbons from the raw material, as well as to drive off any gases generated.

• The process is completed by heating the material over 400 °C in an oxygen-free atmosphere that cannot support combustion.

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Activated carbon• The carbonized particles are then "activated" by exposing them to an

oxidizing agent, usually steam or carbon dioxide at high temperature.• This agent burns off the pore blocking structures created during the

carbonization phase and so, they develop a porous, three-dimensional graphite lattice structure.

• The size of the pores developed during activation is a function of the time that they spend in this stage.

• Longer exposure times result in larger pore sizes. • The most popular aqueous phase carbons are bituminous based جيرية قاعدة

because of their hardness, abrasion resistance, pore size distribution, low cost• Activated carbon is used for adsorption of organic substances and non-polar

adsorbates and it is also usually used for waste gas (and waste water) treatment.• It is the most widely used adsorbent??? most of its chemical (eg. surface

groups) and physical properties (eg. pore size distribution and surface area) can be tuned according to what is needed.

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Activated carbon

Activated Carbon Granular Activated Carbon Powder

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Activated carbon

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Characteristics of Different Adsorbents

Table. Characteristics of Different Adsorbents

TypeCharacteristicsUseDisadvantages

Activated Carbon

Hydrophobic, favors organics over water

Removal of organic pollutants

Difficult to regenerate

ZeolitesHydrophillic, polar, regular channels

Air separation, dehydration

Low total capacity

Silica gelHigh capacity, hydrophilic

Drying gas streamsTrace removal not effective

Activated alumina

High capacity, hydrophilic

Drying gas streamsTrace removal not effective

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Factors Affecting the Rate of Adsorption1. Surface area of the adsorbent:• The rate of adsorption increases with increase in surface area of the adsorbent• Rate of adsorption ά 1/ Diameter of adsorbent for powdered activated carbon 2. Nature of solute (adsorbate):• Solubility of solute: Adsorption ά (1/Solubility of solute in solvent)• Chain length of molecules: Adsorption ά chain length of molecule• Molecular size of solute: Increase in molecular size of solute favors adsorption• Geometry of the molecule: Branched- lesser tendency to get removed, Coiled-

more easily removed because easily entrapped in the interstices.• Degree of ionization: Adsorption ά (1/ Dissociation constant) this is the reason

why salts are not much removed by activated carbon3. Surface tension of the solvent:• Those substances which lower the surface tension of solvent in which they are

dissolved become concentrated in surface layer (eg: organic substances), while those substances which raise the surface tension are less concentrated in the surface than in the bulk of solution. (eg.inorganic ions)

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Ion Exchange• The ion exchange process percolates water through bead-like spherical resin

materials (ion-exchange resins).• Ions in the water are exchanged for other ions fixed to the beads. • The two most common ion-exchange methods are softening and deionization. • Ion exchange materials are insoluble substances containing loosely held ions

which are able to be exchanged with other ions in solutions which come in contact with them.

• Exchanges take place without any physical alteration to the ion exchange material.

• Ion exchangers are insoluble acids or bases which have salts which are also insoluble, and this enables them to exchange either positively charged ions (cation exchangers) or negatively charged ones (anion exchangers).

• Many natural substances such as proteins, cellulose, living cells and soil particles exhibit ion exchange properties which play an important role in the way the function in nature.

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Ion Exchange• Ion exchange uses a resin that removes charged inorganic contaminants like

arsenic, chromium, nitrate, radium, uranium, and fluoride.

• It works best with particle-free water and can be scaled to fit any size treatment facility.

• Ion exchange is most often used to remove hardness (cation resin) or nitrate (anion resin).

• In both instances, it can be regenerated with salt water.

• The use of ion exchange to remove radionuclides (an atom with an unstable nucleus) is complicated by the fact that these materials accumulate in the resin and occur at high levels in the regenerant, greatly complicating operations.

• Activated carbon is generally preferred for removing organic contaminants, whereas ion exchanges often best for removing inorganic soluble molecules

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Types of Ion Exchange Resin / Acid Cation Resins

Strong Acid Cation Resins• Behaving like strong acid• highly ionized in both the acid (R-SO3H) and salt (R-SO3Na) form. • Metal salt can be converted to the corresponding acid.

• They can be used for entire range of pH and are utilized for water softening (calcium and magnesium removal).

• The regeneration takes place by contact with a strong acid solution (hydrochloric or sulphuric)

Weak Acid Cation Resins• Carboxylic acid (COOH) acts as the ionizable group in weak acid cation resins.• They show more affinity for hydrogen ions. This results in the regeneration of

the hydrogen form with less acid than is required for strong acid resins. 

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Types of Ion Exchange Resin / Base Anion Resins

Strong Base Anion Resins• They are suitable for entire pH range. • They deionize water in hydroxide (OH) form. • Acidic nature of the water can be removed and pure water can be obtained. • The reaction can be put forward as:

• for which sodium hydroxide is used as the regenerantWeak Base Anion Resins• In weak base resins, intensity of ionization is affected by pH. • They are incapable to split salts but can absorb acids

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Types of Ion Exchange Resin

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How ion exchange resins work

• The resins are prepared as spherical beads 0.5خرزة to 1.0 mm in diameter.

• These appear solid even under the microscope, but on a molecular scale the structure is quite open.

• This means that a solution passed down a resin bed can flow through the cross-linked polymer, bringing it into intimate contact with the exchange sites.

• The affinity of sulphonic acid resins for cations varies with the ionic size and charge of the cation.

• Generally the affinity is greatest for large ions with high valency.

• For dilute Solutions the order of affinity for some common cations is approximately:

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The general chemical reaction of ion exchange

Cation exchange: nRA + Bn+ = RnBn+ + nA+

Anion exchange: nRA + Bn- = RnBn- + nA-

Where:

R : ionized resin molecule with an attached functional group;

A+ / A- : exchangeable ion;

Bn+ / Bn- : cation/anion dissolved in the liquid;

n+ /n- : electrical charge of the ion B.

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The general chemical reaction of ion exchange

• Suppose a resin has greater affinity for ion B than for ion A. If the resin contains ion A and ion B is dissolved in the water passing through it, then the following exchange takes place, the reaction proceeding to the right:

• When the resin exchange capacity nears exhaustion, it will be in the BR form.

• A mass action relationship applies :

• Q is the equilibrium quotient, and is a constant specific for the pair of ions and type of resin.

• This expression indicates that if a concentrated solution containing ion A is now passed through the exhausted bed, the resin will regenerate into the AR form ready for re-use, whilst ion B will be eluted into the water.

• All large scale applications for ion exchange resins involved such exhaustion and regeneration cycles.

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Uses• To remove unwanted ions from a solution passed through it (heavy metals

from metal wastes - salts from fruit juices)

• To accumulate a valuable mineral from the water which can later be recovered from the resin.

• Strong cation resins in the hydrogen form are used for the hydrolysis of starch and sucrose.

• Used in the laboratory to remove interfering ions during analysis or to accumulate trace quantities of ions from dilute solutions.

• A cation resin in the hydrogen form can be used to determine the total concentration of ions in a mixture of salts. The sample passing through a column is converted to the equivalent quantity of acid and the amount readily found by titration.

• Earliest applications of ion exchange was the separation of rare earth elements (Promethium (element 61) and five new elements in the actinide series).

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WATER TREATMENTThe two major types of treatment applied to water

are:• Water softening - the replacement of ’hard’ ions

such as Ca2+ and Mg2+ by Na+

• Demineralisation - the complete removal of dissolved minerals.

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Water softening• Softening is used primarily as a pretreatment method to reduce water hardness

prior to reverse osmosis (RO) processing.

• The softeners contain beads that exchange two sodium ions for every calcium or magnesium ion removed from the "softened" water.

• In water softening a cation resin in the sodium form is used to remove hard metal ions (calcium and magnesium) from the water along with troublesome traces of iron and manganese

• These ions are replaced by an equivalent quantity of sodium, so that the total dissolved solids content of the water remains unchanged as does the pH and anionic content.

• At regular time intervals the resin is cleaned. This involves passing influent water back up through the resin. In water softening the regenerant is a strong solution of sodium chloride.

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Example of Water softeningA) Sodium cation exchange:• Ca+2 + 2Na.R = Ca.R + 2Na+

• Mg+2 + 2Na.R = Mg.R + 2Na+

Regeneration: using strong brine (NaCl)• Mg. R + 2NaCl = 2Na.R + MgCl2

• Ca. R + 2NaCl = 2Na.R + CaCl2

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Example of Water softening

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Demineralisation = deionization • Complete deionization can be achieved by using two resins. • The water is first passed through a bed of cation exchange resin in hydrogen

ion form• During service, cations in the water are taken up by the resin while hydrogen

ions are released. • Thus the effluent consists of a very weak mixture of acids. • Then water passes through second anion exchange resin in the hydroxide form.• Here the anions are exchanged for hydroxide ions, which react with the

hydrogen ions to form water. • twin bed units will reduce the total solids content to approximately 1-2 mg L-1.• it is usual to pass water leaving the cation unit through a degassing tower.• degassing tower removes the carbonic acid produced from carbon dioxide and

bicarbonate in the feed water and reduces the load on the anion unit.• Without degassing the carbonic acid would be taken up by the anion bed after

conversion to carbonate.

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Mixed resin• Mixed resin produces water with much lower levels of dissolved material than

can be achieved by distillation. • In laboratories, mixed resin is often used in disposable cartridges. These are

only used once, but larger mixed resin units can be regenerated. • After exhaustion the bed is subjected to an up flow of water.• Anionic resin beads are less dense than the cationic ones and they rise to the top

so that the bed is separated into two layers of resin. • Each is regenerated in situ with the appropriate regenerant then rinsed with

clean water.

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Cation and anion exchange and regenerationi) Hydrogen cation exchange:

M+a + aH.R M.Ra + aH+

Examples: Ca+2 + 2H.R Ca.R2 + 2H+

Na+ + H.R Na.R + H+

Regeneration: using strong acidCa. R + H2SO4 2H.R + CaSO4

2Na. R + H2SO4 2H.R + Na2SO4

ii) Hydroxyl anion exchange:A-b + bR.OH Rb.A + bOH

Examples:NO3

-+ R.OH R.NO3- + OH

CO3-2 + 2 R.OH R2.CO3

-2 + 2OHRegeneration: using strong base (caustic soda)

R.NO3- + NaOH R.OH + NaNO3

R2.CO3-2 + 2NaOH 2R.OH + Na2CO3

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Typical ion exchange installation

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The ion exchange system in water treatmenta) Configuration of the ion exchanger• Cylindrical steel tank (Diameter 1-2 m - Height 3-4 m )• ion exchange bed occupies 1-3 meters of the tank height• The water inters from the top (downflow) at the rate of 0.5 to 7 L/s.m2 .• When breakthrough is reached the tank is taken off-line and backwashed by

applying water form the bottom upwards to remove any suspended solids.• After backwashing the regeneration solution is pumped from the bottom up

wards at the rate of 0.7 to 1.5 L/s.m2. • The same influent distributor is used to drain the upflow backwash water and

the regeneration solution ( brine, acid or base). • At the end of the regeneration the bed is washed with clean water to remove

the residual of the regeneration solution• An under drain piping system is installed at the bottom to collect the treated

water, and used to pump the upflow backwash water and the regeneration solution.

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The ion exchange system in water treatmentb) Pretreatment:• The influent to the ion exchanger should be filtered to remove turbidity.

• Dissolved Organic matter should be removed by GAC before the IE because the organic may coat the resin and reduce its exchange capacity.

• The IE is efficient for TDS less than 1000 mg/L.

c) Sizing ion exchanger : depends on the following factors:

• i) Contact time

• ii) Hydraulic loading rate

• iii) resin depth

• iv) number of columns.

d) Multiple tanks Operation:• Ion exchange tanks can be operated in parallel or in series.

• Multiple units permit one or more units to remain in operation while one unit is taken out of service for backwashing and generation or maintenance.

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Exchange capacity of ion exchange resins

• The total solid phase concentration “q0” is termed ion exchange capacity.

• For cation exchange resins, “q0” is in the range of 200 to 500 meq/100 mg of resin.

