Accumulation of heavy metals in anaerobic sludge and its disposal by landspreading

Sludge, a natural, uncalled for byproduct of aerobic/ anaerobic biological digestion of organic matter present in wastewaters, also known as biosolids, is a thick, black and viscous substance comprising of dead and degraded microbial cells that digested the influents. This dead microbial material, accumulates on the bottom of anaerobic lagoons/ reactors, is organic in nature and rich in plant nutrients such as nitrogen, phosphorus and essential trace elements. It must be removed periodically. Therefore, the objective is to use the sludge as a fertilizer on agricultural land. It is an environmentally acceptable and economical method of sludge disposal. The application of organic wastes to farmland has increased over the years as it contributes to the preservation of the environment and results in an improvement of chemical, biochemical and physical properties of soil, although there is an increased risk of soil and ground water being contaminated by pollutants. Sewage sludge may contain heavy metals whose presence in soil may reduce enzyme activities and affect microbial communities in soil. They accumulate in soil and are taken up by crop plants thus posing health hazard. The study was conducted to characterize anaerobic sludge and quantify the concentration of heavy metals in it, to determine the suitability of the sludge as fertilizer for crops. Physicochemical and biochemical analyses showed that anaerobic sludge produced at Upflow Anaerobic Sludge Blanket (UASB) sewage treatment plant at Faridabad (an industrial town), Haryana, India, contained total organic C 15.714 percent, total Kjeldahl N 0.795 percent, available P 8 mu g g(-1)and heavy metals Zn 1500 mu g g(-1), Ni 226 mu g g(-1), Fe 3000 mu g g(-1) and high activity of enzymes like alkaline phosphatase and dehydrogenase in the sludge. These enzymes are important for soil fertility and nutrient cycling. The toxic heavy metals have been accumulated by bacteria from sewage waters. The ideal C/N ratio, P and enzyme activities show its suitability as fertilizer but presence of heavy metals is a matter of concern. Therefore its application as crop fertilizer or onto land has to be carefully managed.