Research Progress in Biofilm-Membrane Bioreactor: A Critical Review

Review

Research progress in biofilm-membrane bioreactor (BF-MBR)-a critical review Wenxiang Zhang, Bing Tang, and Liying Bin Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 25 May 2017 Downloaded from http://pubs.acs.org on May 28, 2017

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Industrial & Engineering Chemistry Research

Research progress in biofilm-membrane bioreactor (BF-MBR)-a critical review Wenxiang Zhang*, Bing Tang, Liying Bin School of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou 510006, PR China

* Corresponding author. E-mail address: [email protected].

Abstract:

Biofilm-membrane bioreactor (BF-MBR), combining a biofilm and a conventional membrane bioreactor (MBR), can sustain the stable organic pollutant removal rate, decrease membrane fouling, reduce production of excess sludge and provide a good living environment for denitrification and dephosphorization process. This mini-review summarized R&D progresses in a BF-MBR. Reports published within the last 15 years are reviewed with respect to BF-MBR studies, analyzing the fundamentals and advantages proposed, identifying the mechanism and performance for pollutant degradation and membrane fouling, and discussing the reactor configurations and designs. Finally, on the basis of reported results, future research perspectives regarding the development of BF-MBR are proposed.

Key words: Biofilm-membrane bioreactor (BF-MBR); Biofilm carriers; Membrane fouling; Denitrification and dephosphorization; Wastewater treatment

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1.Introduction The development of industrialization and urbanization has caused a huge increase in the consumption of water resources and a deterioration of water quality. In an effort to control the effluent limits, to satisfy the increasing strict local water quality standard and upgrade existing overloaded wastewater treatment plants, advanced technologies of wastewater treatment have been proposed1. A membrane bioreactor (MBR) is the combination of membrane module and bioreactor and has been widely used for municipal and industrial wastewater treatment due to high pollutant removal efficiency, better effluent quality, low sludge production rate and small footprint2. However, the inefficient denitrification effect, serious membrane fouling and high operation cost hinder it further application of MBR3. Moving bed biofilm bioreactors (MBBR), as a kind of biofilm wastewater treatment technology, are developed on the basis of a combination of biological contact oxidation and biological fluidized bed and has been proved to be reliable for organic matter and nutrients removal4. Due to high biomass and diversity in bacterial population, MBBR has several advantages, such as stable and reliable operation, strong resistance to shock loading and adaptability, low residual sludge production and high nitrification rate5. Biofilm-membrane bioreactor (BF-MBR), as an alternative way aims to partially solve the fouling concerns regarding conventional MBR and the settle ability issues in relation to MBBR, and was developed by Leiknes and Ødegaard6. In BF-MBR, carriers and membrane module are installed with fluidized or fixed state. Membrane can retain macromolecular substances and solid particles, thus sustain high biomass concentration and result in good effluent quality. On another hand, carriers provide a large surface space for microbial growth, which is benefit for organic pollutant degradation. Compared with conventional MBR, most of biomass in BF-MBR locates in the surface of carriers and form a dense layer of biofilm, leading to the reduction of suspended particles7, thus migrating membrane fouling8. BF-MBR can combine the advantages of both, control activated sludge at low concentration, decrease the energy consumption and improve the process efficiency9. It becomes economically attractive when compact technology is required to accommodate space constraints or when stringent effluent quality requirements are mandatory5a. Currently, BF-MBR is still in the research phase. A comprehensive review on the BF-MBR is still lacking. Therefore, this critical review lists scientific articles published in last 15 years on BF-MBR, from which the current R&D trends on BF-MBR can be demonstrated. Moreover, the fundamentals and advantages, mechanisms and performances, and reactor configurations and designs are summarized. In addition, the possible future research directions were briefly discussed in this mini review. 2


