Solid fuel production through hydrothermal carbonization of sewage sludge and microalgae Chlorella sp. from wastewater treatment plant

This study presents co-hydrothermal treatment of primary sludge (PS) from wastewater treatment plant (WWTP) and Chlorella sp., cultivated using WWTP effluent, and feasibility of using produced hydrochar as solid fuel. The results showed that properties of PS were improved through blending with Chlorella sp., in terms of mixture hydrochar properties (physicochemical composition, calorific value, fuel ratio, product yield, and energy recovery potential). The coalification degree (1.63 of H/C and 0.41 of O/C) and calorific value (5810 kcal kg-1) of hydrochar at 210 °C, defined as suitable hydrothermal carbonization temperature for mixture hydrochar production, were comparable to those of low-ranked coals (i.e., sub-bituminous and lignite). The low ash (<16.01% by dry weight until treatment temperature of 210 °C) and sulfur (0.64–0.78% by dry weight for all treatment temperature) contents of mixture hydrochar make it more attractive solid fuel as clean energy source. The findings suggest that the co-hydrothermal treatment of biomass (generated sludge and cultivated microalgae from WWTP) helps energy self-sufficiency in municipal WWTP.