Table of Contents
1 MEMBRANE BIOREACTORS
1.1 Introduction
The exponential growth of the world population, the increase in resource use and waste production and the demand for more goods of higher quality all produce an urgent need for new, clean and safe technologies and production processes. The use of efficient, selective, precise, flexible technologies, able to operate in mild conditions, such as membrane technologies, will strongly contribute to sustainable development.
In particular, membrane bioreactors have the potential to play a significant role in preventing pollution and allowing resource recovery, chemical purification and energy savings. They are intensified systems in which a biochemical conversion is coupled with a membrane separation process in a single unit operation.
The catalytic action of biocatalysts is extremely efficient and selective compared to conventional chemical catalysts. They can demonstrate higher reaction rates, milder reaction conditions and greater stereospecificity. Therefore, they offer unparalleled possibilities for the operation of precise (bio)chemical transformation without byproduct formation. Most of these properties come from the high molecular flexibility that biocatalysts exhibit.
Compared to more conventional technologies, membrane bioreactors have several intrinsic advantages that make them a sustainable alternative technology [1], such as the possibility of reusing the biocatalyst in a continuous system (which contributes to increasing productivity and probably the economic viability of the process), the continuous selective removal of products from the reaction media and the controlled supply of the reagent to the catalytic reaction environment.
Membrane bioreactors also contribute to greener chemistry by controlling the mass transfer of reactants and products, enhancing overall yields and conversions, minimising solvent use, and maximising mass utilisation. Some of the most critical problems related to alternative energy and clean water production, together with the increasing demand for natural biofunctional products, can find their solution in the use of membrane bioreactors. These can be used in the production of:
- Biodiesel made from natural, renewable sources, such as vegetable oil.
- Pure water, which can be recycled from municipal or industrial wastewater for reuse in a variety of places.
- High-added-value molecules, including from vegetal waste materials.
- Optically pure enantiomers.
The unique properties of membrane bioreactors in terms of process conditions and product quality promote their use as clean production systems of high-quality products that neither create nor discharge waste – hazardous or nonhazardous – to the environment.
Figure 1. A schematic of a simple MBR. Source: https://www.sciencedirect.com/topics/engineering/membrane-bioreactors
Membrane bioreactors also contribute to greener chemistry by controlling the mass transfer of reactants and products, enhancing overall yields and conversions, minimising solvent use, and maximising mass utilisation. Some of the most critical problems related to alternative energy and clean water production, together with the increasing demand for natural biofunctional products, can find their solution in the use of membrane bioreactors. These can be used in the production of:
- Biodiesel made from natural, renewable sources, such as vegetable oil.
- Pure water, which can be recycled from municipal or industrial wastewater for reuse in a variety of places.
- High-added-value molecules, including from vegetal waste materials.
- Optically pure enantiomers.
The unique properties of membrane bioreactors in terms of process conditions and product quality promote their use as clean production systems of high-quality products that neither create nor discharge waste – hazardous or nonhazardous – to the environment.
1.2 The Membrane’s Role
The membrane’s primary role in an MBR system is often to isolate valuable products in situ from unreacted raw materials and biocatalysts. This has many potential advantages. If the membrane successfully separates the produced chemicals, one then avoids additional downstream separation steps.
Synthesis of high-purity pharmaceutical and food product membranes may offer advantages over the more conventional separation techniques (distillation, evaporation, crystallisation, etc.). This is because they are more straightforward, less energy-intensive, and can operate under the calm environment required to maintain enzymatic or biological activity for synthesising heat-sensitive biochemicals.
Membranes can be tailor-made (with respect to their molecular sieving properties and surface specificity through the use of immobilised, ion-exchange or affinity ligands) to be able to separate the products from the biocatalysts. This is important, mainly when pathogenic microorganisms or enzymes are used for bioconversion. Finally, the membrane can independently adjust the residence times in the reactor of products, reactants, and biocatalysts to improve operational flexibility and provide effective process control.
How membranes (in various forms, i.e., cylindrical, coaxial, flat-sheet, spiral wound, hollow fibre, etc.) coupled with the bioreactor depends on the membrane’s role. As with catalytic membrane reactors, the simplest configuration consists of two separate but coupled units: the bioreactor and the membrane module. The biocatalyst (e.g., bacteria, yeasts, enzymes, mammalian cells) could, in this case, be suspended in a solution and continuously circulated through the membrane unit.