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2. Fundamentals and advantages 2.1 MBR Membrane bioreactor (MBR), as the combination of a membrane process like microfiltration or ultrafiltration and a suspended growth bioreactor (Figure 1), and is now widely used for municipal and industrial wastewater treatment with plants up to 80,000 population equivalent (i.e. 48 million liters per day)10. It is possible to operate MBR processes at higher mixed liquor suspended solids (MLSS) concentrations compared to conventional settlement separation systems, thus reducing the reactor volume to achieve the same loading rate. In comparison with conventional activated sludge process, MBR has the following advantages: high quality effluent (very often hygienically highly purified), lower footprint, lower excess sludge production, improved nutrient removal and easy retrofit and upgrade of old wastewater treatment plants. Major disadvantages of conventional MBR are membrane fouling, which limits sustainability and wider applications, higher energy demand mostly caused by air scouring demand, and higher capital costs due to the cost of membranes11. Better understanding of membrane fouling mechanisms, optimization of energy consumption and cheaper membrane materials have overcame some of these disadvantages, making this technology even a more realistic and viable choice by the end of last decade3b.

Figure 1. Schematic diagram of a field-scale MBR system. 3



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2.2 MBBR A moving bed biofilm reactor (MBBR) is a type of biofilm wastewater treatment process that was first proposed by Prof. Hallvard Ødegaardat in the late 1980s12. There is over 700 wastewater treatment systems (both municipal and industrial) installed in over 50 different countries. As shown in Figure 2, the MBBR system consists of an aeration tank (similar to the activated sludge process) with special plastic carriers that provide a surface where a biofilm can grow. The carriers are made of a material with a density close to that of water (1 g/cm3). The carriers will be mixed in the tank by the aeration system and thus will have good contact between the substrate in the influent wastewater and the biomass on the carriers5a. Some of the most important advantages of the MBBR process with relation to the conventional activated sludge process include better oxygen transfer, shorter hydraulic retention time (HRT), higher organic loading rates, lower sludge production, higher nitrification rate and larger surface area for mass transfer13. But, settling ability of biosolids is the largest challenge in MBBR design as the production of filamentous bacteria and poorly settling biomass often hinder solid separation in a secondary clarifier14.

Figure 2. Schematic diagram of a field-scale MBBR system.

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2.3 BF-MBR

Figure 3. Schematic diagram of a field-scale BF-MBR system. In order to reduce the concentration of suspended biomass and membrane fouling without limiting the efficiency of the process, a hybrid system biofilm coupled with a MBR (BF-MBR) is used for the biodegradation of soluble organic matter. As shown in Figure 3, in a BF-MBR, the carriers are added into conventional MBR to provide an attachment surface for the growth of microbial cells. The suspended activated sludge and attached biofilm in the carriers have a synergistic effect for the removal of organic pollutants in wastewater9b. Membrane process can trap suspended microorganisms and organic matters, achieving the purpose of the separation HRT and sludge retention time (SRT). There are three modes for the removal of organic pollutants in BF-MBR: (1) degradation by activated sludge; (2) degradation by attached biofilm; and (3) rejection by membrane. Indeed, the hybrid BF-MBR has the potential of bringing together the advantages of MBBR and MBR. In summary, in comparison to conventional MBR, BF-MBR has the advantage of operating with higher fluxes, being even more compact, having better energetic efficiencies and better membrane fouling control9a. BF-MBR is still under laboratory and pilot, and so far no practical engineering application is reported, however, it has been used to treat various wastewaters (as seen in Table 1), such as urban wastewater, dye wastewater, landfill leachate, pharmaceutical wastewater, food wastewater and paper mill wastewater. In the review paper of Leyva-Diaz et al.15, as shown in Figure 4, BF-MBR can be divided into pure BF-MBR and hybrid BF-MBR. The discrepancy between pure BF-MBR and hybrid one is sludge recycling. A part of purged sludge of hybrid BF-MBR from membrane module recycles back to BF reactor, whereas for pure BF-MBR, there is not recycled sludge. Compared with a pure BF-MBR, a hybrid BF-MBR has both suspended biomass and attached biomass, because of a recycled sludge from the membrane module to the biofilm bioreactor.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Wastewater 20 type 21 22 Urban 23 16 wastewater 24 25 26 27 28 Dye 29 wastewater17 30 31 32 33 34 Landfill 35 leachate18 36 37 38 39 40 41 Pharmaceutical 42 wastewater19 43 44 45 46 47 48 49 Food 50 wastewater20 51 52 53 54 55 56 Paper mill 57 wastewater21 58 59 60

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Figure 4. Schematic description of pure BF-MBR (left) and hybrid BF-MBR (right).

