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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

Section 8: The privy method of excreta disposal design for a village



A sanitary survey and a description of the area where a disposal system will be installed is necessary so the project will ensure the public health. This section covers the design and construction of disposal projects of the privy type. The trainees will learn to construct several types of privy and be able to make the proper selection for a given situation.



Design, plan, and construct a privy system which extends the present system to provide minimum satisfactory service within the community's social and economic limits of acceptability.


1. Establish criteria that proposed system must meet.

2. Define the capacity of the present system.

3. Sketch the location of any disposal facilities with respect to any water supply sources, irrigation systems, and food supply areas.

4. Determine whether these facilities are potential sources of contamination of any of the above.

5. For each disposal facility that is a potential source of contamination, determine whether a feasible means for arresting this situation is available.

6. Estimate population and define the community's capacity requirements for a disposal system.

7. Determine whether the present system is adequate in terms of the community requirements and the design criteria established above.

8. Define the extent to which new installations are necessary.

9. Determine the attitude toward sanitation and the practices of personal hygiene in the community.

10. Determine which types of installations are most likely to be maintained by the community.

11. Determine where any new facilities should be located so that:

a. They do not contaminate any of the existing water supply sources.

b. They are dry, well drained, and above flood level.

c. They are compatible with social attitudes in the community.

12. Identify the types of pit privy systems that satisfy the design criteria.

13. Determine what material and financial resources are available.

14. Prepare a plan to acquire public support toward and finances for the system.

15. Select the privy system that is most feasible.

16. Set up a program to construct this privy system.

17. Carry out this program.


1. Bore a test hole with a hand-held auger.

2. Determine how far faecal borne diseases can be carried in each of the strata or soils encountered in the test hole.

3. Conduct a survey to determine the population of the community.

4. Estimate the volume of pits, holes, and tanks.

5. Determine the practices of hygiene in the community.

6. Identify the components of various types of pit privies.

7. Dig a pit.

8. Mix concrete

9. Construct structures with brick, stone masonry, and rough cut logs.

10. Train local persons in the masonry and carpentry skills needed to build the system.

11. Provide instruction in the maintenance of the system.


1. With a hand-held auger, bore a hole, with horizontal dimensions of from 4 to 12 inches to a depth of 24 feet and determine how far faecal borne diseases can be carried in each of the strata or soils encountered in this test hole.

2. Determine the population of a designated community.

3. In a field exercise, determine the volume of any pits, holes, and tanks.

4. Determine the practices of personal hygiene in a designated community.

5. On a written examination, sketch three different designs for pit privies and list opposite each one of these the materials needed 'or construction.

6. Given the designs and materials needed to construct a pit privy, in a field exercise, construct this privy.


The sanitary survey

In most rural areas. community sanitary surveys are usually necessary to obtain first-hand information concerning local sanitary conditions and needs. Such surveys, undertaken with the participation of local leaders of the community, will be of immense help in program planning and evaluation. Such a survey should cover the following factors.


1. location, topography, climate, character, communications, maps;

2. geology and hydrology, with particular reference to nature of top and underground layers of the soil, its porosity, presence and abundance of ground water ( if any), direction of flow, level of ground-water table, its appearance and portability, estimation of yields of springs, rivers, and so on;

3. population - number, constitution by age-groups and sex, density, growth;

4. industries and agriculture, with particular reference to irrigation, drainage, and soil fertilizing practices;


1. general health of the population, with special emphasis on communicate diseases and on intestinal infections;

2. vital statistics, mortality and morbidity data;

3. health and sanitary administration, with reference to organization, personnel, budget, and activities of voluntary or other agencies in the field of sanitation;

4. existing sanitary conditions in the area, with reference to description of private and public latrines, their distribution and use; to wells, springs, and other systems of water supply (including such information as number of persons served by piped water-supplies, and by wells, the consumption and uses of water, number of dwellings with private water supply, etc.); to wastes collection, disposal, and composting; to milk and food sanitation; to insects (flies, fleas, lice, mosquitoes); to health aspects and standards of housing; and to school sanitation.

5. sociological and cultural patterns, with particular reference to community and family organization, leadership; customs, beliefs, and habits bearing on personal hygiene and community sanitation; present methods (if any) of health education of the public.


1. general economic level of the population; average income per worker;

2. co-operation expected from agricultural, educational, and other agencies or groups for training and health education of the public;

3. housing and vehicle transport for program, vehicle and equipment repair and maintenance facilities; sources or power (electricity, fuel);

4. local construction materials and their costs;

5. local craftsmen and wages;

6. potential resources for self-help;

This information has an important bearing on the project and makes it possible to make a reasonably accurate cost estimate. Such a survey is a useful educational tool and also serves to acquaint the PCV with the families and with their customs, beliefs, interests, and attitudes. In short, it helps to prepare a "social map" of the community.