The alternate layout involves coupling the membrane and bioreactor into the same assembly. In this configuration, the biocatalyst may be immobilised on the membrane’s surface or within its pore structure. The first configuration remains the most popular one. Troubles surround the second configuration, coupling the membrane and bioreactor in the same unit, related to membrane biofouling, especially for whole-cell conversion applications such as fermentation [2].
Additionally, the substrate and/or products need to be carried through the membrane when the biocatalyst is positioned inside the membrane structure. This typically creates additional mass-transport resistances, which may affect the performance of these bioreaction systems. In the most elemental MBR application, the membrane splits the products or metabolites while keeping the biocatalysts, which are then recirculated back to the reactor.
1.3 Process Description
Membrane bioreactors offer a unique opportunity to restrict the physical space of a biocatalyst, which can be a plant/animal cell, a microorganism, or an enzyme. Restricting the physical space of a catalyst, its continuous use, or reuse makes it possible to avoid loss by washing out from the reactor. This feature represents a huge advantage considering the cost of the biocatalyst (or the time and work necessary to reload a reactor) and the fact that the outlet stream will be free of biocatalyst, making the process of product recovery significantly more manageable.
It becomes possible to operate the bioreactor by uncoupling the biocatalyst residence time from the hydraulic residence time [3]. This is particularly important when using microorganisms or cells with relatively slow growth kinetics. When dealing with microbial or cell bioreactors, the adjustment of the biocatalyst residence time (commonly designated as solids residence time, SRT) makes it possible to increase its concentration and control it at a desirable level. This control can be easily implemented by establishing a microbial/cell purge.
There are several ways to restrict the physical space of a biocatalyst. When dealing with enzymes (biocatalyst that does not duplicate), it is common to immobilise them to a membrane. It may also be achieved by entrapment within the porous structure of a supporting membrane. In this case, the biocatalyst is forced through the pores and imprisoned within the void-free space of the membrane.
If asymmetric ultrafiltration membranes are used, this physical confinement may be somewhat effective and long operation periods may be attained without a significant biocatalyst loss to the contacting fluid phase(s). Alternatively, the biocatalyst may be allowed to move freely in suspension— mass transfer limitations may be easily overcome—and a micro-/ultra-filtration membrane is used to avoid its permeation and loss from the reactor. The membrane acts as a permselective barrier that allows products to permeate but retains the biocatalyst in a reduced physical space. In this case, the biocatalyst may be an enzyme, a microorganism, or a eukaryotic cell, active and able to duplicate.
Figure 2. MBR with submerged ultrafiltration module in a plastic tank (membrane: pore diameter ~ 38 nm, filter surface ~ 3.5 m²). Source: “File:Membrane bioreactor (MBR) (5729650689).jpg” by SuSanA Secretariat is licensed under CC BY 2.0.
This type of membrane bioreactor usually assumes two possible configurations: 1—an external loop recycle reactor, or 2—a submerged membrane reactor. In the outer loop recycle reactor, the membrane unit (usually a micro- or an ultrafiltration membrane) is external to the reaction vessel (also known as “side-stream MBR”); the retentate containing the biocatalyst is recycled to the reactor, while the permeate is recovered free of the biocatalyst.
The submerged membrane bioreactors were first proposed in 1988 by Yamamoto and coworkers [4]. This concept involves the direct immersion of the membrane in the bioreactor medium. The permeate is sucked through the membrane—both flat-sheet and hollow fibre configurations are commercially available—and removed from the reactor, fed continuously and operated at constant volume. This configuration is particularly successful for aerobic treatment of domestic wastewaters because aeration assures simultaneously several tasks: provides oxygen to the microbial culture, keeps solids in suspension, improves external mass transfer conditions, and scours the membrane surface, minimising fouling.
One of the most exciting aspects of submerged membrane bioreactors is the fact that the membrane is operated under very gentle transmembrane pressure, imposing a controlled and constant permeate flux to adjust the convective transport of solutes and particulates to the membrane surface. Typically, these systems are operated under subcritical or sustainable permeate flux conditions, allowing for prolonged operation without the need for membrane cleaning.
2 APPLICATIONS
2.1 Membrane Bioreactors for Biochemical Production
Since the 1970s, the idea of coupling response with membrane separation has been used to describe biological processes. MBRs have undergone substantial research, and currently, they are widely used in industry. The widespread use of membranes in the food and pharmaceutical industries naturally led to the creation of MBRs. The dairy industry, in particular, has been a pioneer in the use of microfiltration, ultrafiltration, nanofiltration, and reverse osmosis separators.
