Evaluation of selective clay minerals and biochar as materials for sewage sludge stabilization

Abstract

The objectives of this study were to evaluate sewage sludge’s stabilization with untested until now materials, such as selective clay minerals or biochar in comparison with liming, for enhancing sludge’s fertilization capacity. Dewatered sewage sludge was mixed with bentonite, vermiculite, zeolite, biochar or lime at rates of 0, 15 and 30%, air-dried and analyzed for pathogens and chemical properties. Almost all fecal indicators of treated sludge with 15% bentonite, vermiculite or biochar were reduced by at least one-logarithmic unit (log10) (indicative value of sludge’s stabilization), whereas those of limed sludge were undetectable. Electrical conductivity of all treatments significantly increased, and the highest values were obtained for untreated (6.1 dS m−1) and limed sludge (above 7.0 dS m−1 for both addition rates). The untreated sludge had the significantly highest water-soluble ammonium-nitrogen (2817 mg kg−1) and phosphorus (263 mg kg−1) concentrations followed by sludge treated with bentonite, vermiculite or biochar, whereas limed sludge had the lowest content. Boron concentration of the untreated sludge was similar to the treated sludge. Total concentrations of heavy metals were far below the legislative permissible levels for sludge’s agronomic use. Nutrients’ total content of treated sludge ranged at levels of similar magnitude to the untreated sludge, except for certain cases where they were increased because of the materials’ composition. Consequently, sewage sludge treated with 15% bentonite, vermiculite or biochar seems to be stabilized, retain bioavailable nitrogen and serve as a fertilizer of macro- and micronutrients. However, potential risks of agronomic use, i.e., soil salinization and boron phytotoxicity, should be considered.