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Parallel to the education intervention in 1995, R&D
by ICDDR,B with LGED on human waste based
household connected biogas system started in
Bauniabad. Initially 3 different kinds of biogas
systems were studied; (1) system with a primary
settling tank for the purpose of settling the
untreated wastewater before it enters the biogas
chamber, (2) system with a final settling tank for
the purpose
of settling the treated wastewater
after it exits
the biogas chamber, and (3) system
without
either a primary or final settling tank.
It was concluded
that the inclusion of the tanks
improved the performance, but when a comparison
was made between the costs of operation and
maintenance requirements between systems with
and without tanks (soak pits) in the local context,
it was considered to be quite expensive. However,
later in 1997, it was observed that the environmental
condition of the water bodies surrounding
Bauniabad was deteriorating due to them receiving
raw sewage from the city. It was then decided that
the biogas facilities may be upgraded to include
a soak pit in order to improve the environmental
condition of the surrounding water bodies of
Bauniabad. ICDDR,B then further conducted research
from 1997 to 1999 on the comparison of different
sanitation options including the biogas based
sewerage system, pit latrines and septic tanks in
order to find the most appropriate system that
would be favorable to both the people and the
environmental condition of the surrounding water
bodies of Bauniabad.
From 1997-1999, two approximately 6 m deep
single pit latrines (Agargao, which is another slum
settlement of Dhaka), two 6 m deep twin pit latrines (Agargao and Bauniabad), 3 septic tanks (Agargao and
Bauniabad) and 3 biogas based sewerage systems (Agargao and Bauinabad) were installed and studied.
The pits of the pit latrines were constructed with concrete rings that were about 1 m in diameter and 0.3 m
high. The single pit latrines were used by 3 families and the twin pit latrines by 6 families. Simple drawings and
photographs of tested single pit latrines are presented in Figure 2.2 and Photo 2.3. A schematic drawing of the
twin pit latrine is shown in Figure 2.3.
The rates of decomposition and reduction of the sludge in the pits were slow. The single pits filled-up in about
6 months, and the twin pits filled-up in approximately 8 months. Concerning the twin pit latrine, once the first
pit filled up, a plate was used to shut the connection to the first pit, while another plate initially used to block
the connection to the second pit was removed, allowing the waste to flow into the second pit. However, in all of
the cases, when the second pit was to be used it was already about half filled with wastewater. This may have
been due to either wastewater seeping from the first pit, or due to the water table rising during the rainy season.
Furthermore, during the rainy season, when the water table rose, the filled-up pits became a nuisance. In addition,
another factor contributing to the undesirability of the pit technologies were, as mentioned earlier, the problem
concerning of an appropriate mechanism for the disposal of the sludge.
Thus, the pit technologies were not appreciated compared to the septic tank and biogas facilities, which are both
described in the following section below.
A septic tank is a water-tight single storied, underground tank in which sewage is retained sufficiently long
enough to permit both sedimentation of suspended solids and partial digestion of the sludge by anaerobic
bacteria. A schematic illustration of a septic tank is presented in Figure 2.4.
Three septic tanks were installed and studied in Agargao as a trial to judge its effectiveness before installing the
Figure 2.2 Drawing of tested single pit latrine.
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