Anaerobic Co-digestion of steam pretreated wheat straw and sewage sludge

Abstract: Anaerobic digestion (AD) of wastes and residues is a widely applied and explored process. However, when the process is not optimized, inadequate concentrations of nutrients and inhibitory compounds are common and can lead to low maximum organic loading rates and increased risk of process problems, low organic dry solids degradation rate and reduced methane yields. To overcome process instability and low performance, the simultaneous digestion of two or more substrates, named co-digestion, has been implemented in the last 30 years. Nowadays, the expansion of co-digestion to the use of anaerobically slowly converted agro-waste, via pretreatments, is of major interest due to their availability and energy densities. Among the sustainable feedstocks, wheat straw is a good candidate for co-digestion with sewage sludge, since it has a high concentration of organic compounds, low water content and low concentration of many nutrient elements, which can be provided in higher amount by sewage sludge, being instead characterized by low organic content, high water content and high concentration of several nutrient elements in relation to carbon. This study investigates co-digestion of sewage sludge (combined primary sludge and waste activated sludge) and steam pretreated wheat straw in continuous stirred-tank reactors. Wheat straw was pretreated through acetic acid catalyzed steam explosion at 190°C for 10 minutes, leading to 89% recovery of the total solids. A constant proportion of the two substrates was maintained in the inlet, where 35% of the feedstock volatile solids was provided by wheat straw, giving a C/N ratio of 12.6, and TS content of 10.7%. Digestions of the two single feedstocks were used as controls to compare mono- and co-digestion performance. Micro- and macronutrients were added in the wheat straw controls to reach the same concentrations in the feed as for co-digestion. Similar organic loading rate were applied 2.1 ± 0.1gVS/Ld in all reactors, and the hydraulic retention time was maintained at 22 days. All processes were stable most of the operational time, with methanogenesis inhibition occurring only at the beginning when wheat straw was introduced in the substrate, accompanied volatile fatty acids accumulation. Hence, the microbial population likely adapted to the new substrate. After 26 days the concentration of total volatile fatty acids was maximum only 0.30 g/L within 7 hours after feeding and less than 0.15 g/L at 7 hours after feeding, suggesting that higher organic loads might be tolerated by the system. The average methane yield from co-digestion was 0.27 LCH4 / gVS. This was significantly different only from the methane yield obtained from wheat straw mono-digestion, and a synergistic effect was not demonstrated. Similar degradation degree of volatile solids were obtained from co-digestion and sludge mono-digestion, meaning a good accessibility to degradable compounds in wheat straw was gained from its pretreatment. More experiments need to be performed to assess the digestate quality and its impact on the economy and applicability of the process. Finally, in a full-scale plant digesting sludge only, an increase in annual methane production of 43% was estimated through co-digestion of 35% wheat straw. Due to the high energy density (low water content) of wheat straw, the plant could operate at the same hydraulic retention time as today, with an increase in reactor volume of only 11%.