Improving Septic Tank Performance By A New Rational Design Approach

Project and Seminar Materials for Civil Engineering CE

Improving Septic Tank Performance By A New Rational Design Approach


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.

Chapter One


1.1 Background of the Study

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.

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.

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 to develop a systematic approach to the design and maintenance of septic tanks. Hence, the specific objectives of this research are:

  1. To derive a model to predict the rate of sludge accumulation in septic tanks;
  2. To calibrate the model using field data;
  3. To predict the desludging interval of septic tanks by relating sludge accumulation to reduction in detention time;
  4. To compare the sludge accumulation model to existing models
  5. To expose the unreliability of prevailing design methods and maintenance; and
  6. To present a step by step procedure of how to design a functional septic tank using facts from the research.

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)

Chapter Five

Conclusion and Recommendations

5.1 Conclusion

The septic tank is pivotal to wastewater treatment as well as public health especially in developing countries where central treatment plants are not affordable. Just like every other waste management facility, the septic tank deserves a rational design approach rather than the current haphazard sizing method currently being employed. A preliminary investigation showed that septic tank malfunctioning is common as a result of poor design, construction and maintenance. In order to address this anomaly, a new design approach was developed in this research. This method is based on specifying a desired desludging interval, a minimum residual detention time and a residual depth. A sludge accumulation model was developed and calibrated for the purpose of estimating sludge accumulation per capita. This model shows that sludge accumulation in septic tank is not constant as is commonly assumed.

It has also been shown that because the septic tank is also a storage system, the detention time reduces as sludge accumulates. Charts that show the decline of detention time with sludge accumulation have been produced. These charts show that the usual recommendation that septic tanks be desludged when they are one-third full could be irrational most of the times. It was shown that a tank that is one-third full may still have enough residual detention time for optimal performance. A more rational approach is that septic tanks should be desludged when they no longer have enough detention time for efficient performance. In order to simplify the new design approach, charts have been produced to aid the designer who may not be learned enough to use the equations presented. The reason for this is that preliminary investigation showed most people who undertake the construction of buildings have no formal education in engineering. This set of people will be more at ease with charts. The steps for the new design approach have also been coded into cells in a Microsoft Excel worksheet to facilitate design for the computer literate designer. All the designer needs to supply are the desired desludging interval, wastewater discharge, number of users and a desired length to width ratio. Several septic tank dimensions appropriate for these conditions will be generated and designer can make a choice by applying engineering judgment.

In summary:

  • Every septic tank is unique and therefore should be designed taking cognisance of the number of users, desired desludging interval and expected wastewater flow which is a function of water availability. Users should always know when to expect to desludge their tanks. This should be an intrinsic aspect of the design. Septic tanks should have enough initial volume for long term storage of sludge to avoid frequent desludging. Tanks with small initial volumes soon get silted up with sludge thus requiring frequent desludging;
  • Concrete splash baffles should be completely phased out. Inlet tees should be used instead;
  • All septic tanks should not be designed for a detention time of 24 hours and desludging must not always necessary follow the one third tank volume sludge accumulation specification. The designer should be able to know whether his chosen detention time will maintain the desired efficiency with regard to suspended solids removal efficiency and for how long.

5.2 Recommendations

Based on the outcome of this research, the following recommendations are necessary:

  1. A general awareness campaign on the indispensable role of septic tanks in municipal wastewater management and public health should be mounted by the government and non-governmental organizations. In this campaign, the place of proper design, construction and maintenance should be emphasized.
  2. People should not wait for their septic tanks to be overflowing with sludge before desludging as this reduces the life span of the whole system and also reduces the efficiency of the drain field or soak pit.
  3. The Federal Ministry of Environment should support an extensive test of this design approach and then incorporate it into the Nigerian septic tanks design standard.

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