Co-hydrothermal carbonization of agricultural waste and sewage sludge for product quality improvement: Fuel properties of hydrochar and fertilizer quality of aqueous phase

Hydrothermal carbonization (HTC) is a promising carbon-neutral technology for converting sewage sludge (SS) and agricultural waste into energy. However, HTC-generated aqueous phase (AP) impedes the development of the former. This study investigated the potential of SS with rice husk (RH) and wheat straw (WS) co-HTC to form hydrochar and AP as substitutes for fuel and chemical fertilizer, respectively. Compared with single SS hydro-char, the yield of co-HTC-based hydrochar and higher heating value significantly increased by 10.9%-21.6% and 4.2%-182.7%, reaching a maximum of 72.6% and 14.7 MJ/kg, respectively. Co-HTC improves the safe handling, storage and transportation, and combustion performance of hydrochar. The total nitrogen concentration in AP-SS was 2575 mg/L, accounting for 67.7% of that found in SS. Co-HTC decreased and increased the amine and phenolic components of AP, respectively. AP-SS-RH and AP-SS-WS significantly increased pakchoi dry weight by 45.5% and 49.4%, respectively, compared with AP-SS. The results of the hydroponic experiments with AP instead of chemical fertilizers revealed that AP-SS did not reduce pakchoi dry weight by replacing <20% chemical fertilizers. However, AP-SS-RH or AP-SS-WS replaced 60% chemical fertilizers. Therefore, the co-HTC of SS and agricultural waste increased the AP substitution of chemical fertilizer from 20% to 60%. These findings suggest that the co-HTC of agricultural waste with SS is a promising technology for converting SS into renewable resource products for fuels and N-rich liquid fertilizer while significantly improving fuel and fertilizer quality.

Automated summary

This study investigated the potential of co-hydrothermal carbonization (HTC) of sewage sludge (SS) with rice husk (RH) and wheat straw (WS) to produce hydrochar and aqueous phase (AP) as substitutes for fuel and chemical fertilizer, respectively. The results showed that co-HTC significantly increased the yield and higher heating value of hydrochar, improved its handling and combustion performance, and increased the nitrogen content in the AP. Co-HTC also enhanced plant growth by replacing chemical fertilizers with AP.