Heavy Metal Stabilization in Sewage Sludge Composting Process

The most important factor limiting the soil application of sewage sludge is the presence of heavy metals. This study was conducted to evaluate the effect of composting on nutrient content, heavy metal (Cd, Cr, Ni and Pb) concentration and heavy metal fractions in the sewage composts with different bulking agents (sawdust and coirpith), heavy metal adsorbent (zeolite) and liming materials (lime and flyash). Experimental results revealed that sewage sludge composts were rich in organic carbon and plant nutrients. Total nutrients and heavy metal concentration showed an increasing trend towards the maturity of composts. Heavy metal fractionation studies pointed out that mobile fractions of heavy metals such as exchangeable and carbonate fractions decreased at the end of composting whereas the residual fractions increased which indicates that composting of sewage sludge with heavy metal adsorbent and different bulking agents decreased the mobility and bioavailability of heavy metals. The compost C8 (Sewage sludge + sawdust + zeolite (50:30:20) + flyash) was superior with respect to stabilization of heavy metals studied. The concentration of Cd, Cr and Pb were within the permissible limit as per Fertilizer Control Order (FCO). The addition of coal flyash and zeolite enhanced the stabilization of heavy metals due to the higher alkalinity and ion exchange capacity of flyash and zeolite respectively. Polyphenolic group and organic compounds present in sawdust also have the ability to bind heavy metal. The study revealed that composting is an effective technology for reducing the heavy metal bioavailability and the heavy metal speciation studies of the matured composts revealed that considerable portion of the heavy metals were associated with immobile fractions indicating less bioavailability and ecotoxicity.