• During the exchange, the resin should be electrically neutral thus the all the exchange sites should be occupied either by the original ion (such as Na+) or by the replacing ions ( such as Ca+2 and Mg+2) and the ion exchange occupancy should be equal to “q0” at any time.

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Exchange capacity of ion exchange resins• the Thomas kinetic equation is used for ion exchange columns.

• C = effluent concentrat ion of the ions, mg/l or meq/l• Co = influent concentrat ion of the ions, mg/l or meq/l• k1= rate constant, L/d . eq• q0 maximum solid phase concentrat ion of exchange solute, eq/kg of resin• M = mass of resin, kg• V throughput volume, L ( اإلنتاجية (الطاقة• Q flow rate, L/d

• To apply this equation it is necessary to perform a laboratory column test or pilot scale column to obtain the breakthrough curve.

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Ion Exchange process analysis in the Fixed beda) Mass transfer inside the Ion exchange bed:• When the polluted water is pumped on the ion exchange bed, the pollutant ions

replace the exchangeable ions in the resin.• The area of the ion exchange bed in which the exchange occurs is called the

mass transfer zone (MTZ) • No further adsorption occurs below the MTZ and the water leaving the MTZ

zone contains the minimum concentration value of the pollutant that the bed can produce.

• With time a zone of saturation is created above the MTZ in which the resin has reached its maximum exchange capacity and no further replacement occurs.

• The equilibrium concentration Ce of the pollutant in water in this zone is the same as C0.

• The Zone below the MTZ essentially clean zone and no adsorbed material on it.• With time the saturation zone depth increases and the MTZ is pushed down

until we reach to a point where the clean zone disappears and breakthrough occurs.

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Breakthrough curve• Breakthrough is said to have occurred when the effluent concentration reaches to 5% of

the influent concentration (i.e, Cb = 0.05C0).• After additional time the MTZ start to decrease until it disappears and the bed is called

exhausted. • Exhaustion of the bed is assumed to have occurred when the effluent Concentration is

equal to 95% of the influent concentration (i.e, C = 0.95C0)• The area above the breakthrough curve is equal to the mass of the pollutant adsorbed in

the column

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Calculation of the length of the MTZ ***

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Detection of resin exhaustion• A resin is considered to be exhausted when the ions in the resin have

mostly been replaced by the ions that are being removed from the solution.

• Exhaustion of demineraliser is usually detected by an electrical conductivity cell installed at the outlet.

• When the conductivity rises to indicate ionic breakthrough, a regeneration cycle can be initiated automatically.

• With small units it is possible to incorporate a pH indicator on the anion resin of a mixed bed cartridge.

• Exhaustion can be followed down the side of a transparent cartridge as the alkaline anion resin is converted to the neutralised salt form.

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Advantages and disadvantages in the use of Ion-Exchange Resins

The advantages of ion exchange processes are:• very low running costs. • Very little energy is required, • The regenerant chemicals are cheap and if well maintained resin

beds can last for many years before replacement is needed.

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Disadvantages in the use of Ion-Exchange Resins

Calcium sulphate fouling• Using Sulphuric acid as cation resin regenerant will react with calcium in water

forming calcium sulphate precipitates. • This fouls the resin and blocks drain pipes with a build up of scale

(hydrochloric acid must be substituted).Iron fouling• Aeration allows oxidation of Fe2+ to Fe3+ and consequent precipitation of ferric

hydroxide which clogs resin beads and prevents ion exchange. Iron fouling is the commonest cause of softener failure.

Adsorption of organic matter• the presence of dissolved organic material can become irreversibly adsorbed

within the anion beads, reducing their exchange capacity.• Removal of organics prior to demineralisation is achieved by flocculation with

alum or ferric salts followed by filtration.

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Disadvantages in the use of Ion-Exchange ResinsOrganic contamination from the resin• The resins themselves can be a source of non-ionized organic contamination.

New commercial grade resin often contains organics remaining after manufacture. when removal is needed, the demineralised water can be passed through an ultra filtration membrane.

Bacterial contamination• Resin beds do not act as filters for the removal of bacteria.• Resin beds can generate a culture media for continued growth. • Resins beds can be decontaminated with disinfectants such as formaldehyde• heat or oxidising disinfectants as chlorine must not be used as these damage resins.Chlorine contamination• chlorine damages resins. It is customary to treat such feeds by passing them

through activated carbon which removes chlorine very efficiently.ENVIRONMENTAL IMPLICATIONS• The waste water for disposal after regeneration contains all the minerals removed

from the water plus salt from the spent regenerants.• volume of it is equivalent to 1-5% of the treated water throughput.

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Coagulation and FlocculationCoagulation refers to all the reactions and mechanisms that result in particle

aggregation in the water being treated, including in situ coagulant formation

(where applicable), particle destabilization, and physical interparticle

contacts

• Coagulant formation, particle destabilization, typically occur during and

immediately after chemical dispersal in rapid mixing;

• inter particle collisions that cause aggregate (floc) formation begin during rapid

mixing but usually occur predominantly in the flocculation process.

• The physical process of producing inter-particle contacts is termed

flocculation.

• Flocculation defined as the uses gentle stirring to bring suspended particles

together so they will form larger more settleable clumps (groups) called floc.

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A common classification of particles• Molecules sizes smaller than 1 nm• Colloids generally with

dimensions between 1 nm - 1 μm• Suspended matter having sizes

larger than 1 μm.• Colloids: humic acids, proteins,

colloidal clay, silica and viruses.• Suspended matter: Bacteria,

algae, silt, sand and organic debris.• Suspended matter-when it is larger

than 5-10 μm can be removed quite easily by filtration or sedimentation and filtration.

• The removal of colloids is possible by slow filtration in cases the water is not strongly polluted.

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A common classification of particles

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STABILITY OF PARTICLE SUSPENSIONS• Coagulation process is used to increase the rate or kinetics of particle

aggregation and floc formation• The objective is to transform a stable suspension [i.e., one that is resistant to

aggregation (or attachment to a filter grain)] into an unstable one.• there are forces that tend to pull the interacting surfaces together• The most important attractive force is called the London–van der Waals force.• It arises from spontaneous electrical and magnetic polarizations that create a

fluctuating electromagnetic field within the particles and in the space between them

• The most well-known repulsive force is caused by the interaction of the electrical double layers of the surfaces (“electrostatic” stabilization).

• As particles approach one another on a collision course, the fluid between them must move out of the way.

• The repulsive force caused by this displacement of fluid is called hydrodynamic retardation.

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Electrostatic Stabilization• Origins of Surface Charge. Most particles in water, mineral and organic, have

electrically charged surfaces, and the sign of the charge is usually negative.Three important processes for producing this charge are considered in the following

discussion. • First, surface groups on the solid may react with water and accept or donate

protons.

• Second, surface groups can react in water with solutes other than protons.

• Third, a surface charge may arise because of imperfections التناقص within the structure of the particle; this is called isomorphic replacement, or substitution

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The Electrical Double Layer (EDL)• In a colloidal suspension, there can be no net imbalance in the overall electrical

charge• the primary charge on the particle must be counterbalanced in the system. • If the particle is negatively charged, excess ions of opposite charge (positive)

accumulate in this interfacial region• Ions of opposite charge accumulating in the interfacial region, together with the

primary charge, form an electrical double layer.The diffuse layer results from:

– electrostatic attraction of ions of opposite charge to the particle– electrostatic repulsion of ions of the same charge as the particle, and– thermal or molecular diffusion that acts against the concentration gradients

produced by the electrostatic effects• Because of the primary charge, an electrostatic potential (voltage) exists

between the surface of the particle and the bulk of the solution • This electric potential can be pictured as the electric pressure that must be

applied to bring a unit charge having the same sign as the primary charge from the bulk of the solution to a given distance from the particle surface

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The zeta potential

• It is a scientific term for electrokinetic potential in colloidal systems (ζ-potential)

• It represents the net charge between the primary charge and the counter charge in

the EDL located between the surface and the shear plane

• zeta potential is electric potential in the interfacial double layer at the location of

the slipping plane versus a point in the bulk fluid away from the interface

• zeta potential is the potential difference between the dispersion medium and the

stationary layer of fluid attached to the dispersed particle.

• A value of 25 mV (positive or negative) can be taken as the arbitrary value that

separates low-charged surfaces from highly-charged surfaces.

• The zeta potential indicates the degree of repulsion between adjacent, similarly

charged particles in a dispersion.

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The zeta potential

Zeta potential [mV] Stability behavior of the colloid from 0 to ±5, Rapid coagulation or flocculation

from ±10 to ±30 Incipient instability

from ±30 to ±40 Moderate stability from ±40 to ±60 Good stability more than ±61 Excellent stability

• For molecules and particles that are small enough, a high zeta potential will confer stability, i.e., the solution or dispersion will resist aggregation.

• When the potential is low, attraction exceeds repulsion and the dispersion will break and flocculate.

• Colloids with high zeta potential (negative or positive) are electrically stabilized while colloids with low zeta potentials tend to coagulate or flocculate 

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a net electrostatic repulsion/attraction developed between colloids as a result of EDL

The net resultant force is a result of:• Attractive potential energy (mostly

van der Waals forces).

• Repulsion potential energy

(electrostatic forces)

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Relative strength of forcesDipole-dipole interactions: electrostatic interactions of permanent dipoles in

molecules. These interactions tend to align the molecules to increase the attraction

(reducing potential energy). An example of dipole-dipole interactions can be seen in hydrogen chloride (HCl)

Hydrogen bond : is the attractive interaction between a hydrogen atom and an electronegative atom, such as nitrogen, oxygen, or fluorine. 

The hydrogen bond is a strong electrostatic dipole-dipole interaction. It is stronger than a van der Waals interaction, produces interatomic distances shorter

than sum of van der Waals radiiRelative strength of forces Bond type Dissociation energy (kcal) Covalent 400 Hydrogen bonds 12-16 Dipole-dipole 0.5 - 2 London (van der Waals) Forces <1

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Calculation of the zeta potential***The magnitude of these forces is measured by the zeta potential, which is:

where:

• Z is the zeta potential,

• q is the charge per unit area,

• d is the thickness of the effective charge layer, and

• D is the dielectric constant of the liquid.

The greater the zeta potential, the greater are the repulsion forces between the colloids and, therefore, the more stable is the colloidal suspension

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BriefThere are two major forces acting on colloids:1) electrostatic repulsion (simply, negative colloids repel مع (other negatively charged colloids يتنافر

2) intermolecular, or van der Waals, attraction• Coagulants can be used to reduce the electrostatic repulsive forces. The

electrostatic repulsion reduced by the addition of countercharged ions such as [Al3+]

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Process DescriptionPurpose to aid in the removal of nonsettleable solids from water. Coagulation is defined as: • the destabilization of colloidal solids; • the water treatment process which causes very small suspended solids to attract

one another and form larger particles. Suspended particles in water resist settling for two primary reasons: 1. Particle size; and, 2. Natural forces between particles.• Suspended particles in water normally have a negative (-) charge.• Since these particles all have the same charge, they repel each other, keeping each

other from settling. • This natural repelling force is called the zeta potential. • Coagulation neutralizes the forces (zeta potential), which cause suspended solids

in water to repel each other and resist settling. • Once the repulsive forces have been neutralized these particles can stick together

(agglomerate) when they collide. • The force which holds the floc together is called the van der Waals force.

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Flocculation• After coagulation the destabilized particles can collide, aggregate so flocs can be

formed. This step is called flocculation.

• Flocculation: The process of agglomeration of the destabilized particles to such a size that separation by sedimentation and filtration is possible.

• In flocculation one can make a distinction between peri-kinetic and ortho-kinetic flocculation.

• Brownian motion is the driving force in the agglomeration of destabilized particles up to 1μm-level peri-kinetic flocculation).

• Above ~ 1 μm the influence of Brownian motion on the collision rate of the particles can be neglected, then artificial mixing is necessary to get an efficient flocculation. That part of the flocculation process is called ortho-kinetic flocculation.

