Biogas production and nutrients removal from slaughterhouse wastewater using integrated anaerobic and aerobic granular intermittent SBRs – Bioreactors stability and microbial dynamics

Slaughterhouse wastewater (SWW) was effectively treated in sequential anaerobic and aerobic granular intermittent sequencing batch reactors (ASBR+ISBR) for 665 days at different HRTs (48 h, 32 h, 24 h, and 12 h). The ASBR was stable at each HRT but performed relatively well at 12 h (OLR - 7.8–9.8 kg COD/m3-d) in terms of pollutants removal and biogas production than previously conducted research. The average biogas production was about 17.3 L/day having 70–76 % of CH4 which could subsidize around 52 % of electricity demand while saving 103 US dollars/day if installed at full scale. In the case of post aerobic granular ISBR, carbon and nutrients removal (N&P) was achieved by enriching granules (1.7–2.2 mm) at low DO (0.5–0.8 mg/L) via the nitrite pathway. The ISBR was also well stable at 12 h HRT (average OLR of 2.1 kg COD/m3-d) and met the effluent discharge guidelines recommended by the Central Pollution Control Board of India. During steady-state conditions (12 h HRT), the average removal efficiencies for COD, TSS, O&G, TN, and PO4-P were 98.8 %, 96.4 %, 98.7 %, 93.4 %, and 86.6 % respectively from combined ASBR and ISBR. The microbial analysis confirmed Euryarchaeota, Proteobacteria, Firmicutes, Chloroflexi, Bacteroidetes, Planctomycetes, and Synergistetes as the dominant phyla in ASBR. Methanosaeta (21.56 %) and Methanosarcina (6.48 %) were the prevailing methanogens for CH4 production. The leading phyla observed in ISBR were Bacteroidetes, Proteobacteria, Firmicutes, Armatimonadetes, Verrucomicrobia, Chloroflexi, and Planctomycetes. Heterotrophic AOB (Thauera, Xanthomonadaceae, Pseudomonas, Sphingomonadaceae, and Rhodococcus) were mainly detected in the system for ammonia oxidation besides common autotrophic AOB. Similarly, a known PAO (Accumulibacter) was not identified but other PAO (Rhodocyclaceae, Dechloromonas, Pseudomonas, Flavobacteriaceae, and Sphingobacteriaceae) were prevalent inside aerobic granular ISBR that contributed to both carbon and nutrients removal. The results obtained would help implement the investigated reactor configurations at the pilot and full scale for SWW treatment. [Display omitted] •An HRT of 12 h was found effective for both ASBR and ISBR while treating SWW.•Around 52 % of electricity could be subsidized using ASBR at full scale for SWW.•Nutrients were removed through granulation at low DO (0.5–0.8 mg/l) by SNDPR process.•Methanosaeta and Methanosarcina were the dominant methanogens observed in ASBR.•In ISBR nutrients