Osmotic MBR articles − ten research papers

Below is a selection of abstracts from ten recent papers published in peer-reviewed journals relating to osmotic membrane bioreactor (OMBR) technology, provided by various research groups from the start of 2017. No reviews of OMBRs seem to have been published in that time, but there have been plenty of papers in the following subject areas:

  • aerobic and anaerobic OMBRs, with the latter including a study of draw solution concentration
  • salinity impacts, the increased salt concentration in the biomass being an inevitable consequence of OMBR operation, and
  • a comparison with nanofiltration (i.e. an NFMBR).

There’s also been a few studies of fertilizer-drawn OMBRs where no draw solution recovery is needed, and an example of one such study is included. These pieces of work have all been conducted on a small scale, though, as there are now commercial FO modules available as hollow fibres, it seems likely that pilot-scale OMBR studies will have been conducted by now. The list below is presented in reverse chronological order and no preference is indicated. This list was compiled in May 2020, and may be updated in future as new papers are published.

Ten Osmotic MBR research abstracts

'Sewage can become a valuable source if its treatment is re-oriented for recovery. An anaerobic forward osmosis membrane bioreactor (AnOMBR) was developed for real municipal sewage treatment to investigate performance, biogas production, flux change and mixed liquor characteristics. The AnOMBR had a good treatment capacity with removal ratio of chemical oxygen demand, ammonia nitrogen, total nitrogen and total phosphorus more than 96%, 88%, 89% and almost 100%. Although high DS concentration increased the initial flux, it caused rapid decline and poor recoverability of FO membrane flux. Low DS concentration led to too long hydraulic retention time, thus resulting in a low reactor efficiency. Additionally, it was observed that salt, protein, polysaccharide and humic acid were all accumulated in the reactor, which was not conducive to stable long-term operation. Based on the characteristics of membrane fouling, salt accumulation and AnOMBR performance, the optimal DS of 1 M NaCl solution was selected.' © 2020 Elsevier Ltd

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'The economic feasibility of combining forward osmosis (FO), reverse osmosis (RO) and anaerobic membrane bioreactor (AnMBR) technologies for municipal wastewater treatment with energy and water production was analysed. FO was used to pre-concentrate the AnMBR influent, RO for draw solution regeneration and water production, and AnMBR for wastewater treatment and energy production. The minimum wastewater treatment cost was estimated at 0.81 € m−3, achieved when limiting the FO recovery to 50% in a closed-loop scheme. However, the cost increased to 1.01 and 1.27 € m−3 for FO recoveries of 80% and 90%, respectively. The fresh water production cost was estimated at 0.80 and 1.16 € m−3 for an open-loop scheme maximising water production and a closed-loop scheme, respectively. The low FO membrane fluxes were identified as a limiting factor and a sensitivity analysis revealed that FO membrane fluxes of 10 LMH would significantly improve the competitiveness of FO-RO + AnMBR technology.' © 2020 Elsevier Ltd.

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'The application of the forward osmosis membrane bioreactor (FO-MBR) requires further investigation in the area of process optimization. In current study, different operational parameters for improving the overall efficiency of FO-MBR were studied and optimized in batch and semi-continuous experiments. The effect of osmotic backwashing, cross-flow velocity of the feed solution (activated sludge), and particle size were studied on the FO-MBR flux. All batch experiments were of 8 h duration, comprising 7 h filtration and 1 h osmotic backwash. It was found that osmotic backwashing was ineffective for complete flux recovery in a hollow fibre FO-MBR, due to severity of the fouling caused by the activated sludge feed in ‘active layer facing draw solution’ mode.

A slightly better recovery was observed however with a single 60 min backwash as compared to 2 backwash cycles of 30 min each or 3 backwash cycles of 20 min each. A cross-flow velocity of 0.105 m/s proved optimum. Higher velocities caused a breakdown of flocs and reduced the mean particle size, the latter resulting in lower water flux. Similarly, lower cross flow velocities were not able to provide effective membrane scouring, resulting in lower water flux. The particle size decreased with operational time in the FO-MBR. It was also established that, above the optimum cross flow velocity, flux is inversely proportional to the difference between the initial and final particle size of sludge.' © 2019 Elsevier Ltd.

