Kinetic characterization of acetate utilization and response of microbial population in super fast membrane bioreactor

The study investigated the functional relationships between changes in microbial community induced by different sludge ages (SRT) selected for super fast MBR operation on variable process kinetics, fate of soluble microbial products (SMPs) and system performance. Acetate, a simple and separately identifiable compound, was selected as the sole organic carbon source. MBR operation was monitored at steady-state at extremely low SRT levels of 2.0, 1.0 and 0.5 d and a hydraulic retention time of 1.0 h. Batch experiments were conducted for kinetic evaluation of MBR performance by model evaluation of oxygen uptake rate, polyhydroxybutyric acid and COD profiles. Bacterial DGGE profiles indicated that SRT exerted shifts in the composition of microbial community. Model evaluation of experimental data also confirmed molecular analyses, indicating variable growth kinetics for utilization of acetate and identified high and variable endogenous respiration as a function of SRT. Changes in the microbial community and process kinetics did not affect MBR performance, providing complete depletion of available acetate. Effluent COD remained below 17 mg/L and entirely composed of SMPs. Particle size analysis revealed a bimodal distribution of the wide spectrum of SMPs above 13 nm and below 2 nm. Effective filtration size of the membrane was reduced to 8 nm due to cake filtration effect. The retained fraction of SMPs was in the range of 1–2 mg/L, but accumulated in the reactor volume, same way as biomass. Therefore, the real merit of super fast MBR was the very low levels of SMP generation (14–18 mg COD/L), rather than the effective capture of accumulated COD due to cake filtration.