Land application of sewage sludge: physicochemical and microbial response.

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 from sewage sludge amendment is one major concern worldwide. The health of soils is represented by a composite of their physical, chemical and biological properties. Amending soil with sewage sludge modifies the physicochemical and biological properties of soils. Perhaps the central constituent of soil that is important in the context of sewage sludge amendment is microbial biomass. Soil microbial biomass, the key living part of the soil, is very closely associated with the content of organic matter that exists in arable agricultural soils. When sewage sludge is land-applied, soil enzyme activities may be directly or indirectly affected by the presence of heavy metals. In several studies, results have shown that sewage sludge amendment increased soil microbial and soil enzyme activities; however, reduction in soil enzyme activity has also been reported. When incubation periods of sewage sludge were longer, heavy metal bioavailability increased. Soil pathogenic activity has also been reported to increase as a result of land application of sewage sludges. The level of pathogens in treated sewage sludge (biosolids) depends on the processes used to treat wastewater and sewage sludge. Agricultural application of sewage sludge may result in the transport of pathogens through aerosols downwind of sludge storage or dispersal sites, may contaminate ground water, stock ponds, or may produce food chain contamination from eating food grown in sludge-treated land.