and the Efficacy of Pit Latrine Additives - Water Research Commission

portion remains dissolved in the liquid phase until equilibrium is reached between the dissolved CO 2 in the
liquid phase and the headspace gas CO 2 composition.
A corollary of this result is that if significant CO 2 absorption occurs, the fraction of CH 4 in the gas phase
should be equivalently larger. Incomplete gas composition data was obtained, and generally the variability
observed even between replicates prevents quantitative predictions for CO 2 absorption; however it was
observed experimentally that even when no gas production was recorded it was possible to detect methane
by analysing the gas from the headspace with the GC. The combination of these calculations and
observations support the hypothesis that the decreased gas production observed for increasing moisture
content in the absence of additional NaHCO 3 in the experiments was due to increased absorption of CO 2 in
the moisture.
N CO2 absorbées = f(V H2O ) V g total = f(V H2O )
-4.00E-04
6.0
-3.50E-04
4.0
-3.00E-04
2.0
Nombre de moles
-2.50E-04
-2.00E-04
-1.50E-04
V (ml/btle)
0.0
0 5 10 15 20 25
-2.0
-1.00E-04
-4.0
-5.00E-05
-6.0
0.00E+00
0 5 10 15 20 25
Volume d'eau par bouteille (ml)
N2/CO2 (50/50) N2/CO2 (30/70)
-8.0
H 2O (ml/btle)
N2/CO2 50/50 N2/CO2 30/70
Figure AB.6: Results of PHREEQ simulations of CO 2 absorption and variation in gas production due
to the absorption in moisture.
Thus it is not possible to make any conclusions about whether or not additional moisture affects the rate of
degradation of pit latrine contents under anaerobic conditions.
2.5 Conclusions from Set 1 experiments
It was not possible to draw any firm conclusions about the effect of the treatments on the rate of degradation
in these experiments. This was attributed to the fact that the material used in the tests was already well
degraded at the beginning of the experiments and therefore that the treatments were irrelevant in the context
of the experimental conditions. It was also shown that the presence of additional moisture may have a
confounding effect by absorbing produced CO 2 and thereby confusing the interpretation of gas production
data.
3 Results: Set 2
It was proposed that the experiment should be repeated using a newer (i.e. less well-degraded) source of
material.
3.1 Characteristics of VIP contents: Set 2
The new sample was collected from a rural school outside Pietermaritzburg. This new material comes from
the top layer of a pit thus it was not well-degraded. The colour of this new material was brown whereas the
old one was black. All the experiments for Set 2 were prepared at the same time and the sample was mixed
(as well as it is possible to do so) before filling of the bottles.
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