Modelling the filling rate of pit latrines : a paper presented at the second conference on developments in faecal sludge management in Durban, South...

Excreta (faeces and urine) that is deposited into a pit latrine are subject to biodegradation, which substantially reduces the volume that remains. On the other hand, other matter that is not biodegradable usually finds its way into pit latrines. The net filling rate is thus dependent on both the rate of addition of material and its composition. A simple material balance model is presented which represents the faecal sludge as mixture of biodegradable organic material, un- biodegradable organic material and inorganic material. Measurements made on two pits in eThekwini, South Africa were used to determine parameters for the model. Model predictions were then compared with data from 15 other pits in the same area and filling rate data from previous South African studies. These comparisons indicated that the pits studied exhibited relatively low filling rates resulting from orderly disposal practices. The average composition of the pit (COD, biodegradable material and inorganic fraction) changes with age, which will impact on any subsequent sludge treatment process. Pit filling rates are greatly affected by the disposal of solid waste in addition to the faecal material. For the pits studied, the model predicts that the filling time could have been extended from 15 y to over 25 y if all solid waste had been excluded from the pit. [authors abstract]