Methane production from process water of sewage sludge hydrothermal carbonization. A review. Valorising sludge through hydrothermal carbonization

Hydrothermal carbonization (HTC) has recently emerged as a promising technology for sustainable sludge minimization and solid products valorization. Nevertheless, a not negligible amount of process water remains as by-product, containing organic compounds which solubilize during the process (10–63 g COD L −1 ). Its valorization through anaerobic digestion (AD) has been assessed, but no systematic review has been reported to date. The main parameters, which influence the valorization of HTC process water through AD, are initial feedstocks characteristics, nutrients concentration, HTC process temperature and residence time, inoculum acclimation to toxic compounds and its buffer capacity. Experiences reported in the literature proved that HTC applied to sewage sludge at 180–200 °C at residence times in the range 60–90 min could be a good compromise between hydrochar production and valorization, hydrochar dewaterability, biogas production from AD of process water and energy consumption.