Table 1. BF-MBR for treating various wastewaters. Membrane

Reactor

Carriers type

Filling fraction

Hollow fiber membranes, 0.2 μm

Fluidized bed

Honeycomb carriers

50%

Removal rate COD NH4+-N

TN

95%

50%

After operating 143 days, TMP reached 20 kPa. After operating 2 months, membrane fouling was not serious and foulants only existed on membrane surface.

98%

Hollow fiber membranes

Fixed bed

Polyamide soft carriers

30%~40 %

95%

94%

NA

Polypropylene ultrafiltration membrane, 0.02 μm

Fluidized bed

Polyurethane foam

30 %

98.2%

95%

40%

Membrane fouling

Hollow fiber membranes

Fixed bed

Bio-carriers

NA

56.8%

98.4%

63.2 %

During 1 year-operation, membrane cleaning was conducted twice and membrane permeability could sustain a high level.

Polyvinylidene chloride hollow fiber microfiltration membrane, 0.36 μm

Fixed bed

Spherical bio-carriers

55 %

93%~ 98%.

NA

NA

After operating 135 days, TMP reached 55 kPa.

NA

After operating 30 days, fouling resistance is 6.36 times the membrane resistance.

Polyether sulfone, 0.52 μm

Fixed bed soft

Soft carriers

30 %

90.4%

6


NA

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3. Mechanisms and performances In a BF-MBR, carriers are added into a conventional MBR, then the coexistence bioreactor for activated sludge and biofilm forms. After wastewater enters BF-MBR, the organic pollutants are decomposed by suspended activated sludge and attached biofilm. Aeration can provide the dissolved oxygen for biological process and fluidity power22. With microbial assimilation, the organic matters synthesize cell tissue and are oxidized to carbon dioxide. Ammonia can be decomposed by nitrification on the surfaces of activated sludge and biofilm, then nitrate and nitrite are produced and diffuse into the inside of biofilm or activated sludge floc, after denitrification (main by biofilm), nitrogen is produced and escape23. Comparing with conventional MBR, a BF-MBR has a more diverse microbial community, such as carbonation bacteria, nitrifying bacteria, denitrifying bacteria in activated sludge, and protozoa, metazoan and microbial predators in biofilm. These organisms can effectively control sludge production. 3.1 Pollutant removal Decarbonization In a BF-MBR, biodegradation carried out by the suspended activated sludge and attached biofilm depends on the amount of biomass present in either form, especially for attached biofilm, because the biomass in attached biofilm has a higher activity, and an almost 2/3 higher removal efficiency than in a suspended activated sludge24. Liu et al.16 compared BF-MBR with conventional MBR, and found that BF-MBR had higher removal rates of COD (4.6%) and BOD (3.8%) than conventional MBR. Yang et al.25 reported that COD removal rate of BF-MBR could reach 96.2% for artificial wastewater treatment and was obviously higher that of conventional MBR, indicating that a BF-MBR has an excellent ability to degrade organic pollutants. The filling fraction of carriers, surface area for biofilm growth, biomass activity and MLSS concentration in suspended growth have a significant effect on organic matter degradation. A high surface area of carriers can provide better protection for biofilm growth from intensive detachment mechanisms. Increasing filling fraction of carriers leads to more compact bioreactor. Higher activity of the biomass and more diverse microbial community improve biodegradation5, 25-26. A removal rate of COD between 67.50 and 86.36% operating with filling ratios of 35 and 50% for K1 carrier, but the increase in filling ratio to 50% did not promote the organic matter removal9b,

27

. Moreover, HRT also improves COD

removal, because it can prolong the contact oxidation time of pollutants. Martin-Pascual et al.27 observed that the removal percentages of COD increased with HRT, from 87.62% for an HRT of 10 h to 93.44% for an HRT of 24 h. A similar trend was observed for the removal of BOD5, which increased from 94.41% for an 7



