Influence of sewage sludge on maize yield and quality and soil chemical characteristics

Experiments were conducted during 2003-2005 under a 5-year crop rotation, soybean-wheat-maize-sunflower-wheat, with sewage sludge being applied at rates of 20, 40, and 60 t/ha, combined with different mineral fertilizer rates. Analyzing data on heavy metal contents from soils treated with different rates of applied sewage sludge, it was found that sewage sludge from Iai Water Treatment Station complied with the maximum allowable limits established by European Community countries and Romanian law. The results obtained showed that when raw sewage sludge was applied at a rate of 30 t/ha (18.5 t/ha dry matter), the allowable maximum limits stipulated by Norm 86/278/EEC were not exceeded. When raw sewage sludge was applied at a rate of 60 t/ha (37.0 t/ha dry matter), the limits were exceeded only for zinc content. Sewage sludge application at a rate of 40 t/ha resulted in increased amounts of minerals (especially nitrogen and phosphorus) supplied to the crops. It also resulted in a mean annual supply of 3.76 t/ha highly humificated organic matter; this explains the increase in soil humus content from 2.79 to 3.92%. In maize fertilized with sewage sludge, concentrations of Cu and Ni were within normal limits. Analyses showed the absence of Cr, Co, Cd, Pb, As and Hg in maize plants and grain. These results proved that sewage sludge applied to maize did not affect the quality of the cultivated crops. When sewage sludge was applied at a rate of 24.6 t/ha dry matter, mobile phosphates accumulated in soil at a concentration of 42 ppm and contents of microelements (mobile forms in soil) were 12.1 ppm for copper, 0.35-0.48 ppm for boron, 155 ppm for zinc and 406 ppm for manganese. and on soil respiration. This increased from 0.4 mg CO 2 -C/kg dry soil in the untreated control to 0.7-1.2 mg CO 2 -C/kg dry soil after sewage sludge treatment. Rules on the use of sewage sludge in farming require various sewage sludge treatments (such as pasteurization, mesophilic anaerobic fermentation, thermophilic aerobic digestion, composting, lime treatment, etc.) and limits on the allowed content of polluting substances (heavy metals, organic compounds and pathogens). The conditions regulating the application of sewage sludge on certain soils are set out in Norm 86/278/EEC, supplemented by EC Norm 466/2001 and by internal legislation. Romania has elaborated normative acts where there are established maximum allowable values for the content of heavy metals in sewage sludge and soils (according to Annex 1 of Norm 86/278/ EEC). These normative acts allow the utilization in farming of residual sewage sludge from wastewater treatment stations. The Government Decision forbids the use of sewage sludge when the concentration of one or more heavy metals in the soil exceeds the established allowable limits. When higher rates are detected, the environmental legislative authority allows the application of sewage sludge only after the estimation of heavy metal adsorption by plants, heavy metal excretion by animals, degree of groundwater contamination and long-term effects on biodiversity. The legislation stipulates limitations on sewage sludge application on soils with pH<5, on flooded soils, frozen soils, and where groundwater is near the soil surface, in order to protect soil and water. For the assessment of fields and crops suitable for treatment www.world-food.net