The pit privy


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.


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


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


Example privy designs


This is the simplest recommended latrine or privy, having a hand dug hole, properly mounted slab and a shelter. This is the most widespread and satisfactory type of latrine, when properly designed, built, and located.

Tools and Materials

Materials for building the shelter Handtools for digging the pit, concrete construction and building the shelter.


The pit is round or square, about 3.5 feet in diameter or for each side and usually 3.3 to 10 feet deep. The pit may have to be lined, to prevent caving, with brick, wood, bamboo, etc., even in hard soil. It is good to line the top 3.5 feet of the hole so as to make a solid base for the slab and shelter. 19.5 inches of the top of the hole can be lined with mortar for this purpose.

The following table will help you to estimate the depth of hole to make. The top Part of the table is for a wet-pit privy, where the hole penetrates the water table and the contents are usually quite wet.



Estimated volume and depth* for hole with 10 ft2 area

Pit Type

Years of Service

Personal Cleansing Material





volume ft3 and depth ft

volume ft3 and depth ft






































One and a half feet have been added to the depth since the pit is considered full when material is that distant from the slab.

The base serves as a solid, waterproof support for the floor. It also helps to prevent hookworm larvae from entering. Properly made of a hard, strong material, it helps stop the entrance of by 50% in cases where the types of personal cleansing materials normally employed might indicate that such an allowance is necessary.

It is further recommended that, where practicable, wet-pits should have a minimum depth of 10 ft. With regard to pit storage capacity, it is desirable to design for as long a period as possible, i.e., for 10-15 years, However, it is recognized that, from the standpoint of cost, or because of difficult in supporting pit walls in unstable soil formations, it might sometimes be impossible to attain this objective. Nevertheless, it is strongly recommended that pits should be designed for a life of at least

four years.



Personal cleansing material

Wet Pit

Service life





(cu ft)




(cu ft)



4 year (minimum)





8 year





15 year (maximum)






Personal cleansing material




Dry Pit

Service life


(cu ft)




(cu ft)



4 year (minimum





8 year





15 year (maximum)





Depth given is effective pit depth, and 1-2 ft (30-80 cm) are usually added to obtain overall depth pit

Table I shows the pit volume and dimensions for household latrines for families and gives varying periods of service life based on wet-pit conditions. Table II presents similar data for dry-pit conditions.

These tables show that, where there is little possibility of maintaining water in pits or holes, a pit privy with the largest possible volume is best. From the economic standpoint, the deep pit, although higher in initial cost, will prove to be a profitable investment.

Finally, one factor that also influences the cross-sectional area of the pit, although to a lessor extent, is the size of the floor that covers it. The size of the floor slab depends much on the type of material from which it is built. This matter is discussed in a later section.

The selection of the type of installation best suited to local needs must take into account the element of cost. Water-carried sewerage systems with flush toilets are very expensive and for beyond the economic possibilities of most rural areas. At the ether extreme, it is possible for everyone to relieve himself in the most primitive manner at no cost what-soever; but this method is disastrous in terms of sickness and death and burrowing rodents and of surface water into the pit. The pit lining in most cases will serve as a base although it may need to be strengthed at the ground surface.

A concrete water-seal slab is best and is economical but means added labor and construction. A concrete open-hole slab is the next best, while a wooden floor is adequate. A built-up floor of wood and compacted soil is sometimes used but is difficult to keep clean as it gets soiled and is likely to spread hookworm.

The concrete should not be weaker than l cart cement to 6 parts of aggregate with a minimum of water. It should be reinforced with strips of bamboo about 1 inch wide and with the weaker fibers stripped away. Soak the bamboo in water overnight before use.

Fig. 68 Various Parts of a Sanitary Privy

A = Pit

B = Base

C = Floor

D = Mound

E = House, including door

F = Ventilation

G = Roof

The slabs are cast upside down in one operation. The footrests are shaped by removing part of the wooden form so as to make two separate indentations in the wood. Sheet metal is placed around the form so that the metal extends above the wood to the thickness of the slab. Side walls of the hole and footrests are made with a slight slope so as to come out easily. The form for the open hole is removed when the concrete first sets. Slabs are removed from the forms in about 40 hours and should be stored under water for 10 days or more.

Round slabs can be rolled some distance when carrying is difficult.

