Faecal sludge deposited into the pit latrine is
subject to biodegradation. The aim of this research was to
develop model for the filling rate of faeces in ordinary pit latrine
together with the effect of shape factor on such fillings to prepare
the minds of users on the filling characteristics of the pit based
on contents, usage and environmental factors. Consequently,
faecal sludges sampled from 100 pits already filled were
subjected to laboratory analyses for their physico-chemical and
biological characteristics. Models for the actual filling rate of
faeces in ordinary pit latrines were derived using BOD, COD, VS,
and TS data, calibrated and verified. For a more critical
situation, models were also derived for the filling rates
considering pit shapes that gave the actual pit filling in
comparison with the general condition. From the results, the
actual filling rate was lower than those obtained using BOD, VS
and TS. However, the COD gave filling rate closer to the actual
filling rate than other parameters since TS could not be used to
assess the filling rate of faeces in pit latrines. The pits exhibited
low filling rate in terms of BOD in 2 pits. In terms of COD, the
filling rate was higher compared with that of BOD. Low filling
rate was observed with volatile solids and only 3 pits exhibited
moderately high filling rates whereas total solids had the highest
number of pits with high filling rates. The filling rates for both
the square and rectangular pits increased by 26.5% from 40 – 90
ℓ/capita/year to 50 – 112 ℓ/capita/year due to the shape factor and
this was above the values obtained in existing models while that
of the circular pit remained the same. Reduction in pit volume
and increase in filling rate by faeces was due to dead corners as a
result of clogging, thereby reducing the area available for faecal
infiltration into the surrounding soil. Circular pits are more
stable because of the natural arching effect of the ground around
the hole and there are no corners to concentrate the stresses. Pits
with flat sides are much more likely to develop clogging resulting
to dead corners than the circular pit. From this study, it is
therefore evident that the shape of pit latrine determines the
actual volume and the filling rate of faeces in pit latrine taking
into consideration geological and environmental factors. Thus
for optimal design, our engineers should take note of the pit
shape factor.
Thus, the model can be used to determine the filling rate of
ordinary pit latrines considering soil characteristics, ground
conditions and other favourable conditions.