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close this book Water purification, distribution and sewage disposal for Peace Corps volunteers
close this folder Section 2: Water treatment
View the document Overview:
View the document Self purification
View the document Basic steps in treating water
View the document Sample designs for treatment systems
View the document Lesson plans

Section 2: Water treatment



When the source of water supply is not entirely satisfactory treatment is necessary to insure that the quality of the water meets certain requirements. The trainees are instructed in the basic requirements of water treatment and receive detailed plans for the installation of two simple yet effective treatment systems.



Determine which of the potential water supply sources are the most economically feasible in terms of any treatment process requirements.


1. Define minimal standards of concentration for each pollutant.

2. Identify the nature and extent of pollution for each water supply source.

3. Determine which type of treatment system would most probably be necessary to reduce the pollution level of each source to a safe level.

4. Determine the cost of a treatment process or processes for each source.

5. Select the most economical source(s) in terms of capacity to serve appropriate numbers of people, and treatment process requirements.


1. Describe the four methods used in treating polluted water.

2. Identify the relative costs of different types of treatment systems.

3. Know the effect various pollutants have on different delivery systems.

4. Know what pollutants make water esthetically objectionable.

5. Know what concentrations of chemical pollutants and coliform bacteria constitute health hazards.

6. Identify the factors that influence the future population trends of a given locale.

7. Recognize the relationship between number of water system users and treatment process capacities,


1. Given the designs for various types of treatment systems, calculate with reasonable accuracy the cost of each.

2. Given a list of pollutants, correctly list after each one, if applicable:

a. in what circumstances it can contribute to the destruction of a delivery system.

b. in what concentration it makes water esthetically objectionable.

c. in what concentration it constitutes e health hazard.


Self purification



Under favorable conditions, any polluted body of surface water-stream or river, lake or pond - will rid itself of a certain amount of its pollution by means of nature, processes. This self-purification cannot de depended upon to bring about complete purification, but it may well improve the water quality sufficiently to ease the load on mechanical purification equipment.


When sewage is discharged into water, a succession of changes in water quality takes place. If the sewage is emptied into a lake in which currents about the outfall are sluggish and shift their direction with the wind, the changes occur in close proximity to each other and, as a result, the pattern of changes is not crisply distinguished. If, on the other hand, the water moves steadily away from the outfall, as in a stream, the successive changes occur in different river reaches and establish a profile of pollution which is well defined. However, in most streams, this pattern is by no means static. It shifts longitudinally along the stream and is modified in intensity with changes in season and hydrography.

When a single large charge of sewage is poured into a clean stream, the water becomes turbid, sunlight is shut out of the depths, and green plants, which by photosynthesis remove carbon dioxide from the water and release oxygen to it, die off. Depending on the stream velocity, the water soon turns nearly black. Odorous sulfur compounds are formed and solids settle to the bottom, forming a sludge. The settled solids soon decompose, forming gases such as ammonia, carbon dioxide, and methane or marsh gas. Scavenging organisms increase in number until they match the food supply. The oxygen resources are drawn upon heavily and, when overloaded, become exhausted. Life in such waters is confined to anaerobic bacteria (which exist when no oxygen is available), larvae of certain insects such as mosquitoes, and a few worms. There are no fish; turtles are generally the only forms of higher life present. This condition is known as the zone of degradation.

In a second zone, or zone of decomposition, more solids settle out, the water becomes somewhat clearer, and sunlight penetrates the surface. Oxygen is absorbed from the atmosphere at the air-water interface permitting the establishment of aerobic (oxygen available) conditions. The aerobic bacteria continue the conversion of organic matter into nitrates, sulfates. and carbonates. These, together with the carbon dioxide produced by decomposition as well as by bacteria and plant life, are food sources. With sunlight now penetrating the water, and with abundant food, algae begin to flourish and form a green scum over the surface.

In the third zone, or zone of recovery, algae become more numerous and self-purification proceeds more rapidly. Green plants utilizing carbon dioxide and oxygen will liberate in the say time more oxygen than is consumed, thus hastening the recovery of the stream. Simultaneously, the fish that require little oxygen such as catfish and carp, are also found. As the dissolved oxygen increases, more types of fish appear. After recovery, in the zone of cleaner water, fish find the stream highly favorable, as the algae support various aquatic insects and other organisms on which fish feed. The water is clear or turbid according to concentration of algae, and may have odor for the same reason.

