Effects of lignocellulosic biomass type on the economics of hydrothermal treatment of digested sludge for solid fuel and soil amendment applications

[Display omitted] •Adding biomass and hydrothermal treatment (HTT) increased hydrochar heating value.•HTT of sludge alone had the lowest capital and operational expenditures.•HTT of sludge–cane bagasse led to the highest hydrochar energy recovery.•HTT of sludge–softwood sawdust led to the highest hydrochar revenue.•Saved sludge disposal cost was the most significant factor affecting the economics. Digested sludge is a waste stream from anaerobic digestion (AD) in wastewater treatment plants. Hydrothermal treatment (HTT) of sludge mixed with lignocellulosic biomass is an attractive approach to improve sludge dewaterability and generate value-added products. However, process economics has not been well understood. In this study, firstly, the effect of biomass type on the energy properties of hydrochars was studied. Secondly, two scenarios were simulated to evaluate the effects of biomass type on the economics (processing 50,000 tonnes of sludge per year) of HTT of digested sludge for solid fuel and soil amendment applications. The two HTT scenarios included sludge alone and sludge-biomass mixtures (four cases for four biomass feedstocks) at 180 °C for 60 min. In both scenarios, HTT liquids were returned to existing AD facilities for biomethane production to offset the energy cost of the HTT process. The results showed that the higher heating value significantly increased from 16.0–17.0 MJ kg−1 in the sludge alone case to 18.0–23.0 MJ kg−1 in sludge–biomass mixtures (except for rice husk). With the use of saved transport cost as a revenue source, HTT of sludge–biomass led to a net present value (NPV) range of AU$ 9.9–20.3 million (20 years) and an internal rate of return (IRR) range of 25.0 %–45.2 % for solid fuel application of resulting hydrochar compared to an NPV of AU$ 18.4 million and an IRR of 55.0 % from HTT of sludge alone scenario. HTT of sludge-biomass led to a NPV range of AU$ 4.5–14.5 million and an IRR range of 17.2 %–35.7 % for soil amendment application while the hydrochar from HTT of sludge alone was not recommended for soil application due to the high contents of heavy metals. This study provides useful and critical information for process scale-up and commercialization for integration into wastewater treatment plants.