• Flocculation uses gentle stirring to cause the particles to collide so that they can stick together, for a particle (floc) large enough and heavy enough to settle

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Flocculation Practice in Water Treatment***• Coagulation and Flocculation Process Design

Where,• G = velocity gradient, s-1

• P = power input, W• V = volume of water in mixing tank, m3• μ = dynamic viscosity, Pa.s

• Viscosity is a measure of the resistance of a fluid which is being deformed. The SI physical unit of dynamic viscosity is the pascal-second (Pa·s), which is identical to N·m−2·s

• Velocity gradient velocity gradient is the change in relative velocity between parallel planes with respect to the change in perpendicular distance throughout the depth of the material. Velocity gradient has the dimensions as rate of shear, which is reciprocal seconds.

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Types of Flocculation TanksMechanical Flocculators

Paddle wheel Type (vertical and Horizontal Types)

Foil Type Mixing Blade

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Types of Flocculation Tanks• Hydraulic Flocculators• The axial flow flocculators are typically used because they impart a nearly

constant gradient in each compartment.

• Flocculators are designed to have a minimum of three compartments to provide for tapered (to make smaller gradually) mixing.

• The velocity gradient, G is tapered so that it is larger in the first compartment and less is the other compartments as the floc grows.

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Chemicals are used to neutralize the zeta potential

• These chemicals are coagulants, sometimes called primary coagulants, and

coagulant aids.

• Since most suspended particles in water carry a negative (-) charge, coagulants

consist of chemicals that provide positively (+) charged ions.

Common coagulants are:

• 1. Metal Salts

• a. Aluminum Salts (Alum (aluminum sulfate) - PACs (polyaluminum

chlorohydrate, and other variations)

• b. Iron Salts (Ferric Chloride - Ferric Sulfate - Ferrous Sulfate)

• 2. Polymers (polyelectrolytes)

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Common coagulants / Polymers• Polymers (polyelectrolytes) are extremely large molecules which produce

thousands of charged ions when dissolved in water.

1. Cationic Polyelectrolytes - Have a positive (+) charge. Used as either a primary coagulant or as a coagulant aid. Cationic polymers:

• allow reduced coagulant dose; • improve floc settling; • are less sensitive to pH; • improve flocculation of organisms such as bacteria and algae.

2. Anionic Polyelectrolytes- Have a negative (-) charge. Used primarily as a coagulant aid. Anionic polymers are used to:

• increase floc size; • improve settling; • produce a stronger floc; • They are not materially affected by pH, alkalinity, hardness or turbidity.

3. Nonionic Polyelectrolytes- Balanced or neutral charge. • Used as a primary coagulant or coagulant aid.

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Coagulant aids

Coagulant aids are chemicals which are added to water during coagulation to improve coagulation by:

• building a stronger, more settleable floc;

• overcoming slow floc formation in cold water;

• reducing the amount of coagulant required;

• reducing the amount of sludge produced.

• The key reason coagulant aids are used is to reduce the amount of alum used, which, in turn, decreases the amount of alum sludge produced.

• Alum sludge is difficult to dewater and to dispose of.

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Types of Coagulant AidsActivated Silica • increases the coagulation rate;

• reduces the amount of coagulant needed;

• widens the pH range for effective coagulation;

• strengthens floc

Weighting agents (Bentonite Clay, Powdered Limestone; Powdered Silica) provide additional particles that can enhance floc formation.

They are used to treat water that is:

• high in color; or,

• low in turbidity; or,

• low in mineral content.

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Factors Which Affect How Well a Coagulant Work

Factors Which Affect How Well a Coagulant Work(1) Mixing Conditions (2) pH (3) Alkalinity (4) Water Temperature (5) Turbidity

• If the alkalinity concentration in the water is not high enough, and effective floc will not form when either alum or ferric sulfate is used. Metal salts (alum, ferric sulfate, ferric chloride) consume natural alkalinity.

• Each mg/L of alum will consume 0.5 mg/l total alkalinity (as CaCO3).

• Each mg/L ferric sulfate will consume 0.75 mg/L total alkalinity (as as CaCO3).

• Each mg/L ferric chloride will consume 0.92 mg/L total alkalinity (as CaCO3).

• It may be necessary to add alkalinity to the water (lime, soda ash, caustic soda) to the water in order for the metal salts to work properly. The doses should be confirmed with jar testing.

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Coagulation/Flocculation Facilities

Coagulation/Flocculation Facilities

• Flash Mix - purpose is to distribute the coagulant rapidly and evenly throughout the water.

• Water should be stirred violently for a brief time to encourage the greatest number of collisions between particles as possible.

• Types of Mixers: Mechanical - Pumps and Conduits

• Detention time should be 30 seconds or less (Design Criteria).

• Flocculation - provides for gentle mixing to encourage floc formation.

• Detention time of at least 30 minutes, with a detention time of 45 minutes preferred.

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Process Control

A. Chemical SelectionB. Chemical Application / Solution PreparationC. Monitoring Process Effectiveness

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Process Control / Chemical Selection • A. Chemical Selection - These raw water characteristics should be monitored in

order to do a thorough job of chemical selection. 1. Temperature • Low water temperatures slow chemical reactions, causing decreased efficiency

and slow floc formation. • Higher coagulant doses may be required to maintain acceptable results. 2. pH • Extremes can interfere with the coagulation/flocculation process. • The optimum pH depends on the specific coagulant. 3. Alkalinity • Low alkalinity causes poor coagulation. • May be necessary to add alkalinity (lime, caustic soda, soda ash). 4. Turbidity • Difficult to form floc with low turbidity water, may need to add weighting agents. 5. Color - Indicates presence of organic chemicals which can react with the

coagulant, and with chlorine to form disinfection byproducts.

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Process Control / Chemical Application

B. Chemical Application:

Solution Preparation • For Example when preparing potassium permanganate solutions, a three

percent solution is best. Potassium permanganate has a limited solubility of about five percent at normal temperatures.

In order to prepare the solution needed the following information is required:

• Chemical required

• Volume of water required

• Specific gravity

• Weight of solution

• Concentration

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Process Control / Monitoring Process Effectiveness

C- Monitoring Process Effectiveness • (1) Jar Test • (2) pH • (3) Turbidity• (4) Temperature• (5) Alkalinity

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Process Evaluation and Troubleshooting• Problems with coagulation and flocculation normally show up in subsequent

processes (sedimentation and filtration). Actual plant performance should be monitored for:

Poor or inadequate flash mixing • Gentle flocculation • Adequate flocculation time • Settled- and filtered water quality.Indication of poor or inadequate flash mixing:1. Very small floc (pin floc) 2. High turbidity in settled water should be less than 1.0 NTU when the raw water

turbidity is less than 1 NTU 95 percent of the time; less than 2.0 NTU when the raw water turbidity is more than 1 NTU 95 percent of the time.

3. Too frequent filter backwashing If any of these symptoms occur, check: (1) detention time and mixing energy in the flash mix (detention time should be less

than 30 seconds);(2) stirring speed in the flocculator – should have a peripheral speed of between 0.5

and 2.0 feet per second; (3) flocculation detention time – should be at least 30 minutes, with 45 minutes

being preferred.

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Jar Test• Coagulation/flocculation is the process of binding small particles in the water

together into larger, heavier clumps which settle out relatively quickly. 

• The larger particles are known as floc.

• changing water characteristics require the operator to adjust coagulant dosages at intervals to achieve optimal coagulation.

• Different dosages of coagulants are tested using a jar test, which mimics the conditions found in the treatment plant. 

• The first step of the jar test involves adding coagulant to the source water and mixing the water rapidly (as it would be mixed in the flash mix chamber) to completely dissolve the coagulant in the water. 

• Then the water is mixed more slowly for a longer time period (as flocculation basin conditions and allowing the forming floc particles to cluster together). 

• Finally, the mixer is stopped and the floc is allowed to settle out, as it would in the sedimentation basin.

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Jar Test• A major goal of water treatment is turbidity removal.• The jar test is a simulation of the treatment processes that have been developed to

accomplish turbidity removal • Alum, ferrous sulfate, and ferric chloride are three common coagulants • The best dose will also be a function of pH. The optimum pH for alum coagulation

is usually between 5.5 and 6.5.• There is no way to “calculate” the best dose. It must be determined by trial and

error; hence, the jar test. • The reaction chemistry varies according to the pH and alkalinity of the test sample. Alum coagulation proceeds according to the following equation• if there is enough alkalinity in the water to react with the amount of alum dosed: Al2(SO4)3 • 14H2O + 6HCO3

- ↔ 2Al(OH)3(s) + 6CO2 + 14H2O + 3SO4-2

• If there is insufficient alkalinity, the reaction will proceed according to the equation:

• Al2(SO4)3 • 14H2O ↔ 2Al(OH)3 + 3H2SO4 + 8H2O • An alkalinity test is usually performed before initiating a jar test to determine

whether alkalinity supplements might be required.

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JAR TEST PROCEDURE 1. measure the initial pH, alkalinity, and turbidity of the sample to be tested.

Make any pH adjustment necessary.calculate for the maximum alum dose you plan to use, what alkalinity concentration is required to prevent significant pH reduction.Compare this required amount to the amount measured in step 1, and supplement the sample with alkalinity if necessary.

Al2(SO4)3 • 14H2O + 6HCO3- ↔ 2Al(OH)3(s) + 6CO2 + 14H2O + 3SO4

-2

Decide on six dosages of the chemical(s) include coagulants, coagulant aids, and lime

2. Prepare a stock solution of the chemical(s).3. Collect a two gallon sample of the water to be tested. This should be the raw

water.4. Measure 1,000 mL of raw water and place in a beaker. Repeat for the remaining

beakers.5. Place beakers in the stirring machine.6. With a measuring pipet, add the correct dosage of lime and then of coagulant

solution to each beaker as rapidly as possible.

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JAR TEST PROCEDURE7. With the stirring paddles lowered into the beakers, start the stirring machine and

operate it for one minute at a speed of 80 RPM. 8. Reduce the stirring speed to 20 RPM and continue stirring for 30 minutes 9. Stop the stirring apparatus and allow the samples in the beakers to settle for 30

minutes.10. Determine which coagulant dosage has the best flocculation time and the most

floc settled out. 11. Test the turbidity of the water in each beaker using a turbidometer12. If lime or a coagulant aid is fed at your plant in addition to the primary

coagulant, you should repeat the jar test to determine the optimum dosage of lime or coagulant aid. Use the concentration of coagulant chosen in steps 10 and 11 and alter the dosage of lime or coagulant aid.

13. Using the procedure outlined in step 11, measure the turbidity of water at three locations in the treatment plant - influent, top of filter, and filter effluent.

14. Prepare a graph of alum dose vs. remaining turbidity in order to identify the dosage that produced optimum turbidity removal

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JAR TEST

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Jar Test Example

Low alkalinity High alkalinity

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Explanation of Jar Test Example• Water A had low alkalinity and required less coagulant to achieve good

coagulation and flocculation than the higher alkalinity of Water B• Plots of turbidity versus coagulant dose showed a continual decrease in turbidity

with an increase in coagulant dose.• Water A, with FeCl3, showed a decrease followed by an increase (at 40 mg/L) in

turbidity.• This dictates that adsorption and charge neutralization is taking place due to the

colloids restabilizing and not coagulating. • Addition of coagulant to the low alkalinity waters lead to a drop in the pH of

Water A, which enhanced adsorption and charge neutralization. • higher coagulant doses are needed with high alkalinity waters where pH remains

fairly constant.• Although slightly less alum than FeCl3 was needed to reach an optimum level,

the residual turbidity when using the alum coagulant did not fall below 1 NTU. • This means that even though alum may require a slightly smaller dose, it still

may not be able to meet the desired effluent regulations without the additional help of a filter or polymer

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Filtration• After separating most floc, the water is filtered as the final step to remove

remaining suspended particles and unsettled floc1. Rapid sand filters• use relatively coarse sand and other granular media to remove particles and

impurities that have been trapped in a floc through the use of flocculation chemicals-typically salts of aluminium or iron.

• Water and flocs flows through the filter medium under gravity or under pumped pressure

• Water moves vertically through sand which often has a layer of activated carbon or anthracite coal (a hard, compact variety of mineral coal).