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'Nanofiltration (NF) and commercial HTI Forward Osmosis (FO) membranes were tested in a live membrane bioreactor (MBR) for performance comparison. Inorganic draws (NaCl, Na3PO4), surfactants (TEAB, SDS), and polyelectrolytes (PDAC, PGBE) were all tested as draw solutes; water flux, reverse solute transport (RST), toxicity and viscosity were observed and compared for each. Synthetic municipal wastewater was selected as a feed and Bacillus Subtilis species was inoculated in the solution and grown overnight for development of a monoculture bioreactor. Deionized (DI) water was also used as a feed for control. The results indicated that the NF membrane showed good promise in the FOMBR process when used with high molecular weight draw solutions. In comparison to the FO membrane, it showed a significantly higher water flux with only a slightly higher RST. The NF membrane is recommended for further investigation in the FOMBR, provided fouling control is in place. Although the observed reverse solute transport values were not toxic to the bacteria under observation, long-term accumulation of draw solute in the bioreactor of an FOMBR wastewater treatment plant is an issue which requires further investigation.' © 2019 Elsevier Ltd.

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'Tannery wastewaters are difficult to treat biologically due to the high salinity and organic matter concentration. Conventional treatments, like sequential batch reactors (SBR) and membrane bioreactors (MBR), have showed settling problems, in the case of SBR, and ultrafiltration (UF) membrane fouling in the case of MBR, slowing their industrial application. In this work, the treatment of tannery wastewater with an osmotic membrane bioreactor (OMBR) is assessed. Forward osmosis (FO) membranes are characterized by a much lower fouling degree than UF membranes. The permeate passes through the membrane pores (practically only water by the high membrane rejection) from the feed solution to the draw solution, which is also an industrial wastewater (ammonia absorption effluent) in this work.

Experiments were carried out at laboratory scale with a FO CTA-NW membrane from Hydration Technology Innovations (HTI). Tannery wastewater was treated by means of an OMBR using as DS an actual industrial wastewater mainly consisting of ammonium sulphate. The monitoring of the biological process was carried out with biological indicators like microbial hydrolytic enzymatic activities, dissolved and total adenosine triphosphate (ATP) in the mixed liquor and microbial population. Results indicated a limiting conductivity in the reactor of 35 mS cm−1 (on the 43th operation day), from which process was deteriorated. This process performance diminution was associated by a high decrease of the dehydrogenase activity and a sudden increase of the protease and lipase activities. The increase of the bacterial stress index also described appropriately the process performance. Regarding the relative abundance of bacterial phylotypes, 37 phyla were identified in the biomass. Proteobacteria were the most abundant (varying the relative abundance between 50.29% and 34.78%) during the first 34 days of operation. From this day on, Bacteroidetes were detected in a greater extent varying the relative abundance of this phylum between 27.20% and 40.45%.' © 2018 Elsevier Ltd.

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'Forward osmosis (FO) technology that has been applied to a membrane bioreactor (MBR) is referred to as osmotic MBR (OMBR). The new concept developed in this study aims at retrofitting (partly or fully) existing MBR into OMBR, thus limiting the investment costs, allowing for more flexible and combined operation of MBR and OMBR and to fulfil water quality needs. Submerged FO modules were developed by modifying Kubota microfiltration (MF) modules commonly used in MBR and fitting them with new generation thin-film composite FO membranes. The similar design and water fluxes of both MF and FO modules allowed for an unbiased comparison of OMBR and MBR. For the first time, stable OMBR operation with water fluxes above 10 L.m−2.h−1 was achieved using synthetic seawater as draw solution.

The proof of concept of retrofitting of MBR and OMBR was demonstrated using a 50 L reactor under varying operating conditions. Findings indicated that standalone OMBR operation is challenging due to salinity build-up which impacts bacterial activity and permeation flux. Fouling occurred in the FO modules to the same degree as MF but osmotic backwashing proved to be an efficient cleaning solution for OMBR. The energy reduction benefit of using an osmotic gradient instead of hydraulic pressure to drive permeation was outweighed by the energy required for draw circulation in OMBR. Loss of selectivity of the FO membranes was observed due to superficial (active layer) physical damage. However, the overall high rejection of trace organic contaminants by OMBR (>90%), due to high FO membrane rejection and biological degradation, is a great benefit for OMBR implementation in water reuse schemes.' © 2018 Elsevier B.V.

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'Osmotic membrane bioreactor (OMBR) is an emerging technology with great potentials for wastewater treatment and reclamation. However, the contradiction between the alleviation of salinity build-up and the achievement of a high biomass concentration remains a main obstacle. Here, we attempt to simultaneously achieve a low salinity environment and a high biomass concentration in OMBRs through integrating a microfiltration (MF) membrane system in the OMBR and operating it at a longer sludge retention time (SRT) for controlling the salt accumulation and extending the retention of activated sludge, respectively. The results indicated that the salinity in terms of conductivity in the OMBR assisted with the MF membrane (MF-OMBR) was successfully kept at a low range of 2.5–4.0 mS/cm.

