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

Abstract

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.

Nitrate Removal and Dynamics of Microbial Community of A Hydrogen-Based Membrane Biofilm Reactor at Diverse Nitrate Loadings and Distances from Hydrogen Supply End

“…The gas-based MBfR is usually operated with either open-end or close-end HFMs. In the case of open-end operation, due to the markedly higher gas velocity of advective transport in the intramembrane than the diffusive transfer across the walls of HFMs, the intramembrane gas was uniformly distributed with an elevated concentration level, which enables the high microbial activity in the biofilm along the HFMs [12]. Nonetheless, in addition to the massive loss of gas, the open-end operation is inapplicable to H 2 -based MBfR as it creates an explosive atmosphere.…”

“…Alternatively, the H 2 -based MBfR is extensively equipped with close-end HFMs, on account of electron donor (i.e., H 2 ) saving and operational safety [13][14][15]. Unfortunately, close-end HFMs always suffer from the back-diffusion of inactive gases such as nitrogenous and water vapor gases, especially the N 2 from bulk liquid and hydrogenotrophic denitrification process, which may severely reduce the overall HFM efficiency in terms of H 2 delivery [8,12,16]. According to a previous model-predicted result, the gas transfer rate of HFMs operated at the open-end mode was obviously greater than that of the close-end operation, leading to an approximately 116% increase in the contaminant removal flux of the system [12].…”

“…As displayed in Figure 1, in principle, as a result of the continuous H 2 consumption by the biofilm along the HFMs, accompanied by the back-diffusion of inactive gases (mainly N 2 ) from the biofilm and bulk liquid into the intramembrane as well as the flow of gases toward the distal end of HFMs, the partial pressure of H 2 and N 2 gradually increases and decreases with the increase in distance from the H 2 supply end, respectively; the diffusion of N 2 from the intramembrane into the biofilm is due to its concentration at the end of the HFMs [5]. Until now, to the best of our knowledge, only one single research exists that covers the effects of gas back-diffusion on the gas profiles of biofilm, the gas transfer rate of membrane, and the pollutant removal efficiency of the gas-based membrane biofilm reactor system [12]. No study has been published to give insights into the impacts of gas back-diffusion on the dynamics of a microbial community in H 2 -based MBfR.…”

Hydrogenotrophic Microbial Reduction of Oxyanions With the Membrane Biofilm Reactor

“…When H 2 gas is supplied via hollow-fiber membranes, other dissolved gases in the bulk liquid or gases formed within the biofilm can diffuse into the membrane, diluting the H 2 gas. When operating an MBfR with membranes sealed on one end, these gases concentrate at the distal end of the membrane, possibly decreasing their effectiveness for H 2 transfer and leading to thinner biofilms (Ahmed et al, 2004; Perez-Calleja et al, 2017).…”

“…A better solution is to periodically vent the membrane. By opening the sealed end for a few seconds every few minutes, the H 2 level can be re-established within the membrane with minimal waste (Perez-Calleja et al, 2017).…”