Water treatment

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A safe type of water source that is well protected from pollution is the preferred way to get pure water. An example would be a tube well 20 to 30m up hill from any pollution and equipped with a pump, tight fitting cap and a well designed apron. Unfortunately good sources are not always available and some treatment will be advisable. Three methods will be discussed briefly.

Boiling

A 208 litre oil drum may be cleaned and then mounted horizontally over a brick fire box as shown in Figure 14.21. A tap should be fitted at the bottom of one end and enough clearance to get a bucket under the tap will indicate a proper height for the tank. The tank should not be filled completely and the filler plug should never be installed tightly. Water should be boiled 1 5 to 20 minutes and a litre or two drawn from the tap during the boiling. Once cooled the 200 litres should provide drinking water for several days.

Filtering

There are a number of filter designs that will clarify water and remove some bacteria. They all require periodic cleaning, the difficulty of which depends on the size and type of filter.

Figure 14.21 Water Boiler.

A medium-sized upward-flow sand filter can do a good job of reducing suspended solids and is easy to clean and maintain. The filter containers can be made from 208 litre drums or from 175 to 200 litre concrete tanks made by using a hessian bag filled with sand or sawdust as a form over which mortaris applied. Small-sized tanks will not need reinforcing if good quality mortar is used. A filter cross section is shown in Figure 14.22. Successive layers, first of stones then gravel, coarse sand and fine sand are put in the tank until it is about half full. A layer of charcoal, crushed to about 5mm size, is desirable in that it will contain bacteria, which are helpful in removing disease carrying micro-organisms from the water. The charcoal bed is enclosed with thin cloth and weighted down by a top layer of sand.

Water poured into the top tank flows through the tube to the bottom, where it percolates up through the gravel, sand and charcoal and out the hose to a water jar. Before actual use, some water should be passed through the filter to establish proper filter action. The drain plug at the bottom should be large in size so that when it is removed water will flow rapidly back through the sand and flush away all accumulated sediment. Experience will indicate when back flushing is necessary.

Figure 14.22 Upward-flow water filter for filtering up to 401/day.

Note that each layer should be 20-25cm.

Chlorination

Proper attention to details is needed to do a satisfactory job of chlorination. However, properly done, it can make drinking water much safer. After adding the correct amount of chlorine material, it is necessary to thoroughly mix it into the water and allow it to stand for at least 30 minutes.

Treatment levels are given in parts per million (ppm), 1mg/m equals 1ppm Water that is clear and not suspected of dangerous contamination can be treated with 5ppm of active chlorine. If the water is a little cloudy 10ppm is safer. The sources of chlorine vary considerably in the amount of active chlorine available. Table 14.6 gives information about several materials.

While these quantities may be reduced proportionately for smaller quantities of water, for amounts under 100 litres measurements become more critical and it is advisable to have a chemist weigh out several packages of treating material to match the quantity of water to be treated each time.

Table 14.6 Sources of Chlorine for Water Treatment

Compound Active Chlorine % of weight Quantity to add to 100 litres to get the following concentration (grams)
5ppm 10ppm 15ppm 50ppm
HTH Ca(OCl)1 70 8 15 23 80
Chlorinated lime 25 20 40 60 200
Sodium Hypochlorite 14 38 75 113 380
(NaOCl)
Sodium Hypochlorite 10 48 95 143 480
Laundry bleach 5.25 95 190 285 950

The 50ppm column is shown in the table as being the level at which to treat a new or repaired well or cistern. The dosage is left for 24 hours before flushing out.

Open channel flow

A knowledge of the principles of open channel flow is necessary in designing ditches to carry water into grade level storages and channels to carry away storm water without causing erosion. The same principles apply to the design of irrigation canals, road splashes and drifts. The most common problems are:

The quantity of water flowing in an open drainage channel is the product of the cross-section area of the channel and the speed of flow.

Q = A x V where:

Q is the flow in cubic metres per second (m/s) A is the cross-section area of the channel (m) V is the average velocity of flow (m/s)

If the velocity is checked at any cross section in a channel it will be found that the water is flowing slower along the sides and bottom. This is due to frictional resistance and is more pronounced along vegetated than paved channels. In practice, however, a theoretical average velocity is used.