Applications include processing various natural fluids (blood, milk, fruit juices, etc.), separating whey fractions, including lactose, minerals, proteins, and fats, and the concentration of proteins from milk. These processes are usually carried out using commercial membranes under low pressure and temperature settings.
Figure 3. The Marl Chemical Park is one of the largest chemical sites in Germany and constitutes an example of industrial-scale enzymatic membrane bioreactors in the pharmaceutical field. Source: https://corporate.evonik.com/en/company/locations/europe/germany/marl0
MBR is finding fertile ground for application in biochemical synthesis [5] to produce a broad spectrum of products. These range from liquid fuels (e.g., ethanol), food, and plant metabolites, to fine chemicals, including fragrances, medical products, flavouring agents, food colours, etc.
In the pharmaceutical sector, synthetic biochemical processes are crucial because they enable the creation of complex compounds, such as hormones, which cannot be done safely and effectively using more traditional methods.
Combining biological reactions with membrane separation offers prospective benefits in the field of biochemical synthesis. For instance, biological reactions frequently result in a complicated product combination. However, only a tiny percentage of these compounds are useful; in fact, many often turn out to impair biological or enzymatic activity or even be poisonous. Removing these components in situ through the membrane prolongs biocatalyst life and increases the product turnover rate.
2.2 Membrane Bioreactors for Wastewater Treatment
Nitrogen flow to water bodies has many adverse effects, including eutrophication and harmful algal blooms (HABs) in surface water, as well as poisoning groundwater. The biological treatment of wastewater is the main application of membrane bioreactor (MBR) systems and is the subject of most research.
Biological treatment of wastewater is done by a consortium of microorganisms (mainly bacteria and protozoa), the so-called activated sludge, that together can degrade and consume the organic and inorganic nutrients present in wastewater. In conventional activated sludge (CAS) systems, the liquid effluent is obtained through the sedimentation of solids, which depends on the settling ability of the biomass.
However, by using membranes to retain suspended solids in MBRs, all solids have the same residence time (solids retention time, SRT), which is independent of the hydraulic retention time (HRT). Therefore MBRs can be operated at high biomass concentration and, unlike the conventional activated sludge systems, can easily retain slow-growing organisms with poor settle ability, like nitrifying bacteria [6].
Besides the biological advantages associated with using a membrane, the treated effluent quality is higher in MBRs than in CAS systems. Permeate high quality allows the direct application of the MBR technology when advanced treatments are required, such as for bathing water, sensitive discharge bodies of water, or water reuse. Additionally, due to high biomass concentration and the elimination of settlers, MBRs have smaller footprints than CAS, which can be valuable when a compact system is needed, such as in areas with high population density.
The MBR features are also advantageous in treating industrial wastewater, where the influent may have lower biodegradability and/or comprise toxic compounds, meaning that the microorganisms require more time for adaptation and have lower kinetic rates.
Figure 4. Configurations of nitrogen-removal MBRs: (a) side-stream MBR; (b) submerged MBR; (c) two-chamber MBR; (d) simultaneous biological nitrogen-removal MBR; (e) mobile-bed biofilm MBR; and (f) membrane-aerated biofilm reactor. Source: https://ascelibrary.org/doi/10.1061/%28ASCE%29EE.1943-7870.0001682
The USEPA has set limits for nitrogen in drinking water of 10 mg-N=L for nitrate and 1 mg-N=L for nitrite due to nitrogen’s harmful effects on human health. The excessive development of plants and algae in coastal waterways, known as eutrophication, depletes the dissolved oxygen in the water and suffocates aquatic life. Saxitoxins, brevetoxins, and domoic acid are just a few of the poisonous substances that HABs emit and can have detrimental impacts on both human and animal health [7].
The MBR is a promising technology for developing new methods to remove nitrogen from wastewater. For this application, MBRs typically consist of (1) a bioreactor zone containing microorganisms with functional genes that encode enzymes responsible for the biological transformation of nitrogen into harmless byproducts and (2) a membrane filtration process either as a separate compartment or immersed in the bioreactor that separates the microorganisms and sludge from treated effluent.
