Valorization of sludge pre-treated with hydrothermal carbonization via fermentation: influence of reactor configuration and process parameters

Sewage sludge from wastewater treatment plants poses considerable challenges for treatment and disposal. Hydrothermal carbonization (HTC) offers a promising sludge treatment method, producing process liquid and hydrochar with notable properties such as better filterability and fermentability. This study examines the potential of HTC to enhance volatile fatty acids (VFAs) production from sewage sludge. Four continuous fermentation reactor configurations were evaluated: a sequencing batch reactor (SBR) and three continuously stirred tank reactors (CSTRs). The tests aimed to evaluate VFA yields under varying operational conditions, including hydraulic retention times (HRT) of 4 days vs 8 days, as well as the effects of upstream solid-liquid separation. The results showed that HTC significantly enhanced the potential for methane production from the treated sludge. CSTR reactors operating at an 8-day HRT consistently achieved the highest VFA conversion rates. Although the SBR configuration exhibited the greatest VFA productivity, overall acidification yields showed no significant differences between the reactor types.