Liquid flows through the tank and heavy particles sink to the bottom, while scum (mostly oil and grease) floats to the top. Over time, the solids that settle to the bottom are degraded anaerobically. However, the rate of accumulation is faster than the rate of decomposition, and the accumulated sludge and scum must be periodically removed. The effluent of the septic tank must be dispersed by using a Soak Pit or Leach Field, or transported to another treatment technology via a Solids-Free Sewer.Generally, the removal of 50% of solids, 30 to 40% of BOD and a 1-log removal of E. coli can be expected in a well-designed and maintained septic tank, although efficiencies vary greatly depending on operation and maintenance and climatic conditions.
A septic tank should have at least two chambers. The first chamber should be at least 50% of the total length, and when there are only two chambers, it should be two thirds of the total length. Most of the solids settle out in the first chamber. The baffle, or the separation between the chambers, is to prevent scum and solids from escaping with the effluent. A T-shaped outlet pipe further reduces the scum and solids that are discharged.
Accessibility to all chambers (through access ports) is necessary for maintenance. Septic tanks should be vented for controlled release of odorous and potentially harmful gases.
The design of a septic tank depends on the number of users, the amount of water used per capita, the average annual temperature, the desludging frequency and the characteristics of the wastewater. The retention time should be 48 hours to achieve moderate treatment.The retention time should be 48 hours to achieve moderate treatment.
A variation of the septic tank is called an Aquaprivy. This is a simple storage and settling tank that is located directly below the toilet so that the excreta fall into it. The Aquaprivy has a low treatment efficiency.
This technology is most commonly applied at the household level. Larger, multi-chamber septic tanks can be designed for groups of houses and/or public buildings (e.g., schools).
A septic tank is appropriate where there is a way of dispersing or transporting the effluent. If septic tanks are used in densely populated areas, onsite infiltration should not be used, otherwise, the ground will become oversaturated and contaminated, and wastewater may rise up to the surface, posing a serious health risk. Instead, the septic tanks should be connected to some type of Conveyance technology, through which the effluent is transported to a subsequent Treatment or Disposal site. Even though septic tanks are watertight, it is not recommended to construct them in areas with high groundwater tables or where there is frequent flooding.
Because the septic tank must be regularly desludged, a vacuum truck should be able to access the location. Often, septic tanks are installed in the home, under the kitchen or bathroom, which makes emptying difficult.
Septic tanks can be installed in every type of climate, although the efficiency will be lower in colder climates. They are not efficient at removing nutrients and pathogens.
Users should be careful when opening the tank because noxious and flammable gases may be released.
Operation & Maintenance
Because of the delicate ecology, care should be taken not to discharge harsh chemicals into the septic tank. Scum and sludge levels need to be monitored to ensure that the tank is functioning well. Generally, septic tanks should be emptied every 2 to 5 years. This is best done by using a Motorized Emptying and Transport technology, but Human-Powered Emptying can also be an option.
Septic tanks should be checked from time to time to ensure that they are watertight.
CRITES, R.; TCHOBANOGLOUS, G. (1998): Small and Decentralized Wastewater Management Systems. New York: The McGraw-Hill Companies Inc.
MARA, D. (1996): Low-cost Urban Sanitation. United Kingdom: Wiley.
U.S. EPA (Editor) (n.y.): Septic Tank as a Primary Treatment. United States: United States Environment Protection Agency (U.S. EPA).
This manual provides information on functionality of septic systems containing the modules septic tank and leach field and addresses the user at the household level. Construction, maintenance and operation are described.
This issue presents studies from different regions (Bangladesh, Cameroon, Burkina Faso) that mainly show the non-existence of faecal sludge management. Additionally, the last paper describes a new technological solution (LaDePa) for producing hygienically safe organic fertiliser from sludge from ventilated improved pit toilets (VIPs).
GUTTERER, B.; SASSE, L.; PANZERBIETER, T.; RECKERZÜGEL, T.; ULRICH, A. (Editor); REUTER, S. (Editor); GUTTERER, B. (Editor) (2009): Decentralised Wastewater Treatment Systems (DEWATS) and Sanitation in Developing Countries. Loughborough University (UK): Water Engineering and Deveopment Centre (WEDC). URL [Accessed: 20.03.2014]. PDF
This document speaks about waste water and sanitation strategies in the developing countries. It also advocates the use of DEWATS as sustainable treatment of waste water at a local level backing it up with case studies from different countries. It describes various options available for sanitation and waste water treatment. It gives an idea of planning and executing CBS programs.
NATURGERECHTE TECHNOLOGIEN, BAU- UND WIRTSCHAFTSBERATUNG (TBW) GmbH (Editor) (2001): Decentralised Wastewater Treatment Methods for Developing Countries. GTZ and GATE. PDF
Different operation and maintenance options are presented with respect to sustainable plant operation, the use of local resources, knowledge, and manpower.
The publication presents appropriate technologies for sanitation and highlights socio-economic aspects of planning and implementing. Emphasis is given to household-level sanitation improvements for urban areas, as well as rural areas and small communities. Background information on sanitation, in-depth technical information on the design, construction, operation and maintenance and project planning and development processes involved in projects and programmes complement the book.
Rather old design manual for onsite wastewater treatment options. However, valuable information on established systems such as septic tanks, sand filters, aerobic treatment units (suspended growth and fixed film), disinfection, nutrient removal as well as wastewater segregation and recycling are given. Additional information is given on disposal methods and appurtenances.
