Is nitrification inhibition the bottleneck of integrating hydrothermal liquefaction in wastewater treatment plants?

Sewage sludge management poses challenges due to its environmental impact, varying composition, and stringent regulatory requirements. In this scenario, hydrothermal liquefaction (HTL) is a promising technology for producing biofuel and extracting phosphorus from sewage sludge. However, the toxic nature of the resulting process water (HTL-PW) raises concerns about integrating HTL into conventional wastewater treatment processes. This study investigated the inhibitory effects of HTL-PW on the activity of the main microbial functions in conventional activated sludge. Upon recirculation of the HTL-PW from the excess sludge into the wastewater treatment plant, the level of COD in the influent is expected to increase by 157 mgO2⋅L-1, resulting in 44% nitrification inhibition (IC50 of 197 mg⋅L-1). However, sorption of inhibitory compounds on particles can reduce nitrification inhibition to 27% (IC50 of 253 mg⋅L-1). HTL-PW is a viable carbon source for denitrification, showing nearly as high denitrification rates as acetate and only 17% inhibition at 157 mgO2⋅L-1 COD. Under aerobic conditions, heterotrophic organic nitrogen and organic matter conversion remains unaffected up to 223 mgO2⋅L-1 COD, with COD removal higher than 94%. This study is the first to explore the full integration of HTL in wastewater treatment plants for biofuel production from the excess activated sludge. Potential nitrification inhibition is concerning, and further long-term studies are needed to fully investigate the impacts.