• The top layer removes organic compounds • Most particles pass through surface layers but are trapped in pore spaces or

adhere to sand particles • To clean the filter, water is passed quickly upward through the filter, opposite

the normal direction (called backflushing or backwashing)• compressed air may be blown up through the bottom of the filter to break up the

compacted filter media to aid the backwashing process

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Rapid sand filters

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Rapid sand filters / Advantages and disadvantagesAdvantages• Much higher flow rate than a slow sand filter; • Requires relatively small land area• Less sensitive to changes in raw water quality, e.g. turbidity• requires less quantity of sand• Disadvantages• Requires greater maintenance than a slow sand filter. For this reason, it is not

usually classed as an "appropriate technology,".• Generally ineffective against taste and odour problems.• Produces large volumes of sludge for disposal.• Requires on-going investment in costly flocculation reagents.• treatment of raw water with chemicals is essential• skilled supervision is essential• cost of maintenance is more• it cannot remove bacteria

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Slow sand filters• Slow "artificial" filtration (a variation of bank filtration) to the ground, Water

purification plant

• The filters are carefully constructed using graded layers of sand with the coarsest sand, along with some gravel, at the bottom and finest sand at the top.

• Drains at the base convey treated water away for disinfection

• effective slow sand filter may remain in service for many weeks or even months

• produces water with a very low available nutrient level and low disinfectant levels

• Slow sand filters are not backwashed; they are maintained by having the top layer of sand scraped off

• A 'large-scale' form of slow sand filter is the process of bank filtration in a riverbank.

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Slow sand filters

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Slow sand filters / Advantages and disadvantages• Advantages• require little or no mechanical power, chemicals or replaceable parts,

• require minimal operator training and only periodic maintenance,

• often an appropriate technology for poor and isolated areas.

• simple design

• disadvantages

• Due to the low filtration rate, slow sand filters require extensive land area for a large municipal system.

• Many municipal systems in grown cities installed rapid sand filters, due to increased demand for drinking water.

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Membrane filtration• is a treatment process based on the physical separation of compounds from the

water phase with the use of a semi-permeable membrane.• Most of the membranes used are synthetic membranes made of organic

polymers. Can be divided into two catego ries based on the pore sizes of the membrane:• micro- and ultrafiltration (MF and UF) remove colloidal substances and

microorganisms• nanofiltration and reverse osmosis (NF and RO) remove colloidal substances

and microor ganisms but also dissolved substances like micropollutants and ions• Micro- and ultrafiltration remove substances from the water phase by a sieve

mechanism• Microfiltration removes bacteria and the larger viruses (to a size of 0.05 μm). • Ultrafiltration also removes bacteria, but because of the smaller pore size all the

larger viruses are removed • The removal of suspended solids of MF and UF is at least 99%.

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Membrane filtration• The removal of microorganisms is referred to in log units.• A removal of one log unit corresponds to a 90% removal. The removal of 4 log

units cor responds to a 99.99% removal.• molecular weight cut-off (MWCO) can also be used as an indication of the

ability of membranes to reject compounds• MWCO is the molecular weight of spherical molecules which are 90% rejected

by the membrane’s pores. • The unit of MWCO is Dalton (1 Dalton is the mass of one hydrogen atom =

1.66x10-27kg)• The MWCO for MF/UF is in the range of 10,000 to 500,000 Dalton (10 to 500

kD).

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Different filtration processes and size of compounds removed

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Different filtration processes and size of compounds removed

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Different filtration processes and size of compounds removed

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MF/UF for drinking water• In drinking water treatment, UF can be used in different stages of the process:

– as a pre-treatment of surface water to remove suspended solids, heavy metals, bacteria and viruses in order to prevent pollution of the dunes, or to prevent clogging of the NF/RO membranes

– as treatment of backwash water from rapid sand filtration– treatment of surface water as the first step in drinking water production.

In some cases, the MF/UF installation is preceded by a conventional coagulation/flocculation/floc removal treatment process in order to reduce the risk of membrane fouling

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Log-removal capacity of MF and UF for different microorganisms

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Principle

• The membrane is the barrier responsible for the separation of compounds out of the water phase. The membrane is semi-permeable.

• The pore size determines the removal of different compounds.

• The removed compounds remain at the raw waterside of the membrane and accumulate on the membrane.

• Three water streams can be distinguished:

– the dirty water or raw water is called feed water

– the water passing the membrane is called the permeate or product water.

– the water with the rejected particles is called concentrate or retentate.

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Possibilities for the use of MF and UF for drinking water production

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Membrane and different flows

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Dead-end filtration mode• In dead-end filtration, all the feed water is through the membrane.• The suspended solids remain on the feed side of the membrane. • As a consequence, the resistance of permeation will increase in time. • The water flux decreases if the pressure is constant, or the pressure increases if

the flux is constant.• Periodically the membrane has to be cleaned in order to reduce the resistance of

permeation. • The period of permeation is called filtration time. A filtration run is the filtration

time together with the cleaning time (also called filtration cycle).

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Inside-out filtration• In a configuration with inside-out filtration, feed water enters the inside of the

capillaries or tubular membranes. • The water is pushed from the inside to the outside of the membrane. • Permeate is collected outside the membrane and transported to the permeate

tube.

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Membrane fouling• During filtration the resistance increases as a result of fouling of the membrane

surface.

• The resistance increases because the pores in the membrane are blocked and because caked suspended matter is built up on the membrane surface.

• This resistance increase is referred to as fouling.

• The definition of fouling is: the deposition of suspended or dissolved substances on the membrane surface or in front of the pores or in the membrane pores.

• fouling can be subdivided into different mechanisms:- membrane resistance

- pore blocking

- adsorption in the pores

- cake resistance

- high concentration of dissolved substances near the surface.

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Membrane fouling• Due to the accumulation of solids on and in the membrane during dead-end

filtration, the total resistance increases with time. • The mass of a cake layer is difficult to measure. The thickness of the cake layer

depends on The trans membrane pressure TMP. • The thickness of the cake layer is in the range of several micrometers, depending

on the rejected compounds.

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Cleaning• As a result of the dead-end mode, the membrane has to be cleaned often in

order to remove the rejected compounds.• The cleaning intervals can be constant in time or can be determined by a

maximum pressure.• If possible, cleaning of membranes should be avoided because during the

cleaning no permeate is produced.• Also, permeate and energy are used for the cleaning. With specific cleanings

chemicals are also used.• Different methods or a combination of methods can be used to clean a

membrane module:- forward flush (FF) - back flush (BF)- air flush (AF)- chemical enhanced flush (CEF) or enhanced back flush (EBF)- cleaning in place (CIP) or chemical soaking

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Forward flush (FF) and back flush (BF)

The forward flush is a turbulent cross-flow along the feed side of the membrane surface This is the opposite of the filtration mode where the flow is through the membrane (flow direction perpendicular to the membrane surface)

In the back flush, filtration direction is reversed

After removing the particles from the pores and the membrane surface, the particles and the cake have to be transported out of the module.

A combined back flush and forward flush can be used. First, a back flush is used to clean the pores and to lift the cake. Then, a forward flush is used to transport the dirt out of the module

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Air/water flush• An air/water flush can be used to clean the membrane wall from adhering fouling.

• The air/water flush is actually a forward flush with a combination of air and water.

• The air is used to create a turbulent flow in the membrane under process conditions where no turbulence is attained with the water flow.

• The cleaning efficiency depends on the kind of two-phase flow obtained in the membranes.

• If the water/air ratio is high, only small air bubbles are present in the water and the turbulence is only slightly enhanced.

• When the water/air ratio is too small, the air flows through the middle of the membrane and the cleaning effect is low.

• The best cleaning is obtained with bullet-like air bubbles.

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Air/water flush

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Chemical cleaning• If forward flush, back flush and air flush are not enough to clean the membrane, a

chemical cleaning (often called enhanced back flush or chemical back flush) can also decrease the clean water resistance.

• This kind of cleaning means that the module is soaked with a solution of hypochloric acid, hydrogen chloride, hydrogen peroxide.

• After the soaking, a backwash or a forward flush removes the dissolved dirt.• The main drawback of chemical cleaning is that the membranes age because of the

chemicals, and the lifetime of the membranes, therefore will be shortened.• Also, the chemicals are a cost factor and with a chemical cleaning a chemical

waste stream should be discharged. • Besides the periodical chemical cleaning which is part of the automated process

control of the installation, a more intensive chemical cleaning might also be necessary.

• The so-called “cleaning in place” (CIP) can last from a few hours to several days and is typically not automated.

• If the CIP is not able to clean the membranes, they are replaced by new ones.

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Practice / Module design

Tubular membranes

Capillary membranes

Plate membraneCushion membranes

Spiral Wound Flat Sheet Membrane

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Choosing a module design

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Membrane Processes / Electro-dialysis• What is a Membrane?• The membrane can be defined essentially as a barrier, which separates two

phases and restricts transport of various chemicals in a selective manner.• Membrane Separation Technology• A membrane separation system separates an influent stream into two effluent

streams known as the permeate and the concentrate.• The permeate is the portion of the fluid that has passed through the semi-

permeable membrane.• The concentrate stream contains the constituents that have been rejected by the

membrane.

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Membrane Processes

The main membrane processes are• Dialysis• Electro-dialysis• Reverse osmosis

Driving forces that cause mass transfer of solutes are:• Difference in concentration (dialysis)• Difference in electric potential (electro-dialysis)• Difference in pressure (reverse osmosis)

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Dialysis Theory• Dialysis depends on separating solutes of different ionic or molecular size in a

solution by means of a selectively permeable membrane.• The driving force for dialysis is the difference in the solute concentration across

the membraneThe mass transfer of solute through the membrane is given by:

• M = mass transferred per unit time (gram/hour)• K = mass transfer coefficient [gram/(hr-cm2)(gram/cm3)]• A = membrane area (cm2)• ΔC = difference in concentration of solute passing through the membrane (gram/cm3)• In environmental engineering, Dialysis is not used to an appreciable extent.• In industrial applications, Dialysis can be used to recover Sodium Hydroxide

from textile wastewater.• Dialysis is limited to small flows due to small mass transfer coefficient (K)

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Electrodialysis (ED)• ED is used to transport salt ions from one solution through ion exchange

membranes to another solution under the influence of an applied electric potential difference.

• The cell consists of a feed (diluate) compartment and a concentrate (brine) compartment formed by an anion exchange membrane and a cation exchange membrane placed between two electrodes

• multiple electrodialysis cells are arranged into a configuration called an electrodialysis stack, with alternating anion and cation exchange membranes forming the multiple electrodialysis cells

• dissolved species are moved away from the feed stream rather than the reverse

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Method In an electrodialysis stack, the diluate (D) feed stream, brine or concentrate (C) stream, and electrode (E) stream are allowed to flow through the appropriate cell compartments formed by the ion exchange membranes

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Anode and Cathode Reactions

At the cathode 2e- + 2 H2O → H2 (g) + 2 OH-

At the anode H2O → 2 H+ + ½ O2 (g) + 2e- or 2 Cl- → Cl2 (g) + 2e-

• Small amounts of hydrogen gas are generated at the cathode and small amounts of either oxygen or chlorine gas (depending on composition of the E stream and end ion exchange membrane arrangement) at the anode.

• These gases are combined to maintain a neutral pH and discharged or recirculated to a separate E tank.

• hydrogen gas may be collected for use in energy production.

• ED systems can be operated as continuous or batch production processes

• Continuous process, feed is passed through a sufficient number of stacks placed in series to produce the final desired product quality.

• Batch processes, the diluate and/or concentrate streams are re-circulated through the ED systems until the final product or concentrate quality is achieved

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Electrodialysis Reversal

• An improvement to ED, referred to as Electrodialysis Reversal (EDR)

• EDR utilizes the same concept of ED except that there is periodic automatic reversal of polarity and cell function to reverse the flow of ions across the membrane.

• This returns anions across the anionic membranes and helps break up scale formed on the concentrating face of the membranes and minimizes membrane fouling.

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Application

Some applications of electrodialysis include: • Large scale brackish and seawater desalination and salt production or Small and

medium scale drinking water production

• Water reuse (e.g., industrial laundry wastewater, produced water from oil/gas production, cooling tower makeup & blowdown, metals industry fluids.

