Cover Image
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

Example privy designs

PIT PRIVY

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.

Details

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.

TABLE 10: PIT PRIVY CAPACITY FOR A FAMILY OF FIVE

   

Estimated volume and depth* for hole with 10 ft2 area

Pit Type

Years of Service

Personal Cleansing Material

 

Water

Solid

 

volume ft3 and depth ft

volume ft3 and depth ft

 

4

26

2.6

40

4.0

WET

8

52

5.2

80

8.0

 

15

97

9.7

150

15.0

 

4

40

4.0

60

6.0

DRY

8

80

8.0

120

12.0

 

15

150

15.0

-

-

 

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.

TABLE 11: VOLUME & DEPTH FOR RURAL LATRINE WITH CROSS-SECTIONAL AREA OF 9 SQ. FT. (FOR FAMILY OF LIVE)

 

Personal cleansing material

Wet Pit

Service life

water

solid

 

volume

(cu ft)

depth

(ft)

volume

(cu ft)

depth

(ft)

4 year (minimum)

26

3

40

4.6

8 year

52

5.8

80

9

15 year (maximum)

97

11.8

150

16.5

 

Personal cleansing material

 

water

solid

Dry Pit

Service life

volume

(cu ft)

depth

(ft)

volume

(cu ft)

depth

(ft)

4 year (minimum

49

4.5

90

6.7

8 year

90

9

120

13.3

15 year (maximum)

150

16.6

-

-

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