Co-digestion and mono-digestion of sewage sludge and steam-pretreated wheat straw in continuous stirred tank reactors – nutrient composition and process performance

Background: Wheat straw (WS) constitutes a large biomass resource and can be used for the production of advanced biofuels, for example in the form of methane through anaerobic digestion (AD). Due to the low contents of several nutrient elements and water in harvested WS, the use of sewage sludge (SS), consisting of primary sludge and waste activated sludge, as a nutrient source in co-digestion with pretreated wheat straw (PWS) was investigated theoretically and practically. Results: Process stability and specific methane yield were studied in mono-digestion and co-digestion of PWS and SS in continuous stirred tank reactors (CSTRs) for 208 days. The hydraulic retention time was 22 days and the organic loading rate (OLR) 2.1 ± 0.1 g VS L -1 d -1 . In co-digestion, the OLR was increased to 2.8 g VS L -1 d -1 for one week. WS soaked in acetic acid solution was steam-pretreatment at 190 °C for 10 minutes, ending with an explosive reduction in pressure. Nutrient elements were added to PWS mono-digestion at two different concentration levels. Co-digestion was stable, with a total concentration of short-chain fatty acids (SCFAs) at a secure level below 0.35 g L -1 at both OLRs. This higher OLR would require an increase in reactor volume of 14%, compared to mono-digestion of SS, but would increase the annual production of methane by 26 %. A longer HRT than applied would increase the specific methane yield of all substrates, based on residual methane yields. SCFAs accumulated in the mono-digestion of PWS when using the lower levels of nutrient supplements, concentrations fluctuated with the higher levels. Conclusions: PWS and SS can be co-digested with long-term process-stability, without addition of other chemicals than acetic acid as catalyst in steam-pretreatment and of steam. The specific methane yield from PWS was relatively low, 170 mL gVS -1 , but could likely be increased by optimisation of the steam-pretreatment conditions. The effects of higher concentrations of micro-nutrients in mono-digestion of PWS should be evaluated in future trials.