and the Efficacy of Pit Latrine Additives - Water Research Commission

5.4.1.1 Performance of standard VIP design for sludge accumulation and digestion
The results presented in Chapter 3 showed that in a well constructed and well maintained VIP, good
digestion of organic material could be obtained. It was also shown that the higher the moisture content within
the pit (provided that the surface of the pit contents was not submerged by a free liquid layer) the better the
rate of digestion that could be expected to occur in the buried layers of the pit. As a result, in many pits,
material below the surface of the pit contents was found to be well stabilised, and by corollary, much reduced
in volume from the original material added to the pit. These results indicated that there were no fundamental
flaws in the design of a VIP as a means of accumulating and degrading faecal matter over a period of time.
Where pits did not operate well in degrading faecal matter, there was usually a compounding factor or
factors that may have been responsible.
Integrity of construction
Observations in the course of this research showed that pits with the least smell and fly nuisance were
those which complied with proper VIP construction requirements i.e. those where the ventpipe was properly
mortared in place, the fly-screen was present and intact, the toilet pedestal had a seat and a door which
eliminated light from the pit and where there was no place for flies to enter through the back plate. Many of
the pits visited during this project were built according to accepted VIP design, but poor workmanship or
vandalism had caused one or more of the above elements to be missing. For example, back plates were
often missing, or placed over the pit opening so that light and insects could enter; vent pipes were often
missing, or had been poorly mortared into place and had subsequently slipped down into the pit eliminating
smells at head height; fly-screens were often missing completely and (according to householders) had not
been part of the original construction.
Thus the principle design recommendation is that these elements are given special attention during
construction through reinforcement and careful workmanship. It is equally important that the VIP owners and
users are adequately educated about the importance of these features to ensure that they take responsibility
for addressing minor maintenance issues, and preventing vandalism.
Construction of pit lining
Bester et al. (2000) reviewed a wide range of literature relating to design and construction of VIPs. In the
section relating to pit lining, they describe a range of options and recommendations about design and
materials of construction for pit lining. These are briefly summarised here:
In stable soil conditions (i.e. where there is little risk of collapse), only the upper parts of the pit should
be lined to provide a firm support for the cover slab. In unstable soil conditions where there is a
significant risk of collapse, the pit should be lined from the slab to the base of the pit.
It was recommended that for wet pits 1 the lower quarter and bottom of the pit are lined to retain water,
and that the retention of water would result in more rapid decomposition of organic material.
A range of materials for lining were reported including a variety of blocks, bricks, stones, perforated oil
drums, geofabrics, timber and plaster.
This research has produced two relevant recommendations about pit conditions for extending the life of the
pit: (i) moisture retention increases biodegradation rates; and (ii) contact with soil appears to facilitate or
improve microbial activity. In the light of these recommendations, the three bullet points extracted from
Bester et al. (2000) are considered.
1
Note: this is thought to be a mistake since in wet pits, there should be no need to retain water. It would make sense if this read “dry
pits”
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