Report on Osmotic Membrane Bioreactor

Introduction of Osmotic Membrane Bioreactor

            In general, diminishing supplies of fresh water in arid regions and the rising demand of water in urban centres have promoted a significant and increased interest in direct and indirect reuse of water. Actually, both advanced and conventional treatment processes can be utilised in combination and alone for achieving required quality of water for specific beneficial reutilisation of water. Conventional sludge processes with tertiary treatment have been utilised for reclaiming water for many years for non-potable use. In this paper, the novel technology of osmotic membrane bioreactor will be analysed and its different aspects will be considered (E. R. Cornelissen, Beerendonk, J. J. Qin, Korte, & Kappelhof, 2011). In addition to it, it will be determined whether this technology further requires advancements to be fully utilised in an effective manner or not.

MBR or Membrane Bioreactors

            MBR or membrane bioreactors are a relatively new technology of wastewater treatment that combines ultrafiltration or microfiltration membrane separation with activated sludge process as a clarification substitute. Membrane bioreactors are considered more efficient technologies of treatment for non-potable reuse of water and they also have the capability of serving as pretreatment for direct and indirect use of impaired water. The technology offers a number of benefits over conventional processes of wastewater treatment. These benefits include small physical footprint, automation and ease of operation, consistent quality of water, decreased production of sludge because of high concentration of biomass in the bioreactor, and a high suspended solid rejection which includes pathogenic microorganisms. Generally, for potable reuse, an approach of multiple-barrier treatment is needed for protecting public health.

            Porous membranes might not be suitable or adequate alone for providing this protection due to their limited rejection of trace organic compounds, ions, and viruses. Still, membrane bioreactors can be combined with a DRO or downstream reverse osmosis along with advanced processes of oxidation for complying with more stringent quality regulations of water. A major issue related to the operation of UF or MF membrane bioreactors is referred to as membrane fouling. It normally occurs on the membranes of MBR and might develop on downstream membranes of RO. In particular, DOC or dissolved organic compounds in MBER permeate are capable of causing serve and intense organic fouling on the membranes of RO and offer substrate for the growth of microbes, which might serve to exacerbate biological fouling on the membranes of RO. Productivity is lowered by membrane fouling, operation cost and energy requirements are increased, and water quality might be deteriorated as well (Alturki, et al., 2012).

            For the reduction of fouling and enhancement of small particles and dissolved species, FO or forward osmosis membranes can be used in membrane bioreactors in the configuration of an osmotic membrane bioreactor.

OMBR or Osmotic Membrane Bioreactor

            Actually, OMBR refers to a technology employing multiple barriers and it is suitable for both indirect and direct reuse of water. This method couples a semi-permeable and dense FO membrane with sludge processes utilised for water extraction from sludge with low salinity into a draw and concentrated solution. In comparison with MF and UF membranes utilised in conventional bioreactors, the membranes of FO in OMBR seem to provide the advantage of high rejection of macromolecules particles, and even ions. Membranes of FO have lower propensity of fouling than UF or MF membranes and hence, it needs less cleaning and air scouring. When it comes to its comparison with UF or MF MBR that is followed by a system of RO, the high rejection of inorganic and organic constituents of membrane of FO results in an RO that is influent with higher permeate quality of RO and lower potential of fouling (Wang, Chang, & Tang, 2016).

            In OMBRs, the driving force for flux of water is the difference in the osmotic pressure between activated sludge and draw solution. In general, the daw solution is created with inorganic salts like magnesium chloride or sodium chloride. However, other organic and inorganic salts with the technique at low concentrations can also be utilised as draw solutions. In the FO process, dilution occurs in the draw solution as water seems to diffuse through the membrane from sludge is absorbed into the solution. To the source reservoir, the diluted solution can possibly be returned back or it can be re-concentrated with the use of a secondary separation process for producing draw solution and purified water steams. The re-concentration of draw solution might use membrane, crystallisation, or thermal processes. Usually, in most cases, the re-concentration process of draw solution offers an advanced treatment step, which results in treated water that is suitable and eligible for potable reuse.

            It is important to that one of the limitations of this technique is the consistent solute loss from the draw solution that must be replenished for achieving sustainable and effective operation. Usually, in OMBR and FO applications, solutes tend to reverse diffuse from the developed draw solution into the sludge because of the high difference of concentration across the membrane. In case of OMBRs using RO for re-concentrating the draw solution, solutes are lost in a large volume across the membranes of RO because of the difference in concentration and the RO membrane’s semi-permeable nature. Methods and techniques of reducing the loss of solute and its negative influences are considered quite important.

OMBR Configurations of Osmotic Membrane Bioreactor

            In general, in OMBRs, elimination of inorganic and organic constituents and biological oxidation are accomplished in the bioreactor, and separation of solids is achieved with the use of FO membranes that are semi-permeable. Actually, it is possible to arrange bioreactors in a number of configurations in accordance with the goal of treatment. Membranes’ configuration in association with bioreactors can vary but they are most commonly set or configured as an external skid of membrane or a membrane cassette. When it comes to the configuration of bioreactor, bioreactor’s volume is considered very important because the flow-rates of sludge, influent, and volume seem to dictate and determine the time in which water is properly treated along with the time for which activated sludge is present within the system. The SRT and HRT are considered fundamental parameters of treatment that affect the removal of nitrogen and carbon, and recovery of water in the treatment processes of biologically active wastewater. In bioreactors, the collection of dissolved constituents is quite a unique occurrence in OMBRs. Actually, it adversely influences the processes and it is associated with the ratio of SRT and HRT. However, it can be controlled easily in hybrid systems of OBR (Qiu & Ting, 2013).