Throughout the stages of recovery of self-purification, disease organisms are greatly reduced in number because they lack proper food, and experience unfavorable temperatures and pH values of water. However, the water is still dangerous since all disease organisms have not perished,


Self-purification in lakes and ponds is brought about by the same processes as in rivers arid streams. However, currents are not as strong and sedimentation plays a larger role. Large deposits of sludge, dead algae, and other organic material build up on the bottom. In deep lakes, self-purification is aided by seasonal "overturns."' This is simply an exchange of bottom water for surface water which occurs in the spring and fall, caused by the difference in the temperature of the water at the surface and bottom of deep lakes.


Basic steps in treating water


Turbidity in water consists of finely divided negatively charged colloidal materials which are kept in suspension by mutual repulsion. Turbid water is difficult to clarify by filtration because these fine particles can cause rapid plugging or even pass through a filter. The agglomeration of these colloids into settleable or filtrable aggregates through the action of certain chemicals is called coagulation. Iron and aluminum salts are the most widely used coagulants in water treatment plants.


Plain sedimentation is the natural settling of solids heavier than water without the addition of chemical coagulants. Solids heavier than water are held in suspension while in moving water, but gradually settle to the bottom as the water velocity is reduced. The time required to clarify water by sedimentation depends on the size of the suspended particles and their specific gravity. Large and heavy particles settle in a few minutes once the water has become still, whereas very small particles such as clay and silt may remain in suspension for several days.

Plain sedimentation is not ordinarily used as a separate step in water treatment because the long period required for complete settling would call for an Impractical number of settling tanks. However, in emergency situations, such as the necessity of taking water from a swift flowing stream which is heavily silt-laden after a rainstorm, special sedimentation tanks may be set up as a first step. This initial removal of turbidity reduces the load on the coagulation and filtration steps of the water treatment process, and the frequency of filter back-washing is reduced.


Filtration consists of passing the water through some porous material to remove the suspended impurities. Filtration is one of the oldest and simplest procedures known to man for revoving suspended matter from water and other fludis.

The simplest form of water filter is the sand filter. This filter resembles a small reservoir, the bottom of which is a bed of filter sand which in turn rests on a bed of well-graded aggregate with the largest size aggregate being at the bottom. An underdrain system of tile or brick is provided under the gravel to collect the water from the filter area. The underdrain system consists of a header or main conduit extending across the filter bed. Means are provided for regulating the flow of water out of the filter through this header and also for controlling the rate of flow on to the filter. This allows the filter to be operated at controlled rates which should not exceed 3.0 gallons per minute per square foot of filter area. An average filter bed consists of about 12 to 20 inches of gravel and 20 to 40 inches of sand. The depth of water over the sand bed varies from 3 to 5 feet.


In addition to coagulation, sedimentation, and filtration, water must undergo an additional treatment step: disinfection. This is necessary because no combination of the other three steps can be relied upon to remove all disease producing organisms, the pH and temperature of the water, the presence of interfering substances, and the degree of protection afforded organisms from the disinfecting solution by materials in which -they are imbedded. Therefore, various concentrations of disinfectant are required depending upon the local environmental conditions and the amount of particle removal effected.

Chlorine is the most commonly used chemical for disinfection of water. It is employed in field water supply in the form of calcium hypochlorite, a standard item in the supply system (commercially known as HTH powder). When the calcium hypochlorite is dissolved, the chlorine goes into solution and a calcium carbonate sludge settles out. The chlorine is present in the solution as hypochlorous acid or hypochlorite ion depending on the pH, both of which are powerful oxidizing substances. The chlorine available in either of these two forms rapidly oxidizes the organic and inorganic matter including the bacteria in the water. In this reaction the chlorine is converted to chloride and is no longer available as a disinfectant. The organic matter as well as such material as iron and manganese consume the chlorine. The use of chlorine makes it possible to introduce an accurately measured dosage to insure the destruction of disease-producing organisms as well as provide a readily measured residual to safeguard against recontamination during further handling.

Chlorine Dosage

Dosage is the amount of chlorine added to water to satisfy the chlorine demand as well as to provide a residual after a specified time. The amount required to disinfect water varies with the organic content and pH value of the water the temperature the time of contact and the chlorine residual required The dosage is usually stated in terms of parts per million (ppm) or milligrams per liter (mg/l). In water supply terminology ppm means the same thing as milligrams per liter or "mg/1".

