and the Efficacy of Pit Latrine Additives - Water Research Commission

300
T3 - faeces + inoculum +
Gas produced [mL]
200
100
T2 - faeces + inoculum +
T4 - faeces + inoculum +
lk
T5 - faeces + inoculum +
T1 - faeces + inoculum +
T6 - faeces + inoculum
T7 - faeces alone
0
0 5 10 15 20 25 30 35 40 45
Time [d]
Figure 3.5: Gas production from faeces incubated alone, with anaerobic sludge and with both
anaerobic sludge and additional alkalinity. Solid lines indicate the mean of 3 replicates. Dashed lines
indicate the range of the standard error of the mean.
There was no significant difference in cumulative gas production after 42 days between units with different
concentrations of added alkalinity (student’s t-test, p>0.05). However the difference between cumulative gas
production in any sample with added alkalinity (treatments 1 to 5), and treatment 6 which contained no
additional alkalinity was significant (p0.05).
3.5.1.3 Conclusions
These results give rise to two important conclusions:
Since the gas production from treatments 6 and 7 (faeces and inoculum vs. faeces only) are not
significantly different, it suggests that there is a large micro-organism population present in the faeces
at source; therefore there is an inherent ability for faeces to be naturally broken down. However, as
both of these treatments indicate inhibition, the relative concentrations of micro-organisms cannot be
inferred.
The addition of an alkaline solution to the test units allowed recovery of gas production after a period of
inhibition. The units that were not incubated with additional alkalinity did not exhibit a similar recovery
within the experimental period. This implies that digestion of faecal matter generates higher acid
production rates than can be converted to biogas, or absorbed by available buffering capacity and that
addition of alkalinity may have a significant effect on the rate of stabilisation of organic material in a pit
latrine. As no reference unit was used in which only water was added, it is not possible to infer whether
the recovery in gas production was due to additional moisture in test units 1 to 5, or to additional
buffering capacity due to addition of Na 2 CO 3 .
While these conclusions provide support for the general theory of processes in pit latrines presented in
Section 3.1, specifically, that a significant micro-organism population capable of degrading faeces is
presenting in the faeces themselves, it should be noted that, according to the general theory, fresh faeces
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