Land treatment of wastewater 403In chemistry, an equivalent weight of an ion is equal to its molecular weight
divided by absolute value of ion charge. For example the equivalent weight of
calcium is 40/2 or 20 g equivalent.
When industrial wastes are included in the wastewater to be irrigated,
adjustment of the pH and SAR may be needed. The wastewater to be applied
should have a pH within the range of 6.0 to 9.5 to avoid adverse effects to site
vegetation. Wastewaters with high SAR must be pre-treated to remove Na or
accompanied by special soil management procedures to compensate for the
effect of the sodium. The SAR of wastewaters used for irrigation should be no
more than 8 to 10; otherwise these wastewaters should not be used in irrigation.
Some pre-application methods that may be used to control the chemical
reactions in the soil could include:
- Pre-treatment controls, such as industrial source control or chemical
precipitation, if the amount of potentially toxic elements and
chemicals in the wastewater are likely to exceed the chemical
removal mechanisms in the soil. - Adjustment of wastewater pH and SAR to acceptable levels.
The relationship of potential problems to concentrations of major inorganic
constituents in irrigation waters is shown in Table 8.6.
8.3.3 Biological removal mechanisms
The biological transformations that occur in the soil include organic matter
decomposition and nutrient assimilation by plants. These transformations occur
in the biologically active upper few centimeters of the soil, i.e., the rooting zone.
The numbers of bacteria are large ranging from one to three billions per g of
soil. The great diversity of native organisms enhances the capability of a soil to
degrade the variety of natural and man-made organic compounds in the applied
wastewater.
The presence or absence of oxygen in the soil has a significant effect on the
rate and end products of degradation. The oxygen status of the soil is a function
of soil porosity. Soil properties that allow for rapid infiltration and transmission
of the applied wastewater also yield good oxygen movement; low and
intermittent wastewater application rates used with SR system normally result in
aerobic conditions, rapid organic matter decomposition, and oxidized end
products.