EWE/FS/AS/AS1
LEVEL: C: All
Sanitation: Septic tanks
General
AS1
A septic tank with a drain (absorption of effluent in the soil) is one of the most frequently used treatment
system for waste water in small communities and farms.
The function of the septic tank is to trap the solids in the waste water (by settling) and promote partial
decomposition (by bacteria), to form sludge on the bottom of the tank. The clear liquid (solids removed)
is then discharged (via an overflow) to a sand drain (French drain).
The function of the drain is to ‘treat’ the effluent by filtering (sand or soil). Care must be taken to ensure
that nearby water resources (shallow groundwater or surface water) are not close to the drain system.
The most common septic tank and drain system is illustrated below.
Another version which provides a higher quality effluent is a system with 2 ‘tanks’ or rather 2
compartments as shown below:
In areas where the absorption characteristics of the natural soil is limited (eg. a high clay content) or
where the water table is high, a sand mound system may be required, instead of the normal French
drain. The effluent is pumped to the artificial sand mound (which is constructed on top of the natural
soil) and release through perforated pipes in the sand mound. Refer to EW/FS/AS/AS2 for a typical
sand mound design.
Useful links:
Texas Commission on
Environmental Quality
http://www.tceq.state.tx.us/nav/permits
Related Fact Sheets: AS2, AS3
Contact: ewisa@wamsys.co.za URL: www.waterinfo.co.za
Prepared by: eWISA
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