Problems and perspectives of sludge utilization in agriculture

Sewage sludge utilization in agriculture leads to several advantages through the recycling of nutrient and organic matter to the land.Inorganic nitrogen as NO₃^?1and NH₄⁺ becomes immediately available; organic nitrogen becomes available after mineralization, which depends on soil type and tends largely to diminish with time.The soluble forms of phosphorus are H₂P0₄^? and HPO₄^2? and the percentage P-utilization can easily be predicted from the amount extracted by ammonium chloride.Potassium concentration in sewage sludge is negligible, but crop requirements for it are high and often comparable to those for nitrogen.Organic matter mainly affects soil structure by increasing soil porosity, stability of aggregates and water retention; other properties affected by organic matter addition are pH and cation exchange capacity, which tend to increase.Negative effects due to heavy metals (toxic and phytotoxic) and pathogens can arise following sludge addition to soil. Epidemiological studies in the U.K. indicate that infections can arise, in particular from the presence of Taenia saginata and Salmonella.A recent survey in the European Economic Community (E.E.C.) revealed that nowadays 75% of sewage is treated and 40% of the residual sludges are used in agriculture (about 2 million t (2 M t) of dry solids year^?1), affording a saving in mineral fertilizers of about 206 000 M L It. year^?1 (£82AM; DM 295M; 100M FF). This economic advantage covers the cost of transport over a distance of 8 km and that of spreading the liquid sludge.This paper also compares the legislations of some countries concerning sewage sludge utilization in agriculture and shows that generally the countries with more experience in sludge utilization (France, Germany and the U.K.) permit disposal of higher concentrations of heavy metals.