The mound protects 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 inches beyond the base on all sides. In unusual cases, such as flood plains and tidal areas, the mound may be built much higher than the ground as a protection against floor and high tides. It will normally be built with the earth removed in digging the pit and soil from the surrounding area. A stone facing will help stop it from being washed away by heavy rains. In front of the entrance door, a masonry or brick step can be built to help keep the floor clean.

Fig. 69 Typical round and square bases (built with soil-cement or clay)

Fig. 70 A hewn-log privy base


Latrine for village use

This low cost water seal latrine slab is a single concrete casting. It requires very little space, is sanitary, odorless, easy to install and maintain, and can be used to produce nightsoil fertilizer

Tools and Materials

Foot plate from - See Fig. and

Steel strap iron 2" wide, 1'7" long

3/8' be its 5" long for air vents

Cuter form - made of wood detailed or Fig.

Inner form - made of wood detailed or Fig.

Clay to make water seal form

Cement, sand, stone aggregate up to 1" maximum

Fig. 71 Latrine for Village Use


In villages where space is a premium and the soil can absorb the flushing water, this latrine may be worth serious consideration. A 30" diameter hole eight feet deep is covered with a slab. Most soils have sufficient stability to support the slab. Very loose or sandy soils may require some type Of lining. Any type of simple superstructure can be fitted over it for privacy. If the nightsoil must be used for fertilizer, this method can if used. After the first six months, a new hole is dug, and the slab moved. The fires pit is covered with two feet of dirt. Six months later the night soil in the first pit has been converted to essentially non-pathogenic fertilizer and may be used with reasonable safety. Do not use any nightsoil fertilizer that has not been composted at least three months. The slab is roved back to the first hole and the second covered with two feet of dirt.

The latrine can be cleaned with only 1/2 gallon of water. When this is done, there is no odor nor any flies and it stays quite clean. Thus it is easy to use. Villagers must be urged to provide for a sufficient supply of water to be brought and stored at the latrine in a large container (en. a 4 gallon kerosene tin). A quart container should also be provided. Instructions should be given in the proper method of flushing the latrine. If this is done improperly a large quantity of water will be wasted. Two quarts of water 2-e sufficient to clear the latrine if the water is thrown with a fair amount of force from the narrow end of the latrine.

Installation is so simple that the untrained villager can do it easily. The round one piece construction facilitates moving the slab by rolling it. It is simple to make once the forms and methods are practiced. The materials cost about S1 for a latrine. One trained villager can make three slabs per day, using three forms. The wooden forms cost about $8 each.

A convex foot-plate form about 38" in diameter is made of wood, metal, or concrete. It must be 1" higher in the center than at the edge. See Fig.

Fig. 72 shows the steel ring and inner form in place on the base. The ring is formed of two inch wide strap iron and fastened with a bolt for easy remove, from the concrete slab. The collapsible wooden inner form is detailed in Figure 73 .

The inner form has three pieces Fig. shows the outline of the two side pieces of the form. These must be cut from wood 2 1/4" thick. The 18 1/8" sides and 3 3/4'' sides stay nearly in contact. A wedge shaped piece of wood shown in Fig. holds corner G of the sides one inch apart. the wedge fits along the 9" side. The spring holds the form closed tight against the separation bars while the wedge is inserted and the inner form placed on the base. The dimensions shown for the inner form should only be used as a guide since some inconsistencies have been observed.

Fig. 72 Inner Form of Latrine (Top View)

Two inches of well mixed concrete (cement 1, sand 2, stone chips 2) is placed in the ring and tamped will to compact it. Next the wooden outer form is set up around the inner liner. See Figs 72 and 73. There should be a clearance of not less than 1/2" between the Inner liner and the wooden outer forms. A cement sand mixture (cement 1, sand 2) of plastic consistency is pieced in this inner space and compacted. A 3/8'' bolt through the outer wood form and into the inner form provides an antisyphon vent and helps to hold the loner form in place. See Figs. and

After 48 hours the casting may be pieced on blocks. The clay siphon and wooden inner form removed, and a finish of cement plaster added to cover any imperfections. When this is set a final coat of pure cement is put on. If and a final coat of pure cement is put on. If there Is any defect in the seal it may easily be repaired by putting a little cement slurry (cement and water in creamy consistency) over the defect and adding at once cement plaster to fill the defect.

Fig. 73 Inner form and steel rim in place on base.

Fig. 74A Latrine bowl mold

* is a wooden wedge, used to hold form tight. to remove form from concrete latrine, one knocks out wedge, then removes separation bars; the bottom of the form then contracts.

If the wood is not hard and smooth, a time outer coating may be hammered on to wooden form.

