chapter 2 saad hameed abid
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Chapter Two
Drinking Water Impurities and Treatment
2.1 Introduction
Many areas have water containing impurities from natural or
artificial sources. These impurities may cause health problems, damage
equipment or plumbing, or make the water undesirable due to taste, odor,
appearance or staining. Water related problems will be found primarily in
homes serviced by a private water supply, although occasionally, they
will be found in water from municipal water supplies. Those impurities
which cause health problems should be attended to immediately; other
problems caused by water impurities can be corrected if they are a
nuisance. Before beginning any treatment plan, have water tested by an
independent laboratory to determine the specific impurities and level of
contamination. This will help you select the most effective and
economical treatment method [1].
2.2 Source of Water [4]
The main sources of water are:
Surface water: It includes flowing water (streams and rivers) and
still water (lakes, ponds and reservoirs).
Underground water: It includes water from wells and springs.
Rain water
Sea water.
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2.3 Drinking-water supply agencies
Drinking-water supplies vary from very large urban systems
servicing populations with tens of millions to small community systems
providing water to very small populations. In most countries, they include
community sources as well as piped means of supply.
Drinking-water supply agencies are responsible for quality assurance and
quality control. Their key responsibilities are to rep are and implement
WSPs [6]
To provide water supplies to customers we abstract water from
lochs, reservoirs and boreholes and burns. source waters are treated to
remove impurities to provide safe drinking water. then distribute this high
quality treated water through an extensive network of pipes, pumping
stations and storage tanks for customers to use for drinking, cleaning,
recreation, gardening, or in business processes, as shown in figure (2.1)
[8].
(Figure 2.1) Drinking water supply agent
In many cases, the water supplier is not responsible for the
management of the catchment feeding sources of its supplies. The roles of
the water supplier with respect to catchments are to participate in
interagency water resource management activities; to understand the risks
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arising from potentially contaminating activities and incidents; and to use
this information in assessing risks to the drinking-water supply and
developing and applying appropriate management. Although drinking-
water suppliers may not undertake catchment surveys and pollution risk
assessment alone, their roles include recognizing the need for them and
initiating multiagency collaboration – for example, with health and
environmental authorities. Experience has shown that an association of
stakeholders in drinking- water supply (e.g., operators, managers and
specialist groups such as small suppliers, scientists, sociologists,
legislators, politicians, etc.) can provide a valuable non-threatening forum
for interchange of ideas. [6]
2.4 Community management
Community-managed drinking-water systems, with both piped andnon-piped distribution, are common worldwide in both developed and
developing countries. The precise definition of a community drinking-
water system will vary. While a definition based on population size or the
type of supply may be appropriate under many conditions, approaches to
administration and management provide a distinction between the
drinking-water systems of small communities and those of larger towns
and cities. This includes the increased reliance on often untrained and
sometimes unpaid community members in the administration and
operation of community drinking-water systems. Drinking-water systems
in per urban areas in developing countries – the communities surrounding
major towns and cities may also have the characteristics of community
systems [6].
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Effective and sustainable programmers for the management of
community drinking- water quality require the active support and
involvement of local communities. These communities should be
involved at all stages of such programmers, including initial surveys;
decisions on sitting of wells, sitting of off-takes or establishing protection
zones; monitoring and surveillance of drinking-water supplies; reporting
faults, carrying out maintenance and taking remedial action; and
supportive actions, including sanitation and hygiene practices [6].
