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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

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