Chlorine Demand

The chlorine demand of water is the difference between the quantity of chlorine applied in water treatment and the total available residual chlorine present at the end of a specified contact period. The chlorine demand is dependent upon the nature and the quantity of chlorine-consuming agents present and the pH value and temperature of the water (high pH and low temperatures retard disinfection by chlorination); For comparative purposes, it is imperative that all test conditions be stated. The smallest amount of residual chlorine considered to he significant is 0.1 ppm. The relationship of the demand to the length of the contact period is discussed below. Some of the chlorine - consuming agents in the water are nonpathogenic (non-disease causing organisms), but this bears no relationship to the fact that they contribute to the total chlorine demand of the water.

Residual Chlorine

As indicated above residual chlorine is the amount of unreacted chlorine remaining at a specified time after the chlorine compound is added. Chlorine in aqueous solution is highly unstable. It may change quantitatively and qualitatively under numerous conditions, -including the presence of other elements or compounds. The total residual chlorine in the water can be chemically divided into the following types:

1. Total available residual chlorine. This is the sum of the free available chlorine and the combined available chlorine.

2. Free available chlorine. Refers to hypochlorous acid and hypochlorite ion present in the water. These are the most effective disinfection forms of chlorine. The free available chlorine is a rapid-acting type important because it can be relied upon to destroy bacteria relatively quickly and thus is active during the period immediately following chlorination. The relative amount of each present in the water is dependent upon the pH value of the water. It is important to remember that when the pH is raised the quantity of free available chlorine required to kill the same number of microorganisms increases. With decreasing temperature the same situation of increasing dosage to maintain the same kill is encountered. If the contact time is varied then the dosage applied must also be changed. For example to shorten the contact time the dosage would have to be increased.

3. Combined available chlorine. This results from the presence of ammonia or organic nitrogen that will react to form simple chloramines. Thus the term "combined available chlorine" arises from the fact that the chlorine has combined with another substance. Chloramines are a slower acting and less active form of disinfectant. Therefore, a much higher concentration than that of free available chlorine is needed to produce the same germ destroying effect. The specific chloramines present are also a function of pH.

Disinfecting Time

Chlorine demand in most water is likely to be largely satisfied 10 minutes after chlorine is added. After the first 10 minutes of chlorination, disinfection continues but at a diminishing rate. A standard period of 30 minutes contact time is used to assure that highly resistant or high disease-producing organisms have been applied. Given a sufficiently large chlorine content, and if certain other conditions are met, even such special water purification problems as the presence of amoebic cysts or schistosomes will be solved with the 30-minute contact period.


As has been previously discussed, the efficiency of the chemical disinfection process is dependent upon numerous factors which include the type and concentration of microorganisms, the pH and temperature of the water, presence of interferring substances and whether or not' the organisms are protected from the disinfection solution by being embedded in tissue cells, or clumps of tissue cells, or other material. Therefore, various concentrations of disinfectants are required. Minimum concentrations of disinfectants are prescribed below.

Engineer operated mobile and portable water treatment units employ coagulation and filtration as a part of the treatment process and are capable of a high degree of removal of particulate material. When those units are employed, sufficient chlorine will be added to the water, preferably before coagulation so that the residual in the finished water after 30 minutes of contact will be at least as much as that indicated by the following table.



30 Minute Free Chlorine

Residuals in ppm













If adequate provisions are not made for accurate and frequent measurement of pH, 5.00 ppm must be used.

The following guidelines were used in developing the above table:

1. The water to be treated would be natural surface or ground water of average composition and not grossly or deliberately contaminated.

2. Water temperature would be above the freezing point.

3. The prescribed concentrations of free chlorine should provide a reasonable margin of safety for all bacteria and viruses pathogenic to man, Parasitic ova would have been removed in the coagulation and filtration steps of the treatment process.


Sample designs for treatment systems


Sand filtration does not make polluted water safe for drinking. But a properly built and kept sand filter will prepare water for boiling or chlorination that will make it safe. Trickling sand filters if built properly and cleaned periodically, provide clear water the' must be boiled or treated with chlorine.

The following tools and materials are required:

Steel drum, 2 feet wide by 29 1/2 inches high

Sheet metal to make cover, 29 l/2 inches square,

9.8 feet of wood, 2 x 4 inches

Sand, 7 cubic feet


Blocks and nails

Pipe to attach to water supply

Optional... valve and asphalt roofing compound to treat drum.

Surface water, from ponds, streams or open wells is very likely to contaminated with leaves and other organic matter. A trickling sand filter can remove most of this organic material but will always all virus and other bacteria to pass through. For this reason it is always best to boil or chlorinate water after filtering.