The equation of continuity shows that for a constant discharge, Q. the velocity must change inversely with the section area of the channel.

Q = A1 x V1 = A2 x V2 = A3 x V3

There are two types of flow in a channel which may give the same discharge but at different velocities and depths. A rapid, shallow flow is called super-critical or shooting flow. A deeper, slower flow is called sub-critical flow. An example of each type of flow is found on a dam spillway. The thin layer rushing down the spillway surface is supercritical flow. After hitting the standing wave at the bottom the water moves away much more slowly in a sub-critical flow. In general, super-critical flow should be avoided as erosion will occur in all channels which are not lined with concrete.

The velocity of flow in a channel is determined by the gradient, the shape and size of the cross section and the roughness of the surfaces. It is quite obvious that the velocity will be greater in steep, smooth channels. It is not as obvious that two channels with the same cross-section area but with different shapes can have different velocities. This results from the differing amount of surface contact and frictional resistance.

The effect of cross-section shape is measured by the hydraulic radius of the channel (R). It is found by the equation:

R = A/ P where:

R = hydraulic radius, m
A = Cross-section area, m
P = wetted perimeter, m

The wetted perimeter is the length of the cross section in contact with the water. Figure 14.23 illustrates the effect of shape on the hydraulic radius. Both channels have an area of 24, but a) has a larger R.

Figure 14.23 Channels of equal area but different hydraulic radii.

When other factors are equal, the channel with the larger R will have the higher channel velocity.

The two most common shapes for earth channels are shown in Figure 14.24. The trapezoidal shape has a tendency to gradually change to the parabolic shape over a period of time.

The variables which affect the velocity of flow are related as shown in the empirical equation called the Manning formula for open channel flow.

V = 1 / n x R2/3 x S1/2 where: n

V = velocity, m/s
R = hydraulic radius, m
S = gradient in m/m
n = Manning's roughness coefficient

R = A/P Where:

A = cross-sectional area, m
P = wetted perimeter, m

Table 14.7 Value of Manning's Roughness Coefficient n

(a) Channels free from vegetation n
Uniform cross section, regular alignment, free from pebbles and vegetation, in fine sedimentary soils 0.016
Uniform cross section, regular alignment, few pebbles, little vegetation, in clay loam 0.020
Irregular alignment, ripples on bottom, in gravelly soil or shale, with jagged banks or vegetation 0.025
Irregular section and alignment, scattered rocks and loose gravel on bottom, or considerable weed on sloping banks, or in gravelly material up to 150mm diameter 0.030
(b) Vegetated channels
Short grass (50-150mm) 0.030-0.060
Medium grass (150-250mm) 0.030-0.085
Long grass (250-600mm) 0.040 0.150
(c) Natural stream channels
Clean and straight 0.025-0.030
Winding, with pools and shoals 0.033-0.040
Very weedy, winding, and overgrown 0.075-0.150

Extracted from "Field Engineering for Agriculture Development" by Hudson.

With the Manning formula any three variables can be used to find the fourth. When, for example, R. S and n can be measured or estimated it is possible to calculate velocity.

Open-channel problems may vary in detail, but the principle is usually the same. The designer has some fixed quantities, such as a given discharge to be carried, and some variables such as gradient and velocity which have restricted ranges. Using these, a size and shape can be determined. Usually there is no one unique solution, but a range of satisfactory alternatives.

Example:

An earth or grass-lined channel should be designed with a flow velocity fast enough to avoid sediment deposits but not so fast that erosion will occur. Table 14.8 suggests maximum velocities for various channel soils and vegetative covers.

Table 14.8 Maximum Channel Velocities, m/s (Cover after two seasons)

 

Vegetative cover

Soil Bare Medium grass Good gras
Light silty sand 0.3 0.75 1.5
Coarse sand 0.75 1.25 1.7
Firm clay loam 1.0 1.7 2.3
Coarse gravel 1.5 1.8  
Shale, hardpan 1.8 2.1  
Rock 2.5 - -

Table 14.9 Design velocities for grass waterways, m/s

 

Slope

Soil 0-5% 5-l0% 10%
Erosion resistant veils 2.0 1.75 1.50
Erosion prone soils 1.75 1.50 1.25

Source: Department of Conservation, Government of Zimbabwe

Figure 14.24 Basic dimensions of common channel sections.