A community may already be highly organized and taking action on
health or drinking- water supply issues. Alternatively, it may lack a well
developed drinking-water system; some sectors of the community, such
as women, may be poorly represented; and there may be disagreements or
factional conflicts. In this situation, achieving community participation
will take more time and effort to bring people together, resolve
differences, agree on common aims and take action. Visits, possibly over
several years, will often be needed to provide support and encouragementand to ensure that the structures created for safe drinking-water supply
continue to operate. This may involve setting up hygiene and health
educational programmers to ensure that the community:
— is aware of the importance of drinking-water quality and its relation to
health and of the need for safe drinking-water in sufficient quantities for
domestic use for drinking, cooking and hygiene; — recognizes the importance of surveillance and the need for a
community response;
— understands and is prepared to play its role in the surveillance process;
— has the necessary skills to perform that role;
— is aware of requirements for the protection of drinking-water supplies
from pollution. [6]
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2.5 Chemicals from industrial sources and human dwellings
Chemicals from industrial sources can reach drinking-water directly
from discharges or indirectly from diffuse sources arising from the use
and disposal of materials and products containing the chemical. In some
cases, inappropriate handling and disposal may lead to contamination,
e.g., degreasing agents that are allowed to reach ground-water. Some of
these chemicals, particularly inorganic substances, may also be
encountered as a consequence of natural contamination, but this may also
be a byproduct of industrial activity, such as mining, that changes
drainage patterns. Many of these chemicals are used in small industrial
units within human settlements, and, particularly where such units are
found in groups of similar enterprises, they may be a significant source of
pollution. Petroleum oils are widely used in human settlements, and
improper handling or disposal can lead to significant pollution of surface
water and groundwater. Where plastic pipes are used, the smaller
aromatic molecules in petroleum oils can sometimes penetrate the pipes
where they are surrounded by earth soaked in the oil, with subsequent
pollution of the local water supply [6].
A number of chemicals can reach water as a consequence of disposal
of general household chemicals; in particular, a number of heavy metals
may be found in domestic wastewater. Where wastewater is treated, these
will usually partition out into the sludge. Some chemicals that are widelyused both in industry and in materials used in a domestic setting are
found widely in the environment, and these may be found in water
sources, although usually at low concentrations. Some chemicals that
reach drinking-water from industrial sources or human settlements have
other primary sources and are therefore discussed in other sections of this
chapter. Where latrines and septic tanks are poorly sited, these can lead tocontamination of drinking-water sources with nitrate [6].
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Identification of the potential for contamination by chemicals from
industrial activities and human dwellings requires assessment of activities
in the catchment and of the risk that particular contaminants may reach
water sources. The primary approach to addressing these contaminants is
prevention of contamination by encouraging good practices. However, if
contamination has occurred, then it may be necessary to consider the
introduction of treatment, Table (2.1) Guideline values for naturally
occurring inorganic chemicals that are of health significance in drinking
water [6].
Table (2.1) inorganic chemicals that are of health significance in drinking
water
2.6 Diseases
Sanitation and hygiene, approximately 2.4 million deaths globally
could be prevented each year if every person practiced good hygiene and
had clean drinking water. In developing countries water and sanitation
deficits contribute to almost half of all people’s suffering. This is largely
due to the many diseases that result from drinking unsanitary water.
Waterborne diseases constitute the majority of illnesses that cause
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suffering and death in developing countries. These are diseases that result
from contact or consumption of infected water. Some of these diseases
include malaria, typhoid, cholera, Guinea worm, E. coli, Giardia,
amoebas, and other parasites; but the greatest disease caused by unsafe
drinking water is diarrhea. In fact, diarrhea is currently one of the greatest
killers of children in the world. Pathogenic microorganisms, such as
protozoa or bacteria, in contaminated fresh water may be transferred to a
person through drinking the water, washing with the water, or eating
foods that have been prepared from the unclean water. The person then
becomes infected from the pathogenic microorganisms contained in the
water and develops a waterborne disease [3].
Not only are there waterborne diseases, there are also water-washed
diseases affecting millions of people. Water-washed diseases encompass
those diseases that are removed by merely washing with water. In regards
to health, water is vital to having proper hygiene. Simply washing one’s
hands with soap and water can reduce the risk of endemic diarrhea by upto almost 50% in addition to other respiratory or skin infections [3]
The quality of the water needs to be evaluated not only for
pathogens, but also for harmful chemicals that can contaminate the water
as well. For example, mercury is commonly found in water but is not
harmful in small amounts. However, in large quantities, mercury can be
very harmful to the human body and cause various forms of illness.Chemicals and pathogens immersed in water supplies also cause the
water to taste badly. The taste of the water influences how much of the
water a person drinks; humans need a lot of water and the poor taste of
the water available to a person should not be a factor determining how
much he receives. These issues of water quality are all part of the water
crisis that need to be solved.[3]
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2.7 Impurities of Water [4]
The impurities present in water may be categorized into following
categories:
(1) Dissolved Impurities
(a) Dissolved gases: O2, CO2, H2S etc.
