Effect of Sewage Sludge, Sewage Sludge Compost and Sewage Sludge Biochar on Heavy Metal and It Fractions

This study was conducted to evaluate the effect of sewage sludge (SS), sewage sludge compost (SSC) and sewage sludge Biochar (Sewchar, SC) on the heavy metal content and its fractions. The preparation of sewage sludge compost, Sewchar and incubation study were done in Completely Randomized Block Design. The study was conducted at the College of Agriculture, Department of Soil Science and Agricultural Chemistry, Vellayani, Thiruvananthapuram, India. Experimental method was employed to study the effect of composting and pyrolysis on the total, available and heavy metal fractions. Sewage sludge compost was prepared by using sewage sludge, sawdust and zeolite in the ratio 50: 30: 20 and for the adjustment of pH 2.5 kg flyash was used. Sewchar was prepared through the process of slow pyrolysis at a temperature of 400℃ for 2 hours using the muffle furnace. An incubation experiment was conducted by using different ratios of sewage sludge Biochar, sewage sludge and sewage sludge compost and maintained at field capacity for 180 days. The experimental results showed that conversion of sewage sludge to Sewchar causes the enrichment of total nutrients (except N) and heavy metals in them. Total Cd and Cr in the sewchar increased (Cd – 10.80 mg kg-1 and Cr – 113.20 mg kg-1) during the pyrolysis process and a reduction during the composting (Cd – 5.41 mg kg-1 and Cr – 47 mg kg-1) process was observed. Composting and pyrolysis decreased the available Cd and Cr content in the incubation soil compared to the sewage sludge amended soil. Fractionation studies also showed that there was a reduction in the unstable fractions of Cd and Cr as the incubation days progressed from 0 to 180 days. Composting and pyrolysis can be considered as an effective way to decrease the availability of heavy metals as it can convert the unstable fractions of heavy metals to stable fractions. The increased surface area, porosity and presence of oxygen containing functional groups sewchar can be used for the remediation of polluted soils.