For convenience, Figure 14.25 may be used to solve open channel flow problems. For example, assume a channel is to be designed for a firm clay-loam soil with a medium grass cover (200mm) to be established. A flow of 2.0m/s is the maximum expected and the gradient is approximately 0.025m/m. Choose a channel shape and determine a satisfactory size.

From Table 14.7 read a value for roughness coefficient (n) of 0.030 to 0.085; choose 0.04.

From Table 14.8 read 1.7m/s acceptable velocity.

From Figure 14.25 read 0.30m hydraulic radius.

Arbitrarily choose a parabolic shape.

A=Q/V=2/1.7= 1.18m

P = A/R = 1.18/0.30 = 3.93m

P = t +8d2 / 3t(assume a value for t of 3.75m)
d2 = (P - t) x 3t/8
d2 = (3.93 3.75) x 3 x 3.75/8 = 0.25 d = 0.5m

A = 2/3 td
A = 2/3 x 3.75 x 0.5 = 1.25m

which is close to the previous A = 1.18m

In summary, a parabolic-shape channel 3.75m wide and 0.5m deep will be satisfactory.

Rural sanitation

When dealing with the problems of poor sanitation in rural areas of developing countries, one is tempted to assume that improved technology is the answer and that new latrines will provide the "technological fix". But technology alone does not solve anything, for it has been found that new latrines when built are not fully used, and when used do not wipe out diseases that stem from poor sanitation. Good sanitation depends on people and how they organize hygiene-related activities. It depends on a large "package" of hygiene measures and latrines are only a part of this package.

Technology does have a part to play and many rural communities need basic technical assistance. Latrines may not always be a practical solution but if they are, they must be carefully designed to match local cultural patterns.

Pit Latrines

There are many designs for latrines to be built in areas where more sophisticated sanitary systems are not possible. The simplest design is the pit latrine and there are certain characteristics that are common to the many variations on this design. A latrine should always be dug at least 30m downhill from a well if that is the source of the family water supply. However, in areas where the water table is very high the distance should be increased to 200m or more. The latrine should also be at least 10m from the nearest house or kitchen.

A pit that is a little less than 1 m in diameter is sufficient, but a pit that is a 0.7m by 1.5m oval will provide more convenient space for the person digging. The depth is at

Figure 14.25 Nomograph for Manning's formula.

least partially dependent on the stability of the soil and therefore how deep the hole can be dug without danger of a cave-in. While a depth of 4 to 5m is the normal in stable soil, an increase to 7m will decrease the problem with flies. In areas with a high water table, the depth may have to be decreased as the bottom of the pit should be not less than 1 m above the highest ground water level to avoid pollution. A pit which has a diameter of 90cm and is 5m deep will last for about 5 years if used by a family of 6 persons.

The desired depth and the character of the soil will determine whether a stabilizing liner will be necessary. Most latrines should have a block or brick liner for at least the top metre. To install a stabilizing liner, a hole is dug a little less than 1 m deep and about 1 m in diameter and lined with concrete blocks or bricks. After curing for a few days, the balance of the pit can be dug out being careful not to get the diameter so large as to allow the blocks to sink. If the soil is sandy, then a complete liner may be necessary. Bamboo is one possibility for lining the remainder of the pit sides.

A simple floor to cover the pit can be made of bamboo or timber. However, a much more durable and sanitary slab can be cast of concrete. See Figure 14.27 and the accompanying paragraphs for the design and construction of a two-piece cast concrete slab that includes foot pads and a slope toward the hole.

The type of structure built above the slab to give privacy is largely a matter of personal preference. Bamboo, offcuts, concrete blocks or corrugated steel are all possibilities for wall construction. Corrugated steel or thatch may be used for roofing.

