Modelling the Biodegradability of Sewage in Ordinary Pit Latrines

ABSTRACT. Specific models for the design of pit latrines of different shapes and sizes are non-existent. What are available are general design guides irrespective of the shape and size of latrine which may not give the actual design parameters needed. Moreover, the physico-chemical and biological characteristics of pit latrine sludge samples investigated by some researchers were limited and could not give the actual situation in the pit latrines. The objectives of this research were to derive models for the rational design of pit latrines of various shapes and sizes using data collected from 500 household pit latrines already filled up, carry out laboratory analyses of sludge samples collected from fifteen (15) pit latrines for their physico-chemical and biological characteristics and use the data to derive and verify the model for the pit filling rate. It was therefore necessary that these models be derived and verified using both field and experimental data as well as investigate the physico-chemical and biological characteristics of faecal sludge samples from the pit latrines. In this research, models were derived for the design of various pit latrine shapes and sizes using data obtained through the administration of designed questionnaire on 500 households having pit latrines. These models were calibrated and verified for the rational design of pit latrines of various shapes and sizes using field data so collected. The regression coefficients for calibration were 0.75, 0.65 and 0.50 and for verification were 0.97, 0.98 and 0.99 for square, circular and rectangular pits respectively. Faecal sludge samples collected from fifteen (15) different pit latrines in the study area were subjected to laboratory investigations for their physico-chemical and biological characteristics. From the study, BOD, COD, total solids, suspended solids, volatile solids, moisture contents and plate count showed decreasing trend throughout as the pit depth increased. On the other hand, the pH presented double trends, increasing in some pits and decreasing in others. Temperature presented double scenarios, increasing initially and decreasing afterwards. When BOD, COD, volatile solids and temperature were measured against time and various pit layers, the Figures showed decreasing trend in all the cases. The phosphorous content increased as the pit depth increased in all the 15 pit latrine sludge samples analyzed. The biodegradability of faeces in all the pit latrines sampled in terms of COD alone was 80.33% and 78.26% in terms of combined COD, BOD, total solids, volatile solids and suspended solids. This attested for the reliability of this research. A model for the pit filling rate was derived and verified using data obtained from laboratory analysis. The implication of this research is availing of design models for the construction of various pit latrine shapes and sizes, increased knowledge of physico-chemical and biological characteristics of faecal sludges in ordinary pit latrines and model for pit filling rate. In conclusion, models have been derived and verified for the design of pit latrines of various shapes. Pit filling rate model was also derived and verified. Better devices for measuring the biodegradability of pit latrine sludges for COD and volatile solids as reference parameters should be explored. Studies should be conducted on fresh pits of 1m3 to determine accurately, the time of filling