Energy recovery and pathogen inactivation with dry co-digestion of food waste and pig manure
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Anaerobic digestion is one of the best available technologies for food waste (FW) and pig manure (PM) management by producing methane-rich biogas. However, mono digestion of FW or PM is easily inhibited by high volatile fatty acid (VFA) or ammonia. Co-digestion of FW and PM can provide an effective solution to address these issues due to the buffering interactions between the VFA and ammonia. Compared with wet digestion, dry digestion can reduce the digester volume significantly, thereby decreasing initial capital expenditure and the energy consumption required for heating. In this research, batch dry co-digestion of FW and PM was conducted in laboratory-scale digesters at the total solid (TS) content of 20%. The research objectives were to assess (1) the feasibility and optimal operation conditions of dry co-digestion systems; (2) methane production kinetics and the inhibition mechanisms; (3) the biosafety of digestate, i.e. the inactivation of enteric indicator bacteria, including total coliforms, E. coli, enterococci and Salmonella; and (4) microbial community structure evolution in dry co-digestion systems. The results showed that preferable operation conditions were obtained at a digestate inoculum rate of 50% and a FW/PM ratio of 50:50, with an average specific methane yield (SMY) of 252 mL/g VSadded (volatile solids). Using digestate as inoculum didn’t increase the total amount of SMY but significantly decreased the lag phase from 28 days to 13 days compared with using dewatered anaerobic sludge as inoculum. Total VFA was the main inhibition factor on methane production (P<0.001), and the total VFA concentration was suggested to be < 20.0 g/L to avoid complete inhibition. Dry co-digestion of FW and PM can effectively inactivate the enteric indicator bacteria, as E. coli and total coliform counts decreased below the limit of detection (LOD, 102 CFU/g) within 4-7 days, with free VFA being identified as a significant inactivation factor. Enterococci decreased to below the LOD within 12-31 days, with digestion time the most significant inactivation factor. Salmonella was completely eliminated within 6-7 days. Statistical analysis results showed that pH, VFA type, VFA/ammonia concentration and Salmonella serotype all significantly impacted Salmonella inactivation (P < 0.01). In VFA minimum inhibitory concentration (MIC) tests, the inhibitory effect sequence was in the order of pH > VFA concentration > VFA type > Salmonella serotype; and in ammonia MIC tests, the inhibitory effect sequence was in the order of ammonia concentration > pH > Salmonella serotype. At the same concentration, the inhibition effect of VFA was much greater than that of ammonia. The inoculum was more significant in determining the microbial community structure than the FW/PM ratio. Hydrogenotrophic methanogenesis was an important methane production pathway, with Methanoculleus the dominant methanogen. Significant correlation was observed between the relative abundance of specific microbial taxa and digesters’ physicochemical parameters. Based on correlation analysis, the dry co-digestion associated functions of some previously poorly reported bacteria were predicted here for the first time. The results in this study indicate that dry co-digestion of FW and PM is an effective way for treatment of both substrates, with recovery of methane-rich biogas and safe digestate. The data obtained can provide guidance for on-farm engineering practice. The preliminary prediction on the functionality of previously poorly described bacteria will provide reference for further study.