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close this book Water purification, distribution and sewage disposal for Peace Corps volunteers
close this folder Section 7: Scope of disposal system projects in host communities
View the document Overview:
View the document Public health importance of excreta disposal
View the document How disease is carried from excreta
View the document The characteristics of an adequate system
View the document Possible sanitary measures in rural areas
View the document Soil and ground-water pollution
View the document Location of latrines and other excreta disposal facilities
View the document Sludge accumulation and the life of a pit privy
View the document Community participation
View the document Family participation
View the document Role of health department and other agencies
View the document Public versus private latrines
View the document Human factors
View the document Lesson plans

Sludge accumulation and the life of a pit privy

The life of a pit privy is directly proportional to the time required for a pit to fill. This time is determined by the rate of digestion in the privy. The factors that influence the rate of digestion in a pit are:

(1) the efficiency of bacteriological decomposition.

(2) the degree of abuse to which the pit is subjected (i.e. the stones, sticks, garbage, mud balls thrown into it).

(3) the method of anal cleansing.

The rate of accumulation of digested sludge and of partially digested excrete is not directly proportional to the amount of excrete added each year. This phenomenon is illustrated graphically in Fig. 65. From this graph it will be noted that, after the digestion process has been well established, the actual volume of material in a wet pit might be reduced in time to approximately 10% of the total waste (faeces and urine) deposited. This graph is very approximate, however, and may need to be substantially modified for different areas.

In wet pits, assuming that the daily production of one quart of excrement contains 3.5 oz. of dry solids and that digestion for one year under wet-pit conditions will reduce this mass by 80%, a total of 0.7 oz. of dry solids will remain. Further assuming an 80% moisture content in the digested sludge, one arrives at the figure of 3.5 oz. of wet sludge after one-year digestion period.


Fig. 65 Sludge Accumulation in Pit Latrines

A = Human waste (liquid and solid) deposited in pit by a family of five (at the rats of one litre, or quart pot person per day)

B = Corresponding amounts of solid wastes alone (approximately)

C = Rate of accumulation of sludge by volume, In dry Pit

D = Rate of accumulation of sludge, by volume, In wet pit

This amounts to 1.3 cu. ft. per person per year. Thus, a family of five would require sludge-storage space of 6.5 cu. ft. per year.

It is recommended that for the design of the effective capacity of wet-pit latrines a provision of 1.3 cu. ft. per person per year should be allowed. If cleansing materials such as grass, stones, mud balls, coconut husks, or similar solids are used, it is recommended that this figure be increased by 50% to a total of 2.0 cu. ft. per person per year.

Digestion of solids is about 50% less rapid and less complete in dry-pit latrines than in wet-pit latrines. In designing dry-pit latrines, a provision of 2 cu. ft. per person per year is recommended, to be increased

resulting economic loss. Therefore, between these two extremes a solution should be found that will give the most in health protection and, at the same time, will be within ┬┐he economic possibilities of the people to construct and maintain. Every sanitation worker should carefully consider this aspect of the problem, not only as it applies to privies, but also as it relates to every other type of sanitary improvement. It is relatively easy to decide on a privy campaign for a rural area from a health department office situated far away in the state or provincial capital simply by choosing a design that appears to be satisfactory because it has been used somewhere in the world. This is the kind of privy program which has resulted in empty, abandoned, and unused installations in so many places.