Co-HTL of domestic sewage sludge and wastewater treatment derived microalgal biomass – An integrated biorefinery approach for sustainable biocrude production

Abstract

Microalgal biomass as bioenergy feedstock is gaining wide attention for biocrude production through hydrothermal liquefaction (HTL). However, the availability of feedstock in all seasons is a major challenge. Hence, to ensure a consistent supply of feedstock and transform waste to energy, the present study investigates co-HTL of domestic wastewater treatment derived microalgal biomass (Monoraphidium sp. KMC4) and domestic sewage sludge (DSS) as bioenergy feedstocks. The effects of temperature, feedstock ratio, and residence time were studied and optimised for maximum biocrude yield. The study showed that, co-HTL at optimum operating conditions of 325 °C, 75:25 wt% (KMC4:DSS), and 45 min produced 39.38 wt% biocrude yield at a conversion rate of 83.96 wt%. The optimum biocrude yield was 16% and 79% higher than the individual HTL of KMC4 and DSS respectively. The comprehensive characterizations of co-HTL biocrude showed 76.77%, 10.6%, 8.85%, 3.38% of C, H, N, O and 39.47 MJ Kg−1 of HHV with an energy recovery rate of 77.53%. Meanwhile, co-HTL enhanced the distillation profile of biocrude which had 10.13% of heavy naphtha, 23.92% of kerosene, and 27.09% of gas oil. The FTIR and GC–MS analysis confirmed that the co-HTL biocrude had superior hydrocarbons such as alcohols and esters with limited nitrogen and oxygen heterocyclic compounds. In addition, ICP-AES confirmed a significant decrease in transfer of mineral elements from the co-HTL feedstock to biocrude. This validates the sustainability of the co-HTL process to produce high energy density biocrude with the potential to substitute fossil fuels.