436 Organic waste reuse and recycling: technology and management
Chemical analyses of the soil indicate that while the concentration of sodium
salts is increasing, it has not reached a level injurious to plants and is not
expected to do so in the future. Similarly, nitrates percolating into the
groundwater reserve are not expected to build up to the point of concern.
8.7 Public health aspects and public acceptance
Land treatment of wastewater provides many potential benefits but carries with
it the risk of contamination of human and animal food. Land treatment can
produce an equal or better quality of effluent than a conventional treatment
process, but the public is not aware of it. To be a successful method of
wastewater treatment and reuse, the public acceptance of land treatment as a
safe and suitable method is very important.
The major health concern is possible pollution by nitrogen, heavy metals,
toxic organic compounds and pathogens.
8.7.1 Nitrogen
Excess concentration of nitrate (greater than 10 mg/L, NO 3 -N) in drinking water
will be the major health concern for infants under 6 months of age. The major
pathway of concern in land treatment is conversion of the wastewater nitrogen
to nitrate and percolation to drinking water aquifers. All three land treatment
methods are quite efficient in nitrification of the wastewater nitrogen. Since OF
is a surface discharge system, it is only the SR and RI systems that are of
concern for groundwater impacts. Nitrogen is often the limiting factor for design
of SR system. Appropriate amount of nitrogen in the percolate can be achieved
by selecting proper crop and application rate. The very high loading rates
inherent in RI systems result in the greatest potential for nitrate contamination of
drinking water in the underground aquifers. The extent of nitrification and
nitrate contamination depend on temperature, carbon source, hydraulic loading
rate, loading period and physical properties of the soil. To overcome this
problem the treatment site can be located far away from drinking water sources,
or located above non-drinking water aquifers, or recover the percolate with
wells or underdrains for reuse elsewhere. An alternative that has been proposed
is to use a two step land treatment consisting of OF system followed by RI
system; in this way most of nitrogen could be recovered during the OF process
through biomass uptake, nitrification/denitrification, and volatilization.