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HRT of 10 h to 97.73% for an HRT of 24 h. In addition, temperature positively affects the organic matter removal. Denitrification A BF-MBR has high total nitrogen removal effect, because of the improvement of nitrobacteria growth and the existence of simultaneous nitrification/denitrification (SND)16. In a conventional MBR, as heterotrophic bacteria have a long growth cycle, the nitrobacteria have to compete with carbonation bacteria, but it is at a disadvantage. However, in a BF-MBR, the addition of carriers increases the biomass and microbial community, which benefits the growth of nitrobacteria26. The development of attached biomass on carriers as biofilms resulted in a better interaction between the nitrate and the microorganisms of the nitrogen cycle15. Besides, the anoxic/anaerobic conditions in deeper layers of the biofilm could achieve SND25, on account of the growth of microorganisms in different zones of the biofilm, which carried out the nitrification process in superficial layers of the biofilm and developed the denitrification process in deeper layers of the biofilm. Cuevas-Rodriguez et al.28 analyzed a hybrid BF-MBR under high HRTs (58.8-76.32 h) and MLSS concentration (6390 mg L-1) achieved a high TN removal with an average value of 85.50%. Guo et al.29 reported that the TN removal efficiency was 93.00% in the hybrid BF-MBR and 90.00% in the conventional MBR, which is not a big difference, but this little difference in the hybrid BF-MBR could be due to the development of the biofilm in the inner and outer zones of the carriers, which induced SND. Rafiei et al.30 found that the NH4+-N removal of pure BF-MBR was significantly higher in the hybrid BF-MBR, with an increase of 40% in nitrification compared with the conventional MBR. This was probably due to the increase in SRT for the attached biomass belonging to the hybrid BF-MBR in relation to the suspended biomass in the conventional MBR. This enabled the growth of nitrifying bacteria on the carriers and caused an enhancement in NH4+-N removal. Khan et al.31 compared two BF-MBR systems and observed that different carriers had different total biomass concentration, thus affected TN removal efficiencies. Investigation of an intermittently aerated hybrid BF-MBR showed that SND via nitrite occurred in the same bioreactor under identical operating conditions32. An important factor in achieving short-cut nitrification was the intermittent aeration time. The nitrite accumulation rate increased from 4.5%, with continuous aeration, to 93.3% under an aeration of 2 min and a mixing of 4 min in anaerobic conditions15. Therefore, BF-MBR exhibits improved denitrification efficiency. Dephosphorization In a BF-MBR, phosphorus removal can be achieved by assimilation of biomass growth and 8


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phosphorous accumulating organisms (PAOs). PAOs can absorb excess phosphorus and store in their bodies, then form high phosphorus sludge26. Besides, the quantities of PAOs in the layers of the biofilm were analyzed with fluorescent in situ hybridization (FISH), showing a small quantity of PAOs in the inner layer and a large amount of PAOs in the outer layer32a. The biological system with short SRT and high sludge production rate has good phosphorus removal efficiency. In theory, due to long SRT and low sludge production rate, both BF-MBR and conventional MBR have low phosphorus removal efficiency33. Compared with conventional MBR, for BF-MBR, the addition of biofilm carries and the anoxic/aerobic zones insides deep biofilm layers can increase total phosphorus removal rate from 1.7% to 20.1%6b, 34, and the COD/TP ratio and remove of excess sludge are important for the phosphorus removal efficiency. Moreover, TP removal depends on the alternation time between aerobic and anaerobic conditions in a hybrid BF-MBR32a. In this regard, an average TP removal of 84.10% was obtained when the anaerobic time was 2 h and the aerobic time was also 2 h. Evaluation of simultaneous phosphorus removal under 1, 3 and 6 mg L-1 of DO showed that the optimal DO concentration was around 3 mg L-1, with TP removal rates of 89.50%15. In general, as shown in Table 1, for organic (COD) removal and nitrification, BF-MBR is slightly higher than conventional MBR. But for TN and TP removal, BF-MBR is obviously better than conventional MBR, because of higher microbial activity, smaller floc sizes, more diverse microbial community and anoxic/aerobic zones inside biofilm. 3.2 Membrane fouling mechanism and control strategies Membrane fouling mechanism Membrane fouling plays a significant role in all membrane processes. In a MBR, liquid suspension (i.e., activated sludge and/or surplus of biofilm), including suspended solids (MLSS), colloidal and soluble organic content (i.e., biopolymers, SMP/EPS) and physical properties (i.e., particle size and viscosity), mainly contributes to membrane fouling10, 35. In BF-MBR, the implementation of attached biofilm can enhance filtration performance and reduce membrane fouling. Wang et al.36 reported that the attached biomass decreased 48% total filtration resistance and prolonged three time operational cycles. Adding biofilm carriers decreased EPS values and brought about the significant prolongation of operational cycles (from 57-65 to 92 d)16, 36-37. According to Luo et al.38-39, the hybrid BF-MBR could reduce the membrane fouling propensity in relation to a conventional MBR, as the transmembrane pressure (TMP) development was relatively slow, reaching up to 35 kPa in 89 9



