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close this book Water purification, distribution and sewage disposal for Peace Corps volunteers
close this folder Section 8: The privy method of excreta disposal design for a village
View the document Overview:
View the document The sanitary survey
View the document The pit privy
View the document Example privy designs
View the document Latrine for village use
View the document Thailand water-seal privy
View the document Lesson plans

The pit privy

DESCRIPTION

The pit privy consists of a hand-dug hole in the ground covered with either a squatting plate or a slab provided with riser and seat. A superstructure or house is then built around it.

DESIGN AND FUNCTION OF ITS PARTS

The Pit

The function of the pit is to isolate and store human excrete in such a way that no harmful bacteria can be carried there from to a new host. The pit is usually round or square for the individual family installation and rectangular for the public latrine. Its dimensions vary from 36 in. to 48 in. in diameter or square. Common figures for family latrines are 36 in. diameter or 42 in. square. For public installations, the pit will be 36 in. to 40 in. wide; its length will depend upon the number of holes provided. The depth is usually about 8 ft. but may vary from 6 ft. to 16 ft. In Iran, and elsewhere, some pits have been due to a depth of 23-26 ft. in soils which are very stable.

Lining of the Pit

It is often necessary to provide a pit lining to prevent the sides from caving in. This is true especially in rainy seasons where privies are dug in fine-grained alluvial soils, sandy soils, and similar formations, or when they penetrate deeply into ground water. Even in stable soil formations, it is desirable to line the top 16-24 in. of the pit in order to consolidate it and to prevent it from caving in under the weight of the floor and the superstructure.

Materials commonly used for this purpose include bricks, stones, concrete blocks laterite blocks, adobe materials, lumber rough-hewn logs, split cane, and bamboo. When the firs five materials mentioned above are used, they are laid with pen joints over most of the walls height and with mortar par the top of the walls, the reason being that with these material the lining also serves usefully a, a base for the floor. Brick, linings should preferably be round, not square, as they the, develop arch action and are much stronger for the same all thickness. Wooden lops and bamboo should be used exclusive y to support the walls of the nit, not as a foundation to the floor. Rough-hewn logs will of course, last longer the bamboo linings. If possible both logs and bamboo should be tarred in order to increase their useful life. The use of rot-and termite-resistant woods is recommended wherever possible.

Where a lining is necessary, it is often given to the family by the health department, along with the floor or slab.

The Base

The base serves as a solid, impervious foundation upon which the floor can rest. It also helps to prevent the movement of hookworm larvae. Properly made of a hard, durable material, it helps to prevent the entrance of burrowing rodents and of surface water into the pit. Pit lining in most cases will serve as a base although it may need to be strengthened at the ground surface.

The foundation should be at least 4 in. wide on top in order to provide a good surface for the floor to rest upon, and 6 in. or more at the bottom in order to give a stable contact with the ground. Its shape will be that which will fit the pit. The base should be high enough to raise the floor 6 in. above the level of the surrounding ground, thus, with the mound, protecting the pit from flooding.

The following materials may be used in the construction of the base:

a. plain or reinforced pre-cast concrete - same mix as floors;

b. soil cement - 5%-6% cement mixed with sandy clay soil and tamped at optimum moisture content;

c. clay - tight clay, well tamped at optimum moisture content;

d. brick - mud-dried, burned, adobe, etc.

e. stone masonry;

f. rough-cut logs - hardwood, termite-resistant.

The Floor

The floor supports the user and covers the pit. It should be constructed so as to fit tightly on the base, with a minimum of small cracks and openings between the surfaces. The squat-type plate or slab for pit privies is the most suitable for rural conditions in most parts of the world. However, in many countries a slab provided with a riser and seat may be found to be more acceptable. This aspect of slab design requires careful consideration. An eminent health educator and social anthropologist has stated that customary posture in defaecating is perhaps the single most important fact bearing on the acceptance or rejection of privies.

The floor or slab should normally extend to the superstructure walls, as a peripheral earth strip might be soiled and become a medium for hookworm infestation. It should be made of a durable impervious material with a hard surface which will facilitate cleaning. Materials commonly employed include:

a. reinforced concrete;

b. reinforced concrete with brick filler;

c. wood;

d. built-up floor of small-diameter wooden poles with chinks filled by mud or soil-cement mixture.

The consensus of opinion is that concrete is, in the long run, the most practicable, most acceptable, and cheapest material for the privy floor. Wooden floors come next in the line of preference. Built-up" floors, are less desirable because they are difficult to keep clean and, as they get soiled (especially by children), are likely to spread hookworm.

Latrine slabs or floors may be round, square, or rectangular. When slabs are to be made or cast at a central shop, it is advantageous to adopt a standard stripe and size in order to facilitate production. The size of concrete slabs, which influences to a certain degree the cross-sectional area of the pit and the size of the superstructure, is governed by their weight and by the difficulty of transportation (where this applies).

All factors considered, appropriate dimensions for concrete slabs may be 39 x 39 in. in over-all size. Such a slab will weigh approximately 300 lb. If the average thickness is 2.5 in. Smaller Slabs. 3 x 3 ft. have been bull' where it is easy to complete the floor at the site with a cement surface. Round slabs, 3 ft. in diameter, have also been used. Their advantage is that they may be rolled to the latrine site instead of befog transported.