A desirable feature to include is a vent pipe. A vent will not only reduce odours, but, if screened at the top, will catch numerous flies. The vent hole can be cast in the slab so that the vent is just outside the privy hut. To be most effective the vent should be located on the side with prevailing sunshine, be as large in diameter as possible and painted black, and have a screen over the top. This combination of design features tends to produce a significant air current that carries off the odors and traps the flies. Figure 14.26 shows a latrine of this type. The vent pipe can be made at low cost using hollowed bamboo, but other materials such as masonry, cement/sisal, reeds/mud, PVC or galvanised iron can also be used. A piece of glass fitted at the base of the vent pipe will provide light to attract flies away from the squatting hole and trap them in the vent pipe.

Figure 14.26 Pit latrine with vent pipe.

Latrine Slab

Latrine Slab can be built to cast and finish a perfectly satisfactory 2-piece slab that will be easy to handle. First a small mould is constructed to cast the footpads which should be approximately 10 by 30 by 2cm with rounded corners. They are cast a few days prior to casting the slab and stored in a bucket of water to cure. The form for the slab is then built of 4 boards that are 7 by 1 20cm and any convenient thickness. A round block 5cm thick and 10 to 12cm in diameter and a rectangular block 10 by 20 by 5cm are needed for the hole. If a vent pipe is to be installed another round block will be needed that is 7cm thick with a diameter to match that of the pipe. Two screeds are required, one straight and the other curved enough to be 1 to 2cm low in the middle. Three pieces of polythene are cut to lengths required to serve as separators between the two halves of the slab. See Figure 14.27, section B-B. Six pieces of 8mm reinforcing rod cut to just fit into the form are also needed.

Find a flat surface (floor or leveled earth), spread a piece of polythene, position the form and the wood blocks on it. Mix a 1:3 cement to sand concrete (or 1:2:2 cement to sand to small gravel) using just enough water to get a workable mixture. Position the polythene separators and place a uniform 2.5cm layer of concrete on either side. Place the reinforcing bars as shown in Figure 14.27, Section A-A. Fill the form, and compact and level the concrete with the straight screed. Then, using the curved screed in the middle third of the form, work out the center of the concrete in both directions to give the sloping surface. Smooth lightly with a steel trowel. Place the dampened foot pads in place, working them into the surface slightly. Use any excess concrete to cast a pad to be put just outside the privy entrance.

After all signs of free water have disappeared from the surface, finish the concrete with a steel trowel. Cover and keep damp for several days. Handle with care. There are a number of variations and refinements in latrine use and design that may be considered.

Placing a thick pad of grass in the bottom of a newly dug latrine and then adding some vegetative wastes regularly will turn the latrine into a compost pit with a substantial reduction in odor. When the pit fills, it is necessary to dig a new latrine hole and move the slab and hut. The full hole is covered and left at least six months after which the compost may be removed and used as fertilizer.

Figure 14.27 Concrete stab cast in two pieces.

Aqua Privies

Aqua privies are usually equipped with either a water-trap hole or a discharge that is below water level. Either of these will reduce odors considerably. However, some water must be added daily for complete decomposition of the waste and a soakaway pit is essential to dispose of the effluent that is discharged. See Figure 14.28.

One way to ensure that extra water is added each day is to combine a bath house with the privy. Figure 14.29 shows the plans for such a combination. In the illustration a separate soakaway is shown for the bath as it is combined with a pit latrine. However, if it was combined with an aqua privy, the water would be directed into the privy tank.

Figure 14.28 Aqua privy with soakaway pit.

The bath house is an inexpensive but convenient addition for a family to have either with or without piped water.

The farm home with an adequate and continuous water supply can be equipped with a water closet toilet. A w.c. system uses a much larger quantity of water than the other systems mentioned and requires the installation of a septic tank plus a large soakaway or drainage field to handle the considerable amount of effluent.

Septic Tanks

The septic tank is a large concrete or concrete block tank, the base of which is at least 150cm below the inlet and outlet level. The raw sewage flows into the tank through an open tee and the effluent leaves the tank through a similar tee. The tank is divided by a wooden baffle that extends from 50cm above the bottom to 25cm above the sewage level. A heavy scum forms on the surface and all digestive action is by anaerobic bacteria, i.e. bacteria that live and multiply without the presence of air. Figure 14.30 shows a cross section of a septic tank.

