Effect of total solid content on biogas production from rape stalk and chicken manure with different mixing ratios

Abstract: China has the largest cultivated area of oilseed rape in the word and only the seeds are harvested for human assumption and animal feed, resulting in the production of an estimated 30 Mt every year of stalk materials. Due to the lack if an effective utilization, most of rape stalk (RS) is burned in the open air or left in the field, which could produce carbon dioxide and solid particles, resulting in the serious environmental and safety problems. Anaerobic digestion plays an important role in transforming crop residues into green energy (bio-methane). There are some advantages of RS as feedstock for anaerobic digestion, such as concentrated geographical distribution, facilitating harvest and transport, the low sulphur content and the high cellulose content. Despite these advantages, RS has the low methane yield because of its high carbon content and imbalance C/N ratio for microbes during anaerobic digestion. Chicken manure (CM) is a well-studied co-substrate due to its high buffering capacity and high nitrogen content. Secondly, RS is hardly compatible with traditional wet anaerobic digestion technology (total solid (TS) <10%) in view of its high TS. Solid state anaerobic digestion (TS≥10%) has been claimed to be a number of advantages over wet anaerobic digestion including the minimal biogas slurry generation, the high volumetric methane production and the positive energy balance. There is no literature so far on the evaluation of methane production and anaerobic digestion stability of RS and CM with low C/N ratio with various mix ratios from wet anaerobic digestion to solid state anaerobic digestion systems. In order to obtain the optimal digestion parameters and high methane yield from RS and CM, methane production rate from co-digestion of RS and CM with balance nutrient was investigated at different total solid content (TS) in batch experiments under the temperature of (37±1) ℃ condition, then anaerobic digestion stability and kinetic characteristics of co-digestion of RS and CM were evaluated in this paper. Results showed that the significant synergistic effects were found when the two mixtures at RS:CM ratios of 90:10, 85:15 and 80:20 (based on volatile solid content), and the highest specific methane production rate (SMPR) of 288.7 mL/g was obtained at RS:CM ratio of 90:10, which was 16.1% and 30.4% higher than that of pure CM and RS, respectively. SMPR of substrates were decreased with the increase of TS and the volumetric methane production rate (VMPR) of substrates in TS of 10% and 15%, which were significant higher than those in TS of 5% and 20%. The higher SMPR of 224.4 mL/g and the higher VMPR of 0.50 mL/(mL·d) was achieved in TS=10% at RS:CM ratio of 90:10. The excessive accumulation of volatile fatty acids (VFAs) and the high concentration ammonia nitrogen could disrupt separately the digestion stability of substrates with high C/N ratio or low C/N ratio. VFA/TA and ammonia nitrogen content were the two important evaluation indicators for the digestion stability of substrates with different C/N ratios. Based on the dynamic results, the optimal solid retention time (SRT) was 14 d with TS=10% and RS:CM ratio 90:10 at the start-up stage of continuous anaerobic digestion in the full scale operation. The above results of this study provide useful information to improve the efficiency and stability of co-digestion of RS and CM under different anaerobic digestion conditions.