and the Efficacy of Pit Latrine Additives - Water Research Commission

4 Conclusions from Set 1 and Set 2 experiments
The purpose of these experiments was to determine whether addition of moisture and/or NaHCO 3 to
samples of fresh VIP contents had a statistically significant effect on gas production rate under anaerobic
conditions. The gas production rate was understood to be correlated to the rate of anaerobic degradation of
the VIP contents, and an enhanced gas production rate would thus be an indication of improved stabilisation
rates. Experiments from Set 2 showed that VIP contents alone (with no additional water or NaHCO 3 had
negligible gas production across three replicates; however, addition of water only or water and NaHCO 3
resulted in statistically significant increases in gas production. It was not possible to determine whether the
addition of NaHCO 3 had any effect on gas production rate (either positive or negative)
It was also observed that increasing the amount of water at the beginning of the experiment could have the
opposite effect, i.e. reduced gas production rates as a result of absorption of CO 2 in the moisture (See
Appendix B for details). However, after a certain amount of time, CO 2 absorption ceased as the inorganic
carbon content in the aqueous phase approached saturation with respect to the headspace of the
experiment. 24 Thereafter, the gas production rate (as opposed to the cumulative amount of gas produced)
was greater for experiments with more added water than those with less.
The conclusion of this work were that increasing moisture content of VIP contents has the potential to
increase the rate of stabilisation of buried organic material in the pit.
Significantly greater amounts of gas were produced per test unit for Set 2 units relative to Set 1 units. This
supports the hypothesis that the generally poor performance of all treatments in Set 1 was less a function of
the treatments than that the VIP contents used were already well digested and therefore inherently resistant
to further degradation.
This study supports the motivating hypothesis that the moisture content generally observed in VIP material is
low and may be a limiting factor in the rate of anaerobic digestion that may occur in the buried pit contents.
This implies that increasing the moisture content in the pit has the potential to increase stabilisation rates in
the pit. These broad findings are supported by literature which state that the rate of accumulation in pits is
lower in wet pits than in dry pits (Franceys et al., 1992).
This study indicated that addition of NaHCO 3 resulted in no significant effect on gas production rate, thereby
refuting the hypothesis that there is insufficient buffering capacity in pit latrine contents to support stable
anaerobic digestion. This could be explained in the context of the general theory of processes in pit latrines
(Section 3.1) in that if a significant proportion of the biological conversion of organic material in the pit latrine
happens aerobically on the surface of the pit three effects would reduce the pH buffering requirement in pit
latrine contents:
The buffering capacity requirement for aerobic degradation is less than for anaerobic digestion since
there is no accumulation of acid intermediates in aerobic degradation.
There will be a smaller load of organic material requiring anaerobic digestion, and thus a smaller
overall buffering requirement (than if the entire load of organic material entering the pit were
considered)
The process of aerobic degradation will increase the overall alkalinity of the contents of the pit latrines
24
This is a somewhat simplistic explanation since the “equilibrium” in each experiment will be dynamic: it will depend on pH, headspace
gas composition and pressure, relative rate of production of CO 2 and CH 4 , frequency of headspace venting and aqueous phase
concentrations of a range of chemical species.
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