| Water purification, distribution and sewage disposal for Peace Corps volunteers |
|Section 7: Scope of disposal system projects in host communities|
It should be noted that, in many countries, more than 80% of the population live in rural areas and small communities and, as a general rule, have a low income. In most cases, all the elements or rural sanitation are absent and indiscriminate fouling of the soil with human excrement is common. Such conditions are also found in rural areas near towns, and aggravate the urban sanitation problems. The menace of inadequate excrete disposal is present so long as sanitary privies are lacking in a community.
Rural methods of excrete disposal include privy systems and water-carried sewage systems. Of the privy systems, only the pit privy and sanitary latrines need be considered. 'later-carried sewage systems include cesspools, seepage pits, and septic tanks.
THE PRIVY METHOD
The Pit Privy
The pit privy is used almost exclusively throughout the Western hemisphere and Europe and is common in parts of Africa and the Middle East. With a minimum of attention to location and construction, there will be no soil pollution and no surface-or ground-water contamination. The excrete will not be accessible to flies if the hole is kept covered; but, even when the hole is left open, the fly problem will not be very great since flies are not attracted to dark holes and surfaces. A good superstructure helps to keep the sun's rays and light from shining into the pit. There is no handling of the material. Odors are negligible, and faeces are normally out of sight. The pit privy is simple in design and easy to use, and does not require operation. Its life span will vary from five to fifteen years, depending upon the capacity of the pit and the use and abuse to which it is put. Its chief advantage is that it can be built cheaply, in any part of the world, by the family with little or no outside help and from locally available materials. It has few disadvantages, and it can play a major role in the prevention of filth-borne diseases.
Water Sealed Latrines
Mention may be made here of the water-seal slab which gives its name to a latrine called "water-seal" or sometimes "pour flush" latrine. The water-seal slab may be installed over a pit (such as that of a pit privy). With proper operation and maintenance, the water seal will keep both flies and odors away. For this reason it may be installed as a part of the dwelling, preferably near the back of the house and with an outside entrance. The water-seal slab is extensively used in South East Asia.
WATER-CARRIED SEWAGE SYSTEMS
Experience has shown that, when running water is available, the water-carried system of excrete collection and disposal is most satisfactory and convenient under both urban and rural conditions. It fulfills all sanitary and aesthetic criteria. In particular, contamination of the soil and of surface water is avoided; potentially dangerous wastes are rendered inaccessible to flies, rodents, and domestic animals; and the mechanical transmission of faecal-borne diseases to man is prevented.
One serious disadvantage, however, is the difficulty of disposing of the large volume of wastes resulting from the addition of water. While in cities the liquid wastes are usually carried away by means of sewers, in most rural areas of the world sewerage systems do not exist, and liquid wastes are conveniently discharged into the ground. Since in such areas ground water is often tapped as a source of domestic water-supply, there is an obvious need for proper location and construction of the excrete disposal system, with a full understanding of the hazards involved.
Various methods may be used in rural areas to dispose of liquid wastes. They include the use of cesspools and seepage pits, and the septic-tank systems, which involve settling tanks with single or multiple compartments followed by subsurface irrigation fields, filter trenches or sand or trickling filters. Selection of methods will depend primarily upon the degree of sewage treatment to be provided, upon the location of the system and ether local factors, and, finally, upon costs. Local factors which bear upon the selection and design of the disposal installation include the nature of soil formations, the presence and levels of ground water and the direction of flow, topography, the proximity of sources of water supply, the quantity of sewage, and the area available for the disposal works.
A cesspool is essentially a covered pit which receives raw sewage. It may be of the water-tight or of the leaching type. In some cases, especially in Europe, it is made watertight and is designed to hold the liquid wastes which must be removed periodically, about every six months. The leaching cesspool, on the contrary, is dug into pervious soils in order to allow the liquid portion of the waste to seep off into the ground. The solids then accumulate in the pit and gradually seal the pores of the ground.
Water-tight cesspools are usually designed for a capacity of 15 gal. per person per month, or 90 gal. per person when they are emptied every six months. Leaching cesspools have diameters of 36 in. or more, and are provided with an open joint lining below the inlet level (see Fig. 60). The top part of the lining, which is within 2-3 ft. of ground level, should be impervious and laid with mortar. Covers with inspection manholes are usually provided. After the pores of the ground become clogged and the pit fills, an outlet tee and an overflow pipe lead the supernatant liquid to a seepage pit.