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days of operation. The critical flux was determined in a hybrid BF-MBR and compared with two MBR systems by Achilli et al.40. The hybrid BF-MBR had a critical flux 30% higher than that obtained for the MBR systems (15 L m-2 h-1). But, the overgrowth of filamentous bacteria may cause high fouling rate after addition of carriers38. This indicates the unstable operation of reactor and unhealthy microbial growth lead to enhancement of membrane fouling. In most of membrane processes, cake layer on membrane increases fouling resistance, and the reduction of cake formation can control fouling resistance41. In a BF-MBR, MLSS is the main source that forms cake layer on membrane and operating at low MLSS can reduce cake layer resistance5, 10. Contrary to findings in studies of Lee et al.24 and Yang et al.38, the addition of carriers could reduce the concentration of MLSS, but unexpectedly, higher fouling rates occurred at lower MLSS concentrations, which could be explained as follows. At low MLSS concentration, a dense and less porous cake layer with high fouling resistance formed, while high MLSS concentrations led to formation of a dynamic and loose cake layer, which had a low fouling resistance. Besides, at low SRT, low MLSS may cause high soluble microbial products (SMP) production, thus enhancing fouling. SMP and extracellular polymeric substances (EPS) are main membrane foulants in MBR10. In a BF-MBR, the production of SMP/EPS by attached biomass is lower than conventional MBR, due to the ability of the biofilm to adsorb and bind soluble microbial products, high biological activities and more diverse microbial community16,

42

. However, some studies24,

43

reported that no significantly different

production of SMP/EPS between conventional MBR and BF-MBR. Contrary to these findings, other studies6b, 38 found that the overgrowth of filamentous bacteria, caused by the use of new type of non-woven carriers, had a high content of polysaccharides and proteins in BF-MBR, bringing high membrane fouling. In common membrane processes, the smaller difference between membrane pore size and particle size leads to higher pore blocking and fouling resistance44. The presence of attached biofilm in BF-MBR can result in floc brakeage and produce smaller flocs, whose size is more similar with membrane pore size43, 45. However, compared with conventional MBR, this does not increase membrane fouling, but improves filtration performance. The reason may be that the biofilm carries size and filling fraction play an important role and effect on particle size distributions. The larger carriers and lower filling fractions can flocculate suspended biomass and promote formation of larger flocs, then reducing fouling rates26, 36-37. Generally speaking, with low MLSS concentration and effective scouring effect, BF-MBR has lower membrane fouling than conventional MBR. Meanwhile, it should be noted to avoid the overgrowth of 10


