Septage or septic tank sludge is partially treated sludge that is accumulated and stored in a septic tank. Septage is a by-product from the pretreatment of household wastewater (sewage) in a septic tank where it accumulates over time.
Septage is pumped out of a septic tank or onsite sewage facility with a vacuum truck. Septic tanks can be for residential or non-residential use. Non-residential sources can include wastewater from commercial/industrial development, grease interceptor as well as other sources such as portable toilet, RV, and boat waste.
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Another definition of septage is: "A historical term to define sludge removed from septic tanks." The term "historic" is used because it is nowadays often replaced by the term "fecal sludge" (or faecal sludge in British English), even though "fecal sludge" may have a negative connotation associated with it because it contains the word "feces".
Septic tank sludge or septage is in fact a specific type of "fecal sludge" - a term nowadays widely used in the context of developing countries. Fecal sludge includes the fecal sludge or septage from septic tanks as well as fecal sludge from pit latrines or from public toilets that are not connected to a sewer. Fecal sludge management is a challenge for many cities in developing countries where a large fraction of the population is using on-site sanitation systems. Fecal sludge needs to be managed safely.
Septage is limited to septic tank contents whereas fecal sludge includes contents from other on-site sanitation technologies as well, not only from septic tanks. The term "septic" in septage implies that the sludge has gone through some anaerobic biological degradation and thus is at least partly stabilised. Fecal sludge on the other hand is a wider term and also includes "fresh" sludges from non-sewered public toilets.
Septage consists of the three fractions in a septic tank:
- Scum, which floats to the top and contains fats, oil and grease as well as any floating solid waste.
- Effluent, which is the semi-treated wastewater that comprises the majority of the material in the septic tank
- Sludge, the solids which collect at the bottom of the tank and where most of the bacteria live that are involved in the anaerobic degradation processes
Septage waste is periodically removed (with a frequency depending on tank capacity, system efficiency, and usage level, but typically less often than annually) from the septic tanks by specialized vehicles known as vacuum truck. They pump the septage out of the tank, and transport it to a treatment facility.
Disposing of septage in the United States is becoming an increasing problem as people build homes outside of city sewer systems. Septage waste can be transported to local wastewater treatment plants, used by farmers for fertilizer, or stored in large septage waste storage facilities for later treatment or use on crops.
Services for de-sludging tend to empty a septic tank completely, i.e. take out all septage, while the actual requirement is removal of settled solids, and even this purposefully incompletely so as to leave at least some of the microbial populations in place to continue the anaerobic degradation processes that take place in a septic tank.
- Tilley, E., Ulrich, L., Lüthi, C., Reymond, Ph. and Zurbrügg, C. (2014). Compendium of Sanitation Systems and Technologies (2nd Revised Edition). Swiss Federal Institute of Aquatic Science and Technology (Eawag), Duebendorf, Switzerland
- Linda Strande, Mariska Ronteltap and Damir Brdjanovic. Faecal Sludge Management Systems Approach for Implementation and Operation. IWA Publishing. p. 432. ISBN 9781780404721.