There are several sand filters, but the trickling filter is easiest to set-up and understand. The trickling filter uses sand to strain the organic matter from the water, although this does not always stop small pieces of organic matter or bacteria. But in time, biological growth forms on the top six inches of sand. this slows down the flow of water through the sand but will trap more small organic matter and, at times, up to 95 percent of the bacteria. But if not operated correctly, the sand filter can actually add bacteria to the water.

By removing most of the organic matter, the filter achieves the following results.

1. Removes larger worm eggs, cysts, and cercariae, which are the hardest to kill with chlorine.

2. Allows the use of smaller and fixed doses of chlorine for disinfecting, which results in drinkable water with less taste of chlorine.

Figure 21 Trickling sand filter

3. Makes the water look cleaner

4. Reduces the amount of organic matter, including living organisms and than food, and the possibility of recontamination of the water.

The unit shown in Fig. 21 should give about l quart of water a minute. The drum should be of heavy steel and can be coated with asphalt material so that it will last longer. The 2 millimeter hole at the bottom regulates flow and must not be made larger (slightly less than l/13th of an inch.)

It is important to use clean, fine sand, but not too fine. The sand should be able to pass through a window screen and it is best to wash it.

The following points are very important in assuring that your sand filter operates properly:

1. Keep a continuous flow of water passing through the filter and do not allow the sand to dry out, as this will destroy the microorganisms that form on the surface layer. The best we, to insure a continuing flow is to fix the water intake so that there is always a small overflow. Screen the intake and provide a settling basin to help keep pipes from becoming plugged, which would stop the flow of water. This will also delay your having to clean the filter.

2. Never allow the filter to run faster than 0.6 gallons of water e minute per square foot, as it will prevent the growth of microorganisms in the sand and wash them out through the outlet.

3. Keep light from the sand surface but allow air to circulate, as this will prevent the growth of green plant matter on the surface but help the growth of microorganisms that aid the filtering action.

4. When the flow drops below daily needs, clean the filter This is done by scraping off and discarding the l/2 inch of sand and lightly raking or scratching the surface. After several cleanings, the sand should be raised to its former height by adding clean sand. Before doing this, scrape the old sand down to a clean level. Cleaning should not be more often than every several weeks or even months.


A crude water purification plant is described which uses laundry bleach as a source of chlorine. Although lacking the reliability of a modern water system, this manual plant will provide safe drinking water. Many factors in this system depend upon operating experience. When starting to use the system, it is best to have the assistance of an engineer experienced in water supplies. For construction details see section II, C.


1. Mix concentrated bleach with water in the concentrate barrel with all valves closed.

Fig. 22 Chlorination system

2. Fill the pipe from the mixing barrel to the solution tank with water after having propped the float valve in a closed position.

3. Allow a trial amount of concentrate to flow into the mixing barrel by opening Valve #2

4. Use the measuring stick to see how much concentrate was used.

5. Close valve #2 and open valve #1 so that untreated water enters the mixing barrel

6. Close valve #1 and mix solution in the mixing barrel with a stick.

7. Remove the prop from the float valve of the solution tank so that it will operate properly.

8. Open wide the metering valve and valve #4 to clean the system. Allow a gallon to drain through the system.

9. Close down the metering valve until only a stream of drops enters the funnel.

(steps 2, 8 and 9 may be omitted after the first charging of the system, if the pipe mentioned in the second step is not permitted to empty before recharging the mixing barrel).

10. Open valve #3.

Trial and error must be used to learn how much concentrate should be put in the concentrate barrel, the amount of concentrate to flow into the mixing barrel and the amount of solution to allow past the funnel. The result should be water with a noticeable chlorine taste in the distribution barrel.

The flow into the funnel and the taste of the water in the distribution barrel should be checked regularly to insure proper treatment.


Chlorination, when properly applied, is a simple way to insure and protect the purity of water. These guidelines include tables to give a rough indication of the amounts of chlorine bearing chemicals needed. The amount of chlorine specified will normally make reasonably safe water. Try to have your water treatment system inspected by an expert, and the water itself periodically inspected.

The surest way to treat water for drinking is to boll it - see "Boiler for Potable Water". However, under controlled conditions chlorination is a safe method, and often more convenient and practical than boiling. Water properly treated has residual free chlorine which resists recontamination. The chlorine in water is not harmful since water with a harmful amount of chlorine in it is extremely distasteful. Proper treatment of water with chlorine requires some knowledge of the process and its effects.

When chlorine is added to water, it attacks and combines with any suspended organic matter as well as some minerals such as iron. There is always a certain amount of dead organic matter in water, and almost always live bacteria, virus, and perhaps other types of life. Enough chlorine must be added to oxidize all of the organic matter, dead or alive, and to leave some excess uncombined or "free" chlorine.

