and the Efficacy of Pit Latrine Additives - Water Research Commission

acteria will increase with temperature, and increase with decreasing moisture content. The rate of pathogen
deactivation also depends on the pH value in the treatment system. A thorough review of pathogen die-off in
septic and subsurface soakaway systems may be found in the USEPA Onsite wastewater treatment systems
manual (USEPA, 2002).
Feachem et al. (1983) conclude that pit latrine contents that have been left undisturbed for one year would
be virtually pathogen free “apart from a few Ascaris eggs”. However, these authors caution that the top layer
of the pit contents of a pit in use will contain pathogenic micro-organisms, irrespective of the rate of die-off or
the overall age of the pit. Cotton et al. (1995) quote a review by Lewis in which the key findings were that
2 m of sand and loam between the bottom of a pit and the water table is enough to remove all bacteria, virus
and other faecal organisms
2.2 Non-biological processes in a pit latrine
Two physical processes of a pit latrine can be characterised: (i) the filling of the pit with waste; and (ii) the
hydraulic flow patterns into and out of the reactor via the walls and base of the pit.
2.2.1 Filling rates
There have been several studies on pit filling rates. Still (2002) reviewed four studies (three in South Africa,
including Norris (2000) and one in Tanzania and Botswana). Reported filling rates varied from 10.0 to
120.5 l /(ca.annum) (litres per capita per annum). The most important factors affecting these rates were
understood to be drainage from the pit and the extent to which pits are used for disposal of other household
waste. Number of users was also stated to be a factor; it is assumed that the number of users affects the net
filling rate (l/annum) but has no effect on the specific filling rate (l/ca.annum).
Table 2.2: Pit latrine filling rates (reproduced from Still, 2002, with permission)
Location
Age of
Latrines
Soshanguve approx. 3
years
Number of
Sites
Number of
Visits
11 14 over 28
months
Avg. Pit
Volume
m 3
Range of
Filling Rates
l/ca/annum
1.96 13.1 to 34.0 24.1
Bester’s Camp four years 159 2 or 3 over 3.16 18.3 to 120.5 69.4
25 months
Mbila approx. 5
years
11 1 2.83 10.0 to 33.2 18.5
Gabarone, Dar
es Salaam
Mean Filling
Rate
l/ca/annum
not stated not stated not stated not stated 25 to 30 27.5
(implied)
In a study at Besters Camp, near Durban referenced in Still (2002), a 33% decrease in filling rate was
observed in pits over the 25 month study period for a sample of 159 pits. This was attributed to an increase
in stabilisation rate with time.
Still (2002) also reported that in all pits, the disposal of household solid waste in the pit including rags, cloth,
plastic and glass contributed significantly to the rate of sludge accumulation, by as much as 10 to 20%.
2.2.2 Drainage of liquid out of the pit
The flow of water in and out of the pit via the walls and base depend on the construction of the pit and the
hydrogeology of the surrounding area (Franceys et al., 1992; Cotton et al., 1995). In a sealed pit where the
integrity of the lining is not in any way compromised, the only liquid flow should be associated with urine and
8