WSP (Editor) (2007): Philippines Sanitation Source Book and Decision Aid. pdf presentation. Washington: Water and Sanitation Program. PDF
This Sanitation Sourcebook distils some of the core concepts of sanitation in a user-friendly format so that the book can serve as a practical reference to sanitation professionals and investment decision-makers, particularly the local governments. The annexe contains a practical collection of factsheets on selected sanitation system options.
WSP (Editor) (2008): Technology Options for Urban Sanitation in India. A Guide to Decision-Making. pdf presentation. New Delhi: Water and Sanitation Program (WSP). URL [Accessed: 26.03.2010]. PDF
These guidance notes are designed to provide state governments and urban local bodies with additional information on available technologies on sanitation. The notes also aid in making an informed choice and explain the suitability of approaches.
Anaerobic Reactors is the forth volume in the series Biological Wastewater Treatment. The fundamentals of anaerobic treatment are presented in detail, including its applicability, microbiology, biochemistry and main reactor configurations. Two reactor types are analysed in more detail, namely anaerobic filters and especially UASB (upflow anaerobic sludge blanket) reactors. Particular attention is also devoted to the post-treatment of the effluents from the anaerobic reactors. The book presents in a clear and informative way the main concepts, working principles, expected removal efficiencies, design criteria, design examples, construction aspects and operational guidelines for anaerobic reactors.
MOREL, A.; DIENER, S. (2006): Greywater treatment systems for hotel premises, Sri Lanka. In: MOREL, A.; DIENER, S. (2006): Greywater Management in Low and Middle-Income Countries, Review of Different Treatment Systems for Households or Neighbourhoods. Duebendorf, 80.
This case study presents different cost-effective on-site treatment technologies with main focus on hotel greywater and blackwater treatment systems. The first greywater treatment plant was constructed for the Swiss Residence Hotel. Based on the experience gained by the Swiss Residence Hotel, further systems were implemented at the hotels Ivy Banks and Coral Sands.
ROBBINS, D.; STRANDE, L.; DOCZI, J. (2012): Opportunities in Fecal Sludge Management for Cities in Developing Countries: Experiences from the Philippines. North Carolina: RTI International . URL [Accessed: 15.01.2013]. PDF
In July 2012, a team from RTI International deployed to the Philippines to evaluate four FSM programs with the goal of reporting on best practices and lessons learned. The four cases—Dumaguete City, San Fernando City, Maynilad Water for the west zone of metro Manila, and Manila Water from the east zone of metro Manila—were chosen to highlight their different approaches to implementing FSM.
U.S. EPA (Editor) (1987): It's Your Choice: A Guidebook for Local Officials on Small Community Wastewater Management Options . United States Environmental Protection Agency, Office of Water Office of Research and Development. PDF
This booklet was prepared by the United States Environmental Protection Agency U.S. EPA for officials from communities of less than 10000 people to present them available options and thus to enable an informed choice.
U.S. EPA (Editor) (2000): Decentralized Systems Technology Fact Sheet - Septic System Tanks. Washington, D.C.: United States Environmental Protection Agency, Office of Water Office of Research and Development (US EPA). URL [Accessed: 15.04.2010]. PDF
Four-page factsheet by the United States Environmental Protection Agency (U.S. EPA), describing septic tanks, their applicability and the design of septic tank systems for household-level black and greywater treatment.
U.S. EPA (Editor) (2000): Decentralized Systems Technology Fact Sheet - Septic Tank Leaching Chamber. Washington, D.C.: United States Environmental Protection Agency, Office of Water Office of Research and Development.. URL [Accessed: 15.04.2010]. PDF
Seven-page factsheet by the United States Environmental Protection Agency (U.S. EPA), describing the system, its applicability and the design of a leaching field following a septic tank.
U.S. EPA (Editor) (2000): Decentralized Systems Technology Fact Sheet: Septic Tank Systems for Large Flow Application. Washington, D.C.: United States Environmental Protection Agency, Office of Water Office of Research and Development.. URL [Accessed: 15.04.2010]. PDF
Ten-page factsheet by the United States Environmental Protection Agency (U.S. EPA), describing the system, its applicability and design of a septic tanks for large-flow applications.
Booklet that describes how a septic system works and what homeowners can do to help their systems treat wastewater effectively.
EAWAG/SANDEC (Editor) (2008): Sanitation Systems and Technologies. Exercises: Septic Tank. Duebendorf: Swiss Federal Institute of Aquatic Science (Eawag), Department of Water and Sanitation in Developing Countries (Sandec). PDF
Exercise on the dimensioning of a septic tank.
EAWAG/SANDEC (Editor) (2008): Sanitation Systems and Technologies. Lecture Notes . Duebendorf: Swiss Federal Institute of Aquatic Science (EAWAG), Department of Water and Sanitation in Developing Countries (SANDEC). PDF
Lecture notes on technical and non-technical aspects of sanitation systems in developing countries.
EAWAG/SANDEC (Editor) (2008): Sanitation Systems and Technologies. Presentation. Duebendorf: Swiss Federal Institute of Aquatic Science (Eawag), Department of Water and Sanitation in Developing Countries (Sandec). PDF
PDF presentation on the technical and non-technical aspects of sanitation systems in developing countries.
In this document, you will find more information on the various technical options that meet the requirements for basic sanitation. These need to be considered within all the sustainability requirements, e.g. affordability, operation and maintenance. The options are divided into two categories: Dry non-water reliant on-site systems and wet systems (that do require water for operation).
http://water.epa.gov/infrastructure/septic/ [Accessed: 05.08.2014]
This site offers valuable information and resources to manage onsite wastewater systems in a manner that is protective of public health and the environment and allows communities to grow and prosper.