As an example, MBRs can be used to implement conventional nitrification/ denitrification processes as described previously. In the nitrification zone of the bioreactor, first, ammonia is converted into nitrite by a series of autotrophic microorganisms under aerobic conditions. Then nitrite is oxidised into nitrates by a separate population of microorganisms. Subsequently, the bacterial community in the denitrification zone of the bioreactor converts the nitrates mostly into inert nitrogen gas under anoxic conditions.
Increasingly, MBR approaches serve as a platform for integrating novel microbial pathways and processes for removing nitrogen from wastewater, such as anaerobic ammonium oxidation (ANAMMOX) and simultaneous nitrification and denitrification (SND).
2.3 MBRs from A Microalgae Perspective
Photobioreactors (PBRs) are robust cultivation systems that employ effective cultivation techniques providing all the essential elements, i.e., light, nutrients, temperature and mixing, for the healthy growth of microalgae. The operating conditions are monitored, regulated, and controlled for the high yield of algal biomass [8].
Microalgae can go through different metabolisms like autotrophic, heterotrophic and mixotrophic. Autotrophic culture is the most common mode of microalgae cultivation. In this cultivation process, microalgae directly convert the inorganic carbon present into organic matter through the photosynthetic process. Most microalgal cells efficiently harness solar energy and utilise CO2 as the carbon source, contributing to the abatement of CO2. Hence autotrophic mode is considered an environmentally sustainable and economical mode of microalgae cultivation.
In the heterotrophic mode of cultivation, microalgae use organic carbon for their growth and do not require light. Therefore acetate, glycerol, and volatile fatty acids (VFAs) may be utilised as organic carbon sources for biomass synthesis. In heterotrophic cultivation, the biomass yield is better than in autotrophic cultivation, but the organic carbon is expensive compared to CO2. Accordingly, organic carbon must be utilised from wastewater to overcome this limitation.
Mixotrophic cultivation is to counter the limitations of autotrophic and heterotrophic cultivation modes. So, in this mode of cultivation, microalgae is utilising inorganic carbon through photosynthesis as well as organic carbon. The mixotrophic cultivation mode is divided into two stages. In the first stage, the growth is heterotrophic, and the microalgae cells consume organic carbon present in the medium; when it is consumed up to a certain limit, autotrophic growth is induced, and organic carbon assimilation starts taking place.
The selection and design of PBRs for microalgae cultivation depends upon various factors such as mixing strategy, orientation, surface area to volume ratio (S/V), illumination, air/CO2 supply, accumulation of oxygen and temperature. PBRs can be closed or open systems having various advantages and disadvantages but closed PBRs have the upper hand due to better control of the operational parameters. The prospects and limitations of both types of systems are summarised in Table 1.
Table 1. Advantages and disadvantages of open and close PBRs. Source: https://iopscience.iop.org/article/10.1088/1757-899X/1142/1/012004
It is almost impossible to decouple the retention time of microalgae from the dilution rate in an ordinary PBR merely using settling, but this can be achieved by using a membrane photobioreactor. Using MPBR serves two purposes, i.e., they help to hold the biomass inside the reactor (preventing washout) and assist CO2 transfer. MPBR provides complete retention of microalgae cells while the medium passes through (as permeate), thereby enhancing biomass concentration in the bioreactor.
The biomass concentration can be better controlled by partly returning the retentate. As the biomass is concentrated in the retentate stream, the permeate can be reused as a feed medium. The membrane will act to separate solid and liquid, thereby helping to isolate the microalgal cells from the effluent.
Furthermore, the bioreactor’s hydraulic retention time and solids retention time can be controlled independently during the culture, and the microalgal biomass concentration is not affected by the hydraulic loading and the growth of algal cells. Consequently, improved performance in terms of nutrients/pollutant removal and algal biomass productivity can be achieved in membrane PBRs.
2.4 Petrochemical Wastewaters
Oily and petrochemical wastewaters represent one of the most concerned pollution sources due to their toxic and refractory characteristics. This type of wastewater originates from various sources such as crude oil production, oil refinery, petrochemical industry, metal processing, lubricant and cooling agents, and car washing. Tightening effluent regulations and increasing need for the reuse of treated water have generated interest in the treatment of oily and petrochemical wastewater using MBR processes.
