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close this book Water purification, distribution and sewage disposal for Peace Corps volunteers
close this folder Section 5: Construction techniques
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View the document Scheme
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View the document Construction at the source
View the document Construction at the pumping station
View the document Construction at storage facility
View the document Construction on supply line
View the document Lesson plans

Construction at storage facility

STORAGE TANKS

Storage tanks can be built on high grounds in which case they are termed ground-level reservoirs, or they are elevated reservoirs.

Ground-level reservoirs are usually built of masonry, mass concrete, or reinforced concrete, according to the materials and local skill available (see below)


Fig. 44 Ground-Level Reservoirs

A = Cross-sections of reservoir

B = Types of walls for reservoirs

C = Sketch detail of manhole opening in reservoir cover

D = Typical valve arrangement for ground-level reservoir with two compartments

Aa = Effluent

Bb = Supply

Cc =Overflow

Dd = Drain


Typical Spring Collection-Chamber For Towns

A = Ground level

B = Water-bearing formation

C = Impervious stratum

D = Collection chamber

E = Opening protected by a stone-and-gravel pack in order to exclude sand and debris

F = Collecting room

G = Measuring weir

H = Measuring rod, bottom of which is level with lower edge of weir

I = Outlet pipe to reservoir or town

J = Floor drainage

K = Locked entrance door

L = Screened opening through door for ventilation purpose

M = Diversion ditch for surface run-off. Should be at least 15 m (49 ft) away from the collection structure


Property Protected Spring (I)

A = Protective drainage ditch to keep drainage water a safe distance from spring

B = Original slope and ground line

C = Screened outlet pipe: can discharge freely or be piped to village or residence

Springs can offer an economical and safe source of water. A thorough search should be made for signs of ground-water outcropping. Springs that can be piped to the user by gravity flow should be checked.


Property Protected Spring (II)

A = Protective drainage ditch to keep drainage water a safe from spring

B = Screened outlet pipe: to discharge freely or be piped to village or residence

In order to prevent leakage in the reservoirs, the following should be done:

1. Build concrete walls with as few joints as possible

2. Copper or polyethylene strips should be built in vertical joints if possible.

3. Paint the whole inside surface with a bitumen compound or with a solution of sodium silicate (water glass).

4. Render interior surface with about 3/4 inch thickness of mortar composed of water-proof cement and sand, after thoroughly rough ending the surface to be rendered to ensure a good key.

Elevated reservoirs may be of reinforced concrete or-of steel. Reinforced concrete is suitable when many tanks of similar size are to be built in a series of villages, so that the system is used over and over again. The construction techniques involved are the same as for ground-level storage, except that the elevating walls should be built first.

Steel reservoirs are suitable for single reservoir plans. The tank can be ordered from the manufacturers and comes complete with the accompanying assembly manual which is easy to follow. The tower foundations are to be locally built of concrete.

Steel reservoirs can also be used for ground-level tanks on rocky sites or in areas where masonry rocks are scarce. In such cases, the tank must be slightly elevated to allow painting of lower parts. Elevated storage tanks have valves to stop overflowing. When a float valve is used to control the level in the tank, the overflow should never come into action if the valve is working properly. In the case of a "floating" tank it is usual to control the inflow through a float valve and the outlet joins the delivery pipe through a non-return (see Fig. 48 ). A depth gauge operated by a float and wire shows the amount of water within the tank, and is visible from the outside.


Fig. 48 Elevated Storage Tank

Outlet always taken from 6 in above tank floor; wash-out at extreme bottom of tank

A = Diagrammatic arrangement of pipes when overhead tank acts as balancer ( floating tank ). Not suitable for use with reciprocating pumps.

B = Diagrammatic arrangement of pipes when pumping direct to storage tank

When a float valve is not used, there is no control on the depth of water except the intelligence of the operator of the supply pump and the overflow, and carelessness in adjusting the hours of pumping to the draw-off can result in considerable waste, while the farther the tank is from the pump-house the easier it is to overlook such waste. The simple indicator shown below is one way of reducing this to the minimum as, properly sited, it can be seen for a considerable distance. However, the nearer the tank is to the pump-house the easier this control becomes.


Fig. 49 Water-Level Indicator For Elevated Storage Tanks

A = Suitable indicator for top two three feet of water in tank

B = Appearance of indicator from a distance; it should be orientated so that it appears against the skyline from observation point; it can be seen clearly a mile away.

C = Section at a, showing construction and operation of lower indicator

In the construction of storage facilities, the following provisions should be made:

1. Manhole covers must be tightly fitting to prevent surface water from entering the reservoir . They should be locable.

2. Surface covers must be water-tight and light-proof to prevent algae growth.

3. Ventilation must be included to let out air as water fills the tank. These must be covered with fine-mesh wires (not less than 18-mesh).

4. Inlet and outlet pipes, overflow and wash-out pipes should have mesh at their open ends. The outlet pipe should be 6 in. above the bottom of the tank. If the tank has concrete floors, the floor should slope towards the wash-out pipes to enhance cleaning. The diagrams below illustrate the proper design for a concrete storage tank.


Fig. 50 Storage Tank


Fig. 51 Manhole covers


Fig. 52 Typical valve and toy


Fig. 53 Piping Installation

WATER PURIFICATION SYSTEM

Water purification systems are usually incorporated in the storage tanks. Where only disinfection (chlorination) is required, the treatment tank can act as distributing reservoir. The cistern is a typical storage-purifier combination.

