Indirect application of sludge for recycling in agriculture to minimize heavy metal contamination of soil

Abstract

In this study, an indirect application method was developed for safe recycling of nutrients from municipal sewage sludge (MSS) for use in agriculture. Permeable net bags were filled with MSS to prepare sewage-sludge net bags (SSN). The SSN were stacked next to banana, papaya, and corn crops. Cadmium (Cd), copper (Cu), lead, and zinc levels in the soil, crops, and MSS were monitored. After application of the SSN for 9 years, the levels of the four heavy metals did not exceed soil standard limits. By comparison, direct application of MSS resulted in soil Cd and Cu levels that exceeded China's soil standards in the 3rd and 4th years. The crop yields obtained with the SSN were not significantly different to those obtained using mineral fertilizer, and the heavy metal concentrations in the crops met the required food standards. With the SSN, more than 94% of the heavy metals were retained in the MSS, and these were mainly present in the residual fraction. In conclusion, indirect application of MSS could considerably reduce heavy metal pollution of soil and safely supply nutrients for crops.