Investigating the impact of grey water on the performance of a septic tank.

Abstract

A septic tank is an underground water-tight chamber designed to receive, and partially treat wastewater before it is released into a soak pit or drain field for secondary treatment. Septic tanks are the commonest type of on-site sanitation all over the world because they are simple in concept and their use is on the rise in Uganda. In Uganda, residents either have a septic tank that receives both black and grey water or only black water. Grey water especially from the kitchen, introduces oil and grease from the kitchen into a septic tank. However, the impact of grey water on the performance of septic tanks is still not well understood. This study investigated the impact of oil and grease on the performance of septic tanks. In order to attain this, fieldwork which involved collecting samples from the septic tanks receiving only black water and also those receiving both black and grey water was conducted. The collected samples were analyzed for various physicochemical pollutants. These were BOD, COD, pH, nitrogen, E. coli, ammonia, TDS, TSS, Total phosphates and FOG. These helped distinguish the difference in performance of the septic tanks. pH in the sampled septic tanks remained constant in both influent and effluent which according to recent studies favours microbial activity that led to reduction of pollutants in the septic tanks. The laboratory results indicated that septic tanks receiving both black and grey water contained more FOG than those receiving only black water which is not surprising since the kitchen wastewater flowed in the former. Septic tanks receiving both black and grey water compared to septic tanks receiving only black water exhibited better removal efficiencies in nitrogen (10.19>4.1%), ammonia (11.82>8.3%), BOD (7.42>5.26%), TSS (5.88>4.42%), and TDS (3.24>1.17%). Based on these results, septic tanks receiving both black and grey water are considered to be perform better than those that receive only black water. This is an indicator that grey water has a positive impact on septic tank performance. However, this study also revealed that all of the sampled septic tanks had very high contents of FOG in the range of 115-135 mg/L compared to acceptable amounts recommended by past studies. This means that these septic tanks are prone to blockages which could hinder septic tank performance hence need for regular maintenance.