Pit emptying systems
Practical Action
Pit Latrines – Encompassing VIP and simple pit latrines. Householders will save money in the
long term if pit latrine slabs do not need to be dismantled to allow emptying. Manual
emptying is likely to require better access then mechanical emptying as a person may need to
climb inside. It is possible for the slab to contain removable segments (if used these should
be well sealed to prevent flies), or simply allow it to be lifted completely off when emptying is
required. Similarly the superstructure of the latrine must facilitate access, by easy removal or
pit access provided outside the superstructure. The dry and thick nature of waste in pit
latrines (unless pits are lined (see below)) can make removal difficult and put strain on
mechanical pumps. Some users may also prefer to use solid anal cleansing materials which
could block emptying equipment and cause faster sludge accumulation.
Pour Flush Latrines and Offset Pits – The inclusion of water will increase sludge liquidity and
make emptying slightly easier. The water seal prevents the use of solid anal cleansing
material and thus protects emptying machines. However, the water seal also prevents direct
emptying, thus some form of separate access to the pit must be supplied. Water seals have
sometimes been smashed in order to gain access, negating their advantages. Many pour-
flush latrines have offset pits which are advantageous as access can be easier (as long as the
pit is located in an accessible area).
Unlined vs Lined Pits – The decision to line pits or not is discussed in the technical brief
‘Types of Toilet and Their Suitability’. When planning for emptying systems lined pits are
likely to hold more water and make sludge easier to remove. Unlined pits constructed in
unstable ground are likely to collapse when emptied, posing a risk to manual workers.
Septic Tanks – Septic tanks should be inspected periodically to ascertain if emptying is
required. They should not be emptied when full, but when the solids component of the waste
fills between one half and two thirds of the tank. If the tanks are allowed to fill with solids
then the retention time of the tank will not be long enough and the effluent will contain
unacceptable levels of pathogens. Furthermore, septic tanks should not be completely
emptied, and a small amount of digesting sludge should be kept in the bottom (Franceys et
al, 1992).
It is also important to plan for the costs incurred after construction. If householders are able
to afford a latrine, are they able to fund the recurring emptying costs and what are these costs
likely to be. It would also be ideal during the planning of a project to ascertain what emptying
systems are available and discuss and advise customers of their options and corresponding
costs at the time of construction.
Ground Conditions
It is also possible for the ground conditions to affect emptying. Groundwater could have an
affect on liquidity with dry conditions making sludge harder to pump and wet conditions
making emptying the liquefied sludge easier. The local drainage conditions will also affect the
liquidity, if flooding occurs emptiers are likely to wait for flood waters to assist with sludge
removal. The permeability of the ground can also affect the liquidity, with more permeable
soils resulting in drier waste and impermeable ground resulting in a wetter pit due to seepage
restriction.
Filling Rates and Composition
The rate at which pits fill and the composition of the waste can vary drastically between
households. Further research into these topics is required, but controlling the factors is
extremely problematic. Some households may dispose of household waste into the latrine
which could vary the filling rate and adjust the regularity with which services should be
provided. Furthermore, the inclusion of greywater and solid anal cleansing materials can
cause greater variance. Table 1 shows estimated accumulation rates.
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