Studies on septic tank effluent quality and soakage

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.