When a pit latrine fills in rural areas families tend to dig another pit, relocate or rebuild the super-structure, and close the old pit. The contents of the old pit bio-degrade over time. However, in urban slums the density of dwellings is much greater and the number of people using each latrine is high. Pits tend to fill much faster and in many areas there is no longer space to build a replacement pit and latrine. Pits are often emptied manually, with the waste either buried on-plot or disposed of in public spaces.
Sanitation no longer becomes on-site per se (i.e. waste is no longer treated in-situ) - latrines become one part in a staged process, in which the first is merely provision of access to a facility (the building or improving of pits, or construction of a toilet block). Two more stages become apparent. The second is the removal of excreta from the facility, which may then be buried close by or dumped into the immediate environment. Alternatively a third stage takes place, this being the transport and eventual treatment of the waste.
Sanitation agencies need therefore to look beyond the provision of toilet facilities to the requirements for sustainable usage and maintenance. Unless the linkages between building pit latrines (access), emptying the pits (removal) and safely disposing of the pit sludge (treatment) are addressed, provision of additional toilets does not solve the challenge of human waste in poor urban communities.