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IMPROVING SEPTIC TANK PERFORMANCE BY A NEW RATIONAL DESIGN APPROACH

ABSTRACT

This study was aimed at developing a rational approach to septic tank design in order to reduce health risks associated with improperly treated effluent especially in developing countries. To this end, several research tools including questionnaires, pilot scale study and model formulation were employed. Questionnaires were used to conduct a preliminary study with a view to ascertaining people‟s perception with regard to septic tank design, use and maintenance. This preliminary study revealed that the septic tank is a poorly designed and grossly overlooked but indispensable waste management facility. Pilot scale studies were conducted to monitor physicochemical and microbial parameters. A sludge accumulation model was formulated from first principles by applying material balance to a model septic tank. The model was calibrated using data from three different septic tank audits spanning 3 years, 5 years and 8 years respectively and involving over 1000 septic tanks. A correlation coefficient of R = 0.98 was obtained between measured and calculated sludge accumulation data. The sludge accumulation model showed that sludge does not accumulate at a constant rate as is usually assumed but rather at a reduced rate over time. The sludge accumulation model was compared with two existing but purely empirical models namely: Weibel‟s model derived in 1955 for the US Public Health Service and Bound‟s (1995) model. Finally a rational approach to septic tank design was developed. Design charts and a Microsoft Excel based design programme were produced to aid the unlearned designer and the computer literate designer respectively.

INTRODUCTION

The septic tank system is the most widely used onsite treatment system for domestic wastewater. In fact, most developing countries (Nigeria inclusive) lack the technology and economic power to construct and operate sewerage systems for conveyance of domestic wastewater to central sewage treatment facilities, so a greater population rely on the septic tank system for sewage treatment. It is an enclosed receptacle designed to collect wastewater, segregate settleable and floatable solids (sludge and scum), accumulate, consolidate and store solids, digest organic matter and discharge treated effluent (Bounds, 1997). In the United States only, over 50 million people use the septic system (Collick et al., 2006). According to Fidelia (2004, in Burubai et al., 2007), over 46% of the Nigerian population use the septic tank system. The septic tank system was once thought to be a temporary solution to domestic wastewater treatment and disposal. This was true until 1997 when the United States Environmental Protection Agency and Congress officially recognized the system as a sustainable, long-term solution for treating wastewater.

The septic tank is an anaerobic reactor due to the insufficiency of oxygen concentration to act as electron acceptor. The wastewater is degraded by micro-organisms aerobically while the C, CO2 SO4 act as electron acceptors to form CO2, H2, CH4 and S2- (sulphides). At the same time, most of the organic N is converted to NH+4 (inorganic). The effluent flows into the drain field where aerobic degradation occurs due to abundance of oxygen in the unsaturated soil layer. The C in the wastewater is now oxidized to CO2 while NH4+ is oxidized to NO2- thus raising the nitrate level of the sewage to about seven times the limit acceptable for dumping water (10mg/l). The H+ released from the oxidation of NH4+ now reduces the pH of the effluent.