Land treatment of wastewater 3998.2.4 Combined processes
Rapid infiltration can be used after overland flow to further reduce
concentrations of BOD 5 , suspended solids, and phosphorus. Because of the
increased reliability and overall treatment capability, the application rates for the
overland flow process could be higher than normal. This is shown schematically
in Figure 8.5 (a).
In another scheme, Figure 8.5 (b), the rapid infiltration process precedes slow
rate treatment. The recovered renovated water should meet even the most
restrictive requirements for use on food crops. The unsaturated zone can be used
for storage of renovated water to be withdrawn on a schedule consistent with
crop needs.
8.2.5 Groundwater recharge
Recharging underground aquifers with treated wastewater is one of the generally
accepted forms of water reuse. The water disappears from the site and is usually
diluted by other sources of water. Furthermore, a long time ordinarily elapses
before recharged water is extracted, so most microbial contaminants will die off.
Basically, there are two different methods for recharging an aquifer. Either
water may be allowed to enter the aquifer from the surface through spreading
beds or spray irrigation, or it may be pumped under pressure through to down
wells which will penetrate a deep aquifer.
Water applied to the soil surface will be filtered and biologically oxidised,
and many chemical constituents will be adsorbed or precipitated on the soil
particles. The treatment given by surface infiltration can be equivalent to, or
better than, conventional biological treatment followed by filtration and
disinfection.
8.3 Wastewater renovation mechanisms
Any wastewater treatment system, including a land treatment system, is
designed to convert the wastewater into an acceptable effluent and to dispose of
the solids removed and produced in the system. The basic approach is to
determine the characteristics of the untreated waste and to utilize the capabilities
of various treatment processes to achieve the desired effluent quality.
The effectiveness of a land treatment system is related to the characteristics
of the soil and the resultant pollutant removal mechanisms. When wastewater is
applied to the soil, some constituents may pass through the soil to the
groundwater, some are utilized by growing plants, some are metabolized by the
soil micro-organisms, and others are retained within the soil. The design of a