Impact of decreasing hydraulic retention times on the specific affinity of methanogens and their community structures in an anaerobic membrane bioreactor process treating low strength wastewater.

Maximum specific growth rate (μ) and substrate saturation constant (K) are widely used in determining the growth of microorganisms. The ratio (μK), also referred to as specific affinity, a, is a better parameter to assess the advantage in competition for substrates by bridging microbial growth and the kinetics of enzymatic substrate uptake, but is not well studied. This study investigated the effect of hydraulic retention time (HRT) on the a of anaerobic sludge from an anaerobic membrane bioreactor (AnMBR), associated microbial communities and the overall wastewater treatment performance. The AnMBR was fed with acetate wastewater (~500 mg COD/L) and operated at fixed solids retention time (45 d) while HRT continued to decrease. There was no significant difference in K (ranging from 170 to 243 mg COD/L) at different HRTs. However, a increased from (4.0 ± 0.2) × 10 to (4.9 ± 0.2) × 10 and to (6.5 ± 0.1) × 10 L/mg COD/d as HRT decreased from 24 h to 12 h and further to 6 h, respectively. This was accompanied by the increase in acetoclastic methanogens (mainly Methanosaeta) from 3.85 × 10, 8.82 × 10 to 1.05 × 10 cells/L, respectively. The fraction of Methanosaeta in the anaerobic biomass increased from 33.67% to 61.08% as HRT decreased from 24 h to 6 h. Correspondingly, effluent quality was improved, as evidenced from the COD concentrations of 32 ± 6, 21 ± 4, and 13 ± 5 mg/L at the HRTs of 24 h, 12 h, and 6 h, respectively. The results confirm that microorganisms are able to adapt to growth conditions by adjusting their kinetic properties and suggest that short HRTs in the AnMBR favor the growth and accumulation of Methanosaeta with high specific affinity likely because they can compete for acetate at low concentrations by increasing substrate uptake rate and thus specific microbial growth rate.