Soakaway Trenches

The effluent from a properly operating septic tank will be almost free of solids and further biological activity in the soakaway trench or pit will be aerobic in nature, i.e. some air needs to be present. Because of this, trenches with a depth of about 50cm are preferred over deep pits.

Before a tank and soakaway system are installed, it is important to check with local authorities concerning design specification requirements. If there are no specific rules, the information given in Table 14.9 may be used.

Percolation time is found by digging a hole 30cm square and 60cm deep. Fill the hole with water and let it drain completely. Refill and then measure the seconds/ mm rate at which the water level falls.

Figure 14.29 Bath house and latrine (All dimensions in cm).

Figure 14.30 Septic tank and distribution box.

Figure 14.31 Tank and soakawaypeld.

Table 14.10 Septic Tank and Sookaway Trench Sizes

No of people regularly in home Tank indside dimensions(cm below drain level)

Soakaway trench (m)* with percolation rates seconds/mm)

  L x W x D 10-30 30 60 60-100 100-140
24 200X 100 x 150 10 30 60 100
6 250 x 125 x 150 15 45 90 150
8 250 x 125 x 150 20 60 120 200
10 250 x 150 x 150 25 75 150 250

*Trenches should be 100cm wide and 50cm deep.

The outlet from the septic tank should be approximately 50cm below ground level. However, site gradients and the need to install the tank low enough so that the sewerage lines will drain into it sometimes makes this difficult. The soakaway field is ordinarily close to the tank but may need to be separated by some distance because of site conditions.

The soakaway tench should be approximately 100cm wide and 50cm deep and with very little slope. A layer of gravel or broken stone is placed in the bottom of the trench, and then 100mm clay tile or 100mm perforated PVC pipe is laid in the trench. The maximum slope of the soakaway lines is 1:200. If, because of a sloping site, lines have to be installed at different levels, leakproof pipe or tile should be used to carry the effluent from one level to the other, but the seepage lines themselves should always be nearly level. Gravel or stone is added until the lines are covered. Hay, grass or newspapers can be put over the stones before backfilling to prevent the soil from filling the open spaces between the stones. See Figure 14.31. Although both the aqua privies and septic tanks need to be cleaned out periodically, if they are built large enough, the period between clean-outs can be up to two to three years depending on how heavily the system is used.

Further reading

Bachmann A., Nirman J., Manual for Water Systems and Pipe Work, Kathmandu, Nepal, Swiss Association for Technical Assistance, 1980.

Hudson N.W., Field Engineering for Agriculture Development, Oxford University Press, Clarendon Press, 1975.

Koegel R.G., Self-Help Wells, FAO Irrigation and Drainage Paper no. 30, Rome, Food and Agriculture Organization of the United Nations, 1977.

Longland F. (ea. P. Stern), Field Engineering: An Introduction to Development Work and Construction in Rural Areas, London, Intermediate Technology Publications, Ltd., 1983.

Mann H.T., Williamson D., Water Treatment and Sanitation, Simple Methods for Rural Areas, London, Technology Publications Ltd., 1976.

Nissen-Petersen, Rain Catchment and Water Supply in Rural Africa: A Manual, London, Hodder and Stoughton, 1982.

Pacey A., Rural Sanitation: Planning and Appraisal, An Oxfam Document, London, Intermediate Technology Publications Ltd., 1980.

SKAT, Manual for Rural Water Supply, with Many Detailed Constructional Scale-drawings, Publication No.8, St. Gallen, Swiss Centre for Appropriate Technology, SKAT, 1980.

VITA, Using Water Resources, Mt. Rainier, M.D., Volunteers in Technical Assistance, 1977.

Waterhouse J., Water Engineering for Agriculture, London, B.T. Batsford Ltd., Academic and Educational, 1982.

Watt S.B., A Manual on the Hydraulic Ram for Pumping Water, London, Intermediate Technology Publications Ltd., 1977.

Watt S.B., Ferrocement Water Tanks, and Their Construction, London, Intermediate Technology Publications Ltd.,1978.

Watt S.B., Wood W.E., Hand-dug Wells and Their Construction, London, Intermediate Technology Publications Ltd., 1979.


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