Pit emptying systems
Practical Action
Table 1: Suggested maximum sludge accumulation rates (litres/person/year) [Source:
Franceys et al, 1992]
Accumulation
rate
Wastes retained in water; degradable anal cleansing material used
40
Wastes retained in water; non-degradable anal cleaning materials used
60
Waste retained in dry conditions; degradable anal cleaning materials used
60
Wastes retained in dry conditions; non degradable anal cleaning materials
90
used
Note: ‘in water’ means the groundwater level is above the top of the sludge
It is widely accepted that filling rates vary and that they should be estimated for the area
under consideration. Accumulation rates from different research locations compiled by Still
(2002) show rates varying between 18 and 70 litres/person/year (l/p/y). There are no
standardised rules and Harvey (2007) suggests even higher rates for emergencies, with 0.5
litres/person/day (l/p/d) (182.5 l/p/y) for solids and 0.8 l/p/d (292 l/p/y) for urine
accumulation (increasing to 1.3 l/p/d (474.5 l/p/y) if water is used for anal cleansing). If
possible estimates of local rates should be used; otherwise the figures in table 1 provide a
suitable estimate.
The equation in box 1 was adopted by Harvey et al (2002) to calculate latrine volume. This
equation helps with the design of new pit latrines, whereas the equation in box 2 shows how
to calculate the duration between emptying operations for an existing latrine.
Box 1 – Equation for volume of pit
Volume of Pit, V = (N x S x D) + 0.5A
1000
Box 2 – Equation for emptying frequency
Design Life, D = (V – 0.5A) x 1000
(N x S)
Where V = volume of pit (m3); N = number of users; D = design life (years); S = sludge
accumulation rate (litres/person/year); and A = pit base area (m2).
As an example, if a household of 8 people were using one latrine, with a pit base area of
1.5m2, the conditions were dry, and stones were used for anal cleansing. From table 1 a
sludge accumulation rate of 60 l/p/y would be selected. If it is known that the volume of the
latrine is 4m3, then the emptying period (design life) can be derived from the equation in box
2. Using these values the design life is found to be 6.8 years.
There are still many knowledge gaps in this area. For example, accumulation rates do not
specifically take into account soil infiltration rates, which will affect the extent to which urine
and water contributes to the accumulation volume. Furthermore, it is accepted that once a
large volume pit has filled once it will not be completely emptied (due to gradual compaction
of sludge), reducing its effective volume and the subsequent design life. Wherever possible
local information should be sought on what filling rates people observe and these values
should be assessed against calculations.
Introduction of solid anal cleansing materials (such as corn cobs and textiles) into latrines
can potentially block or damage emptying equipment. Prior to introducing an emptying
technology, establishing with users what material is likely to be in pits could mitigate risks.
Institutional Environment
To ensure the safe transfer of excreta from a household level to adequate disposal and
treatment a variety of stakeholders are likely to be involved. If implementing collection
systems alone it would be advisable to assess how excreta will be transferred to an end
disposal point. In some cases municipalities may assist with moving the waste on from some
form of collection point, if this is the case responsibilities should be clear.
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