In the meanwhile, a high concentration of mixed liquor suspended solid (MLSS) in the range of 8.09 ± 1.02 g/L was achieved in the MF-OMBR through operating at a longer SRT of 30 d. A high MLSS concentration enhanced the removals of ammonia nitrogen (NH4+-N) CHECK and total organic carbon (TOC) in the mixed liquors, and consequently increased the water quality of MF and forward osmosis (FO) permeates. In addition, the high MLSS concentration strengthened the removal of some trace organic chemicals (TrOCs) such as Trimethoprim by the activated sludge in the MF-OMBR system. Moreover, a high MLSS concentration did not induce severe FO membrane fouling compared with a low MLSS concentration in OMBRs, which might be due to a low fouling tendency of FO membrane operating at a low water flux.' © 2018 Elsevier B.V.

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'In this study, the feasibility of utilizing an anaerobic osmotic membrane bioreactor (OMBR) for the treatment of a refractory acid dye, Lanaset red G.GR, is demonstrated. The experimental results show that an increased sludge concentration and reversed salt accumulation exacerbate membrane fouling, which leads to flux decline. The excellent rejection performance of the forward osmosis (FO) membrane and salt accumulation could lead to a reduction of microbial activity and an increase in soluble microbial product (SMP) and extracellular polymeric substance (EPS) contents. These consequences will affect the OMBR performance. Moreover, the FO membrane demonstrated a limited rejection of aniline-type intermediates. These overall findings suggest that the OMBR process is a good option for the treatment of dyeing wastewater. Further improvements on the membrane materials and membrane surface properties to alleviate fouling, salt reverse osmosis as well as the remaining color issues are still necessary before practical application becomes possible.' © 2018 The Royal Society of Chemistry.

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'Anaerobic osmotic membrane bioreactor (AnOMBR) has aroused growing interests for its low energy demand, ability to efficiently process low ionic strength wastewater and high effluent quality. However, salt accumulation remains a main obstacle for causing severe water flux decline, fouling aggravation and inhibitory on the microbial activity. Here, we report a novel microfiltration (MF) assisted AnOMBR (AnMF-OMBR) for mitigating salt accumulation. The results indicated that the MF membrane effectively prevented salt accumulation in the bioreactor. The stable salinity level (within the range of 2.5–4.0 mS/cm) enabled the AnMF-OMBR to achieve a long-term continuous operation together with a higher methane production in comparison with a conventional AnOMBR.

The forward osmosis (FO) permeate from the AnMF-OMBR had excellent water quality, while the MF permeate required further treatment (e.g., phosphorus precipitation and activated carbon adsorption) before its beneficial reuse. A thick fouling layer combining biofouling and inorganic scaling was existed on the FO membrane. Further confocal laser scanning microscopy (CLSM) revealed the dominance of polysaccharides and microorganisms over proteins. The current study demonstrated that the AnMF-OMBR can be a promising and sustainable wastewater treatment technology for its simultaneous energy recovery (in the form of biogas) and water reuse (from both FO and MF membranes).' © 2017 Elsevier B.V.

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'In this study, the behavior of organic micro-pollutants (OMPs) transport including membrane fouling was assessed in fertilizer-drawn forward osmosis (FDFO) during treatment of the anaerobic membrane bioreactor (AnMBR) effluent. The flux decline was negligible when the FO membrane was oriented with active layer facing feed solution (AL-FS) while severe flux decline was observed with active layer facing draw solution (AL-DS) with di-ammonium phosphate (DAP) fertilizer as DS due to struvite scaling inside the membrane support layer. DAP DS however exhibited the lowest OMPs forward flux or higher OMPs rejection rate compared to other two fertilizers (i.e., mono-ammonium phosphate (MAP) and KCl). MAP and KCl fertilizer DS had higher water fluxes that induced higher external concentration polarization (ECP) and enhanced OMPs flux through the FO membrane.

Under the AL-DS mode of membrane orientation, OMPs transport was further increased with MAP and KCl as DS due to enhanced concentrative internal concentration polarization while with DAP the internal scaling enhanced mass transfer resistance thereby lowering OMPs flux. Physical or hydraulic cleaning could successfully recover water flux for FO membranes operated under the AL-FS mode but only partial flux recovery was observed for membranes operated under AL-DS mode because of internal scaling and fouling in the support layer. Osmotic backwashing could however significantly improve the cleaning efficiency.' © 2017 Elsevier B.V.

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