A number of possibilities exist for using fecal sludge beneficially. Data provided by Water Services Authorities indicate that most pits are filling in five to nine years. This suggests that of the more than a million ventilated improved pit latrines that have been built in the past decade many will soon reach capacity. Studies of pit filling rates across a number of communities indicate that pits generally fill at a rate of 40 liters per capita annum, with 60 liters per capita annum providing a safe margin for planning pit design and emptying programs (Link-download is free but registration required).
STILL, D. FOXON, K. (2012): How fast do pit toilets fill up? A scientific understanding of sludge build up and accumulation in pit latrines. (= Tackling the Challenges of Full Pit Latrines , 2 ). Gezina, South Africa: Water Research Commission URL [Accessed: 15.01.2013]