An Austrian team investigated the feasibility of an MBR process to treat petrochemical-contaminated wastewater by using synthetic wastewater containing either fuel oil or lubricant oil and a surfactant [9]. The removal efficiency of 99.9% for fuel oil as well as lubricant oil could be achieved at the conditions of an HRT of 13.3 hr, influent with 500–1000 mg/L hydrocarbons. The permeate quality was so high that the effluent could be reused in the industrial process. When we compared it with those from plain membrane filtration, the results clearly showed that the MBR process enhanced permeate quality due to the biodegradation of pollutants.
Figure 5. The Musa Bay is home to the Bandar Imam Khomeini petrochemical complex, Iran’s largest supplier of petrochemicals to the international market. Its western shore is a potential source of wastewater and oil tankers discharge. Source: “Musa Bay, Iran” by NASA Earth Observatory is licensed under CC BY 2.0.
Another important study upgraded a full-scale facility from chemical de-emulsification to a UF process followed by an MBR system. The facility was used to treat oil-contaminated wastewater from an automobile engine manufacturing plant. After the upgrading, 90% COD and complete oil, grease, and phenolics removals were achieved, in contrast to the previous widely fluctuating results.
Although most of the reported studies have been conducted on aerobic MBR and excellent COD removal has been obtained, utilisation of anaerobic MBRs in oily and petrochemical wastewater treatment has some inherent benefits. The use of external anaerobic MBRs (AnMBRs) for oily and petrochemical wastewater has only been reported by Van Zyl et al.164. This study observed an OLR of up to 25 kg/m3/day with an effluent COD normally lower than 500 mg/L, corresponding to over 97% COD removal. The OLR was much higher (0.5–3 kg/m3/day) than that obtained in aerobic ones.
A perusal of the literature shows that some signs of progress have been made in the field. However, the present work has been mainly confined to batch studies. Also, there is a scarcity of economic evaluation applied in the area based on full-scale applications. These limitations should be overcome to push the development of MBR technology in oily and petrochemical wastewater treatment
3 ENHANCING MBRs
3.1 From Conventional to Enhanced MBRs
MBR systems essentially consist of a combination of membrane units responsible for the physical separation and biological reactor systems accountable for the biodegradation of the waste compounds [10]. These systems are implemented based on two primary configurations: external/side-stream configuration and submerged/immersed configuration.
External configuration, which involves the recirculation of the mixed liquor through a membrane module outside the bioreactor, usually employs high cross-flow velocity (CFV) along the membrane surface to provide membrane driving force and control membrane fouling. As a result, this configuration provides more direct hydrodynamic control of membrane fouling and offers the advantages of easier membrane replacement and high fluxes but at the expense of frequent cleaning and high energy consumption (2–12 kWh/m3 product).
For submerged configuration, membrane modules are directly placed in the mixed liquor. The driving force across the membrane is achieved by pressurising the bioreactor or creating negative pressure on the permeate side. Several distinct advantages of submerged MBRs are their much lower energy consumption and less rigorous cleaning procedures.7,11–13 Moreover, the operating conditions are much milder than in external MBR systems because of the lower tangential velocities. To date, both configurations have been extensively employed for industrial wastewater treatments.
In recent years, many efforts have been made on the development of air-lift side-stream MBRs. The concept applies the side-stream air-lift principle using a robust and reliable side-stream configuration while incorporating all the advantages of the low energy-consuming submerged systems. MBRs with this configuration have been tested for the treatment of toilet wastewater, landfill leachate, pharmaceutical wastewater, and municipal wastewater.
3.2 Enhancing MBRs Through Electrochemical Processes
Interest in membrane bioreactors and electrochemical processes has recently surged because these techniques can be used to remove several pollutants from different types of wastewater. However, both methods still face some drawbacks, and recent studies have been directed toward these drawbacks. MBRs are faced with high-cost requirements resulting from membrane fouling, aeration, excess sludge management, and removal of phosphorus and heavy metals.
Firstly, membrane fouling in MBRs contributes to the cost of membrane cleaning and replacement. Critical compounds that contribute immensely to membrane fouling in MBRs are soluble microbial products or soluble extracellular polymeric substances that are produced during the metabolism and lysis of the microbes in the bioreactor. One of the ways to mitigate this problem is a coupling with additional techniques, e.g. multi-stage processes.
Secondly, many levels of coarse aeration are required for membrane scouring in MBRs, which affects the overall operating costs. Aeration might contribute up to 35-50% of the overall operating cost in MBRs.
