Effect of composting and vermicomposting on potentially toxic element contents and bioavailability in sewage sludge digestate

Vermicomposts and composts prepared from sewage sludge digestate and additives (spent mushroom compost, straw, biochar) after 43 days pre-composting followed by 90 days vermicomposting with Eisenia fetida or by compost maturing were investigated regarding the potentially toxic element (PTE) As, Co, Cr, Cu, Mo, Ni, Pb and Zn contents. The average increment in the total PTE concentration for the entire process was ten times higher (104 %) compared to the increment solely in the composting or vermicomposting (9.3 and 9.5 %, respectively) after pretreatment. Compared to the untreated digestate the As and Co concentrations in the final mixtures were 26 and 51 % higher, respectively while for the other PTEs 26 +/- 9 % average decrease was observed. Total PTE content was the same in composts and vermicomposts. Average PTE bioavailability (water soluble/total concentration) was statistically the same in vermicomposts (2.5) and composts (2.7), but lower in mixtures with biochar (2.5) than without it (2.8).