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