(b) Inorganic salts:
(i) Cations: Ca++, Mg++, Na+, K +, Fe++, Al+++ etc.
(ii)Anions: CO3 – , Cl
– , SO4
– , NO3
– etc.
(c) Organic salts.
(2) Suspended Impurities
(a) Inorganic: Clay and sand.
(b) Organic: Oil globules, vegetables, and animal material.
(3) Colloidal Impurities
Finally divided clay and silica Al(OH)3, Fe(OH)3, organic waste products,
coloring matter, amino acids etc.
(4) Microscopic Matters
Bacteria, algae, fungi etc.
2.8 Sources of Impurities in Water [4]
Following are the sources of impurities in water:
Gases (O2, CO2 etc.) are picked up from the atmosphere by
rainwater.
Decomposition of plants and animals remains introduce organic
impurities in water.
Water dissolves impurities when it comes in contact with ground,
soil or rocks.
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Impurities are also introduced in water when it comes in contact
with sewage or industrial waste.
River water contains dissolved minerals like chlorides, sulphates,
bicarbonates of sodium, magnesium, calcium and iron. It also contains
suspended impurities of sand, rocks and organic matter. The composition
of river water is not constant. The amount of dissolved impurities in it
depends on its contacts of the soil. Greater the duration of contact, more
soluble is the minerals of soil in it. Lake water has high quantity of
organic matter present in it but lesser amount of dissolved minerals. Its
chemical composition is also constant. Rain water is obtaining as a result
of evaporation from the surface water. Probably it is the purest form of
natural water. But during its downward journey through the atmosphere it
dissolves organic and inorganic suspended particles and considerable
amount of industrial gases like (CO2, NO2, SO2 etc.). Rain water is
expensive to collect and is irregular in supply [4].
Underground water is free from organic impurities and is clearer in
appearance due to the filtering action of the soil. But it contains large
amount of dissolved salts. Sea water is very impure due to two reasons:
1. Continuous evaporation increases the dissolved impurities content,
which is further increased by the impurities thrown by rivers as they joinsea.
2. It is too saline for most industrial uses except cooling.
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2.9 Treatment Methods of Water [5]
A. Internal Treatment
B. External Treatment
A- Internal Treatment Method
1. Phosphate Conditioning:
- Small amount of phosphate ions are added to precipitate Ca ions.
- Chosen depending on the pH conditions of boiler.
2. Colloidal Conditioning:
-
Using kerosine, tannin, starch etc
-
Get coated over the scale forming particles
- Removed by Blow down Process
3. Carbonate conditioning:
- Na2CO3 is added to precipitate Ca salts as CaCO3
-
Removed by Blow down Process
-
Used in low pressure boilers
4. Calgon Conditioning:
-
Scale forming salts are converted into soluble complexes.
- E.g. Sodium Hexameta Phosphate (Na2PO3)6 is added…reacts with
Ca and forms Calcium Hexameta Phosphate (Ca2PO3)6
- Prevents Scale formation
5. Radioactive conditioning:
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-
Adding radioactive tablets
- Emits radiation energy which prevents Scale formation
6. Electrical Conditioning:
- Mercury bulbs placed in boiler
-
Emits electrical discharge
-
Prevents Scale formation
B- External Treatment Method (or) Water softening Method
• Removal of hardness causing substances from water
Methods:
1.
Zeolite process
2.
Ion Exchange Process
3.
Mixed Bed deionization
Zeolite (or Permutit) Process: are Hydrated sodium alumino Silicate
Na2O. Al2O3 X SiO2 Y H 2O (X= 2-10, Y= 2-6 )
Natural Zeolites:
1. Natrolite - Na2O. Al2O3 4SiO2 .2H 2O
2.
