CHAPTER ONE

INTRODUCTION

1.1 Background

The septic tank is one of the most widely used on-site systems for treating domestic wastewater. Many developing countries, Nigeria included, lack the technology and financial capacity to build and run sewerage networks that carry wastewater to central treatment plants, so a large share of the population relies on septic tanks. Globally, an estimated 3.1 billion people use improved on-site sanitation facilities, and about 1.5 billion of them use septic tanks (Shaw & Dorea, 2021). A conventional septic tank is a watertight receptacle designed to collect wastewater, separate settleable and floatable solids into sludge and scum, accumulate and store those solids, digest part of the organic matter, and discharge a clarified effluent that may be treated further in a dispersal field (Shaw & Dorea, 2021).

The tank itself is largely an anaerobic reactor, because the oxygen supply is too low for aerobic degradation to dominate. Under these conditions, anaerobic microorganisms break down part of the organic matter in the retained solids, producing gases such as carbon dioxide and methane, while organic nitrogen is converted mainly to ammonium. Roelants et al. (2026) note that septic tanks provide effective solids retention and partial carbon breakdown under anaerobic conditions, but also that they are a significant source of methane. The effluent then passes to a drain field, where the unsaturated soil provides oxygen and further aerobic treatment, including the oxidation of ammonium to nitrite and nitrate. This oxidation releases hydrogen ions and lowers the pH of the effluent. A properly functioning system should reduce the pollution load of wastewater to levels that meet local and international standards for disposal.

Despite their wide use, the biological processes inside septic tanks are not well understood. In a systematic review and meta-analysis, Shaw and Dorea (2021) found that few studies provide data on the anaerobic microbiology and activity in conventional tanks. Although the evidence supports the presence of anaerobic conditions, they concluded that it cannot yet be confirmed that a septic tank treats waste through anaerobic biodegradation. They also observed that many design guidelines rely only on physical parameters, such as solids retention time and storage volume, so that tanks function essentially as solids separation and holding units.

When a septic tank is poorly designed, constructed or maintained, it ceases to protect health and instead becomes a pathway for pathogens and nutrients into the environment. Poor degradation in the tank contributes to solids accumulation and soil clogging, and to contamination of ground and surface water with pathogens and nutrients (Shaw & Dorea, 2021). This is a particular concern where people depend on shallow wells and boreholes. In Ibadan, Nigeria, Sosanya and Remi-Esan (2022) detected Escherichia coli and total coliforms above the WHO guideline in groundwater sampled around schools with septic tanks. In Paiko, north-central Nigeria, Aweda et al. (2022) reported microbial contamination and elevated nitrate in hand-dug wells and linked the poor water quality to improper sewage systems and waste disposal. Odewade et al. (2025) likewise recorded faecal coliforms and pathogenic bacteria in well water in Funtua, Katsina State, in connection with seepage from on-site sanitation and the burden of waterborne disease.

1.2 Statement of the Problem

If wastewater entering a septic tank is not adequately treated, it passes to the soil and groundwater largely unnoticed, with consequences for public health. Groundwater is the main source of drinking water in many communities and is often used untreated because it is assumed to be clean. The result is continued exposure to waterborne diseases such as typhoid fever, diarrhoea, giardiasis, gastroenteritis, hepatitis, salmonellosis and dysentery, as well as to nitrate-related health risks.

The operating practices that keep a tank working properly are not well grounded in science. A key concern of this study is desludging. Because no deterministic equation is generally used to predict the desludging interval, tanks are often emptied either too frequently or too rarely. Excessive accumulation of sludge reduces effective volume and treatment efficiency, while overly frequent desludging raises operating costs and can disrupt the biological processes that develop within the tank, which reduces the decomposition of solids (Gill et al., 2018). Reporting sludge accumulation as a constant rate has also been questioned, since microorganisms continuously consume part of the solids (Al Momani, 2015).

In addition, what is called design is often little more than lumped sizing based on physical parameters rather than a systematic and rational approach (Shaw & Dorea, 2021). The problem addressed by this research is therefore the lack of a systematic, rational approach to septic tank design and maintenance that accounts for anaerobic processes and sludge accumulation, in order to protect public and environmental health and to support anaerobic treatment.

