Assessing on-site systems and sludge accumulation rates to understand pit emptying in Indonesia

Despite 85% urban residents in Indonesia using on-site sanitation, demand for pit emptying is low and there is a lack of empirical data on local conditions. The type of system, sludge accumulation rate and pit emptying frequency was analysed from 190 household surveys, measured content of 107 pits and government interviews in six Indonesian cities. The sludge accumulations rates were on the low end of existing literature, with an average rate of 25l/p.y. 83% of the sample were single pits with an unsealed base receiving only blackwater and 22% had an overflow to a waterway or drain. The majority of systems had never been emptied. First emptying at 45% sludge depth occurred after an average 16 years, however subsequent emptying occurred more frequently at 2-4 year intervals. Planning on the basis of actual sludge accumulation rates could lead to more viable pit emptying business models and appropriately sized treatment plants.