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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 The groundwater aquifer acts as a storage for the treated wastewater, which is usually withdrawn for reuse.

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)).

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