• Pre-demineralization (e.g., boiler makeup & pretreatment, ultrapure water pretreatment, process water desalination, power generation, semiconductor, chemical manufacturing, food and beverage)

• Food processing

• Agricultural water

• Glycol desalting (e.g., antifreeze / engine-coolants, capacitor electrolyte fluids, oil and gas dehydration, conditioning and processing solutions, industrial heat transfer fluids, secondary coolants from heating, venting, and air conditioning.

• Glycerin Purification

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Example Treatment Train• The conventional EDR treatment train typically includes raw water pumps,

debris screens, rapid mix, slow mix flocculator, basin or clarifier, gravity filters, EDR membranes, chlorine disinfection, and clearwell storage.

• Microfiltration (MF) could be used in place of flocculation, sedimentation, and filtration.

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Benefits

• ED and EDR can operate with minimal fouling or scaling, or chemical addition.

• Low pressure requirements.

• ED and EDR facilities are quieter than RO.

• Long membrane life expectancy.

• Unaffected by non-ionic sealants such as silica1.

• Low chemical usage for pretreatment1.

• Ability to treat feed water with higher SDI, TOC and silica concentrations, and

more turbidity than RO.

• Can operate with up to 0.5 ppm of free chlorine in the feed water to control the

biological matter in the feed water.

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Maintenance• ED membranes are durable, can run under a wide range of pH conditions (pH 2

– 11), and endure high temperatures during cleaning.

• They can be removed from the unit and scrubbed if necessary.

• If operated properly, membranes have an average life of 12 to 15 years.

• Solids can be flushed out by turning the power off and letting water circulate

through the stack.

• The ED stack must be disassembled, mechanically cleaned, and reassembled at

regular intervals.

• They can also be cleaned using a 5% hydrochloric acid solution

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Limitations• Non-charged, higher molecular weight, and less mobile ionic species will not

typically be significantly removed.

• less economical when low salt concentrations in the product are required

• large membrane areas are required to satisfy capacity requirements for low concentration feed solutions.

• ED systems require feed pretreatment to remove species coat, precipitate onto, or foul the surface of the ion exchange membranes.

• Fouling decreases the efficiency of the ED system. Species of concern include calcium and magnesium hardness, suspended solids, silica, and organic compounds. 

• Water softening can be used to remove hardness, and micrometre or multimedia filtration can be used to remove suspended solids.

• EDR systems seek to minimize scaling by periodically reversing the flows of diluate and concentrate and polarity of the electrodes.

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Reverse OsmosisPrinciples of Reverse Osmosis

A. Osmosis:• Osmosis is the natural passage or diffusion of a solvent such as water through a

semi-permeable membrane from a week solution to a stronger solution. This natural phenomena is explained in many ways as follows:

• The movement is due to the difference in the vapor pressure of the two solutions separated by the membrane.

• The vapor pressure of the pure solvent is higher than that of the solution with dissolved solids. Thus the solvent moves from the higher pressure to the lower pressure side.

• Others say that the solvent moves from the less concentrated (higher-potential) solution to the more concentrated (lower-potential) one to reduce the solution concentration.

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Osmosis• The solvent continues to move and water rises in the concentrated solution side

to a level with a hydrostatic pressure ( Δπ ) equivalent to the difference in vapor pressure of two solutions. At this level the system is said to be at equilibrium.

• ( Δπ ) is called the Osmotic Pressure ( is the driving force for osmosis to occur). • The osmotic pressure of a solvent depends on many factors such as the

characteristics of the solvent, the dissolved solids concentration, and temperature.

• The osmotic pressure of any solution can be approximated by the following equation:

• c = summation of the molar concentration of the dissolved ions• R = Universal gas constant (82.05746 cm3 atm K−1 mol−1 or 8.314 J/K.mol)

T = Temperature in degrees Kelvin.• ci = concentration ion (g/m3)• Mi = molecular weight ion (g/mol)• zi = valence ion (-)

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Osmosis

In osmosis, water moves across the membrane from the dilute to the concentrated solution.

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Reverse Osmosis (RO)B. Reverse Osmosis (RO):

• Reverse Osmosis is the forced passage of a solvent (e.g. water) through a

membrane against the natural osmotic pressure to accomplish separation of the

solvent from a solution of dissolved solids.

• If a pressure equal to the Osmotic pressure ( Δπ ) is applied to the side of higher

salt content, the water flow from lower to higher salt concentration will stop.

• If an additional pressure is exerted the water flow will be reversed and to the

direction from high to low salt concentration producing fresh water.

• The membrane allow the passage of the solvent while blocking the passage of

salt ions. The salt ions or the dissolved matter are called Solutes.

• Some salts move with water since each membrane has a rejection efficiency

that is less than 100%.

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Reverse Osmosis AppliedIn reverse osmosis, pressure is applied to the concentrated solution reversing the natural direction of flow, forcing water across the membrane from the concentrated solution into the more dilute solution

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Principle Reverse Osmosis (RO)

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Reverse Osmosis Treatment (RO)

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Reverse Osmosis Treatment (RO)

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Osmotic pressure Calculation

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Osmotic pressure Calculation• Example 2: In water from surface water (18o), the following ions are present

at the given concentrations:• [HCO3-] 135 g/m3 M = 61.0 g/mol• [SO42-] 63 g/m3 M = 96.1 g/mol• [Cl-] 95 g/m3 M = 35.5 g/mol• [Na+] 52 g/m3 M = 23.0 g/mol• [Ca2+] 60 g/m3 M = 40.1 g/mol• [Mg2+] 11 g/m3 M = 24.3 g/mol• R= 8.314 J/K.mol• Calculate the osmotic pressure of the water.

Example 3Why is the osmotic pressure in the concentrate higher than in the feed?

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Solution

Notes: By comparison, the osmotic pressure of brackish groundwater (2000 mg/l NaCl) is 1.7 x 105 Pa (= 1.7 bar), the osmotic pressure of sea water (35.000 mg/l NaCl) is 30 x 105 Pa (= 30 bar).

Example 3: AnswerThe feed is separated into permeate and concentrate flows. The concentrate flow contains the same amount of salts as the feed flow, however, they are dissolved in less water. A higher salt concentration means a higher osmotic pressure.

Example 2: Answer

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Osmotic pressure Calculation

atm = 14.6959488 Psi (pound per square inch

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Membrane configuration

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TheoryMass balanceThe water mass balance for a membrane element is given by:

Qf = Qc + Qp in which: • Qf = feed flow (m3/h)

• Qc = concentrate flow (m3/h)

• Qp = permeate flow (m3/h)

Also, the dissolved material of mass balance can be derived by:QfCf = QcCc + QpCp

in which: • Cf = concentration of dissolved material in feed water (g/m3)

• Cc = concentration of dissolved material in con centrate (g/m3)

• Cp = concentration of dissolved material in per meate (g/m3)

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Recovery• The recovery indicates the overall production of the system. It is the relationship

between permeate and feed flow (water recovery rate):

• A recovery of 80% means that 80% of the feed flow is produced as permeate. This also means that the concentration of salts in the concentrate is 5 times higher than the concentration in the feed flow, assuming that all salts are retained.

• The recovery of one element is between 1 and 10%, therefore more elements should be placed in a series to obtain the desired recovery of 80%.

• For sea water desalination, the maximum achievable recovery is about 50%. This recovery is limited by the possibility of scaling caused by high salt concentrations.

• For groundwater, however, recoveries up to 95% can be obtained.

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Rejection• RO has a very high efficiency of inorganic chemicals.

• However, it also has a very high efficiency in removing dissolved organic matter 90-99%, but it is preferred to remove these materials using other methods such as carbon adsorption. This is due to the fouling nature of organic matter.

• RO is capable of removing more than 99% of microorganisms.

Rejection indicates the amount of material rejected by a membrane. Rejection is calculated by:

in which:

• R = solute rejection rate

X 100

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RO Contaminant Rejection efficiency

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Estimate Quantity and Quality of Waste Stream From Reverse Osmosis Facility

Example 4: Estimate Quantity and Quality of the Waste Stream, and total quantity of water that must be processed from reverse osmosis facility that is produce 4000 m3/d of water to be used for industrial cooling operations. Assume that both the recovery and rejection rates are equal to 90 % and that concentration of feed stream is 400 g/m3

Answer:1. Determine the flow rate of the concentrated waste stream and the total amount of

water that must be processed • Qc = Qp (1-r)/r• Qc = (4000 m3/d)(1-0.9)/0.9 = 444 m3/d2. Total amount of water that must be processed to produce 4000 m3/d of RO water • Qf = Qp + Qc = 4000 m3/d + 444 m3/d = 4444 m3/d3. Determine the concentration of the permeate stream. • Cp = Cf (1-R) = 400 g/m3 (1-0.9) = 40 g/m34. Determine the concentration of concentrated waste stream • Cc = (QfCf – QpCp) / Qc

• Cc = {(4444 m3/d)(400 g/m3) – (4000 m3/d)(40g/m3)} / (444 m3/d)• Cc = 3643 g/m3

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Checking the need for pretreatment• RO membrane may be fouled by many ways, such as colloidal mater, bacterial

activity, iron and manganese, chlorine, scale from calcium carbonate.

• Particulate matter will be retained and is an ideal nutrient for biomass, resulting in biofouling.

• Both fouling processes (scaling and biofouling) should be avoided as much as possible to efficiently operate reverse osmosis.

• Many parameters have been used to assess the need for pretreatment of feed water before the RO units.

• The most common indexes are the silt density index (SDI), and the modified fouling index (MFI).

• Fouling indexes are determined from simple membrane tests. The samples are passed through a 0.45μm Millipore filter at a gage pressure of 210 kPa . The time needed for the test varies between 15min-2hr depending on the fouling nature of the water. The same equipment is used for the two indexes.

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Checking the need for pretreatment

SDI Calculation

Where

Q = average flow, L/s

A = constant

MFI = modified field fouling index, s/L2

V = volume of water filtered in the test

The equation of the MFI

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Checking the need for pretreatmentExample 4: Determine the silt density index for a proposed feed water from the

following test data. If a spiral wound RO membrane is to be used, will pretreatment be required?

• Total run time = 30 min • Initial 500 mL = 2 min • Final 500 mL = 10 min

Answer: • SDI = 100{(1 – (ti/tf)} / t• SDI = 100{1 – (2/10)}/30 = 2.67

Compare the SDI to the acceptable criteria Calculated SDI value 2.67 is less than 3; therefore no further pretreatment would

be needed normally. As a practical matter, because the SDI value is close to 3.0 it may prudent to consider some of pretreatment to prolong the filtration cycle

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Nanofiltration• It is not always necessary to remove all dissolved ions. For example, when water

has to be softened nanofiltration will be sufficient.

• Nanofiltration removes divalent ions (like Ca2+ , Mg2+ and SO42-), while

monovalent ions are not rejected.

• Nanofiltration membranes have larger pores than reverse osmosis membranes,

resulting in a lower resistance for filtration and also lower operational pressures

(2 - 10 bar).

• The pores of nanofiltration are smaller than ultrafiltration pores.

• Nanofiltration membrane modules can be constructed as spiral-wound

membranes, and now as capillary membranes as well.

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Softening • Due to the long residence time in the subsoil, groundwater is in chemical

equilibrium (i.e., calcium carbonate equilibrium).

• Groundwater comes in contact with the atmosphere when it is pumped up or

discharged into surface water. When carbon dioxide disappears from the water, it

is not in calcium carbonate equilibrium anymore.

• Also, when water is heated the equilibrium is changing, the Ca2+ and HCO3- - ions

will precipitate in the form of calcium carbonate (CaCO3) leads to calcium

carbonate scaling (e.g., deposits in water boilers).

• To prevent precipitation of calcium carbonate at the customers’ taps, calcium ions

are partially removed from the water by drinking water com panies. This is called

softening.

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Why softening?

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Water quality and softening

• The hardness of water is classified from very soft to very hard.

• The produced water always needs to comply with the standards.

• The most important water quality parameters influenced by softening are acidity, hardness, bicarbonate, sodium, and the solubility potential for metals like copper and lead.

