The effect of different additives on the immobilization of heavy metals, the stability of organic matter, and the ecotoxicity of municipal sewage sludge

Abstract

There is an ongoing search for the most environmentally friendly use of various materials as additives to improve sludge properties. The aim of this study was to analyze the effect of additive type and dose of biochar (BC), dolomite (DL), bentonite (BN) or diatomite (DT) on heavy metal immobilization, organic matter stability, biochemical activity and ecotoxicity of municipal sewage sludge (SS). Risk Assessment Code (RAC) and Ecological Risk Factor (ERF) values indicated low risk associated with heavy metal (HM) mobility. Higher non-hydrolyzing C contents, compared to SS control, were found in SS to which 1% BC (by 3.6%), 1% DL (by 38.0%), 1% BN (by 25.5%) and 1% DT (by 20.6%) were added. Higher ratio C humic acids/C fulvic acids (Ckh/Ckf) values (more than 30% on average) were obtained when 1% BC, DL or BN were added compared to the 3% addition. Compared to the SS, the Ckh/Ckf values in SS with the addition of BC, DL or BN were on average more than 19% higher. The least varied values of dehydrogenase activity and respiration activity of SS (within dose) were obtained after the application of DT and BC, while the values of both parameters differed the most when BN was added to SS. Based on the results, it is not possible to clearly state which of the applied additives had the most significant effect on reducing the biochemical activity of SS. Sewage sludge and its mixtures generally showed low phytotoxicity to Sorghum saccharatum and Lepidium sativum, as root growth inhibition was 8–56% and 17–48%, respectively.