Evaluation of system stability and anaerobic conversion performance for corn stover using combined pretreatment

A novel pretreatment method combining freezing-thawing and ammonia was developed, and the system stability and anaerobic conversion performance of corn stover (CS) were investigated. The CS was pretreated by water freezing-thawing (water-FT) and ammonia freezing-thawing (ammonia-FT), and then pretreated and untreated CS were anaerobically digested in three continuously stirred tank biodigesters (CSTBRs) at three organic loading rate (OLR) of 1.6, 1.8 and 2.0 gVS L^-1 d^-1. The analyses showed that pH value and volatile fatty acids (VFAs) concentration of three systems were in reasonable ranges of 6.8-7.5 and 160-330 mg L^-1, respectively. Total ammonia nitrogen (TAN) in R3 system ranged from 600 mg L^-1 to 1300 mg L^-1, which was conducive to maintain system stability. The removal amount of soluble chemical oxygen demand (SCOD) of CS by ammonia-FT pretreatment was 17.3-20.5% higher than that of water-FT pretreatment. The results also showed that CS pretreated by ammonia-FT achieved 27.8-32.4% and 13.9-16.1% more methane yield (250-267 mL g_vs^-1) than those of untreated and water-FT pretreatment, respectively. Correspondingly, ammonia-FT pretreated CS in R3 obtained the highest conversion rates of 47.7%, 56.9%, 42.7%, and 48.8% for TS, VS, cellulose, and hemicellulose, respectively, and the lowest cost was of 0.0336 USD m³ CH₄^-1 and the highest energy recovery of 2428.1 kWh kg_vs^-1. The study demonstrated that ammonia-FT pretreatment is one of efficient and cost effective method for stable CS bioconversion.