Primary sewage sludge filtration using biomass filter aids and subsequent hydrothermal co-liquefaction

Hydrothermal liquefaction (HTL) is a promising technology for biofuel production and treatment of wastewater sludge. The current study investigates a novel utilization of biomass-assisted filtration of primary sludge to obtain high dry matter (DM) content sludge. Drastic improvements in filtration speed are achieved using different types of lignocellulosic biomass filter aids prepared via mechanical pre-treatment. The combined sludge-biomass filter cake is subsequently used as a feedstock for HTL and shows superior bio-crude yields and properties compared to their individual counterparts. The chemical energy recovery to bio-crude is increased to 75% compared to 46% for biomass and 67% for sludge on its own. The increased DM content of filter cakes (∼25%) compared to primary sludge (5%) increases the energy efficiency of HTL of primary sludge by a factor of 4.5. Introducing a biomass filteraid-HTL combination to a wastewater treatment plant would reduce the organic carbon load to treat by 62%. By combining sludge with lignocellulosic biomass the use of alkali catalyst can be avoided entirely which represents a major cost factor in HTL of lignocellulosics.