Wastewater

MID TERM TEST

1 st MARCH 2012

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Wastewater Wastewater Treatment SystemTreatment System

Ms Noor Rosyidah Binti Sajuni

School of Engineering

[email protected]

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Content

• Regulation and standard for wastewater

• Principle of wastewater treatment in industrial and domestic sewage,

• Primary treatment methods,

• Secondary treatment and

• Tertiary treatment methods

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Sources of Wastewater

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Wastewater Collection System

• Sewers are underground conduits for conveying wastewaters by gravity flow from urban areas to points of disposal.

• Storm waters enter a storm drainage system through inlets located in street gutters or depressed area that collect natural drainage.

• Sanitary sewer transport domestic and industrial wastewaters by gravity flow to treatment facilities.

• Sanitary sewers are placed at sufficient depth to prevent freezing and to receive wastewater from basement.

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Principle of wastewater treatment in industrial and domestic sewage

• Municipal wastewater treatment systems are designed to serve the needs of towns and cities

• The steps in treatment:

– Collection

– Preliminary treatment

– Primary treatment

– Secondary treatment

– Advanced treatment

– Disposal of residue

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Conventional Wastewater Treatment Process

Pretreatment involves:Screening Grit RemovalOil separationFlow equalization

Disinfection can use:Chlorine compoundsBromine ChlorideOzoneUV Radiation

Chemical Treatment is used in conjunction with the physical and chemical processes:Chemical precipitation Adsorption

Sludge Treatment and Disposal involves:grinding, degritting, blending, thickening, stabilization, conditioning, disinfection, dewatering, heat drying, thermal reduction, ultimate disposal

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Considerations in Plant Design

• Effluent quality: normally govern by local government authority. Example:

– Environmental Quality (Sewage & Industrial Effluents) Regulation 1979

• Specify the requirement for new sources of discharges requiring written permission of the Director General of Environment.

• It regulates the acceptable conditions of discharge into inland waters by specifying parameter limit of effluent.

• There are 2 standard: Standard A & Standard B

• Standard A: discharge that into any inland water within catchment areas.

• Standard B: discharge into any other inland water.

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• Design Loading.

– Wastewater quantity used for sizing basins and equipment varies with the nature of the equipment and whether the process is hydraulic or loading limited.

– The selection of the flow and load values must take into account hourly, daily, and seasonal variations.

– Flow and load values are typically expressed in terms of peak hour, max. day, min. day, max. average month; and annual average.

– Loading may be lbm of BOD, SS, concentration.

– Flow values are important during the sizing pumps, pipes, and hydraulically limited equipment.

– The loading value are important in sizing aeration and digestion equipment to meet peak demands.

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Typical Design Criteria for Treatment Processes

Process Loading

Flow measurement Peak hrly flow

Bar Screen Peak hrly flow

Pump Peak hrly flow

Min. hrly flow

Grit chamber Min. hrly flow

Max. monthly flow

Primary settling Max. monthly flow

Biological treatment

Max. monthly BOD loading

Peak hrly BOD loading

Final settling Max. monthly flow

Disinfection Peak hrly flow

Thickening Max. daily sludge flow, max. solid loading

Digestion Max. monthly volatile solid load, max. monthly sludge flow

Dewatering Max. sludge flow, max. solid loading

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On-site Disposal Systems

• About 85-90% of on-site wastewater disposal system are conventional septic systems.

• A conventional septic system consists of 3 parts: septic tank, a distribution box and absorption field.

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• Septic tank: to remove large particles and grease, which would otherwise clog the tile field. Heavy solids settle to the bottom and undergo decompositions. Grease float on the surface and trapped.

– The size of the tank should able to accommodate a holding time for water in a septic tank of at least 24 hrs. For individual homes : 3m3 tank min., 4m3 for 3 bedrooms house; 5m3 for 4 bedrooms house, and 6m3 for 5 bedrooms house.

– Bacteria action in the tank helps to degrade the organic matter in the wastewater.

• Distribution box: to distribute the septic tank effluent throughout the absorption field.

• Absorption field: Consists of a series of trenches that contain perforated PVC pipes that are about 10cm in diameter.

– The pipes are placed over 15cm deep layer of drainrock and then buried with an additional layer of drainrock.

• Most septic system will fail eventually. The normal lifetime of an absorption field is 20-30 years.

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• Water-tight sewer lines• Septic tank• Disposal system

– Subsurface drain field• Absorption trenches• Infiltration chambers

– ET/Absorption trenches– Aerobic system with sprinkler– Total retention lagoons– Pre-approved alternative disposal systems

• A wastewater system is made up of several connected components. Water-tight sewer lines convey wastewater from the house to the septic tank and from the tank to the disposal system.