Laumontite - CaO. Al2O3 4SiO2 .4H 2O
3.
Harmotome - (BaO.K 2O). Al2O3 5SiO2 .5H 2O
- Capable of exchanging its Na ions.
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As noted above, where a health-based guideline value cannot be
achieved by reasonably practicable treatment, then the guideline value is
designated as provisional and set at the concentration that can be
reasonably achieved through treatment. Collection, treatment, storage and
distribution of drinking-water involve deliberate additions of numerous
chemicals to improve the safety and quality of the finished drinking-water
for consumers (direct additives). In addition, water is in constant contact
with pipes, valves, taps and tank surfaces, all of which have the potential
to impart additional chemicals to the water (indirect additives) [7].
Water treatment transforms raw surface and groundwater into safe
drinking water. Water treatment involves two major processes: physical
removal of solids and chemical disinfection [7].
Water treatment must go through these steps:
1- Coagulation:
Coagulation removes dirt and other particles suspended in water.
Alum and other chemicals are added to water to form tiny sticky particles
called ―floc‖ which attract the dirt particles. The combined weight of the
dirt and the alums (floc) becomes heavy enough to sink to the bottom
during sedimentation, as shown in figure (2.2) [7].
Figure (2.2) Coagulation
Coagulation
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2- Sedimentation:
Coagulated particles fall, by gravity, through water in a settling tank
and accumulate at the bottom of the tank, clearing the water of much of
the solid debris and clear water moves to filtration, as shown in figure
(2.3) [7].
Figure (2.3) Sedimentation
3- Filtration, Disinfection & Storage:
Filtration: The water passes through filters, some made of layers of
sand, and charcoal that help remove smaller particles [7].
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Disinfection: A small amount of chlorine is added or some other
disinfection method is used to kill microorganisms that may be in the
water [7].
Storage: Water is placed in a closed tank or reservoir for disinfection
to take pace. The water then flows through pipes to home and business in
the community [7].
Figure (2.4) Filtration, Disinfection & Storage
Purpose of disinfection: To make Drinking water free of any disease
causing bacteria and microbes.
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Methods of disinfection :
There are 3 mainly used disinfection methods at large scale.
CHLORINATION
OZONATION
ULTRAVIOLET RADIATION
Chlorine is the most common cost-effective means of disinfecting
water. The addition of a small amount of chlorine is highly effective
against most bacteria, viruses, and protozoa. But cysts (durable seed-like
stages) formed by parasitic protozoa such as Cryptosporidium and
Giardia can survive chlorine [7].
Chlorine is applied to water in one of three forms: elemental
chlorine (chlorine gas), hypochlorite solution (bleach), or dry calcium
hypochlorite. All three forms produce free chlorine in water [7]
Ozonation: OZONE is Strongest oxidant/disinfectant available. More
effective against microbes than chlorination. But, costly and difficult to
monitor and control under different condition [7].
Ozonation process:
Ozone (o3) is generated on-site at water treatment facilities by
passing dry oxygen or air through a system of high voltage electrodes [7].
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Figure (2.5) Complete Cycle of Water Treatment
2.10 Hardness in Drinking Water
Hardness in drinking water is defined as those minerals that
dissolve in water having a positive electrical charge.
The primary components of hardness are calcium (Ca++) and magnesium
(Mg++) ions. Dissolved iron (Fe++) and manganese (Mn++) also satisfy
the definition of hardness, but typically make up only a very small
fraction of total hardness. Minerals are composed of either atoms or
molecules. An atom or molecule that has dissolved in water is called an
―ion.‖ Positively charged ions are called cations and are noted as (+). A
double sign would indicate a plus two electrical charge. Contaminants
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having a similar positive charge would be removed by a matching type of
ion exchange resin, i.e. water softening [9].
2.11 Health Effects of Hardness
The presence or absence of the hardness minerals in drinking water is not
known to pose a health risk to users. Hardness is normally considered an
aesthetic water quality factor. The presence of some dissolved mineral
material in drinking water is typically what gives the water its
characteristic and pleasant taste. At higher concentrations however,
hardness creates the following consumer problems [9].