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Water quality and softening

• Acidity (pH): guideline = 8.0 < pH < 8.3

• Directly after the softening process, acidity of the water is higher than the above-mentioned guideline. By means of pH-correction (acid dosing), pH is decreased to the desired value.

• Hardness: guideline = 1.5 < hardness < 2.5 mmol/l

• Hardness is defined as the sum of the concentration of dissolved calcium and magnesium ions.

• Bicarbonate concentration: guideline > 2 mmol/l

• The bicarbonate concentration should be higher than 2.0 mmol/l, resulting in water with sufficient buffering capacity (pH stability);

• Sodium concentration: guideline = as low as possible

• Because sodium influences blood pressure and therefore, indirectly, heart and vascular diseases, the concentration should not be higher than 120 mg/l.

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Water quality and softening• Solubility potential• Softening also works to reduce the solubility of metals from pipe material. For

drinking water the most important metals are lead (Pb) and copper (Cu), because these metals have a health impact.

• guideline Cu2+ < 2 mg/l - guideline Pb2+ < 0.01 mg/l• The values for copper and lead solubility are determined by a pipe test. The pipe

test is performed in stagnant water and takes 16 hours. Empirical relationships have been derived to give a rapid indication about the release:

• Cumax = copper dissolving capacity (mg/l)• TAC = Total An organic Carbon (mmol/l)• Pbmax = lead dissolving capacity (mg/l)• SO4

2- = sulfate concentration (mmol/l)• T = temperature (oC)• TAC concentration can be influenced by the softening process (mainly caused by

the decrease in HCO3-).

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Softening processes• Groundwater extracted from calcareous subsoils can have a high degree of hardness.

• Water extracted from deep sand layers is, on the other hand, fairly soft (0.5 mmol/l).

• The hardness of surface water is normally from 2.0 - 3.0 mmol/l.

• The hardness of water can be decreased by means of different processes:

1. Dosing of a base: (NaOH, Ca(OH)2 or Na2CO3 ) - Because of a shift in the

calcium carbonic acid equilibrium, spontaneous crystallization occurs. By dosing the

base in a reactor with seeding grains (sand), crystallization will occur on the surface

of the seeding grains, forming limestone pellets (softening in a pellet reactor)

2. Ion exchange: Ca and/or Mg are exchanged with other ions (Na is used the most).

3. Membrane filtration:the hardness is removed partly (nanofiltration) or fully (RO).

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Softening in a treatment plant• Softening installation (including post treatment) into an existing groundwater

treatment process, the following possibilities are considered:• Softening of raw water• Softening of aerated water• Softening after rapid filtration

Softening of raw water• If iron and manganese are present in dissolved form in the water (anaerobic

water), these substances will be trapped in the CaCO3 grains. • The advantage of this is that the loading on the sand filters is reduced. Disadvantage:• CaCO3 grains become less pure• high dosage of base is needed due to the high concentration of carbon dioxide

in raw water. So before the softening reaction starts, the carbon dioxide needs to be converted to HCO3

- and CO32-.

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Softening in a treatment plant

• Softening after an aeration:

• lower chemical dose will be sufficient, because some of the carbon dioxide is

removed during aeration.

• An additional (possible) cost advantage of softening (aerated) raw water is that,

in many cases, existing filters that have been used for iron and manganese

removal up till this point, can also be applied as ‘carry-over’ filters. When

softening after filtration is applied, the purest pellets are formed. Iron and

manganese are removed by the filters.

• A disadvantage is that after softening a new (expensive) rapid filtration step

must be added to the process to remove the ‘carry-over.’

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Theory / Equilibrium• The calcium carbonic acid equilibrium determines whether calcium carbonate

precipitates:• In groundwater abstracted a high concentration of calcium ions is present.• Given that there is no CO3

2- in the water , calcium does not precipitate but remains in dissolved form in the water, resulting in water with a high degree of hardness.

• This reaction only happens if sufficient CO32- ions are present:

Ca2+ + CO32- = CaCO3

• To realize this the pH has to be increased.• The equilibrium reactions of carbonic acid shift to the right and the CO3

2- concentration increases (carbonic acid equilibrium):

CO2 + 2H2O = H3O+ + HCO3-

H2O + HCO3- = H3O+ + CO3

2-

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Theory / Equilibrium

• The increase of pH is realized by adding a base solution that binds the H3O+ - ions.

• Sodium hydroxide (NaOH caustic soda ) is used to achieve this. • The more NaOH is added, the more the pH rises and the stronger the softening.

• The above-mentioned reactions are irreversible; in reality, equilibrium will be set.

• Also for the bases Ca(OH)2 and Na2CO3, similar reaction equations can be formulated.

• By dosing a base, the carbonic acid equilibrium shifts to the left, forming calcium carbonate. The Saturation Index exceeds 1. At a similar SI, crystallization of calcium carbonate occurs, forming a deposit on the seeding grains present in the reactors.

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Langelier Saturation Index (LSI)• LSI is a calculated number used to predict the calcium carbonate stability of

water. • It indicates whether the water will precipitate, dissolve, or be in equilibrium with

calcium carbonate. • In 1936, Wilfred Langelier developed a method for predicting the pH at which

water is saturated in calcium carbonate (called pHs). • The LSI is expressed as the difference between the actual system pH and the

saturation pH: LSI = pH (measured) - pHs• For LSI > 0, water is super saturated and tends to precipitate a scale layer of

CaCO3.• For LSI = 0, water is saturated (in equilibrium) with CaCO3. A scale layer of

CaCO3 is neither precipitated nor dissolved.• For LSI < 0, water is under saturated and tends to dissolve solid CaCO3.• If the actual pH of the water is below the calculated saturation pH, the LSI is

negative and the water has a very limited scaling potential. • If the actual pH exceeds pHs, the LSI is positive, and being supersaturated with

CaCO3, the water has a tendency to form scale. • At increasing positive index values, the scaling potential increases.

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Langelier Saturation Index (LSI)• In practice, water with an LSI between - 0.5 and + 0.5 will not display

enhanced mineral dissolving or scale forming properties.

• Water with an LSI below -0.5 tends to exhibit noticeably increased dissolving abilities

• water with an LSI above +0.5 tends to exhibit noticeably increased scale forming properties.

• LSI is temperature sensitive. The LSI becomes more positive as the water temperature increases.

• This has particular implications in situations where well water is used. The temperature of the water when it first exits the well is often significantly lower than the temperature inside the building served by the well or at the laboratory where the LSI measurement is made. This increase in temperature can cause scaling, especially in cases such as hot water heaters.

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Langelier Saturation Index (LSI)

• LSI is defined as: : LSI = pH (measured) - pHs• Where: pH is the measured water pH• pHs is the pH at saturation in calcite or calcium carbonate and is defined as:

212

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Langelier Saturation Index (LSI)• As an example, suppose the drinking water supplied to animals has the following analysis:• pH = 7.5• TDS = 320 mg/L• Calcium = 150 mg L-1 (or ppm) as CaCO3

• Alkalinity = 34 mg L-1 (or ppm) as CaCO3

• The LSI index is calculated at two temperatures, i.e. 25oC (room temperature) and 82oC (cage wash cycle). The colder incoming water will warm to room temperature in the manifolds. Residual water in the rack manifold can be heated to 82oC when the rack is in the cage washer.

• LSI Formula: LSI = pH - pHs

• pHs = (9.3 + A + B) - (C + D) where: • A = (Log10[TDS] - 1)/10 = 0.15• B = -13.12 x Log10(oC + 273) + 34.55 = 2.09 at 25°C and 1.09 at 82°C• C = Log10[Ca2+ as CaCO3] - 0.4 = 1.78• D = Log10[alkalinity as CaCO3] = 1.53• Calculation at 25oC:• pHs = (9.3 + 0.15 + 2.09) - (1.78 + 1.53) = 8.2• LSI = 7.5 - 8.2 = - 0.7• Hence No Tendency to Scale• Calculation at 82oC:• pHs = (9.3 + 0.15 + 1.09) - (1.78 + 1.53) = 7.2• LSI = 7.5 - 7.2 = + 0.3• Hence Slight Tendency to Scale213

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Disinfection

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Background: Current Methods of Disinfection

• Large-Scale:– Chlorination– Ozone– UV irradiation

• Small Scale:– Boiling– Iodine tablets– Filters

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Use of Disinfectants as Chemical Oxidants

Oxidation is a chemical reaction where electrons are transferred from one species

(the reducer) to another species (the oxidant)

Disinfectants are used for more than just disinfection in drinking water treatment.

While inactivation of pathogenic organisms is a primary function, disinfectants

are also used oxidants in drinking water treatment for several other functions:

1. Minimization of Disinfection Byproducts formation : Several strong oxidants,

including potassium permanganate and ozone, may be used to control DBP

2. Prevention of re-growth in the distribution system and maintenance of biological

stability;

– Removing nutrients from the water prior to distribution;

– Maintaining a disinfectant residual in the treated water; and

– Combining nutrient removal and disinfectant residual maintenance

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Continue: Use of Disinfectants as Chemical Oxidants

3. Removal of color: Free chlorine is used for color removal. A low pH is favored. Color is caused by humic compounds, which have a high potential for DBP formation

4. Improvement of coagulation and filtration efficiency;a. Oxidation of organics into more polar forms;b. Oxidation of metal ions to yield insoluble complexes such as ferric iron complexes; c. Change in the structure and size of suspended particles.

5. Oxidation is commonly used to remove taste and odor causing compounds. Because many of these compounds are very resistant to oxidation, advanced oxidation processes (ozone/hydrogen peroxide, ozone/UV, etc.) and ozone by itself are often used to address taste and odor problems. The effectiveness of various chemicals to control taste and odors can be site-specific.

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Continue: Use of Disinfectants as Chemical Oxidants

6. Removal of Iron and Manganese

7. Prevention of algal growth in sedimentation basins and filters: Prechlorination will prevent slime formation on filters, pipes, and tanks, and reduce potential taste and odor problems associated with such slimes.

OxidantIron (II)(mg/mg Fe)

Manganese (II) (mg/mg Mn)

Chlorine Cl20.620.77

Chlorine Dioxide, ClO21.212.45

Ozone, O30.430.88*

Oxygen, O20140.29

Potassium Permanganate, KMnO4

0.941.92

Source: Culp/wesner/Culp, Langlais et al., 1991Optimum pH manganese oxidation using ozone is 8-8.5 source Reckhow et

al.,

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Factors affecting disinfection effectiveness

• Time

• pH

• Temperature

• Concentration of the disinfectant

• Concentration of organisms

• Nature of the disinfectant

• Nature of the organisms to be inactivated

• Nature of the suspending medium

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CT Factor• One of the most important factors for determining or predicting the germicidal

efficiency of any disinfectant is the CT factor, a version of the Chick-Watson

law (Chick, 1908; Watson, 1908).

• The CT factor is defined as disinfectant contact time, the mathematical

product of C x T, where C is the residual disinfectant concentration measured

in mg/L, and T is the corresponding contact time measured in minutes.

• CT values for chlorine disinfection are based on a free chlorine residual.

• Chlorine is less effective as pH increases from 6 to 9.

• In addition, for a given CT value, a low C and a high T is more effective than

the reverse (i.e., a high C and a low T).

• For all disinfectants, as temperature increases, effectiveness increases.

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CT Values for Inactivation of Viruses

Various levels of deactivation can be achieved. This is often expressed as a log reduction:1 log reduction = 90% deactivation2 log reduction = 99% deactivation3 log reduction = 99,9% deactivation4 log reduction = 99,99% deactivation

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Chick's Law• The death of microorganisms is first order with respect to time • Thus, the remaining number of viable microorganisms in water, N,

decreases with time, t, according to:

where k is an empirical constant descriptive of the microorganism, pH and disinfectant used.

• Integrating with respect to time, and replacing limits (N = No at t = 0) yields:

• Integrating with respect to time, and replacing limits (N=No a t = 0) yields:

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Chick-Watson Law• Incorporates the Concentration-time Product concept into Chick's Law.