            Methods or processes are incorporated by hybrid OMBRs for resolving salts and other particles from bioreactors. Some examples of processes which have been integrated with OMBRs include porous membrane systems of MF and UF that are capable of extracting salts with the MF or UF permeate. Additionally, a method of maintaining low salinity of bioreactor that has been considered is to utilise a decant step, involving sludge settling and clear supernatant draining from the system.

Bioreactors of Osmotic Membrane Bioreactor

            Most of the studies associated with OMBR have been performed with the use of an individual aerates bioreactor that is combined with either a frame and plate cassette and a cross-flow membrane cell or a membrane bundle of hollow-fibre. The design is simplified by an individual reactor. However, it might not be the best for consistent treatment due to the fact that nitrate and ammonia available in the sludge diffuse across the membranes of FO into the draw solution and might compromise the quality of final product, in accordance with the re-concentration process of draw solution considered. The concentration of species of nitrogen can be decreased in the design of a single reactor through alternation between aerobic and anoxic conditions but it is quite tough to evade nitrogen’s accumulation in the draw solution if there is continuous extraction of water from the sludge through the membrane of FO in an individual reactor.

            Generally, an alternative to the design of individual reactor that could prevent the nitrogen species’ accumulation into the solution is concerned with treating the wastewater biologically for the removal of nutrient and carbon in a batch reactor with membranes of FO submerged in a different tank for filtration. Usually, in this operating scheme and configuration, complete de-nitrification and nitrification can be achieved in a possible batch reactor with changing or alternating aerobic and anoxic phases before all water is extracted from the sludge through the membranes of FO. It would not be wrong to say that this design of batch-reactor is quite an effective and large upgrade over the configurations of individual reactor, which can draw contamination in solution. Another typical arrange of bioreactor for the removal of nitrogen include de-nitrification’s anoxic zone followed by nitrification’s anoxic zone. For treatment of municipal wastewater, this arrangement is effective because for de-nitrification, no addition of chemical carbon is required. In addition to it, aeration requirements are decreased because organic carbon load’s portion is eliminated during the process of de-nitrification. It is, however, important to note that this configuration of bioreactor has to be re-designed for systems of OMBR because there will be a continuous diffusion of nitrate into the draw solution if membranes of FO extract water from the initial stage of nitrification (Holloway, Achilli, & Cath, 2015).

Conclusion and Advancement of Osmotic Membrane Bioreactor

            From the above analysis, it can be said that the OMBR is quite an effective technology of treatment for direct or indirect pot reuse of water and recovery of nutrient because of various treatment barriers which are inherent to its specifications and design. In spite of the obvious benefits of this method for rejecting and removing TOrCs and nutrients, there are still a number of challenges that need to be overcome before it can be a commercial and viable technology of water treatment. It is important for future studies and researches to concentrate on further investigating and studying mass transport in the technique, developing new modules of membrane, generation evolution of the technique, and recovery of nutrients along with hybrid configurations. Thus, advancements should focus on:

·         Creating design parameters of OMBR like suitable spacer design and the aeration demand for configurations.

·         Standardising the operating conditions of OMBR like the HRT and SRT for minimising the accumulation of salt but taking benefits of high operating conditions of SRT that have seemingly become the hallmark of conventional MBRs of UF and MF utilised in water treatment at full-scale.

·         Determining salinity build-up’s effect on the community of microbes during the treatment of OMBR. In addition, characterising this community in an OMBR and how it seemingly might different low-salinity processes of sludge treatment.

·         Developing the strategies aimed at cleaning membrane that enable clean in-place for the reduction of system down time and chemical costs.

·         Producing and designing polyamide TFC membranes of FO with reverse salt and reasonable water flux characteristics, and that are chemically stable and durable for withstanding the effects of working in a microbial and chemical aggressive environment.

·         Testing and selecting organic solutions which are capable of being biodegraded for preventing the accumulation of salt in the bioreactor.

References of Osmotic Membrane Bioreactor

Alturki, A., McDonald, J., Khan, S. J., Hai, F. I., Price, W. E., & Nghiem, L. D. (2012). Performance of a novel osmotic membrane bioreactor (OMBR) system: flux stability and removal of trace organics. Bioresource technology, 113, 201-206.

E. R. Cornelissen, D. H., Beerendonk, E. F., J. J. Qin, H. O., Korte, K. F., & Kappelhof, J. W. (2011). The innovative osmotic membrane bioreactor (OMBR) for reuse of wastewater. Water Science and Technology, 63(8), 1557-1565.

Holloway, R. W., Achilli, A., & Cath, T. Y. (2015). The osmotic membrane bioreactor: a critical review. Environmental Science: Water Research & Technology, 1(5), 581-605.

Qiu, G., & Ting, Y.-P. (2013). Osmotic membrane bioreactor for wastewater treatment and the effect of salt accumulation on system performance and microbial community dynamics. Bioresource technology, 150, 287-297.

Wang, X., Chang, V. W., & Tang, C. Y. (2016). Osmotic membrane bioreactor (OMBR) technology for wastewater treatment and reclamation: Advances, challenges, and prospects for the future. Journal of membrane science, 504, 113-132.