Hydrothermal liquefaction of sewage sludge with recirculation of pretreated aqueous phase from activated carbon.

Abstract: The management and optimization of the aqueous phase are the major challenges that hinder the promotion of hydrothermal liquefaction (HTL) technology on the commercial scale. Recently, many studies noticed the accumulation of the N-content in the bio-crude with continuous recycling of the aqueous phase from high protein-containing biomass. In the present study, sewage sludge was processed at 350 °C via HTL. The produced aqueous phase was treated with activated carbon and its subsequent recycling effect on the properties of the bio-crude and aqueous phase was investigated. By contacting the aqueous phase with activated carbon, 38-43 % of the total nitrogen was removed from the aqueous phase. After applying the treated aqueous phase recycling, the energy recovery of the bio-crude increased from 50 to 61 % after three rounds of recycling in an autoclave. The higher heating values of the bio-crudes were calculated in the range of 31 to 33 MJ/kg. From overall carbon/nitrogen recoveries, 50 to 56 % of the carbon was transferred to the bio-crude phase and more than 50 % of the nitrogen remained in the aqueous phase. GC-MS analysis highlighted that the aqueous phase is composed mostly of N&O-heterocyclic compounds, small chain organic acids, and amides. ICP-AES analysis showed that more than 90 % of the inorganic elements like Al, Ca, Fe, Ni. Cu, Pb, and Zn were concentrated into the solid phase.