Hydrolysis and Methanogenesis in UASB-AnMBR Treating Municipal Wastewater Under Psychrophilic Conditions: Importance of Reactor Configuration and Inoculum

Abstract

Three upflow anaerobic sludge blanket (UASB) pilot scale reactors with different configurations and inocula: flocculent biomass (F-UASB), flocculent biomass and membrane solids separation (F-AnMBR) and granular biomass and membrane solids separation (G-AnMBR) were operated to compare start-up, solids hydrolysis and effluent quality. The parallel operation of UASBs with these different configurations at low temperatures (9.7 ± 2.4°C) and the low COD content (sCOD 54.1 ± 10.3 mg/L and pCOD 84.1 ± 48.5 mg/L), was novel and not previously reported. A quick start-up was observed for the three reactors and could be attributed to the previous acclimation of the seed sludge to the settled wastewater and to low temperatures. The results obtained for the first 45 days of operation showed that solids management was critical to reach a high effluent quality. Overall, the F-AnMBR showed higher rates of hydrolysis per solid removed (38%) among the three different UASB configurations tested. Flocculent biomass promoted slightly higher hydrolysis than granular biomass. The effluent quality obtained in the F-AnMBR was 38.0 ± 5.9 mg pCOD/L, 0.4 ± 0.9 mg sCOD/L, 9.9 ± 1.3 mg BOD₅/L and <1 mg TSS/L. The microbial diversity of the biomass was also assessed. Bacteroidales and Clostridiales were the major bacterial fermenter orders detected and a relative high abundance of syntrophic bacteria was also detected. Additionally, an elevated abundance of sulfate reducing bacteria (SRB) was also identified and was attributed to the low COD/SO₄ ^2- ratio of the wastewater (0.5). Also, the coexistence of acetoclastic and hydrogenotrophic methanogenesis was suggested. Overall this study demonstrates the suitability of UASB reactors coupled with membrane can achieve a high effluent quality when treating municipal wastewater under psychrophilic temperatures with F-AnMBR promoting slightly higher hydrolysis rates.

Keywords: anaerobic membrane bioreactor; hydrolysis; microbial community; municipal wastewater; psychrophilic temperature; upflow anaerobic sludge blanket.

FIGURE 1

Schematic representation of pilot scale UASB with flocculent biomass (F-UASB), and 2 anaerobic membrane reactors with flocculent and granular biomass (F-AnMBR and G-AnMBR).

FIGURE 2

(A) sCOD (mg/L); (B) sCOD removal (%); (C) pCOD (mg/L); and (D) pCOD removal (%) for F-UASB, F-AnMBR and G-AnMBR.

FIGURE 3

(A) TSS (mg/L); (B) TSS removal (%); and (C) total methane production (mL CH₄/L wastewater) for F-UASB, F-AnMBR, and G-AnMBR.

FIGURE 4

Total suspended solids mass balance. Error bars represent standard deviation. Different subscripts represent the statistically differences using Tukey HSD comparisons (p < 0.05).

FIGURE 5

Percentage of hydrolysis and methanogenesis. Error bars represent standard deviation. Different subscripts represent the statistical differences using Tukey HSD comparisons (p < 0.05).

FIGURE 6

Solids hydrolyzed in the different reactors over specific time intervals. Error bars represent standard deviation. Different subscripts represent the statistically differences using Tukey HSD comparisons (p < 0.05).

FIGURE 7

Principal component analysis (PCA) of (A) Bacteria and (B) Archaea 16S rRNA sequencing profiles for each reactor.

FIGURE 8

Taxonomic assignment of sequencing reads from bacterial community at 85% confidence level (order) (A) and at 95% confidence level (genera) (B). Relative abundance was defined as the number of reads (sequences) affiliated with that taxon divided by the total number of reads per sample. Phylogenetic groups with relative abundance lower than 1% were categorised as “others”.

FIGURE 9

Taxonomic assignment of sequencing reads from archaeal community at 85% confidence level (order) (A) and at 95% confidence level (genera) (B). Relative abundance was defined as the number of reads (sequences) affiliated with that taxon divided by the total number of reads per sample. Phylogenetic groups with relative abundance lower than 1% were categorised as “others”.