A cesspool should be located downhill from a well; in any case, a distance of 50 ft. will prevent bacterial pollution of the well. To prevent chemical pollution, too, the distance between a well and a cesspool placed directly uphill from it should be not less than 150 ft. Cesspools of the leaching type should be located at least 20 ft. away from dwelling foundations. Their construction is not permitted by health authorities in densely inhabited communities where wells are used as sources of drinking-water supply.
The Seepage Pit
The seepage pit receives the effluent from aqua privies, cesspools, and septic tanks and allows it to percolate away into the ground. It is sometimes used for the disposal of laundry, bathroom, and kitchen wastes. In the latter case, a grease trap may be necessary on the house sewer-line. The seepage pit may also be built at the lower ends of subsurface disposal tile lines in order to catch the septic-tank effluent which may have gone through without percolating away.
As shown in Fig. 3, the seepage pit consists merely of a round hole in the ground dug deep enough to penetrate 6 ft. or more into a porous layer of the earth. Diameters of 39100 in. and depths of 7-16 ft. are common. The side walls are lined with bricks or stones laid without mortar below the level of the inlet pipe. The hole may be filled with stones, in which case a lining is not required. The seepage pit should be closed with a tight cover which will prevent access to mosquitoes and files and to surface water as well.
If the soil through which the pit has been dug is not sufficiently porous, the effluent will slowly accumulate and will ultimately overflow. Even in porous soils such a situation is common, as the pores of the earth walls become choked by the deposit of the finely divided matter carried by the effluent and by the solids built up by the life activities of zoogloeal organisms which thrive on the grains of the soil in contact with this effluent. These phenomena, in fact, govern the life span of a seepage pit, which should normally last for several years (perhaps 6-10 years) if the effluent is only slightly turbid as a result of efficient primary treatment of the raw sewage.
When a seepage pit ceases to operate, a new one should be dug several meters away. In order to increase the life span of the disposal system, it is possible to dig two or three seepage pits and to connect them at the top. The distance between any two pits should be at least three times the diameter of the larger pit.
The obvious disadvantage of seepage pits is the danger of pollution of ground water. For this reason they should be carefully located. Seepage pits should preferably be located downhill and, in any case, at least 50 ft. away from drinking-water sources and wells. As in the case of cesspools, the construction of seepage pits is not usually permitted by health authorities in closely built communities where ground water is used for domestic purposes.
A = Ground Water
B = Depth 7 ft. or more to pervious soil
C = Inlet Pipe
D = Outlet pipe to another cesspool This pipe serves only when first cesspool is clogged and stops working
E = 6-in. layer of coarse gravel
F = Inspection manhole, (20 in. x 20 in.)
G = Stones laid without mortar
H = Stones laid with mortar
I = Ground Level
A = Variable depth of soil
B = Cement Joints
C - Open Joints
D = Rock Fill, 6 in. or more
THE SEPTIC TANK
The septic tank is the most useful and satisfactory unit among all water-carried systems of disposal of excrete and other liquid wastes from individual dwellings, small groups of houses, or institutions located in rural areas out of reach of sewer systems. It consists of a covered settling tank into which the raw sewage is led by the building sewer (See Fig. 62,63). The processes which take place inside the septic tank constitute the "primary treatment" of the raw sewage, and those which occur in the disposal field form the "secondary treatment". All liquid wastes, including those from bathrooms and kitchens, may be sent to the septic tank without endangering its normal operation.
A = Inlet
B = Outlet
C = Baffle
D = Floating scum
E = Sludge
F = Scum-clear space
G = Sludge-clear space
H = Depth of water in tank
I = Clearance
J = Depth of penetration of baffle
K = Distance of baffle to wall, (8-12 in.)
L = Top of baffle 1 in. below roof for ventilation purposes
M = Tank covers, preferably round
N = Ground level, less than 12 in. above tank (if less, raise tank covers to ground surface).
A = Private house or public institution H - Pipes laid with light joints
B = House sewer
C = Building sewer
D = Grease interceptor on pipe line from kitchen
E = Manhole
F = Septic tank
G = Dosing chamber and siphon
I = Distribution box
J = Drop-bores or terracotta L's
K = Absorption tile lines
L = Seepage pit, when required
M = Slope of ground surface
N = Topographic contour lines