Decontamination of sludge by the METIX-AC process. Part I: Effects on sludge quality and leaching of chemicals

Sludge decontamination removes metals, and then generates biosolids that can be safely recycled in agriculture. The research currently presented is probably one of the initial comparison of the agro-environmental impact of decontaminated versus non-decontaminated sludge. Four different sludge were tested at two rates (80 and 160 kg/ha N), before and after their decontamination by a new process (METIX-AC), and subsequently compared to non-amended and inorganically fertilized soil, for maize cultivation under greenhouse conditions. Results presented in this paper reveal that the METIX-AC process very successfully extracted several metals from sludge (e.g., up to 87%, 72% and 66%, respectively for Cd, Cu and Zn), while preserving satisfactory levels of nutrients (e.g., less than 7% of lost TKN). The proposed process was, however, inefficient for extracting Cr and Pb. The mechanisms of metal removal from sludge and their leaching into drainage water are discussed. Decontaminated sludge ameliorated the water holding capacity of soil, reduced the total quantities of leachates, and produced drainage water of an acceptable quality. Yet, leaching of N-compounds was not significantly (Pr ⩽ 0.05) reduced by the use of decontaminated as opposed to non-decontaminated sludge.