The thickness of slabs also varies a great deal in practice. In order to reduce weight, the tendency, of course, has been to reduce the thickness to a minimum consistent with safety. In his respect, however, much depends on the quality of the concrete and the reinforcement available. When these factors are satisfactory the slab may be 2.5 in. - 3 in. thick on its edges and 2 in. thick at its center. A slab l yd. square will then weigh approximately 286 lb. The surface of the slab will slope towards the hole, which is an advantage in Asian countries where water is used for anal cleansing. Where solid cleansing materials are used, the slab may be of uniform thickness throughout, but not less than 2.5 in. thick.

Where it is not possible to cast concrete slabs in place and where the problem of transportation is serious, the possibility of casting the slab in four parts may be considered.

With respect to the shape and size of squatting plates, the following are important considerations:

1. The opening should be large enough and shaped so as to minimize-or better, prevent - soiling of the floor. An opening having an effective length of about 15 in. preferably more, will satisfy this requirement.

2. It should not be so large that small children may fall into the pit. An opening having an effective width or diameter of 7 in. or less will satisfy this requirement.

In communal installations, the number of openings will depend on the number of people to be served. It is good practice to provide one hole for not more than 15 users, preferably one for each 10-12 persons.

It is often recommended that squatting plates should be provides with slanting foot-rests to minimize the possibility of soiling the floor. Foot-rests usually form an integral part of the squatting plate and should be designed to be used by both adults and children. When foot-rests are not properly built - for instance, when they join the floor at a sharp angle or are excessively long, etc. - they make it difficult to clean and scrub the floor.

Another factor affecting the acceptance or rejection of a privy by the users is the free distance from the opening to the back wall of the latrine. When this distance is too small, the back of the user will rest against the wall, which may not at all times be very clean and free from ants or other insects. Also, there is a chance that excrete may soil the upper portion of the pit wall. Yet this distance should not be too large; otherwise there is a likelihood that the back part of the floor will be soiled. The minimum distance between the rear edge of the opening and the superstructure wall should be no less than 4 in. preferably 6 in. and maximum of 7 in.

The Mound

The function of the mound is to protect the pit and base from surface run-off which otherwise might enter and destroy the pit. It should be built up to the level of the floor and be very well tamped. It should extend 20 in. beyond the base on all sides. In exceptional cases in flood plains and tidal areas, the mound may be built up considerably above the ground for protection against tides and flood waters. It will normally be built with the earth excavated from the pit or surrounding area, and may be consolidated with a stone facing to prevent it from being washed away by heavy rains. In front of the entrance door, it may be preferable to supplement the mound with a masonry or brick-built step. This helps to keep the latrine floor clean .

In the Philippines, where the dwelling is often built above the ground on piles, the latrine floor is also elevated; and a drop- pipe 'cads the excrete downward to the covered pit below. this is called the "antipole" system.

The House or Superstructure

The house affords privacy and protects the user and the installation from the weather. Fig. 67 shows various types of houses, and a typical wooden house frame for use in rural areas. From the sanitary viewpoint, the house is less important than the pit or the floor. For this reason,, when latrine programs are undertaker on a campaign basis, the house is often left for the people to erect in the manner which is most satisfactory to them with only general advice being offered by you. Standardized superstructures are desirable, however, from many standpoints, among which economy of construction and durability are most important.

A properly built superstructure should conform to certain rules, the most significant of which are:

1. Size. It should preferably fit the dimensions of the floor or slab and should never be too large, lest people be tempted to defaecate on any part of the floor at times when the area around the opening has been soiled by previous users. The height of the roof over the slab near the entrance door should be 6.5 ft. or more.

2. Ventilation of superstructure. It is desirable to provide openings 4-6 in. wide at the top of the house's walls to facilitate constant ventilation.

3. Lighting. Natural light should be available wherever possible. However, the superstructure should provide sufficient shade over an uncovered seat or hole in order not to attract flies.

4. Cleanliness. A superstructure which is left dirty and in a constant state of disrepair will soon be abandoned and unused as a latrine. It is therefore extremely important that the house be Kept clean at all times, both inside and outside, and that no poultry or animals be housed in it. White or colored washings of the superstructure should be encouraged, and the vegetation immediately surrounding it should be trimmed. The roof should cover the house completely and have a large overhang to protect the mound and the walls from rain and roof drainage. One of the duties of the health department staff, especially the sanitarians and health educators, is to provide constant advice to the family regarding the cleanliness and the proper use of the latrine.

Example Privy Shelters

Drawings of several designs are provided on the next page. The structures shown have been found satisfactory in many parts of the world.

Tools and Materials needed for construction include:

A sheet of corrugated sheet metal roofing, 4 ft. x 4 ft. or larger

Wooden posts 2 in x 2 in., 66 feet long

Boards, 8 in. wide, 3/4 in. thick, 132 feet long.

Nails, handtools, paint (2 quarts)


Fig. 67 Types of Privy Shelters

COMPLETED PRIVY SHOWING PALM THATCH WALL AND ROOF COVERING


Fig. 67 Wattle house with palm thatch roof


House of cut lumber with corrugated metal or asbestos cement roof


The of superstructure recommended by as public health service


House of brick with tile roof