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filamentous bacteria caused by the non-woven carriers and the unsuitable operation of reactor is of great significance. Membrane fouling control strategies During the operation of BF-MBR, several membrane fouling control strategies including aeration, scouring effect of carriers and temperature, were utilized to alleviate membrane fouling7, 46. Rahimi et al.47 analyzed the process performance and membrane fouling in a hybrid BF-MBR with different aeration rates in the bioreactor, i.e., 42, 85, 151, 296 and 380 L h-1, and several aeration rates in the membrane tank, i.e., 0.4, 0.8, 1.2 and 1.6 m3 m-2 h-1, expressed as specific aeration demand per membrane area (SADm). According to the authors, the optimum combination occurred at an aeration rate in the bioreactor of 151 L h-1 and a SADm of 0.8 and 1.2 m3 m-2 h-1. Under these aeration conditions, the membrane permeability was the highest, the foulant concentration was the minimum and the performance of SND was the highest. Therefore, the aeration rate was a major factor in the nutrient removal and membrane fouling control. Besides, the carriers in BF-MBR have a scouring effect on membrane fouling control by hitting the membrane surface. For many traditional fouling control strategies, such as hydrodynamic turbulences and air bubbles, they cannot reach the membrane surface, due to the protection of laminar boundary layer. But, the scouring effect of carriers could directly and effectively break the boundary layer, thus reducing fouling layer48. Additionally, compared with other fouling control strategies, the scouring effect of carriers has much lower energy consumption, because of void additional reinforcement for shear rate and aeration49. The effect of temperature on the performance of BF-MBR was studied by Martin-Pascual et al.50. A multiple linear regression was utilized to model the permeability of the membrane as a function of the temperature (ranging from 10 to 35 oC) and the dynamic viscosity (varying from 1 to 5 cP). The results showed that permeability augmented with the temperature and permeability reduced when the dynamic viscosity enhanced. The variations of permeability because of the temperature and dynamic viscosity in the working ranges were similar (higher than 60% for both variables). In conventional MBR, many other fouling control strategies, including coagulation pretreament51, bubble52, filtration mode53, hydrodynamic enhancement54, relaxation and backwashing55, were employed to reduce membrane fouling. In fact, these fouling control strategies can be also studied and employed to alleviate membrane fouling of BF-MBR in future investigations. 11



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4. Reactor configurations and designs BF-MBR has various configurations (shown in Table 2). According to the way of carrier installation, a BF-MBR can be divided generally into two main groups: a fluidized BF-MBR and a fixed BF-MBR. In a fluidized BF-MBR, the carriers with similar density with water are not fixed, and can flow with airflow in the reactor. In a fixed BF-MBR, the carriers are fixed at the specific locations of the reactor. Besides, the novel BF-MBRs, including activated carbon BF-MBR and worm BF-MBR, are also designed to improve membrane fouling control and pollutants removal. Table 2. Advantages of various BF-MBRs. Fixed BF-MBR

Activated carbon BF-MBR

Worm BF-MBR

Pollutants removal

High filling fraction and large effective specific surface area improve the biofilm growth, biomass activity and pollutant degradation efficiency

High stability of biofilm improves organic pollutant degradation

Large specific surface area of activated carbon enhances the growth of attached biomass. Strong adsorption capacity improves organic macromolecules removal

Worm improves the sludge characteristics and reduces excess sludge production

Membrane fouling control

The carriers can collide with membrane surface and increase the shear stress on membrane surface, then reduce membrane foulant deposition

Less shedding of biofilm and lower foulant Activated carbon absorbs concentration foulants and reduces improve membrane fouling. membrane fouling control

Worm reduces EPS content, and destroys aromatic protein and tryptophan protein

Fluidized BF-MBR

4.1 Fluidized BF-MBR The main carriers of fluidized BF-MBR include honeycomb carriers and diatomite. Fluidized state, as shown in Figure 5, with high filling fraction and large effective specific surface area, can improve the biofilm growth, biomass activity and pollutant degradation efficiency. Reboleiro-Rivas et al.56 used a fluidized BF-MBR to treat real urban wastewater and found that the filling fraction and aeration hydraulics could improve biofilm growth and increase organic pollutant removal rate. Yang et al.25 reported that the removal rates of TN and NH4+-N in a BF-MBR could reach 70% and 80% for the urban wastewater with COD/TN=8.9 and TN loading=7.85 mg/L. Besides, at optimized aeration condition, TN and NH4+-N removal rate could rise to 60％ and 100％38. Fluidized state can also improve membrane fouling control. During the fluidized state, the carriers can 12


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collide with membrane surface and increase the shear stress on membrane surface, then reducing membrane foulant deposition and membrane fouling. Moreover, the flow of carriers affects the microbial growth and changes EPS properties. Yang et al.57 found that fluidized state could affect MLSS concentration and attached biomass for controlling membrane fouling.

