Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>
4.1.4 Septic tank
A septic tank is a water tight tank, usually located just below
ground, and receives both blackwater and greywater (Figure 2.18). It can be used
with pour flush toilets or cistern flush toilets. It functions as a storage tank
for settled solids and floating materials (e.g. oils and grease). The storage
time of the wastewater in the tank is usually between 2 and 4 days. About 50 %
removal of BOD and Suspended Solids (SS) is usually achieved in a properly
operated septic tank due to the settling of the solids during wastewater
A septic tank can be constructed of bricks and mortar and
rendered, or of concrete. Its shape can be rectangular or cylindrical. A septic
tank can be partitioned into two chambers to reduce flow short circuiting and
improve solids removal.
The overflow from a septic tank is directed to a leach pit or
trench. A leach pit (Figure 2.19) is similar to the pit of a pit latrine or pour
flush latrine. The pit must be sized to allow percolation of the volume of
wastewater generated. A pit works well in soils with high permeability. In soils
with lower permeability a trench can provide the larger surface area of
percolation (Figure 2.20). The trench is usually filled with gravel and a
distribution pipe for the wastewater is placed in this gravel layer. Soil is
then placed above this gravel layer to the ground surface.
A leach pit or trench does not work when the soil permeability
is too low (e.g. clayey soil or hard rock). In regions where annual evaporation
is high, trees and shrubs can be used to help pump the water into the atmosphere
by evapotranspiration. An evapotranspiration bed can be designed similar to a
leach trench, but a suite of suitable local vegetation species tolerant of high
nutrients and water are planted above and surrounding the trench (Figure 2.21).
The trench should be sized to store water during the rainy season or low
A leach pit or drain does not work either when the groundwater table is close
to ground surface. In this case off-site disposal is necessary using a settled
sewerage system (Section 2 (3)). If the groundwater table is not too close, an
inverted leach drain as described under Improved On-site Units below (4.1.5) can
The organic solids in a septic tank undergo anaerobic bacterial decomposition
just as in the pit of a pit latrine. The sludge needs emptying, and the period
between emptying is usually designed to be between 3 to 5 years. The sludge has
to be further treated before reuse or disposal (Section 2 (5)).
The septic tank overflow undergoes further bacterial decomposition as it
percolates through a leach pit or trench. The decomposition is undertaken by
soil bacteria, usually under aerobic conditions. The BOD of the wastewater can
reach a low figure (<20 mg/L) if the distance between the bottom of the pit
or trench to the groundwater table is greater than 2 m. Nutrients are not
significantly removed by the bacteria and usually pollute the groundwater.
Pathogenic bacteria are removed by die-off or filtration by the soil, but
viruses may travel further in the soil or groundwater.
Percolation of septic tank overflow is much slower compared to rainwater
percolation. This is because a layer of bacterial slime grows on the surfaces of
the soil particles, restricting flow. Two leach pits or trenches used
alternately, say every 6 months, are better than a single leach pit or trench of
the same total area for percolation, because as one is used the other will
recover its percolation rate.