Anaerobic Digestion of Dairy Manure in a Fixed Bed CSTR: Methane Production Performance and Microbial Diversity

To improve the fixed bed and continuous stirred-tank reactor (CSTR) technology relative to the rate of treatment of dairy manure (DM), a continuous stirred tank-treated DM with immobilized carrier biofilm was investigated for 20 days (hydraulic retention time (HRT) was 10 d). Methane productivity, biofilm characteristics, and microbial diversities of the biofilm and digestate were measured. The highest content of extracellular polymeric substances (EPS), proteins, and polysaccharides occurred at 15 days of digestion. An agglomeration phenomenon was observed using a scanning electron microscope on the biofilm. This indicated that the biofilm consisting of EPS was stable during the second HRT. Microbial diversities in digestate and carrier biofilm were characterized using a 16S rRNA sequencing technique. Results showed that the dominant bacterial communities were Pseudomonas (17% to 26%), Clostridium (13% to 21%), and Bacteroidetes (7% to 16%), while the archaea communities were Methanocorpusculum (25% to 37%), Methanosarcina (15% to 33%), Methanoculleus (11% to 15%), and Methanosaeta (13% to 18%). The methane production rate was significantly correlated with bacterial communities (Pseudomonas, Clostridium, Altererythrobacter atlanticus), archaeal communities (Methanosarcina, Methanoculleus, Methanosaeta, and Methanoplanus), and biofilm characteristics (chemical oxygen demand (COD) and EPS). These findings showed that a carrier biofilm could efficiently increase methane production.