Figure 5. Schematic description of fluidized BF-MBR (left) and fixed BF-MBR (right). 4.2 Fixed BF-MBR The main carriers of fixed BF-MBR include soft carrier, semi-soft carrier and composite carrier. Among them, a honeycomb carrier is the most widely used. In fixed BF-MBR, as shown in Figure 4, due to the fixed configuration of carriers, the biofilm is not easily to fall off, thus decreasing foulants. Chen et al.58 compared the multi-level cycle BF-MBR (fixed bed with honeycomb carriers) and conventional MBR, and reported that during 4 months operation, their mean COD removal rates were 92.2% and 85.3%, suspended solids (SS) removal rates were 93.8% and 85%, and NH4+-N removal rates were 84.1% and 65.3%, respectively. Furthermore, a fixed bed configuration contributed to membrane fouling control. Ng et al.20 studied the organic pollutant removal and membrane fouling characteristic, and found that COD removal rate fluctuated within 93%~98%, and exceed 70% SMP particles were small than 100 Da, as well the membrane (mean pore size 0.036 μm) fouling could be reduced to a large extent. Sponge is another effective soft carrier. Nguyen et al.59 added a sponge into a MBR, and showed that COD, NH4+-N and phosphate removal rates reached 95%, 83.6% and 75.5%, while MLSS concentration and membrane fouling reduced.

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4.3 Novel BF-MBRs 4.3.1Activated Carbon BF-MBR As illustrated in Figure 6, activated carbon, as an excellent carrier with a large specific surface area and strong adsorption capacity, can not only provide high surface area for the growth of attached biomass, but also absorbs organic macromolecules, which induces removal of refractory organics and membrane fouling control. However, activated carbon can cause pore blocking and the excessive addition may enhance membrane fouling. Ma et al.60 utilized activated carbon as biofilm carrier to fabricate BF-MBR for treating low pollution surface water at 10 oC. The addition of activated carbon can effectively control the irreversible fouling and increase removal rates of COD, NH4+-N, and UV254 to 75%, 93% and 85%, respectively. Li et al.61 utilized an activated carbon-MBR to treat trace pollutants and found that before adding activated carbon, the removal rates of carbamazepine and sulfamethoxazole were 0% and 64%, however, after adding activated carbon (1.0 g/L), their removal rates were 82% and 92%. The improvement of pollutant removal is due to the adsorption of activated carbon for some refractory organics. Nguyen et al.62 reported that activated carbon-MBR could effectively remove hydrophobic and stable chemical substances. Villamil et al.63 found that activated carbon could absorb proteins smaller than 1 mm, which caused the irreversible fouling of the membranes, thus increase critical flux of 25 % and sustain operation TMP stable for long term operation. Kim et al.49 systematically studied the scouring agents in membrane cleaning of MBR, and found that the scouring agents could hit the membrane surface, whereas neither hydrodynamic turbulences nor air bubbles can reach the membrane surface which is protected by the laminar boundary layer. In addition, the use of adsorbents, such as activated carbon, as mechanical scouring agent provides additional benefit of sorption compared to other inert granular materials. Thus, activated carbons can improve fouling control and membrane flux, but steady replacement of aged activated carbon is vital, otherwise, fouling could be worse than that of without activated carbon system. In addition, Aslam et al.48 also found that activated carbon was an energy-efficient way and it had much lower energy cost than other fouling control strategies, such as shear-enhanced, intensive aeration and ozone oxidation.

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Figure 6. Schematic illustration of the experimental equipment of the high concentration activated carbon -MBR process. 1-Raw water tank; 2-feed pump; 3-membrane module; 4- activated carbon; 5-vacuum gauge; 6-electromagnetic valve; 7-effluent pump; 8-electromagnetic valve; 9-backwash pump. Figure reprinted with permission from60. 4.3.2 Worm BF-MBR Worms are many different distantly related animals that typically have a long cylindrical tube-like body and no limbs. In worm BF-MBR, the worm inoculation in biochemical pool can prolong food chain. Worm swallows zoogloea, as shown in Figure 7, improves the sludge characteristics and reduces excess sludge production. Guo et al.64 found that worm could reduce 45%~60% excess sludge. Tian et al.65 reported that the addition of worm caused clear reduction of EPS content and destroyed aromatic protein and tryptophan protein, therefore alleviating membrane fouling. However, the worm inoculation is not easy to control and its effect on previous crafts is unclear66, which needs more studies in future.