• All systems will have a septic tank to digest organic solids. The effluent, or liquid waste discharged from the tank goes to some type of disposal system.

• The most common disposal system is a subsurface drain field. Traditionally this has been a perforated pipe buried in a gravel-filled trench. Recently polyethylene infiltration chambers have been used in place of the traditional absorption trench.

• In the case of sites where soils are not suitable for absorption trenches (usually because of poor infiltration rates, but also because of shallow water tables or rock outcrops) other types of disposal systems may be required.

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Unit Operations of Pretreatment

• Pretreatment have little effect in reducing BOD5.• Physical unit operations are normally the 1st treatment methods to be used. • Screening is generally as the 1st unit operation in the wastewater treatment

plant.– A screen is a device with openings, generally uniform size, that is

used to retain solids found in the influent wastewater to the plant or combined wastewater collection systems.

– The principle role of screening is to remove coarse materials from the flow stream that could:i. Damage subsequent equipmentii. Reduce overall treatment process reliability and effectivenessiii. Contaminate waterways

– Sometimes, fine screens are used in place of or following coarse screens where greater removals of solid are required to:i. Protect process equipmentii. Eliminate material that may inhibit the beneficial reuse of

biosolids

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Classification of Screens

Screening

Coarse Screen

Microscreen Fine screens

6-150 mm< 0.5 μm

< 6 mm

Hand cleaned

Mechanically cleaned

Static wedgewire

drum Step

Chain-driven

Reciprocating rake

CatenaryContinuous

belt

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• Coarse Screen (Bar Racks)

– To remove large objects that would damage or foul pumps, valves, and other mechanical equipment.

– Hand-Cleaned Coarse Screens

• Used frequently ahead of pumps.

– Mechanically cleaned bar screens

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Design Factor for Coarse Screens• Design consideration

– Location

– Approach velocity

– Clear opening between bars and mesh size

– Headloss through screens

– Screen handling, processing and disposal

– Control

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Fine Screens

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• Grit Chambers

– Grit chambers are basin to remove the inorganic particles to prevent damage to the pumps, and to prevent their accumulation in sludge digestors.

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• Equalization

– Flow equalization is not a treatment process but a technique that can be used to improved the effectiveness of both secondary and advance wastewater treatment processes.

– Wastewater does not flow into a municipal wastewater treatment plant at a constant rate.

– The purpose of flow equalization is to dampen the variation so that the wastewater can be treated at a nearly constant flow rate.

– For existing plant, it can improve the performance.

– For new plant, it can reduce the size and cost of the treatment unit.

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Primary Treatment• With the screening completed and the grit removed, the wastewater still

contains light organic suspended solids.

• Some of these can be removed from sewage by gravity in a sedimentation tank. The mass of settled solids is called raw sludge.

• Sludge can be removed from sedimentation tank by mechanical scrapers and pumps.

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Sedimentation

• Sedimentation or clarification is the removal of particulate matter, chemical floc, and precipitates from suspension through gravity settling.

• The common criteria for sizing settling basins are detention time, overflow rate, weir loading, and with rectangular tanks, horizontal velocity.

– Detention time, expressed in hour as:

– Overflow rate (surface loading):

• There are 2 types of sedimentation tank: circular and rectangular tanks

hourtdaymflowdailyaverageQmvolumebaVwhereQ

Vt ),/(),(sin 33

areasurfaceAdaymmVwhereA

QV oo ,/ 23

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Type of Sedimentation

• Type I: Discrete particle settling - Particles settle individually without interaction with neighbouring particles.

• Type II: Flocculent Particles – Flocculation causes the particles to increase in mass and settle at a faster rate.

• Type III: Hindered or Zone settling –The mass of particles tends to settle as a unit with individual particles remaining in fixed positions with respect to each other.

• Type IV: Compression – The concentration of particles is so high that sedimentation can only occur through compaction of the structure.

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Comparison of Type I and II sedimentation

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Coagulant

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Coagulation is the destabilization of colloids by addition of chemicals that neutralizes the charges of the colloids. The chemicals are known as coagulants, usually higher valence cationic salt (Al3+, Fe3+ etc.) Flocculation is the agglomeration of destabilized particles into a large size particles known as flocs which can be effectively removed by sedimentation or floatation. The addition of another reagent called flocculant may promote the formation of the floc.

