3.1 Effect of mixing ratio in anaerobic digestibility of FVW in co-digestion:
Various authors have investigated the efficiency of co-digestion of sewage sludge with fruit and vegetable waste 3. The higher COD, TS and VS content of FVW compared to MWAS (Table 2), assures higher biodegradable organic content and makes it suitable for anaerobic digestion. However, the high VS content of FVW favors quick hydrolysis, consequently followed by increased acidification which could threaten methanogenesis 31. In the current study, the mixing ratio of Beenyup mixed waste activated sludge with fruit and vegetable waste were optimized in two-stage anaerobic digesters at constant HRT. Co-digestion (3:1) achieved 1.6 times higher cumulative methane yield compared to co-digestion (1:1) (Figure 1a). Two-stage anaerobic digestion accelerates rate-limiting hydrolysis step in the thermophilic phase, followed by the methanogenic population in the mesophilic phase. The CH4/CO2 ratio in both digestions was observed to be less than 0.5 during the initial thermophilic phase indicating a higher build-up of carbon dioxide in the reactors (Figure 1b). At higher organic loading, the addition of FVW increases the production of carbon dioxide, hydrogen and acetate production by acidogenic microbial population in the thermophilic phase32. No significant amount of methane production was observed in co-digestion (1:1) signaling unsuitable conditions for methanogenic population inside the digester. Co-digestion (3:1) achieved 9.1-fold higher methane production during the initial thermophilic phase, as a courtesy of improved pH stability. Moreover, the reduction of organic loading by changing the FVW fraction in sludge feedstock didn’t significantly curb the accumulating carbon dioxide in the initial phase. However, transfer of substrate to phase II, increased methane production by 1.49-fold in co-digestion (3:1). Besides, during this phase methane percentage increased significantly and a greater reduction of carbon dioxide compared to co-digestion (1:1) was observed (Figure 1b). This could be due to the increased stability achieved in mesophilic phase-II with improved balance of nutrients.
Furthermore, the FVW fraction and OLR have been reported to have a significant effect on biogas production and process stability in co-digestion with sludge 33–35. The HRT (Hydraulic retention time) of the thermophilic phase was 2 days in both the systems and the corresponding OLR were 13 gVS/l.day and 11.5 gVS/l.day in co-digestion (1:1) and (3:1) respectively. The VS removal percentage was almost similar in both the systems during the thermophilic phase. However, a 33% higher VS removal was observed in co-digestion (3:1) compared to co-digestion (1:1), in the mesophilic phase II. The overall OLR of the digestions were 1.3 gVS/l.day and 1 gVS/l.day in co-digestion (1:1) and co-digestion (3:1), respectively. Di Maria et al., reported a higher OLR (>2 kgVS/m3.day) with an excessive fraction of FVW in co-digestion with waste mixed sludge resulted in microorganism washout and decrease in biogas production under mesophilic condition 8. Whereas, in the current study we observed that 50% of FVW in co-digestion with sludge leading to an OLR of 1.3 gVS/l.day itself had a detrimental effect on the methanogenic population under thermophilic condition. Besides, the reduction of OLR to 1 gVS/l.day with 25% FVW showed improvement in overall process performance and biogas production.
Even though thermophilic condition favors hydrogenotrophic methanogens (methane production from H2 and CO2), poor methanogenesis was observed in phase I anaerobic digestion due to the accumulation of VFAs and poor buffering capability of FVW (Figure 2a and b). FOS/TAC is the ratio of volatile organic acid to alkaline buffer capacity, and it is often widely used to assess the process stability of an anaerobic digester. Literature studies report that FOS/TAC values of stable anaerobic digestion lie in the range of 0.2-0.66,36,37. FOS/TAC values beyond 0.6 indicate unsuitable operating conditions for anaerobic microorganisms and results in a decrease in biogas production. In the current study, the average values of FOS/TAC in phase I thermophilic reactor were 1.91 and 1.1 in co-digestion (3:1) and co-digestion (1:1) respectively (Figure 2b). Considering that waste activated sludge is characterized with low C/N ratios and high buffer capacity, increase in WAS fraction in co-digestion has certainly improved the FOS/TAC ratio38. As the thermophilic condition has higher organic degradation rates, along with increased organic loading, the phase-I anaerobic digestion resulted in accumulation of VFAs resulting in unstable anaerobic digestion in co-digestion (1:1) (Figure 2a). However, transfer of substrate to Phase-II significantly improved the stability of anaerobic digestion and resulted in average FOS/TAC values of 0.51 and 0.38 in co-digestion (3:1) and co-digestion (1:1) respectively (Figure 2b). Improved process efficiency was observed in co-digestion (3:1) in phase-II, with 21% higher reduction in VFAs and 1.6-fold increase in methane yield compared to co-digestion (1:1), at the end of 20 days of HRT.