Efficient conversion of sewage sludge into hydrochar by microwave-assisted hydrothermal carbonization.

The treatment of sewage sludge (SS) is an environmental problem worldwide. In recent years, hydrothermal carbonization (HTC) of SS for hydrochar (HC) has attracted extensive attention. This study preliminarily explored the microwave-assisted HTC of SS for the first time. Increasing the reaction temperature (150-250 °C) and reaction time (0-120 min) resulted in a decrease in the HC yield, and it gradually increased with the rising solid-liquid ratio (0.03-0.25 g/mL). Compared with raw SS, the HC products possessed higher aromaticity, carbonization degree, porosity, and polarity, and lower content of soluble nutrients (N/P/K) and leachable heavy metals (Cu, Zn, Pb, Cd, Cr, and Ni), indicating a lower risk of nutrient and heavy metal loss. Attention should be paid to the total contents of Zn and Cd in HC exceeded the permitted value for use in cultivated land with edible crops. The use of CaO as a catalyst improved the yield of HC, made the HC and process water (PW) weakly alkaline, and further passivated the heavy metals in the HC. In the case of H3PO4, although the conversion of SS was enhanced (lower content of volatile organic matter in HC), the contents of soluble nutrients (N/P/K) in HC/PW increased, and the migration of Zn and Cd into process water was enhanced. The HCs obtained in this study had poor combustion properties, but higher ignition temperatures than raw SS. PW must be properly treated or recycled because it still contained high contents of organic matter and nutrients. This fundamental study provides basic insights into the microwave-assisted HTC of SS.

Effects of rice straw/wood sawdust addition on the transport/conversion behaviors of heavy metals during the liquefaction of sewage sludge.

Lignocellulosic biomass has been widely introduced into the liquefaction process of sewage sludge (SS) to improve the yield/quality of liquefaction products (bio-oil/biochar). This study explores the effect of adding rice straw (RS) and wood sawdust (WS) on the transport/conversion behaviors of heavy metals (HMs) during the liquefaction of SS. The introduction of lignocellulosic biomass, especially for RS, substantially lowers the total content of HMs in biochar. Most HMs (except Cd) still remain in biochar, although the introduction of RS/WS enhances the transport of HMs into bio-oils. The addition of RS/WS raises the percentage of HMs in active form, but the contents of bioavailable/leachable HMs are not considerably increased and even decreased in some cases, especially when RS is introduced. The overall pollution degree and environmental risk of HMs in biochars are lowered to a certain extent with the addition of RS/WS. Considering that the pollution degree and environmental risk of HMs present in biochars are still at a considerable level, appropriate pollution management measures should be undertaken when using such biochars for agricultural use.