Energy and nutrient recovery by spent mushroom substrate-assisted hydrothermal carbonization of sewage sludge

•Co-HTC of sewage sludge and spent mushroom substrate was carried out.•The energy yield of hydrochar from co-HTC increased by 12.1–44.8%.•The AP of co-HTC increased the biomass of pakchoi by 25.7–40.9%.•Co-HTC can reduce the inhibitory effect of AP on pakchoi growth.•The APs with EC of 0.15 and 0.2 mS cm−1 were more beneficial for pakchoi growth.