Disinfection can be modeled using the Chick-Watson Law:

where

• N = number of microorganisms at time t

• N0 = number of microorganisms at time = 0

• k = inactivation constant

• C = disinfectant concentration, mol/L

• n = constant of dilution, usually close to 1.0

• t = time, min

tkCNN n

0

ln

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Chick LawExample. The specific lethality of hypochlorous acid is

5L.min/mg for the Polio virus. Determine the time required to obtain 99.99% inactivation using 1.0 mg/L of HOCl.

Solution: N/No = 1 - Removal = 1 - .9999 = 0.0001

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t (sec)N/No

40.07690

80.00633

120.00050Solution:1. Plot the -Ln (N/N0)

against tine2. Execute linear regression

for experimental points. This yields the slope of the line (k = 0.634/s).

3. The time required for a 10,000 fold reduction is:

The following table shows the disinfection of poliomyelitis virus using hypobromite as a disinfectant. Determine Chick's constant and the time required to reduce the concentration of viable polio virus to 1/10,000 of the original concentration

Example

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ChlorineChlorine has many attractive features that contribute to its wide use in the industry.

Four of the key attributes of chlorine are that it:

• Effectively inactivates a wide range of pathogens commonly found in water;

• Leaves a residual in the water that is easily measured and controlled;

• Is economical; and

• Has an extensive track record of successful use in improving water treatment operations

There are, however, some concerns regarding chlorine usage that may impact its uses such as:

• Chlorine reacts with many naturally occurring organic and inorganic compounds in water to produce undesirable DBPs;

• Hazards associated with using chlorine, specifically chlorine gas, require special treatment and response programs; and

• High chlorine doses can cause taste and odor problems.

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Chlorine purposes in water treatment

• Taste and odor control;

• Prevention of algal growths;

• Maintenance of clear filter media;

• Removal of iron and manganese;

• Destruction of hydrogen sulfide;

• Bleaching of certain organic colors;

• Maintenance of distribution system water quality by controlling slime growth;

• Restoration and preservation of pipeline capacity;

• Restoration of well capacity, water main sterilization; and

• Improved coagulation by activated silica.

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Chlorine Chemistry• Chlorine gas hydrolyzes rapidly in water to form hypochlorous acid

• Hypochlorous acid is a weak acid (pKa of about 7.5), meaning it dissociates slightly into hydrogen and hypochlorite ions

• Between a pH of 6.5 and 8.5 this dissociation is incomplete and both HOCl and OCl- species are present to some extent (White, 1992). Below a pH of 6.5, no dissociation of HOCl occurs, while above a pH of 8.5, complete dissociation to OCl- occurs.

• As the germicidal effects of HOCl is much higher than that of OCl-, chlorination at a lower pH is preferred.

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Effect of pH on relative amount of hypochlorous acid and hypochlorite ion at 20°C.

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Commonly Used Chlorine SourcesSodium hypochlorite and calcium hypochlorite are the most common sources of

chlorine used for disinfection of onsite water supplies.

• Sodium Hypochlorite (common household bleach)

• Sodium hypochlorite is produced when chlorine gas is dissolved in a sodium

hydroxide solution.

• Clear to slightly yellow colored liquid with a distinct chlorine odor.

• Common laundry bleach - 5.25 to 6.0 percent available chlorine, when bottled.

• Do not use bleach products that contain additives such as surfactants, thickeners,

stabilizers, and perfumes.

• Always check product labels to verify product content and use instructions.

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Sodium Hypochlorite (common household bleach)

• Higher concentrations of chlorine in sodium hypochlorite solutions are generally not available.

• Above 15 percent, the stability of hypochlorite solutions is poor, and decomposition and the concurrent formation of chlorate is of concern

• Sodium hypochlorite solutions are of an unstable nature due to high rates of available chlorine loss

• Over a period of one year or less, the amount of available chlorine in the storage container may be reduced by 50 percent or more.

• Solutions more than 60 days old should not be counted upon to contain the full amount of available chlorine originally in solution

• Swimming pool chlorine - 10.0 to 12.0 percent available chlorine.

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Sodium Hypochlorite (common household bleach)

• The stability of hypochlorite solutions is greatly affected by heat, light,

pH, initial chlorine concentration, length of storage, and the presence of

heavy metal cations

• These solutions will deteriorate at various rates, depending upon the specific

factors:

1. The higher the concentration, the more rapidly the deterioration.

2. The higher the temperature, the faster the rate of deterioration.

3. The presence of iron, copper, nickel, or cobalt catalyzes the deterioration

of hypochlorite. Iron is the worst offender

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Commonly Used Chlorine Sources• Calcium hypochlorite is formed from the precipitate that results from

dissolving chlorine gas in a solution of calcium oxide (lime) and sodium

hydroxide.

Calcium Hypochlorite

• Dry white powder, granules, or tablets - 60 to 70 percent available chlorine -

12-month shelf life if kept cool and dry - If stored wet, looses chlorine rapidly

and is corrosive.

• A chlorine test kit should be used to check the final chlorine residual in a

prepared chlorine solution to assure that you have the concentration intended.

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Which is Best, Sodium Hypochlorite or Calcium

Hypochlorite?• Sodium hypochlorite is more effective

• This may be associated with the quality of the ground water in the well being treated rather than with the source of the chlorine itself.

• If there is an abundance of calcium based materials in both bedrock wells. Calcium hypochlorite already has a high concentration of calcium.

• At 180 ppm of hardness, water is saturated with calcium to the point that it precipitates out of the solution, changing from the dissolved state to a solid state.

• Introducing a calcium hypochlorite solution into a calcium rich aquifer can cause the formation of a calcium carbonate (hardness) precipitate that may partially plug off the well intake.

• Sodium hypochlorite does not have the tendency to create the precipitate.

• If the calcium carbonate concentration in the ground water is above 100 ppm (mg/l), the use of sodium hypochlorite is recommended instead of calcium hypochlorite.

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Typical Chlorine Dosages at Water Treatment Plants

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Chlorine AddedInitial chlorine concentration added to water

Chlorine DemandReactions with organic material,

metals, other compounds present in water prior

to disinfection

Total ChlorineRemaining chlorine

concentration after chlorinedemand of water

Free ChlorineConcentration of chlorineavailable for disinfection

Combined ChlorineConcentration of chlorine

combined with nitrogen in thewater and unavailable for disinfection

Chlorine Addition Flow Chart

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DISINFECTANT DEMAND REACTIONS

• Reactions with Ammonia• In the presence of ammonium ion, free chlorine reacts in a stepwise manner to

form chloramines

• monochloramine (NH2Cl), dichloramine (NHCl2 ), and trichloramine (NCl3), each contribute to the total (or combined) chlorine residual in a water.

• The terms total available chlorine and total oxidants refer, respectively, to the sum of free chlorine compounds and reactive chloramines, or total oxidating agents.

• Under normal conditions of water treatment, if any excess ammonia is present, at equilibrium the amount of free chlorine will be much less than 1 percent of total residual chlorine.

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Chlorine residual• Chlorine persists in water as ‘residual’ chlorine after dosing and this helps to minimize the

effects of re-contamination by inactivating microbes which may enter the water supply after chlorination. It is important to take this into account when estimating requirements for chlorination to ensure residual chlorine.

• The level of chlorine residual required varies with type of water supply and local conditions.

• In water supplies which are chlorinated there should always be a minimum of 0.5mg/l residual chlorine after 30 minutes contact time in water.

• Where there is a risk of cholera or an outbreak has occurred the following chlorine residuals should be maintained:

– At all points in a piped supply 0.5mg/l

– At standposts and wells 1.0mg/l

– In tanker trucks, at filling 2.0mg/l

• In areas where there is little risk of a cholera outbreak, there should be a chlorine residual of 0.2 to 0.5 mg/l at all points in the supply. This means that a chlorine residual of about 1mg/l when water leaves the treatment plant is needed.

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Combined ChlorineWhat is it?• Free chlorine that has combined with ammonia (NH3) or other nitrogen-

containing organic substances.• Typically, chloramines are formed .

Where does NH3, etc come from?• Present in some source waters (e.g., surface water).• Contamination; oxidation of organic matter• Some systems (about 25% of U.S. water supplies) actually Add ammonia. Why would you want to Add ammonia?• Chloramines still retain disinfect capability (~5 % of FAC, Free Available

Chlorine)• Chloramines not powerful enough to form THMs.• Last a lot longer in the mains than free chlorine,

– Free chlorine + Combined chlorine = Total Chlorine Residual• Can measure “Total” Chlorine• Can measure “Free” Chlorine• Combined Chlorine can be determined by subtraction

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pH Effect on Chlorine• Chlorine is a more effective disinfectant at pH levels between 6.0

and 7.0, because hypochlorous acid is maximized at these pH levels • Any attempt to disinfect water with a pH greater than 9 to 10 or

more will not be very effective.• The pH determines the biocidal effects of chlorine.• Chlorine will raise the pH when added to water.• By increasing the concentration of chlorine, and subsequently

raising the pH, the chlorine solution is actually less efficient as a biocide.

• Controlling the pH of the water in the aquifer is not practical. However buffering or pH-altering agents may be used to control pH in the chlorine solution being placed in the well.

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Free Chlorine Distribution with pH

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Temperature Effect on Chlorine• As temperatures increase, the metabolism rate of microorganisms

increases.

• With the higher metabolic rate, the chlorine is taken into the microbial cell faster, and its bactericidal effect is significantly increased.

• The higher the temperature the more likely the disinfection will produce the desired results.

• Virus studies indicate that the contact time should be increased by two to three times to achieve comparable inactivation levels when the water temperature is lowered by 10°C (Clarke et al., 1962).

• Steam injection has been used to elevate temperatures in a well and the area surrounding the Well bore

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Contact time• Time is required in order that any pathogens present in the water are inactivated.

• The time taken for different types of microbes to be killed varies widely.

• it is important to ensure that adequate contact time is available before water enters a distribution system or is collected for use

• In general, amoebic cysts are very resistant and require most exposure. Bacteria, including free-living Vibrio cholerae are rapidly inactivated by free chlorine under normal conditions.

• For example, a chlorine residual of 1mg/l after 30 minutes will kill schistosomiasis cercariae, while 2mg/l after 30 minutes may be required to kill amoebic cysts.

• Contact time in piped supplies is normally assured by passing the water, after addition of chlorine, into a tank from which it is then abstracted.

• In small community supplies this is often the storage reservoir (storage tank). In larger systems purpose-built tanks with baffles may be used. These have the advantage that they are less prone to "short circuiting" than simple tanks.

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Germicidal Efficiency of Chlorine• The major factors affecting the germicidal efficiency of the free

chlorine residual process are: chlorine residual concentration - contact time – pH - water temperature.

• Increasing the chlorine residual, the contact time, or the water temperature increases the germicidal efficiency. Increasing the pH above 7.5 drastically decreases the germicidal efficiency of free chlorine.

• Chlorine dissolved in water, regardless of whether sodium hypochlorite or calcium hypochlorite is used as the source of the chlorine, generally exists in two forms, depending on the pH of the water:

- HOCl - hypochlorous acid (biocidal)- OCl - hypochlorite ion (oxidative)

• Hypochlorous acid is the most effective of all the chlorine residual fractions

• Hypochlorous acid is 100 times more effective as a disinfectant than the hypochlorite ion

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Distribution Diagram for Chloramine Species with pH****

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Breakpoint Chlorination****

• The type of chlorine dosing normally applied to piped water supply systems is referred to as breakpoint chlorination. Sufficient chlorine is added to satisfy all of the chlorine demand and then sufficient extra chlorine is added for the purposes of disinfection.

• As the applied Cl2:N ratio increases from 5:1 to 7.6:1, breakpoint reaction occurs, reducing the residual chlorine level to a minimum.

• Breakpoint chlorination results in the formation of nitrogen gas, nitrate, and nitrogen chloride.

• At Cl2:N ratios above 7.6:1, free chlorine and nitrogen trichloride are present.

Distilled water and rainwater (no Cl2 demand) will not show a breakpoint.

Breakpoint

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Breakpoint- why should we care****?The importance of break-point chlorination lies in the control of:

The killing power of chlorine on the right side of the break point is 25 times higher than that of the left side

taste,

odor,

and increased germicidal efficiency.

