Environmental Engineering Reference
In-Depth Information
as a soil amendment promotes microbiological activity, but may have the opposite
outcome if toxic heavy metals are present in the applied sludges. Researchers have
studied both the positive and negative effects of land application of sewage sludges
and have reported dissimilar effects from sewage sludge on soil microbial biomass
and activity. In some studies, appreciable concentrations of heavy metals in sewage
sludge did not appear to have any negative inluence on soil microbial biomass and
enzyme activities. However, other reports have clearly illustrated that the heavy
metals present in sewage sludge do decrease the proportion of microbial biomass C
in total soil organic matter.
Increases in soil microbial biomass from sewage sludge amendments mainly
result from stimulation of the indigenous soil microbes by microbes present in the
sludge organic residues. Microbial biomass also increases from the addition of sub-
strate-C. The effect of heavy metals on soil microbes depends on the characteristics
of the soil as well as on the character and rates of sewage sludge applied. It is known
that that biosolids originating from wastewater treatment plants contain a wide vari-
ety of pathogens. Several plant-disease causing pathogens have also been reported
to occur in the biosolids.
Our main conclusions from reviewing the cogent literature and from preparing
this review are as follows:
1. Although the application of sewage sludge in agricultural practice may be
beneicial, it also may contaminate ground water, drinking water from wells, and
the food chain
2. Land applications of sewage sludge may result in transport of pathogens through
aerosols to areas of human habitation
3. Considering the foregoing, the physicochemical analysis of sewage sludge is
necessary before a decision is made to use it for land application
4. To reach a clearer conclusion on the value of sewage sludge disposal by land
application under diverse conditions, more research is required. In particular,
research is needed on application to different soil types and at sewage sludge
amendment rates to evaluate effects on soil microbial biomass
5
Summary
In the present review, we address the effects of sewage sludge amendment on soil
physicochemical properties and on soil microbial biomass. Sewage sludge is a
by-product of sewage treatment processes and is increasingly applied to agricultural
lands as a source of fertilizer, and as an alternative to conventional means of
disposal. The particular characteristics of sewage sludge depend upon the quality
of sewage from which it is made, and the type of treatment processes through which
it passes. Sewage sludge may substitute for inorganic fertilizers because it is rich in
organic and inorganic plant nutrients.
However, the presence of potentially toxic metals and pathogens in sewage
sludge often restricts its uses. Ground water and food chain contamination resulting
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