EFFECT OF SHAPE FACTOR ON MODELING PIT VOLUME AND THE FILLING RATE OF FAECES IN PIT LATRINES

Computation of pit latrine filling rate and volume without considering actual pit shapes lead to wrong values.
This study investigated effect of shape factor on volumes and filling rates of pit latrine sludges in 3 different pit shapes in Aku community using household questionnaire administered in 200 households. Data collected included pit shapes, dimensions, soil types, user-population, usage frequency, lifespan, mass of initial biodegradable faecal matter, final mass of unbiodegradable faecal matter at varying times and depths and mass of biodegraded matter at the end of specified period. Models for pit volumes and filling rates incorporating shape factors were derived and calibrated with existing models. Shape factors obtained for square, rectangular and circular pit latrines were 0.79, 0.79 and 1.0 respectively resulting to 21% volume reduction in square and rectangular pits without volume change in circular pit. Filling rates obtained for square and rectangular pits were higher than those of existing models while that of the circular pit remained unchanged.Volume reduction and filling rate increase were attributed to stagnant corners that developed in pit walls during pits filling, thereby reducing available area for faecal infiltration into surrounding soil. this showed that circular pits are more ideal for pit latrine design.
The new and existing Ugwu (2015) models were calibrated with the result showing good correlation between them. Correlation coefficients for the square pit latrines was 0. 996 for the Ugwu model, 0.996 for that of Oluwafemi (1984), 0.991 for the CASSAD (2005) model and 0.992 for the model with shape factor. For circular pit latrine, the value was 0.966 for Ugwu model, 0.994 for that of Oluwafemi (1984), 0.994 for the CASSAD (2005) model and 0.938 for the model with shape factor. For rectangular pit , the value is 0.996 for Ugwu model, 0.996 for Oluwafemi (1984), 0.966 for CASSAD (2005) and 0.997 for the model with shape factor.
Models derived were verified using filed data. Coefficient of verification for the square, circular and rectangular pit latrines were 0.97, 0.98 and 0.99 respectively and for verification were 0.97, 0.98 and 0.99 for square, circular and rectangular pits respectively.
Both actual and derived values correlated positively with high correlation coefficients. The correlation coefficient for the rectangular pit was the highest with value 0.990 followed by the circular pit with the value, 0.959 and lastly the square pit with value 0.939. This showed that the data collected from the field were reliable.
It is recommended that models incorporating shape factors should be used in designing pit latrines of desired shapes and volumes. This is to forestall the pits filling earlier than necessary. Field data should be fitted into any pit shape model desired by the household considering the population of users, frequency of usage, design capacity, expected lifespan, and soil conditions.