Complaints of “chlorine” odor and “burning eyes” from pools/ spas that people usually attribute to over-chlorination is actually due to chloramines! (i.e. UNDER-chlorination)

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The “Breakpoint”…another look****

Chlorine is reduced to chlorides by easily oxidizable stuff (H2S, Fe2+, etc.)

Chloramines broken down & converted to nitrogen gas which leaves the system (Breakpoint).

Cl2 consumed by reaction with organic matter. If NH3 is present, chloramine formation begins.

At this point,THM formation can occur

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• Measure Free and Total chlorine• Bump up chlorinator to increase chlorine dose a certain

known amount• On the following day, re-test Free and Total chlorine.• If Total increases but Free does not, you are NOT at

breakpoint.• Repeat process until both Total and Free chlorine increase

similarly upon adjustment

****Ensuring you are at Breakpoint

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1.Chlorine is a health concern at certain levels of exposure.

2.Drinking water containing chlorine well in excess of drinking water standards could cause irritating effects to eyes and nose.

3.Some people who drink water containing chlorine well in excess of standards could experience stomach discomfort.

4.Drinking water standards for chlorine protect against the risk of these adverse effects.

5.Little or no risk with drinking water that meets the drinking water standard level and should be considered safe with respect to chlorine.

Can you have too much chlorine****?

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Advantages and Disadvantages of Chloramine Use****

Advantages• Chloramines are not as reactive with organics as free chlorine

in forming DBPs.• The monochloramine residual is more stable and longer lasting

than free chlorine or chlorine dioxide, thereby providing better protection against bacterial regrowth in systems with large storage tanks and dead end water mains. However excess ammonia in the network may cause biofilming.

• Because chloramines do not tend to react with organic compounds, many systems will experience less incidence of taste and odor complaints when using chloramines.

• Chloramines are inexpensive.• Chloramines are easy to make.

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Advantages and Disadvantages of Chloramine Use****

Disadvantages• The disinfecting properties of chloramines are not as

strong as other disinfectants, such as chlorine, ozone, and chlorine dioxide.

• Chloramines cannot oxidize iron, manganese, and sulfides.• When using chloramine as the secondary disinfectant, it

may be necessary to periodically convert to free chlorine for biofilm control in the water distribution system.

• Excess ammonia in the distribution system may lead to nitrification problems, especially in dead ends and other locations with low disinfectant residual.

• Monochloramines are less effective as disinfectants at high pH than at low pH.

• Dichloramines have treatment and operation problems.• Chloramines must be made on-site.

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Chlorine Residual Testing****

The presence of chlorine residual in drinking water indicates that:

• a sufficient amount of chlorine was added initially to the water to inactivate the bacteria and some viruses that cause diarrheal disease; and,

• the water is protected from recontamination during storage. The presence of free residual chlorine in drinking water is correlated with the absence of disease-causing organisms, and thus is a measure of the potability of water.

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Analyze samples for chlorine immediately after collection.

Free chlorine is a strong oxidizing agent; unstable in natural waters.

It reacts rapidly with various inorganic compounds and more slowly oxidizes organic compounds.

Factors including reactant concentrations,sunlight, pH, and temperature influence decomposition of free chlorine in water.

Avoid plastic containers may have a large chlorine demand.

Chlorine Sampling Issues****

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Pretreat glass sample containers to remove any chlorine demand Soak in a dilute bleach solution for at least 1

hour Dilute bleach solution =1 mL bleach to 1

liter of deionized water. Rinse thoroughly with deionized or distilled

water. Common error in chlorine testing is obtaining an

unrepresentative sample. If sampling from a tap, let the water flow for

at least 5 minutes to ensure a representative sample.

Let the container overflow with the sample several times, then cap the sample containers so there is no headspace (air) above the sample.

Chlorine Sampling Issues****

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Restricted Water Use During Chlorination****

1. Do not drink the water and avoid all body contact.2. Water use should be minimized to assure that

chlorine remains in the well during the minimum contact period.

3. If strong chlorine odors are detected, ventilate the effected area immediately, and minimize exposure to the fumes.

4. Avoid doing laundry, filling fish tanks, watering plants and using water for other purposes where the chlorine may have an adverse effect.

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Dechlorination ****• Dechlorination (removing residual chlorine from disinfected wastewater

prior to discharge into the environment/sensitive aquatic waters or in a treated water to be lowered prior to distribution

• the chlorinated water can be dosed with a substance that reacts with or accelerates the rate of decomposition of the residual chlorine.

• Compounds that may perform this function include thiosulfate, hydrogen peroxide, ammonia, sulfite/bisulfite/sulfur dioxide, and activated carbon;

• Hydrogen peroxide is not frequently used because it is dangerous to handle

• only the latter two materials have been widely used for this purpose in water treatment (Snoeyink and Suidan, 1975).

• (1) SO3-2 + HOCl = SO4

-2 + Cl- + H+

• (2) SO3-2 + NH2Cl+ H20 = SO4

-2 + Cl- + NH4+

• On a mass basis, 0.9 parts sulfur dioxide (or 1.46 parts NaHSO3 or 1.34 parts Na2S2O5) is required to dechlorinate 1.0 part residual chlorine.

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Dechlorination / ADVANTAGES AND DISADVANTAGES ****

Advantages• Protects aquatic life from toxic effects of residual chlorine.• Prevents formation of harmful chlorinated compounds in drinking water

through reaction of residual chlorine with water born organic materials.

Disadvantages• Chemical dechlorination can be difficult to control when near zero levels

of residual chlorine are required.• Significant overdosing of sulfite can lead to sulfate formation, suppressed

dissolved oxygen content, and lower pH of the finished effluent.

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Ozone Generation• 3O2 2O3• Ozone Generator

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Primary Uses of OzoneOzone is used in drinking water treatment for a variety of purposes including:

• Disinfection;

• Inorganic pollutant oxidation, including iron, manganese, and sulfide;

• Organic micropollutant oxidation, including taste and odor compounds, phenolic pollutants, and some pesticides; and

• Organic macropollutant oxidation, including color removal, increasing the biodegradability of organic compounds, DBP precursor control, and reduction of chlorine demand.

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Pathogen Inactivation and Disinfection Efficacy

• Ozone has a high germicidal effectiveness against a wide range of pathogenic organisms including bacteria, protozoa, and viruses.

• Ozone cannot be used as a secondary disinfectant because the ozone residual decays too rapidly.

• Ozone disinfection efficiency is not affected by pH although because of hydroxyl free radicals and rapid decay, efficiency is the same but more ozone should be applied at high pH to maintain “C”.

• Inactivation of bacteria by ozone is attributed to an oxidation reaction. The first site to be attacked appears to be the bacterial membrane. Also, ozone disrupts enzymatic activity of bacteria

• The first site of action for virus inactivation, particularly its proteins and RNA• aqueous ozone penetrates into the Giardia cysts wall and damages the

plasma membranes, additional penetration of ozone eventually affects the nucleus, and ribosome

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CT Values for Inactivation of Giardia Cysts by Ozone (pH 6

to 9)

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CT Values for Inactivation of Viruses by Ozone (pH 6 to 9)

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Advantages and Disadvantages of Ozone Use

Advantages

• Ozone is more effective than chlorine, chloramines, and chlorine dioxide for

inactivation of viruses, Cryptosporidium, and Giardia.

• Ozone oxidizes iron, manganese, and sulfides.

• Ozone can sometimes enhance the clarification process and turbidity removal.

• Ozone controls color, taste, and odors.

• One of the most efficient chemical disinfectants, ozone requires a very short

contact time.

• In the absence of bromide, halogen-substitutes DBPs are not formed.

• Upon decomposition, the only residual is dissolved oxygen.

• Biocidal activity is not influenced by pH.

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Advantages and Disadvantages of Ozone Use

Disadvantages

• DBPs are formed, particularly by bromate and bromine-substituted DBPs,

in the presence of bromide, aldehydes, ketones.

• The initial cost of ozonation equipment is high.

• The generation of ozone requires high energy and should be generated on-site.

• Ozone is highly corrosive and toxic.

• Biologically activated filters are needed for removing assimilable organic

carbon and biodegradable DBPs.

• Ozone decays rapidly at high pH and warm temperatures.

• Ozone provides no residual.

• Ozone requires higher level of maintenance and operator skill

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Fundamentals of UV• History of UV.• What is UV?• Terms • Concept of UV Dose• Types of UV Reactors• Inactivation mechanism• Environmental Effects• Advantages

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Ultra Violet - Beyond the Violet

100 400

Cosmic Rays

Gamma Rays X Rays Ultra

VioletVisible Light Infrared Micro

WavesRadio Waves

Electromagnetic Spectrum

200 315

UV C UV B UV A Visible Light

254 280 400

800

Expanded Scale of Ultraviolet Radiation

Germicidal Wavelengths 245-285 nm

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UV Dosage D = I · tWhere: D = UV Dose, mW×s/cm2

I = Intensity, mW/cm2 t = Exposure time, s

Terms • UV Output - Energy Delivered (W/lamp)• UV Intensity - Rate of Energy Delivery (mW/cm2(• UV Transmittance - Ability of water to transmit UV light

Terms and Dosage

Two main factors affect ultraviolet intensity:WATER QUALITY - Water quality refers to the clarity of the water to be treated and the degree to which it allows ultraviolet light to pass through it unobstructedLAMP OUTPUT - Proper lamp output is easily maintained by regular cleaning of the quartz sleeve that encases the lamp (generally every six months) and by lamp replacement once per year.

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Measuring UV Dosage• Intensity (Irradiance)

– actinometry (instruments used to measure the heating power of radiation) is a chemical system or physical device which determines the number of photons in a beam integrally or per unit time.

– Radiometers or UV sensors

– Mathematical models

• Time: The time of exposure to ultraviolet light (retention time) is directly related to the flow rate of water passing through the disinfection chamber. By changing the retention time for a given ultraviolet intensity, the dosage can be increased or decreased as needed.

• Bioassay Methods: (is a type of scientific experiment to measure the effects of a substance on a living organism )

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Lamp ConstructionThe lamps typically used in UV disinfection consist of a quartz tube filled with an inert gas, such as argon, and small quantities of mercury.

Tungsten Electrode Tungsten Electrode

Mercury Vapor and Argon Quartz Envelope

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UV applicationPath of UV Light in Water • Lamp Encasement - Water Quality - Target Organism

Pretreatment

Raw Water

Filtration UV Reactor

To Distribution

UV Treatment Application

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UV Applications in Drinking Water

• Disinfection of Surface Water

• Disinfection of Ground Water

• Oxidation of Organic Chemicals

• Oxidation of NOM

• By Using UV methods: no natural physiochemical features of the water are changed and no chemical agents are introduced into the water.

• formation of THM or other DBPs with UV disinfection is minimal

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Inactivation Mechanism• UV radiation is efficient at inactivating vegetative and sporous forms of

bacteria, viruses, and other pathogenic microorganisms.

• Electromagnetic radiation in the wavelengths ranging from 240 to 280nm

effectively inactivates microorganisms by irreparably damaging their nucleic

acid.

• The germicidal effects of UV light involve photochemical damage to RNA and

DNA within the microorganisms.

• DNA damage irreversible over time so UV contactors should be designed to

either shield the process stream or limit the exposure of the disinfected water to

sunlight immediately following disinfection.

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THE MECHANICS OF DISINFECTION

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Limitations of UV treatment“Point” Disinfection• UV units only kill bacteria at one point in a watering system and do not

provide any residual germicidal effect downstream. If just one bacterium passes through unharmed (100% destruction of bacteria cannot be guaranteed.

• Cells Not Removed• Bacteria cells are not removed in a UV unit but are converted into pyrogens.

The killed microorganisms and any other contaminants in the water are a food source for any bacteria that do survive downstream of the UV unit.

• Due to these limitations, the piping in a watering system treated by UV disinfection will need to be periodically sanitized with a chemical disinfectant.

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UV Effectiveness

Crypto/Giardia 10-20 4 logBacteria 20-40 4 logVirus 50-150 4 log

Dose Removal/Inactivation (mJ/cm2(

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• Effective for Crypto/virus• No chemicals added to

water• Small footprint• Pressurized system• Cost Effective• No DBPs• Oxidize organic chemicals

Benefits of UV

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Applicability of Alternative Disinfection Techniques