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<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>
4.2.4 Land based treatment
Land based treatment of wastewater relies on the action of soil
bacteria to degrade the organic wastes in the wastewater. In what is termed
'Soil Aquifer Treatment' wastewater is applied to unlined basins in cycles of
flooding and drying of approximately one week each (Figure 2.28). During
flooding wastewater percolates through the soil beneath the basin to the
unconfined groundwater aquifer. Organic substances are consumed by soil
bacteria. Suspended solids are trapped at the bottom of the basin, and the
percolation rate decreases. During drying the layer of solids accumulating at
the bottom of the basin are degraded by bacteria and also undergo drying. The
percolation capacity for wastewater is therefore rejuvenated.
Soil aquifer treatment is also known as rapid-rate land
application. It works well when the soil permeability is high (> 1 m/day),
and the highest groundwater table is at least 2 m below the bottom of the basin.
Upon reaching the groundwater the SS and BOD of the water is generally low.
Furthermore if the soil beneath the basin contains clay minerals, pollutants
like heavy metals may be adsorbed by the clay minerals. The groundwater aquifer
acts as a storage for the treated wastewater, which is usually withdrawn for
Figure 2.28: Soil aquifer treatment or rapid-rate land application system
In what is termed 'slow-rate land application system' wastewater
is applied to land through channels in the upper part of the gradient and treated
wastewater is collected in channels in the lower part of the gradient of a slightly
inclined ground (Figure 2.29). The application is intermittent and its rate
is dependent on the permeability of the soil and the loss of water due to evaporation.
The organic substances in the wastewater are biodegraded by soil bacteria at
the surface of the soil and during percolation through the soil. Vegetation
is usually part of the treatment process. It takes up nutrients (nitrogen and
phosphorus) released from the degradation of the organic substances. The vegetation
(usually grasses) is harvested by grazing animals (cattle or sheep). Note that
in New Zealand the successful disposal of treated wastewater has been achieved
by spray irrigated into forests and for crops growth. Again the trees and crops
take up the disposed nutrients and use then to promote growth. This is mainly
for disposal purposes and not for re-uses. Crops (usually grass) are harvested
as silage and then fed to live stock. This disposal system is referred to as
"cut and carry" for the livestock do not graze the irrigated paddocks.
The silage is of good quality and there is a demand for it. Sub-surface irrigation
disposal of wastewater for silage is also being promoted.
When the soil is saturated with water (e.g. during the rainy
season), 'overland flow' or 'grass filtration' mode of operation is used. In
this case wastewater flows over the soil surface and the organic substances are
removed by bacteria attached to the vegetation and soil surface (Figure 2.30).
Raw wastewater can be used in any of the above land based
treatment system provided that the application rate is small. Settled wastewater
needs to be used for higher rates of application. Land application treatment
systems work well in arid or semi-arid regions, where the soil is generally not
saturated with water over much of the year, and reuse of wastewater for
agriculture is attractive. Particular attention has to be given to public health
requirements (see Section 2 (6)).