Width of mold a A & B are 4-1/4 and 51/2 in. respectively. The separation bars at A & B are 2-1/2 & 4 in. long respectively.

Finished bowl-seal form, ready for placing on wooden base. The water-seal form has been molded from clay, by hand, and placed on top of the wooden bowl form. The size and configuration of the water-seal form must be shaped carefully, as shown. This is not difficult.

Fig. 74B Latrine bowl mold

Fig. 74C Latrine bowl mold

Fig. 74D Latrine bowl mold

Fig. 75 Concrete slab has been poured; part of the exterior sectional mold has been placed in its position.

Fig. 76 Rough exterior knock-apart mold made to fit around the clay core with a clearance of 1/2 in. to 3/4 in.

Wooden constituents of above mold:

1. 4" x 4" x 36' - 2 pieces

2. 3" x 4" x 16" - 2 pieces

3. 3" x 4a x 16" - 2 pieces

4. 3" x 3" x 21" - 1 pieces

5. 2" x 5" x 13" - 1 pieces

5. 4" x 4" x l" - 1 pieces

7. 5" x 13" x 1" - 1 piece

8. 3" x 4" x 4" - 1 piece

Fig. 77 Sectional view after pouring the cement in bowl and trap. Note the concave shape of the base plate.

Fig. 78 Transverse section of the casting with forms in place.

Fig. 79 Section of the casting after removal of the forms.

Fig. 80 Completed casting set up on bricks where the wooden inner form is removed and clay siphon lining dug out. The final finish of cement plaster and neat cement polish is applied.

Fig. 81 The completed casting from above showing the dimensions.


Thailand water-seal privy

This concrete water-seal slab is most useful for wide-scale privy programs. It is used to cover an ordinary pit privy. This method represents the collected experience of a long established privy program in Thailand. The general method should be applicable to other water-seal slab designs.

Tools and Materials

Master molds - Can be purchased from Village Health and Sanitation Project, Ministry of Public Health, Department of Health, Bangkok, Thailand. This aluminum master mold weighs 24 pounds and costs $7.50 plus shipping charges.

Concrete making materials

Wood for platform forms

Reinforcing rod and wire


Crankcase oil

Beeswax and kerosene (optional)

3/4" x 3/4" x 5" steel bars

The baste method used for making these water-seal slabs is to cast the slab, bowl, and water-seal trap using three forms :

1. A wooden form for shaping the slab.

2. A concrete bowl core for shaping the inside of the bowl.

3. A concrete core for shaping the inside of the water-seal trap.

Fig. 82 Sketch of Finished Privy

Since the three parts of the stab are all cast at one time, the finished privy slab is quite strong. The water-seal trap is curved hack under the bowl as shown in Fig. 83 A

This makes flushing more difficult, but prevents erosion of the back of the pit on loose soil. The same general method could be used to make a forward flushing trap, Fig. 83 B.

The forms used when making a slab must stay in place till the concrete has gained enough strength to allow their removal. This is usually 24 hours. For this reason, many sets of forms are necessary if a reasonable number of slabs are to be cast every day. Here is where the master molds are needed. One is used to cast the bowl core, and two are needed to cast the trap core.

Casting the Bowl Core

1. Oil the inside of the master bowl mold and insert a 3/4" x 3/4" x 5" steel bar into the bottom.

2. Add a fairly loose mixture of cement and water, called neat cement, to a depth of about 6". Then fill to brim with a 1:1 cement sand mixture. The 1:1 should be firm, not runny, and should be laid into the loose neat cement without stirring to insure a smooth finish on the bow, core.

3. After the bowl core has become firm enough, scoop a depression into the surface to install the two steel hooks made from the reinforcing rod. They should be about 9" apart, and should not protrude above the surface of the concrete. See Fig.

4. Allow the concrete to set at least 24 hours before removing the bowl core from the master mold. The bowl core can be used to make another master mole and vice versa.

Fig. 83A Backward Flushing Trap

Fig. 83B Forward Flushing Trap

Casting the Trap Core

Make the trap core using the pair of master molds, which consist of the trap master mold and the insert mold.

1. Add about 1" of 1:1 cement sand mix to the oiled trap master mold and put in some wire for reinforcing. Then fill it with 1:1 almost to the brim.

2. Put the oiled insert mold into place and scrape off excess.

3. After 45 minutes remove the insert and add a square sheet metal pipe 3/4" high made by wrapping sheet metal around a 3/4" x 3/4" steel bar.

4. Remove the finished trap core by gently tapping the master mold with a wooden block

Construction of the Wooden Slab Form

1. Make a frame of 1 1/2" x 1 1/2" wood with an inside diameter of 80 cm x 80 cm. A notch and single nail on each corner works we 1. See Fig. 86 .