The cistern filter is a sand filter which keeps organic matter from entering the cistern. The water may then be disinfected and stored in the cistern. The diagrams below show the construction design for such a filter.


Fig. 54 Cistern Filters

A catchment area always collects leaves, bird droppings, road dust, insects, etc. A cistern filter removes as much of these as possible before the water enters the cistern.

The sand filter is usually built at ground level and the filtered water runs into the cistern, which is mostly underground. The largest pieces, such as leaves, are caught in the splash plate. The splash plate also serves to distribute the water over the surface of the filter, so that the water does not make holes in the sand. A piece of window screen forms the splash plate.

Most filters are made too small to handle the normal rush of water from rainstorms. This results in the filter always overflowing or a channel being dug in the sand, which will ruin the filter. The filter area should be not less than one-tenth of the catchment area. A typical filter area would be 4 feet by 4 feet for a family-sized unit with average rainfall intensity.

About every 6 months, the manhole cover to the filter must be removed and the filter cleaned. Remove all matter from the splash plate and scrape off and remove the top half-inch of sand. When the depth of sand becomes only 12 inches, rebuild it with clean sand to the original depth of 18 inches.

A simple way to discard the first runoff from the roof, which is usually mostly leaves and dirt, should be provided. This will make your filter last longer between cleanings. The easiest way is to have a butterfly valve (like a damper in a stovepipe) in the downspout. After the rain has washed the roof, the valve is turned to allow the runoff water to enter the filter. A semiautomatic system is shown in Fig.

When building the filter, it is important to insure easy cleaning and to use properly-sized sand and gravel. The filter is usually mounted right on the cistern but can also be close to it. It rust have a screened overflow.

Water Purification Plant

Tools and Materials

3 barrels, concrete tanks or 55-gallon drums

1 eight inch funnel or sheet metal to make a funnel

2 smaller tanks, about 5 gallon or 20 liters in size, equipped with float valves

4 shut-off valves

1 throttle or needle valve (clamps may be used instead of the valves, if hose is used) some pipe or hose with fittings hypochlorite of lime or sodium hypochlorite (laundry bleach)

This plant can be used in small systems, using laundry bleach as a source of chlorine.

The water purifier should be made as in the drawing. The two large barrels on top of the structure are for weakening the bleach. The two smaller tanks on the shelf below are for holding equal amounts of weakened bleach solution and of water, at a constant pressure. This makes a constant flow of the solution water, at the same speed, into the hoses leading to the mixing points. The mix is further controlled by the valves and may be seen through the open funnel. If a throttle valve is not available, a shut-off valve may be used and a throttle action obtained by this valve and valve #4 in series.


Fig. 55 Chlorination system

Placing the two barrels at a height of 10 feet causes a pressure of only about five pounds a square inch. Thus the plumbing does not have to be of high quality except for valve #1 and the float valve of the water holdup tank, if the rain water supply is under higher pressure.

Sometimes special chlorinators are required; in which case when hypachlorinators are ordered, the following data should be furnished to the manufacturers:

If water is pumped:

1. Sketch of pumping installation

2. Number and type of pumps

3. Manual or automatic operation

4. Pumping rate (liters/second or gallons/minute) and total water pumped per day (cubic meters or gallons)

5. Electric current available (volts, phase, cycle)

6. Pressure on pump discharge (minimum and maximum)

7. Suction lift

8. Sizes of suction and discharge pipes

9. Other data (space available for installation, sizes of foot valves, check valves, etc.)

For gravity system:

1. Sketch of system, indicating source of water supply and distances

2. Size of main

3. Size of meter, if any, giving make and description

4. Pressure at meter or point of installation (minimum and maximum)

5. Rate of flow (minimum and maximum)

6. Average daily flow (cubic meters or gallons per day)

7. Fire flow, if any (liters/second or gallons/minute)

8. Allowable loss of pressure (m or ft)

9. Other data (space available for installation, etc.)

Boiler for Potable Water

Sometimes it is easier to boil drinking water than to disinfect. The following design can provide enough safe water for a smell community with a distribution system, since it would require a lot of fuel to boil enough water for the system.

Tools and Materials

1 - 55 Gallon drum

1 - 3/4" Pipe Nipple 2" long. Quantity of bricks for two layers of bricks to support drum.

1 - bag of cement plus sand for mortar and base of fireplace.

1 - large funnel and filter medium for filling.

1 - metal plate to control draft in front of firebox.

1 - 3/4" valve, preferably all metal such as a gate valve to withstand heat.


Fig. 56 Boiler for Potable Water

This drum for boiling of drinking water is intended for use in your residence to provide a convenient method for preparation and storage of sterile water. The fireplace is simple, oriented so that the prevailing wind or draft goes from front to back of the drum between the bricks. A chimney can be provided but is not necessary.

The unit has been tested in many Friend's workcamps in Mexico and elsewhere. A 55 gallon drum would normally last a 20 person camp group for an entire week, and certainly would provide adequate safe water supply for two or three individuals for a much longer time. Water must boll at least 15 minutes with steam escaping around the completely loosened filler plug. Be sure that the water in the pipe nipple and valve reach boiling temperatures by purging about two liters of water out through the valve while the drum is at a full boil.