Abstract:
The study is an experimental investigation aimed at looking into i
the effluent quality of septic tanks designed for different combination of
domestic wastewater. The study also looked into the matter of soil
absorption capacity of effluent of different quality.
In Bangladesh, sanitation programs are largely limited to on site
options and. exclude conventional sewerage technology,. because of its
high initial costs. The primary on-site sanitation options include septic
tanks system. But many of the septic tank systems malfunction or fail for
various reasons.
Septic tanks are designed normally either to receive toilet wastes
or all type of wastes. The design criteria is the volume of sewage only. No
consideration is given on the quality of septic tank effluent which is
discharged to soakwells. In this study, effluent characterestics are
determined. in three different composition of. domestic wastewater.
These are toilet wastes only,toilet and kitchen wastewater, and toilet
kitchen and bathroom wastewaters .
.The test results show that qualitatively toilet wastes together with
kitchen wastewater produce better effluent than toilet wastes only. The
waste quality parameters like BOD, COD,TOe and SS are reduced to 40%
of the original value Similarly all purpose septic tanks receiving toilet,
kitchen and bathroom wastewater produce much superior quality
effluent. In addition, organic contaminant removal efficiency of these
septic tanks are very high, about 70%. But the major disadvantage of this
combination is that the size of the septic tank and soak wells which
increase enormously compared to other cases.
The most important factor to be determined when considering
absorption system is whether the soil is suitable for the absorption of the
effluent. Soil percolation capacity tests were conducted for determining
the absorption capacity of soil. Percolation tests conducted with the three
(ypes of effluent on the same type of soil, as mentioned earlier. The
absorption rate is more for effluent with toilet andkitchen;toilet, kitchen
and bathroom than with toilets only. The toilet wastewater. contain high
amount of BOD,COD,TOCand SS which forms slime which deposit in the
infiltrative layer and gradually reduce the absorption rate.
Finally an attempt has been made t~ propose a new approach for
the design of septic tank where effluent i quality has been taken ihto .
. consideration. New approach is based on toilet and kitchen wastewater
septic tank with 3 days detention time. This will ensure better
functioning of the system and chances of failure of soakage pit will be
reduced. In addition possibility of contamination of surface and ground
water would be reduced as the effluent quality improves using this
method. The bathroom wastewater may be discharged to surface drains as
.it contain insignificant quantity of contaminent. However all wastewater
septic tank is recommanced when situation permits.
