Removal of heavy metals from sewage sludge using combined hydrothermal pretreatment and chelate extraction

The removal of heavy metals from sewage sludge using the combined influence of hydrothermal pretreatment and chelate extraction was tested. The classical method of batch extraction with chelates and the advanced hydrothermal pretreatment (HTP) extraction process with chelates were compared. Both experiments with removal of selected HMs were performed with 0.1 M solutions of 5 different chelating agents: citric acid (CA), S-carboxymethyl-L-cysteine (SCLC), ethylenediamine disuccinic acid (EDDS), methylglycine diacetic acid (MGDA) and ethylenediaminetetraacetic acid (EDTA). The sequential order of potential extraction efficiency at chelates was found as EDDS> EDTA> MGDA >>> SCLC> KC. The mixture of sewage sludge with high sand content soil (A) had in all cases the higher removal efficiency in comparison to the sewage sludge mixture with clay soil (B). The removal of the tested HMs from mixture A and B was better than from the sewage sludge alone. HTP extraction method showed better removal efficiency and significantly shorter time of the process. Also, this extraction method had the higher potential of efficiency in the mixture of soils and sewage sludge. More detailed research on this topic is desirable.