2. Make a wooden platform 90 cm x 90 cm out of 1" thick planks. Gouge '/2" deep footrests if these are desired. See the outline in Fig.

Fig. 84 Bowl Core Handles

Casting the Slab

With these three forms finished you are ready to cast the first water-seal slab.

1. Use a paint brush to coat the bowl core and the trap core with a layer of wax about 1/8" thick. Prepare the wax by dissolving 1 kilogram of melted beeswax in 1/2 liter of kerosene. The wax coating will last 5 or 6 castings adding it to the cost of each slab. Wax makes removing the cores much easier, but isn't absolutely necessary.

2. Place the bowl core on the wooden slab form and fill all cracks with clay.

3 Oil the bowl, platform and frame.

4. Apply a 1/4" thick coat of pasty cement and water mixture to the bowl core and platform. (Many Thai people prefer to spend 25ยข more for an attractive polished slab. To do this, instead of using a mixture of cement and water, use a mix of 5 cement : 5 color : 1 granite chips. After tile forms are removed, polish with a carborundum stone and plenty of water).

5. Cover the bow' core with a mixture of l cement : 2 sand, to total thickness of 1/2". Notice the smooth lip made on the cement 3/8" from the top of the bowl core. This lip is your water-seal. Use fairly dry cement and allow it to set for 15 minutes before cutting this lip.

6. Place the trap core on the bowl core and seal the crack with clay. Also add a little clay on each side of the form to prevent cement from getting to 'he front lip.

7. Cover with 1 : 2 cement sand mixture to a thickness of l/2". Do not exceed the 1/2" thickness below the trap core or you will not be able to remove this core.

8. Fill the slab form with a mixture of 1 cement : 2 sand 3 clean gravel or crushed rock almost to the top. In preparing the concrete, first mix cement and sand, then add gravel and water. Use water conservatively. The looser the mixture, the weaker the concrete will be.

9. Press In 4 pieces of 1/4" steel rod reinforcing.

10. Fill to top of frame and smooth. Allow at least 24 hours for setting.

11. Remove the frame by tapping lightly with hammer.

12. Turn the slab form over on a wooden stand and use simple levers to remove the bowl core. You must remove the bowl core before the trap core.

13. Tap the trap core gently and slip it out. Add a little water and check to see if your seal is 3/8".

14. Keep the slab damp and covered for a minimum of 3 days and preferably a week to gain strength.

Fig. 85 Privy Frame

Fig. 86 Privy Slab Outline



Lesson plans




LESSON OBJECTIVE: Outline the factors that must be considered in designing a village privy system.




Sanitary Survey

Recall the need for a sanitary survey.

WHO Monograph Series #39

p. 158-155.


Outline the essential elements of this survey.


Demonstrate estimating the volume of pits, holes and tanks.


The Pit Privy

Discuss the function of the basic components of a pit privy.

WHO Monograph Series #3

p. 43-76.


Illustrate the designs and components for three types of pit privies.


Sanitary Survey

Discuss how to determine the adequacy of the present system.

See Section 7


Assemble students to conduct a survey of a rural area. This survey must


1) designate the population and its distribution


2) determine the practice of hygiene in this erect indicated from the present type(s) of facilities implemented


3) state where present facilities are improperly located, poorly maintained and not used


4) estimate the health of the community from records of communicable diseases, intestinal infections (diseases caused by worms) and acute conjunctivitus in infects (inflammation of the mucus membrane around the eyeball) and cite the death and debility statistics


5) identify what type(s) of privies are presently used (public or private; distribution and use) and determine the location of these facilities with respect to water supply systems.


6) designate the number of new installations needed and where they should be located


7) define the average income of the community members


8) estimate the cooperation available from agencies or groups for training and health education of the public


9) determine what materials are available locally, their costs, whether potential resources for self-help are available and if the needed craftsmen are available locally and their wages


10) state what type(s) of privies would be the most economically feasible.


Assemble students for discussion of survey findings.






LESSON OBJECTIVE: Set up and carry out a program to construct a privy system.




Selection of Privy System

Assemble students to visit the present facilities to determine the type(s) of facilities presently implemented.


Review the criteria to be used in the selection of a village privy system.


Select the most feasible privy designs.


Discuss how to prepare a plan toward the finances for the system.


Instruction techniques

Outline the logical sequences of construction for the privy type selected.

For construction techniques see Section IIC


Have students assemble the tools and materials for the construction of this privy type.


Set up a program (allocate construction tasks to the students) for the construction of a privy of the type selected.


Supervise students in the construction of this privy