OHSOHsOHAlOHSOAl 24232342 83)()(214)(

ClHsOHFeOHFeCl

OHClCOsOHFeHCOOHFeCl

3)()(3

733)()(37

323

223323

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Biological Treatment

• Optimize primary treatment efficiency. BOD removed by primary treatment requires less energy than BOD removed by biological treatment, also surplus sludge should not accumulate in the primary clarifier to avoid dissolving and carry-over to the secondary process.

• Control amount of aeration to avoid excessive dissolved oxygen.

• Improve aeration system efficiency. For example, fine-bubble diffusers can increase oxygen transfer efficiency in comparison to coarse-bubble diffusers.

Types:

Aerobic ProcessesAnoxic ProcessesAnaerobic ProcessesCombined Aerobic-Anoxic-Anaerobic ProcessesPond Processes

Attached GrowthSuspended Growth Combined Systems

Aerobic MaturationFacultativeAnaerobic

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• Air activated sludge is an aerobic process in which bacteria consume organic matter, nitrogen and oxygen from the wastewater and grow new bacteria. The bacteria are suspended in the aeration tank by the mixing action of the air blown into the wastewater. There are many derivations of the activated sludge process, several of which are described in this section.

• High purity oxygen activated sludge is an aerobic process very similar to air Activated sludge except that pure oxygen rather than air is injected into the wastewater.

• Aerated pond/lagoon is an aerobic process very similar to air activated sludge.

Mechanical aerators are generally used to either inject air into the wastewater or to cause violent agitation of the wastewater and air in order to achieve oxygen transfer to the wastewater. As in air activated sludge, the bacteria grow while suspended in the wastewater.

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• Trickling filter is a fixed film aerobic process. A tank containing media with a high surface to volume ratio is constructed. Wastewater is discharged at the top of the tank and percolates (trickles) down the media. Bacteria grow on the media utilizing organic matter and nitrogen from the wastewater.

• Rotating biological contactor (RBC) is a fixed film aerobic process similar to the trickling filter process except that the media is supported horizontally across a tank of wastewater. The media upon which the bacteria grow is continuously rotated so that it is alternately in the wastewater and the air.

• Oxidation ditch is an aerobic process similar to the activated sludge process. Physically, however, an oxidation ditch is ring-shaped and is equipped with mechanical aeration devices.

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Major Aerobic Biological Processes

Type of Growth

Common Name Use

Suspended Growth

Activated Sludge (AS) Carbonaceous BOD removal (nitrification)

Aerated Lagoons Carbonaceous BOD removal (nitrification)

Attached Growth

Trickling Filters Carbonaceous BOD removal. nitrification

Roughing Filters (trickling filters with high hydraulic loading rates)

Carbonaceous BOD removal

Rotating Biological Contactors

Carbonaceous BOD removal (nitrification)

Packed-bed reactors Carbonaceous BOD removal (nitrification)

Combined Suspended & Attached Growth

Activated Biofilter ProcessTrickling filter-solids contact processBiofilter-AS processSeries trickling filter-AS process

Carbonaceous BOD removal (nitrification)

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Activated Sludge Process• The aeration tank contains a suspension of the wastewater and microorganisms, the

mixed liquor. The liquor is mixed by aeration devices (supplying also oxygen)• A portion of the biological sludge separated from the secondary effluent by

sedimentation is recycled to the aeration tank• Types of AS Systems: Conventional, Complete-Mix, Sequencing Batch Reactor,

Extended Aeration, Deep Tank, Deep Shaft

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Advantages/Disadvantages

Advantages

• Flexible, can adapt to minor pH, organic and temperature changes

• Small area required

• Degree of nitrification is controllable

• Relatively minor odor problems

Disadvantages

• High operating costs (skilled labor, electricity, etc.)

• Generates solids requiring sludge disposal

• Some process alternatives are sensitive to shock loads and metallic or other poisons

• Requires continuous air supply

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Trickling Filters

• The trickling filter or biofilter consists of a bed of permeable medium of either rock or plastic

• Microorganisms become attached to the media and form a biological layer or fixed film. Organic matter in the wastewater diffuses into the film, where it is metabolized. Periodically, portions of the film slough off the media

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Advantages

• Good quality (80-90% BOD5 removal) for 2-stage efficiency could reach 95%

• Moderate operating costs (lower than activated sludge)

• Withstands shock loads better than other biological processes

Disadvantages

• High capital costs

• Clogging of distributors or beds

• Snail, mosquito and insect problems

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Rotating Biological Contactors• It consists of a series of circular disks of polystyrene or polyvinyl

chloride that are submerged in wastewater and rotated slowly through it

• The disk rotation alternately contacts the biomass with the organic material and then with atmosphere for adsorption of oxygen