Thirdly, vast amounts of sludge are ejected from some MBR systems as waste. The excess sludge production arising from biomass retention in MBRs leads to high sludge handling and management costs. Sometimes, the cost of sludge management is even higher than that of aeration.
Fourthly, adequate phosphorus removal in MBRs is not achieved by phosphorus accumulating organisms; hence there is a need to incorporate enhanced phosphorus removal systems with MBRs. Furthermore, process biokinetics can be inhibited by the presence or accumulation of heavy metal ions.
Figure 6. Experimental setup of an encapsulated self-forming dynamic biomembrane (e-ESFDM)e-ESFDMBR (a). Cross-section of the bioreactor (b). Photographs of the e-ESFDM module before and after operation in the bioreactor and finally showing the formed ESFDM (c, Philippines). Source: https://www.nature.com/articles/s41545-022-00184-z#Fig2
Electrochemical processes can be implemented for the efficient removal of hazardous compounds, such as soluble products and metal ions, from wastewater [11]. In addition, these processes significantly influence the destruction of microorganisms in treated effluents. Electrolysis gases that can be used to provide aeration for enhanced reaction kinetics are generated from these processes.
Many electrochemical systems produce coalesced and dense waste sludge that is more convenient to handle. However, the electrical energy consumption per mass of pollutant removed during the process is a drawback. Electrochemical systems designed to remove medium to high-strength wastewater might consume a lot of electrical energy. Passivation and electrode corrosion over time also pose some constraints to electrochemical processes.
To mitigate the drawbacks mentioned above, the integration of electrochemical reactors with MBR has been proposed in recent studies. The coupled technologies have been demonstrated to facilitate the removal of SMP, curtail membrane fouling, improve the physicochemical characteristics of sludge, and enhance the removal of phosphorus and heavy metal ions.
3.3 Other Recent Developments
The original architecture of a conventional wastewater treatment MBR consists of an aerated sludge reactor with an external membrane filtration unit (external MBR) or a submerged membrane with suction (submerged MBR). Revised MBR architecture has been proposed to enhance the removal performances in wastewater treatment [12].
Modern technologists proposed an adsorption MBR which put down an adsorption tank between an activated sludge tank and an MBR to receive raw wastewater so the nitrifiers can be enriched. They noted enrichment of nitrifiers and improvement of sludge filterability by the adoption of the adsorption-MBR system. Others demonstrated the supreme process performance of a pilot-scale staged anaerobic fluidised membrane bioreactor when treating primary-settled domestic wastewater. Over wide temperature variation, the proposed system produced >90% biochemical oxygen demand (BOD) and chemical oxygen demand (COD) removal at low energy consumption.
Moreover, some studied the microbial community shifts in operating an anoxic– oxic-MBR. The incorporation of anoxic and oxic zones altered microbial community structure and enhanced the nitrogen removal capability of an MBR. lite) to increase the concentration of mixed liquor suspended solids, reduce the quantity of soluble microbial products in suspension and reduce the transmembrane pressure drop. The presence of zeolite was proposed to enhance wastewater treatment performance and ease reactor operations.
Teams in Korea and Australia evaluated the performance of a pilot scale reactor consisting of a granular activated carbon (GAC)-sponge fluidised bed bioreactor and a submerged MBR. They noted that the tested pilot system could remove 90% dissolved organic carbon (DOC), 95% NH4+-N, and about 70% PO4-P. The transmembrane pressure for the studied system remained low over the experimental period. Other researchers removed 22 trace organic contaminants by GAC and powdered activated carbon (PAC)+MBR. The application of MBR + GAC (two stages) and the MBR-GAC (one step) processes enhanced the removal of seven hydrophilic and biologically persistent compounds.
Chinese technologists investigated the performance of a pilot-scale MBR system with PAC addition for enhancing micropollutant removal from surface waters. They also mitigate membrane fouling by adding GAC to a membrane-coupled expanded granular sludge bed reactor. Since the activated carbon can adsorb various extracellular substances, its presence alleviated membrane fouling efficiently.
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[6] Membrane Bioreactors for Nitrogen Removal from Wastewater: A Review
[8] Ahmad, I., Abdullah, N., Koji, I., Yuzir, A., & Muhammad, S. E. (2021, April). Evolution of Photobioreactors: A Review based on Microalgal Perspective. In IOP Conference Series: Materials Science and Engineering (Vol. 1142, No. 1, p. 012004). IOP Publishing.
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