Heavy metals stabilization by different amendments in sewage sludge composting

This paper focuses on the effect of different amendments on the stabilization of heavy metals during the composting process. These amendments, including lime, coal fly ash, and natural zeolite, were selected on the basis of their availability, potential efficiency and market-pnice. Due to the natural zeolite has ability of adsorption and ion exchange for absorbing the ammonia (NH3) and cation. In this study, the addition of natural zeolite may decrease the pH value and electrical conductivity (EC). However, lime and coal fly ash are both alkaline materials for increasing the pH value in the composting process. Based on the variation of temperature, the different amendments have inhibitory effect on the microbial activity. Meanwhile, in this study, the most pathogens will be destroyed in mesophilic and thermophilic phase for 28 days. The volatile solid (VS) content, EC, cation exchange capacity (CEC) and C/N ratio in final products were all complied with the agricultural use thresholds. On the other hand, according to the results of germination index (GI), the compost has no inhibitory effect on the plant rooting. In this study, characterization and toxicity of heavy metals were considered as a good index for evaluating the sewage sludge compost application. ne total concentrations of heavy metals in the compost were for below the limit values for the regulation and agricultural use. The experimental results of compost shown all complied with the critieria for sludge compost even thought there was no Cd detected in the compost. Meanwhile the results of toxicity characteristics leaching procedure (TCLP) analysis for the tested compost were nearly in compliance with current EPA regulatory limits. The results of DTPA-metals showed that the addition of lime, coal fly ash, or natural zeolite may inhibit the mobility for targeted metals. As a result of the physicochemical characteristics and heavy metal stabilization, the tested amendments were shown to improve the sewage sludge compost stabilization and enhance feasibility of compost for the agricultural applications.