Some organisms are more resistant to chlorine than others. Two particularly resistant varieties are amebic cysts (which cause amebic dysentary) and the cercariae of schistosomes (which cause schistosomiasis). These, among others, require much higher levels of residual free cholrine and longer contact periods than usual to be safe. Often special techniques are used to combat these and other specific diseases. It always takes time for chlorine to work. Be sure that water is thoroughly mixed with an adequate dose of the dissolved chemical, and that it stands for at least 30 minutes before consumption.

Since both combined and uncombined chlorine has an unpalatable taste, it is best tend safest) to choose the clearest water available. A settling tank, and simple filtration can help reduce the amount of suspended matter, especially particles large enough to see. Filtration that can be depended upon to remove all of the amebic cysts, schistosomes, and other pathogen normally requires professionals to set up and operate. NEVER depend on home-made filters alone to provide potable water. However, a home-made slow sand filter is an excellent way to prepare water for chlorination.

Thus, depending on your water, different amounts of chlorine are needed for adequate protection. Measuring the amount of free chlorine after the 30 minute holding period is the best way to control the process. A simple chemical test using a special organic indicator (orthotolidine) can be used. When this is not available, Table 3 may be used.


Water Condition

Initial Chlorine Dose in Parts Per Million (ppm)


No hard-to-kill organisms suspected

Hard-to-kill organisms present or suspected

Very clear, few minerals

5 ppm

Get expert advice; in an emergency boil and cool water first, then use 5 ppm to help prevent recontamination. If boiling is impossible, use 10 ppm.

A coin in the bottom of an 8 oz. glass of the water looks hazy

10 ppm

Get expert advice; in an emergency boil and cool first If boiling is impossible use 15 ppm.


In the chart, parts per million or "ppm" means the ratio of:

Weight of active material (chlorine)/Weight of water

In water supply terminology, ppm means exactly the same thing as milligrams per liter or "mg/1"

The second chart, Table 4. gives the amount of chemical to add to 1000 gallons of water to get a solution of 1 ppm. Multiply the amount of chemical shown in Table 4 by the number of ppm recommended in Fig. 3 to get the amount of chemical you should add to 1000 gallons of water. Usually it is convenient to make up a solution of 500 ppm strength which can then be further diluted to give the chlorine concentration needed. The 500 ppm solution must be stored in a sealed container in a cool dark place, and should be used as quickly as possible since it does lose strength. Modern chlorination plants use bottled chlorine gas, but this can only be used with expensive machinery by trained experts.



% by weight of active material

Quantity to add to 1000 gallons of water to get a 1 ppm solution

High Test (Calcium hypochlorite) Ca(OCl)2


1/5 ounce

Chlorinated lime


1/2 ounce

Sodium hypochlorite(NaOCl)


1 ounce

Sodium hypochlorite

10% ounces

1.3 ounces

Bleach - a solution of chlorine in water

usually 5.25%

2.6 ounces

Fig. 23 Chlorination System



Lesson plans



LESSON OBJECTIVE: Describe and demonstrate how to estimate the coat of the four methods used in treating water.






Discuss the process of self-purification.

List and describe the four basic methods of water treatment:

1) coagulation

2) sedimentation

3) filtration

4) disinfection.

Outline the essential components of sand filtration and two chlorination units.

Estimate the costs of construction, operation, and maintenance for these units.

Manual of Individual

Water Supply Systems

p. 64-83.

WHO Monograph Series #42

p. 171-193.

Small Water Supplies

p. 26-47.




LESSON OBJECTIVE: Define the pollutants that must be eliminated;

(a) to provide esthetically pleasing and safe water; and,

(b) to prolong the life of the delivery system.




Permissible Levels of Chemical, Physical and Bacteriological Pollutants

List and describe pollutants that are

1). esthetically objectionable

2) health hazards

3) contribute to the destruction of a delivery system.

Establish permissible levels of concentration for each of these pollutants.

Recall the processes that will eliminate or reduce these pollutants.

From a list of pollutants, have each student state the undesirable property (ies) of each and recall a method of elimination (if applicable).

WHO Monograph Series #42

p. 46-54.



LESSON OBJECTIVE: Define the criteria that must be applied in selecting the most economically feasible source.




Criteria of Water Supply Selection

Review the basic requirements of a water supply source.

Small Water Supplies

p. 8-14


Outline the criteria to be used in the selection of water supply sources.