Sludge-derived char : utilisation as a metal sorbent in dilute wastewaters

Abstract: Metal concentrations in soil and water have increased since the Industrial Revolution, which may have negative health and environmental effects. Metal pollution occurs, for instance, in municipal wastewater, industrial wastewater, and stormwater. Concentrations are often low, due to dilution. A common technology for metal removal is sorption. Char produced from pyrolysis of municipal sludge (SDC), has been pointed out as a potential low-cost sorbent. However, the scientific literature mainly focuses on experiments using artificial solutions at concentrations much higher than those found in said wastewaters (in a Swedish context). The goal of this study was to investigate SDC use for the removal of metals, focusing on reuse of SDC in primary treatment (PT) of municipal wastewater and with some attention to other applications. The investigation was performed through experimental laboratory studies, modelling, and literature review/assessment. The data obtained from the literature review indicated that sorption is suitable for enhancing removal of nickel (Ni), copper (Cu), cadmium (Cd), lead (Pb), and zinc (Zn) during PT. With respect to Ni and Cu; from ~7–50% and ~10–70%, respectively (no SDC), to ~65–98% and ~45–85%, respectively (with SDC). Experimental results showed that SDC could remove >95% of Pb, Cd, Cu, Cr, Ni, Hg, and Ag from artificial solution, at concentrations similar to those in raw municipal wastewater. However, sorption of Cu and Ni was inhibited in real wastewater (a decrease from >99 and 99%, respectively, to 68 and 40% respectively). The inhibition was linked to the presence of dissolved organic matter. Modelling, based on raw wastewater composition indicated that SDC addition in PT may enhance the removal of Cd and Cu (from 39% to 79% and 30 % to 43%, respectively). Experiments showed that the investigated SDC had a larger Pb sorption capacity, compared to activated carbon and wood-derived char. Experimental investigations and modelling (sorption isotherms) indicated that literature data did not give satisfactory estimations of the Pb sorption capacity onto SDC at concentrations considered; the available data was generally valid for much higher concentrations. The experimentally determined Cd sorption capacity of SDC produced from primary sludge exceeded that of SDC produced from digested sludge. However, given the loss of biogas production the theoretical energy balance of primary sludge pyrolysis was negative. Finally, the local demand for Cd-sorbent in the Västerås region could potentially be covered by the SDC generated locally.