Optimization of two-stage thermophilic anaerobic digestion of dairy wastewater: Effect of carrier material on process performance and microbial community

Although two-stage anaerobic digestion (TSAD) is not a novel process, information of the process stability, microbial community and effective operating conditions is limited and often contradictory. In this work, the influence of different carrier materials (polyurethane foam, carbon felt, Raschig rings and a combination of carbon felt and Raschig rings) on the performance of the thermophilic TSAD of dairy wastewater was studied. The organic loading rate (OLR) in the acidogenic reactor (RH) was gradually increased from 13.74 to 32.56 g COD/(L d), and from 0.64 to 11.46 g COD/(L d) in the methanogenic reactors by correspondingly reducing the hydraulic retention time (HRT). The highest hydrogen production rate of 1280.3 mL/(L & sdot;d) and hydrogen yield of 93.2 mL/g COD were achieved at OLR of 13.74 g COD/(L & sdot;d), but hydrogen production stopped at higher OLR. The main soluble metabolites in RH were butyrate and lactate, and the microbial community was dominated by Streptococcus, Thermoanaerobacterium, Veillonellales-Selenomonadales and Pseudomonas. The highest methane production rate (2674 mL/(L & sdot;d) at HRT of 0.5 days) was observed in the reactor with polyurethane foam, while the highest methane yield (305.5 mL/g COD at HRT of 1.5 days) was obtained in a reactor containing carbon felt. Spirochaetaceae, Desulfomicrobium, Anaerolineaceae, Candidatus Caldatribacterium and Cloacimonadaceae W5 were linked to these materials and explained the highest methanogenic performance.