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 S^2- (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.

A properly functioning septic tank system should be able to reduce the pollutional level of wastewater to such a level as is within local and international standards for wastewater disposal. The septic tank system consists of a water tight tank for removal of solids and partial digestion of organic matter, and a drain field which is a secondary treatment system. The tank is an anaerobic system while the drain field is mostly aerobic which further treats the effluent before channeling it to the groundwater. In some cases, the drain field could be a gravity type or a dosing type.

All things being equal, the septic tank system does not pose much problem and requires little maintenance. However when the system is not working properly, it merely serves as a route for recycling pathogens and deadly chemicals through the ecosystem. According to Cogger (1988), nearly 40% of groundwater attributed disease outbreaks can be traced to the failure of onsite disposal systems. Weissman et al. (1976), Bidgman et al. (1995) and Taylor et al. (1981) among others, reported cases of disease outbreak resulting from groundwater contamination due to septic tank failure. In Africa where most people depend on streams, shallow wells and boreholes, the case is even more severe.

1.2 STATEMENT OF PROBLEM

If wastewater flowing into the septic tank does not receive adequate treatment, it is simply passed on to the groundwater unnoticed thus wreaking havoc on public health. Researchers have shown that most septic tanks especially in developing countries do not even attain an average performance throughout their lifetime. The result is that most septic tanks only act as a conduit for conveying raw / under treated sewage into the soil leading to massive fouling of our groundwater. And because the groundwater is the main source of potable water in most communities, man constantly stands the risk of water borne and water related diseases. Most times, the groundwater is used without treatment on the common assumption that it is “always clean”. The menace of such diseases as typhoid fever, diarrhea, giardiasis, gastroenteritis, hepatitis, methemoglobinamia, samonellosis, dysentery, etc will continue to plague humanity until a systematic approach to the design, construction and maintenance of the septic tank system is adopted.

The foregoing indicates that the septic tank system requires proper design, construction, use and maintenance. The cardinal aspect of septic tank maintenance which is of interest in this research is desludging. The absence of a deterministic equation for the prediction of desludging interval has usually led to too frequent desludging or excessive accumulation of sludge in the septic tank. Too frequent desludging increases cost of operation while excessive accumulation of sludge drastically reduces the efficiency of the septic tanks. The problem at the heart of this research is to develop a systematic and rational approach to the design of septic tanks and also to provide suitable guidelines for the maintenance of the septic tank system in order to ensure the protection of public and environmental health as well as enhance anaerobic processes.

1.3 OBJECTIVES OF THE STUDY

Most of the existing methods of septic tank design are not based on extensive scientific research and have so far proved inadequate. Most times what is referred to as design is mere lumped sizing instead of systematic and rational design. Therefore, the main objective of this research is the investigation of anaerobic processes in septic tank as a wastewater treatment option via developing a systematic approach for the design and maintenance of septic tanks for better health outcomes.

Hence, the specific objectives of this research are:

To derive a model to predict the rate of sludge accumulation in septic tanks;
To calibrate the model using field data;
To predict the desludging interval of septic tanks by relating sludge accumulation to reduction in detention time;
To compare the sludge accumulation model to existing models

1.4 SCOPE OF WORK

The core of this research shall concentrate on only the septic tank and not any of its secondary complements such as the soil absorption field, mound system, wetland, waste stabilization pond, etc. The reason is that, under normal circumstances, the tank itself is the limiting factor of performance in the septic tank system. Because most homes in developing and even developed countries still use the conventional septic tanks, this study will not extend to modified septic tanks. However, some of these systems will be highlighted during literature review for the sake of completeness.

1.5 JUSTIFICATION OF THE STUDY

The septic tank is pivotal to public health and yet one of the most overlooked and least maintained waste treatment facilities. Research has shown that most outbreaks of water borne epidemics result from fecal contamination. In most developing countries, people are not aware of the crucial role of the septic tank as they merely view it as a sewage pit that needs no special design, construction and maintenance considerations. The result is the ubiquity of malfunctioning septic tanks. This is why this subject deserves a serious intellectual attention.

1.6 LIMITATIONS OF THE STUDY

Several researches conducted on the septic tank system have shown that sewage is very difficult to work with. Characteristics of sewage vary from place to place and from septic tank to septic tank depending on the activities of users. Sewage is very inhomogeneous, consisting of a liquid phase, settled and partly settled solids, scum, dissolved solids such that it is difficult to obtain a representative sample (Heinss et al., 1999)