• Excess solids are removed by shearing forces created by the rotation mechanism

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Advantages

• Short contact periods

• Handles a wide range of flows

• Easily separates biomass from waste stream

• Low operating costs

• Short retention time

• Low sludge production

• Excellent process control

Disadvantages

• Need for covering units installed in cold climate to protect against freezing

• Shaft bearings and mechanical drive units require frequent maintenance

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Major Anaerobic Biological Processes

Type of Growth

Common Name Use

Suspended Growth

Anaerobic Contact Process Carbonaceous BOD removal

Upflow Anaerobic Sludge-Blanket (UASB)


Attached Growth

Anaerobic Filter Process Carbonaceous BOD removal, waste stabilization (denitrification)

Expanded Bed Carbonaceous BOD removal, waste stabilization

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Anaerobic Contact Process

• Untreated wastewater is mixed with recycled sludge solids and then digested in a sealed reactor

• The mixture is separated in a clarifier

• The supernatant is discharged as effluent, and settled sludge is recycled

Advantages/DisadvantagesAdvantages

• Methane recovery

• Small area required

• Volatile solids destruction

Disadvantages

• Heat required• Effluent in reduced chemical form

requires further treatment• Requires skilled operation• Sludge to be disposed off is

minimal

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Upflow Anaerobic Sludge Blanket• Wastewater flows upward through a sludge blanket composed of biological

granules that decompose organic matter• Some of the generated gas attaches to granules that rise and strike degassing

baffles releasing the gas• Free gas is collected by special domes• The effluent passes into a settling chamber

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Advantages

• Low energy demand

• Low land requirement

• Low sludge production

• Less expensive than other anaerobic processes

• High organic removal eficiency

Disadvantages

• Long start-up period• Requires sufficient amount of

granular seed sludge for faster start-up

• Significant wash out of sludge during initial phase of process

• Lower gas yield than other anaerobic processes

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Major Anoxic and Combined Biological Processes

Type of Process

Type of Growth

Common Name Use

Anoxic Suspended Growth

Suspended Growth Denitrification

Denitrification

Attached Growth

Fixed-film Denitrification

Denitrification

Combined Aerobic, Anoxic, and anaerobic Processes

Suspended Growth

Single- or multi-stage processes, various proprietary processes

Carbonaceous BOD removal, nitrification, denitrification, phosphorus removal

Attached Growth

Single- or multi-stage processes

Carbonaceous BOD removal, nitrification, denitrification, phosphorus removal

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Pond Treatment Processes

Common Name Comments Use

Aerobic Stabilization Ponds

Treatment with aerobic bacteria; oxygen is supplied by algal photosynthesis and natural surface re-aeration; depth of 0.15 to 1.5 m


Maturation (tertiary) Ponds

Use aerobic treatment; applied loadings are low to preserve aerobic conditions

Secondary effluent polishing and seasonal nitrification

Facultative Ponds

Treatment with aerobic, anaerobic and facultative bacteria; the pond has 3 zones: a surface aerobic zone, a bottom anaerobic zone, and an intermediate zone partly aerobic-anaerobic


Anaerobic Ponds Treatment with anaerobic bacteria; depths of up to 9.1 m to conserve anaerobic conditions

Carbonaceous BOD removal (waste stabilization)

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Suspended Growth Separate Stage Nitrification

Single State Nitrification

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Attached Growth

Attached Growth Nitrification following Act. Sludge

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Biological Denitrification

• A modification of aerobic pathways (no oxygen)

– Same bacteria that consume carbon material aerobically

• Denitrifying bacteria obtain energy from the conversion of NO3- to N2 gas, but

require a carbon source

NO3- + CH3OH + H2CO3 C5H7O2N + N2 + H2O + HCO3

-

• Need low (no) oxygen (< 1 mg/L)

• Need carbon source (BOD in Wastewater)

• Neutral pH (pH 7)

• Conc of nitrate

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Phosphorus Removal

• Chemical Precipitation

– Calcium (lime) addition at high pH (>10)

• Reacts with alkalinity

– Alum (Aluminum Sulfate) precipitation

– Iron precipitation

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Sewage Treatment

Tertiary Treatment (Physicochemical Process)• Precipitation

• Filtration

• Chlorination

• Treated water is discharged to waterways

• Used for irrigation

• Recycled into drinking water

expensive process, sharply reduces inorganic nutrients (PO4, NO3)

Wastewater

Engineering

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