Mass transfer enhancement and improved nitrification in MABR through specific membrane configuration.

One of the main energy consumptions in wastewater treatment plants (WWTPs) is due to the oxygenation of aerobic biological processes. In order to approach to an energy self-sufficient scenario in WWTPs, Membrane Aerated Biofilm Reactors (MABRs) provide a good opportunity to reduce the impact of aeration on the global energy balance. However, mass transfer limitations derived from poor flow distribution must be tackled to take advantage of this technology. In this work, in order to improve mass transfer between biofilm and bulk water, a specific configuration was developed and studied at laboratory scale, aimed at compactness, energy efficiency and high nitrification rates. Nitrification rates were higher in the innovative configuration than in the conventional one, achieving a Volumetric Nitrification Rate (VNR) as high as 575.84 g NH4-N m-3 d-1, which is comparable with confirmed technologies. Regarding energy consumption due to aeration, a reduction of 83.7% was reached in comparison with aeration through diffusers with the same Oxygen Transfer Efficiency (OTE). These results highlight the importance of hydrodynamic conditions and the membranes configuration on treatment performance.

Periodic venting of MABR lumen allows high removal rates and high gas-transfer efficiencies.

The membrane-aerated biofilm reactor (MABR) is a novel treatment technology that employs gas-supplying membranes to deliver oxygen directly to a biofilm growing on the membrane surface. When operated with closed-end membranes, the MABR provides 100-percent oxygen transfer efficiencies (OTE), resulting in significant energy savings. However, closed-end MABRs are more sensitive to back-diffusion of inert gases, such as nitrogen. Back-diffusion reduces the average oxygen transfer rates (OTR), consequently decreasing the average contaminant removal fluxes (J). We hypothesized that venting the membrane lumen periodically would increase the OTR and J. Using an experimental flow cell and mathematical modeling, we showed that back-diffusion gas profiles developed over relatively long timescales. Thus, very short ventings could re-establish uniform gas profiles for relatively long time periods. Using modeling, we systematically explored the effect of the venting interval (time between ventings). At moderate venting intervals, opening the membrane for 20 s every 30 min, the venting significantly increased the average OTR and J without substantially impacting the OTEs. When the interval was short enough, in this case shorter than 20 min, the OTR was actually higher than for continuous open-end operation. Our results show that periodic venting is a promising strategy to combine the advantages of open-end and closed end operation, maximizing both the OTR and OTE.

Comparison between a fixed bed hybrid membrane bioreactor and a conventional membrane bioreactor for municipal wastewater treatment: a pilot-scale study.

A hybrid membrane bioreactor (HMBR) was developed, by adding biofilm support media into a conventional membrane bioreactor (CMBR), and operated in parallel with a CMBR. Results showed that effluent quality was significantly better with the HMBR. The removal efficiencies of COD, BOD5, NH4(+)-N and TN with the HMBR were 84%, 98%, 97% and 75%, respectively, as compared to 80%, 96%, 93% and 38% with the CMBR. There were no differences in phosphorus removal. The membrane fouling rate in the HMBR was on average only 57% of that in the CMBR. The lower concentration of colloidal biopolymer clusters in the HMBR sludge, probably due to their retention by the biofilm, could be partially responsible for this difference. Filterability and settleability of the sludge were also better in the HMBR. Consequently, it is concluded that the addition of fixed support media for biofilm growth can improve the performance of CMBRs.

Evaluation of a hybrid vertical membrane bioreactor (HVMBR) for wastewater treatment.

A new hybrid membrane bioreactor (HMBR) has been developed to obtain a compact module, with a small footprint and low requirement for aeration. The aim of this research was to assess its performance. The system consists of a single vertical reactor with a filtration membrane unit and, above this, a sponge fixed bed as support medium. The aeration system is located under the membrane unit, allowing for membrane cleaning, oxygenation, biofilm thickness control and bulk liquid mixing. Operated under continuous aeration, a bench-scale reactor (70 L) was fed with pre-treated, raw (unsettled) municipal wastewater. BOD(5) and suspended solids removal efficiencies (96 and 99% respectively) were comparable to those obtained with other membrane bioreactors (MBRs). Total nitrogen removal efficiencies of 80% were achieved, which is better than those obtained in other HMBRs and similar to the values reached using more complex MBRs with extra anoxic tanks, intermittent aeration or internal deflectors.