1.3 Objectives of the Study

The main objective of this research is to investigate anaerobic processes in the septic tank as a wastewater treatment option by developing a systematic approach to its design and maintenance for better health outcomes. The specific objectives are to:

1. derive a model to predict the rate of sludge accumulation in septic tanks;
2. calibrate the model using field data;
3. predict the desludging interval of septic tanks by relating sludge accumulation to the reduction in detention time; and
4. compare the sludge accumulation model with existing models.

1.4 Scope of Work

The research concentrates on the septic tank only, and not on secondary components such as the soil absorption field, mound system, wetland or waste stabilisation pond. Under normal circumstances, the tank is the limiting factor in the performance of the septic tank system, and the quality of its effluent also affects how well downstream components work (Shaw & Dorea, 2021). Because most homes in both developing and developed countries still use conventional septic tanks, the study does not extend to modified tanks, although some of these systems are highlighted in the literature review for completeness.

1.5 Justification of the Study

The septic tank is central to public health, yet it is one of the most neglected and least maintained wastewater treatment facilities. Evidence from Ireland illustrates the point: Gill et al. (2018) found that most septic tanks and packaged systems there are rarely emptied, and they noted a lack of understanding among homeowners of how their systems operate. In many developing countries, people view the tank as a simple sewage pit that needs no special design, construction or maintenance, which explains why malfunctioning tanks are so common. Better knowledge of anaerobic processes and sludge accumulation would support more reliable design and rational desludging schedules, and would help to prevent groundwater contamination. It would also contribute to understanding the greenhouse gas emissions of septic tanks (Roelants et al., 2026). These reasons make the subject worthy of serious scientific attention.

1.6 Limitations of the Study

Sewage is difficult to work with. Its characteristics vary from place to place and from tank to tank, depending on the activities of users, and the influent is generally more concentrated and more variable than municipal wastewater (Shaw & Dorea, 2021). Sewage is also inhomogeneous, consisting of a liquid phase, settled and partly settled solids, scum and dissolved solids, so it is hard to obtain a representative sample. Data on sludge are scarcer than data on influent and effluent, and sampling and analysis are demanding, particularly in resource-constrained settings (Shaw & Dorea, 2021). These factors may limit the precision with which field data can be used to calibrate and validate the model.

References

Al Momani, F. (2015). Mathematical model for predicting sludge accumulation in septic tanks based on operational parameters: Effect of hydraulic mixing. Water and Environment Journal. https://doi.org/10.1111/wej.12135

Aweda, A. K., Amadi, A. N., Kyari, R., Samuel, A., & Samson, B. A. (2022). Investigation of groundwater quality from selected wells in Paiko, northcentral Nigeria. Journal of Geography and Geology, 12(1), 1.

Gill, L., Mac Mahon, J., Knappe, J., Gharbia, S., & Pilla, F. (2018). Desludging rates and mechanisms for domestic wastewater treatment system sludges in Ireland (EPA Research Report 253). Environmental Protection Agency. https://www.epa.ie/publications/research/reports/Research_Report_253.pdf

Odewade, L. O., Imam, A. A., Adesakin, T. A., & Odewade, J. O. (2025). Assessment of human faecal contamination on groundwater quality and reporting consequent waterborne diseases in Funtua Metropolis, Katsina State, Nigeria. Frontiers in Water. https://doi.org/10.3389/frwa.2025.1561777

Roelants, T., Zheng, M., Saerens, B., Van Hal, M., van Loosdrecht, M. C. M., Yuan, Z., & Volcke, E. I. P. (2026). Reconsidering septic tanks. ACS ES&T Engineering, 6(6), 1741–1753. https://doi.org/10.1021/acsestengg.6c00129

Shaw, K., & Dorea, C. C. (2021). Biodegradation mechanisms and functional microbiology in conventional septic tanks: A systematic review and meta-analysis. Environmental Science: Water Research & Technology. https://doi.org/10.1039/d0ew00795a

Sosanya, P. A., & Remi-Esan, I. A. (2022). Septic tanks contamination on groundwater quality around elementary schools in Ibadan, Oyo State, Nigeria. Federal Polytechnic Ilaro Journal of Pure and Applied Sciences, 4(2), 85–95.