Co-digestion of steam pretreated wheat straw and municipal sewage sludge at varying concentrations of solids

Wheat straw is one of the most abundant agricultural residues in the world and can be used for production of energy carriers, e.g. methane through anaerobic digestion (AD). In previous studies, stable co- digestion of steam-pretreated wheat straw (PWS) and combined sewage sludge (cSS), composed of primary sludge (PS) and waste activated sludge, was demonstrated for over 200 days. The aim of the study presented here was to explore if the specific methane production can be increased by using more concentrated PWS, through removal of part of the liquid after steam pretreatment. Further on, the aim was to evaluate if the organic loading rate (OLR) can be increased by concentration of PS and if co-digestion of PWS and concentrated PS (PSc) results in a higher process stability than mono-digestion of PSc. By removal of water from the PS it would be possible to add PWS, with maintained substrate flow, to existing AD reactors at wastewater treatment plants. Wheat straw was ensiled with acetic acid for two weeks, steam-pretreated at 180 °C for 10 min and fractionated. Four parallel anaerobic processes in completely stirred tank reactors were evaluated: A control reactor (R1) fed only with cSS (with an OLR of 2.1 g_VS L?¹ day?¹ and HRT of 19 days) and three co-digestion reactors (R3, R4 and R5). Specifically, R3 was supplied with cSS and the solid fraction (SF) of PWS, (with an OLR of 2.1 g_VS L?¹ day?¹ and HRT of 27 days); R4 received cSS plus PWS (with an OLR of 2.1 g_VS L?¹ day?¹ and HRT of 25 days); and R5 was fed with PWS and PSc (8.8% TS) with an HRT of 47 and 35 days. In R5, the OLR was gradually increased from 2.1 to 2.8 g_VS L?¹ day?¹. All reactors started at room temperature (around 22 °C) and were then gradually raised to 37 °C. Preliminary results (after 2 sludge retention times) indicate a significantly higher specific methane yield from co-digestion of PWS and cSS, than previously reported. Like in previous studies the co-digestion of PWS and cSS is stable, with low concentrations of volatile fatty acids, without the use of any additives. Further on, co-digestion of PSc and PWS gives the highest specific methane production and methane productivity, so far. Co-digestion of PWS and cSS is a promising possibility for increased production of renewable methane from residues, using the existing infrastructure.