• Produces soap scum most noticeable on tubs and showers.
• Produces white mineral deposits on dishes more noticeable on
clear glassware.
• Reduces the efficiency of devices that heat water. As hardness
deposits build in thickness, they act like insulation, reducing the
efficiency of heat transfer.
It has also been observed that areas of higher hardness in drinking water
maybe associated with lower incidents of heart disease. This possible
relationship is being investigated [9].
2.12 Water Softening
Water softening uses an ion exchange process. Sodium typically is
put into the water while hardness and certain other minerals are
proportionally removed. A private home water softener typically has two
tanks. The taller tank contains the purifying media called a cation ion
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exchange resin, while the smaller tank contains the sodium or potassium
salt used to regenerate the exchange resin. During normal operations, raw
water passes through the ion exchange resin media in the tall tank. The
calcium (Ca++), magnesium (Mn++), iron (Fe++), or manganese (Mn++)
ions and other ions in the water are ―exchanged‖ for sodium (Na+) or
potassium (K+) ions which have been temporarily stored in the pores of
the exchange resin. A recent improvement in softeners is the introduction
of an equipment configuration that backwashes when needed rather then
by time clock.
As the softener removes hardness minerals from the water supply,
sodium or potassium will be given back to the water proportionally.
Shown below is the concentration of either sodium or potassium that
would be added to the existing raw water concentration, if 10 mg/L of
hardness is removed. To determine the increase for your situation, divide
your total hardness by 10 and then multiply that result by the appropriate
number to the right of the equal sign [9].
Eventually the removal capacity of the resin media becomes
exhausted and the resin will need to be regenerated. The regeneration
process begins by a rapid backwashing of the resin to remove any fine
particulate material that may originate in the well. The process then
continues at a slower rate by ―brining‖ which is the adding of salt
solution to the resin. During this process the sodium or potassium from
the brine enters the resin pores and displaces the previously removed
hardness ions or iron/manganese. After approximately 20 minutes, the
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remaining brine, along with the concentrated displaced hardness ions and
other ions are flushed out of the device and disposed of into an approved
dry well, septic tank, or sewer [9].
Some owners of water softeners have expressed concerns relative to
the affect of the waste brine on their leach fields. Studies by the Water
Quality Association (WQA) indicate that concentrated waste brine and
purged contaminants do not injure leach fields or septic tanks. The WQA
is the professional association of the small water treatment device
industry. Additional studies are now underway [9].
Sodium and chloride do not disappear when disposed into the
ground. Sometimes this disposal can contaminant wells downhill of the
party using the softener. Thus, reducing salt usage as much as possible is
desirable [9].
2.13 Advantages of Water Softening [9]
• Softener resin can be regenerated and re-used.
• Ion exchange can consistently remove hardness from water to
extremely low levels.
• Softening removes dissolved iron and manganese (ie colorless).
Other water quality factors, such as pH and alkalinity, are notcritical to removing iron and manganese.
• Conventional softening can also remove other health related
contaminants.
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2.14 Disadvantages of Water Softening [9]
• Adds sodium or potassium to your drinking water depending on
which ―salt‖ you use. For those concerned with elevated sodium
levels in their drinking water, potassium chloride (KCl) can be
substituted in place of sodium chloride (NaCl). The process is
equally as efficient, however the cost of potassium chloride is
higher than sodium chloride.
• Softening will not operate satisfactorily if particulates such as
iron bacteria, clay particles, rusty colored water exists, even
occasionally. If any solids are present, a particle (sediment)
filter must be installed before the media tank.
• Water softeners require a location to dispose of waste brine. If
you do not have sewer service, disposal of the waste brine will
likely be into the ground. This creates the potential of
contaminating the groundwater, and subsequently your own
well or those wells of your neighbors down hill. When
potassium chloride is used, the potassium should be recognized
as a soil nutrient, being one of the three components of typical
manmade fertilizer.