Enhancing protein pre-hydrolysis via effluent recirculation towards high-rate household food waste anaerobic digestion using an expanded granular sludge bed reactor

Abstract

Anaerobic digestion (AD) of household food waste (HFW) can be promoted greatly using a high-rate reactor like expanded granular sludge bed (EGSB), but the low hydrolysis efficiency of the solid organics limits its practical applications. In this study, protein hydrolysis was identified as the limiting factor for achieving high-rate AD in the EGSB reactor, and then, EGSB effluent recirculation was implemented to enhance its pre-hydrolysis. An EGSB reactor treating HFW was operated with an increasing organic loading rate from 2.0 to 9.0 g COD/L/d to identify the key hydrolysis-limiting solid organic component (carbohydrate or protein) of HFW. The results showed that the protein removal efficiency was 54–64%, much lower than that of carbohydrate (89–99%). Protein hydrolysis was implied as the bottleneck restricting high-rate AD performance for treating HFW in the EGSB reactor. On this basis, effluent recirculation from the EGSB to a pre-hydrolysis reactor at a ratio of 0.5 was applied to enhance the hydrolysis of HFW. It was found that the carbohydrate hydrolysis efficiency increased from 86% to 93%, while the hydrolysis efficiency of protein increased by a factor of two (from 27% to 53%) at pH 6.5. The enrichment of Proteiniphilum (from 0.3% to 39.0%) enhanced protein hydrolysis in the recirculated pre-hydrolysis reactor. These findings suggest that EGSB effluent recirculation is an effective and practical way to enhance the protein pre-hydrolysis of HFW, thereby enabling the high-rate AD of HFW using the EGSB reactor.