This study was carried out to investigate the relationship between the methane producing pathways and the characteristics of anaerobic granules treating municipal wastewater. For this purpose, two pilot scale upflow anaerobic sludge blanket reactors with different granule size distribution (1-2 mm and 3-4 mm) were investigated at operating temperatures of 20 degrees C and 28 degrees C for 239 days. There was an increased and stable biogas production when temperature was elevated to 28 degrees C likely due to reduction in methane solubility. Larger granules had multi-layered internal microstructures with higher acetoclastic methanogenic activities (250-437 mL CH(4)g(-1) VS d(-1)) than smaller granules (150-260 mL CH(4)g(-1) VS d(-1)). The relative abundance of acetoclastic methanogens of larger granules was higher, confirming acetoclastic methane producing pathway was more prominent. However, there was no significant difference in the performance of the two reactors because they were operating below their capacities in terms of organic loading rate to volatile solids ratio.

The influence of hydraulic retention time (HRT of 3-5 h) and temperature (20-25 degrees C) on performance and microbial dynamics of two pilot-scale upflow anaerobic sludge blanket (UASB) reactors with different granule size distribution (UASB1 = 3-4 mm and UASB2 = 1-2 mm) were investigated for 217 days. Increasing the HRT to 5 h even at a lower temperature of 20 degrees C enhanced COD removal and biogas production with average of 59 +/- 16% (up to 85%) and 73 +/- 9 L/(m3 center dot d) (up to 102 L/(m3 center dot d)) for UASB1; 63 +/- 16% (up to 85%) and 75 +/- 9 L/ (m3 center dot d) (up to 90 L/(m3 center dot d)) for UASB2, respectively. This is explained by sufficient contact time between microorganisms and substrate. Acetoclastic methanogenic activity was higher in UASB1 because Methanosaetaceae (produces methane from acetate) dominated (64 +/- 4%). However, Methanoregulaceae (29 +/- 3%) and Methanomicrobiales_unassigned (20 +/- 6%) which produce methane from H2/CO2 and formate were significant in UASB2. The extent of change in the microbial dynamics with HRT and temperature was more obvious in the smaller granule reactor.

Volatile fatty acids (VFAs) are intermediates of anaerobic digestion with high value and wide range ofusage. Co-digestion of sewage sludge and external organic waste (OW) for VFA production can helpachieve both resource recovery and ensure sustainable and innovative waste management. In view ofthis, the effect of substrate proportions on VFA production from co-digestion of primary sewage sludgeand OW is studied. Long-term operation in a semi-continuous reactor was performed to assess the resilienceof such a system and the VFA-rich effluent was tested for its ability to be used as carbon source fordenitrification. Co-digestion was initially carried out in batch reactors with OW proportion of 0%, 25%,50%, 75%, 100% in terms of COD and scaled up in a semi-continuous reactor operation with 50% OW.In the short-term operation in the batch mode, acetic acid dominated, however, increasing OW fractionresulted in increased valeric and caproic acid production. Moreover, in the long-term semi-continuousoperation, caproic acid dominated, accounting for 55% of VFAs. The VFA-rich effluent from the semicontinuousreactor achieved the highest denitrification rate as a carbon source when compared withacetic acid and methanol. The results demonstrate that co-fermentation can increase VFA yield and shiftproducts from acetic acid to caproic acid in long-term operation and the VFAs can be used withinwastewater treatment plants to close the loop.

Alkaline co-fermentation of primary sludge and external organic waste (OW) was studied to elucidate the influence of substrate ratios and long-term system robustness and microbial community dynamics using batch and semi-continuous reactors. Volatile fatty acid (VFA) production increased with increasing OW fraction in the substrate due to synergistic effects of co-degradation. VFA production at pH 10 increased up to 30,300 mgCOD/L (yield of 630 mg COD/gVS_fed) but reduced over time to ≈10,000 mgCOD/L. Lowering pH to 9 led to the restoration of VFA production with a maximum of 32,000 mg COD/L (676 mg COD/g VSfed) due to changes in microbial structure. VFA was composed mainly of acetic acid, but propionic acid increased at pH 9. The microbial community was dominated by Bacillaceae (34 ± 10%) and Proteinivoracales_uncultured (16 ± 11%) at pH 10, while Dysgonomonadaceae (52 ± 8%) was enriched at pH 9. The study demonstrated a zero-waste strategy that turns organic wastes into bio-based products.