Figure 7. Waste sludge (left) versus worm faeces (right) in worm BF-MBR. Figure reprinted with permission from66. 15



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5. Conclusions and recommendations 5.1. Key outcomes of the review In this paper, the fundamentals and advantages, the mechanisms and performances for pollutant degradation, membrane fouling and fouling control strategies, and the reactor configurations and designs in BF-MBRs were reviewed. The main conclusions of this review are listed below: Fundamentals and advantages: In BF-MBR, carriers were added into MBR to provide an attachment surface for the growth of microbial cells. The fundamental of pollutant removal is the synergistic effects of activated sludge and attached biofilm. In comparison with a conventional MBR, the BF-MBR has the advantage of operating with higher fluxes, being even more compact, having better energetic efficiencies and better membrane fouling control. Mechanisms and performances: For organic pollutants removal, the addition of attached biofilm can improve biodegradation efficiency. The high surface area of carriers and optimized carrier filling fraction increase the biomass activity and lead to more diverse microbial community, then enhancing decarbonization. Furthermore, the improved growth of nitrobacteria, caused by the increases of biomass and diverse microbial community, and SND, produced by the anoxic/anaerobic conditions in deeper layers of the biofilm, can promote denitrification capacity. In addition, the addition of biofilm carries and the anoxic/aerobic conditions inside biofilm also enhance dephosphorization. With respect to membrane fouling, the implementation of attached biofilm can decrease EPS/SMP content, flocculate suspended biomass and promote formation of lager flocs, then reducing membrane fouling. Besides, the scouring effect of carriers on membrane surface also alleviate fouling layer by hitting boundary layer on membrane. Reactor configurations and designs: BF-MBR has two main groups: fluidized BF-MBR and fixed BF-MBR. In fluidized BF-MBR, the carriers can flow with airflow in the reactor, so it has high filling fraction and large effective specific surface area, as well as high organic pollutant degradation efficiency. For fixed BF-MBR, the carriers are fixed and biofim is not easy to fall off, leading to low membrane fouling. In addition, the novel BF-MBRs (activated carbon BF-MBR and worm BF-MBR) are also designed to improve membrane fouling control and 16


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pollutants removal. 5.2. Recommendations for further research To further understand BF-MBR mechanisms, improve pollutant degradation efficiency and membrane fouling control as well as overcome existing obstacles, more studies should focus on the following problems: (1) Pollutant biodegradation kinetics of different carriers. Based on the characteristics of various carriers, the pollutant biodegradation kinetics and biodegradation efficiency with different carrier types, filling modes and filling fractions should be studied. (2) Membrane fouling mechanism. Membrane fouling is an important factor for membrane application. The study of fouling mechanism is helpful to improve fouling control and sustain long-term stable operation. There are many studies about MBR fouling mechanism and control. But, to date, no data was reported on the fouling mechanism and fouling control strategies of BF-MBR. (3) Relationship between carriers and membrane fouling. Until now, many studies focus on the addition of carriers on pollutant removal. However, the addition of carriers also has a significant influence on membrane fouling. The effects of the attached biofilm and scouring between carriers and membrane on membrane fouling are not clear and need further investigation. (4) Economic evaluation and operation cost. Economic evaluation and cost control are significant to assess the feasibility of new technology. So far, no study was reported on the economic evaluation and cost control of BF-MBR. (5) Integrating with other wastewater treatment technologies. By integrating with other wastewater treatment technologies, such as using forward osmosis as post-treatment, trace pollutant can be removed and reuse water is produced. (6) Practical application. There have been many BF-MBR studies based on lab experiments. Further research should focus on scaling up and treating real water types with BF-MBR. Acknowledgements The authors would like to acknowledge the financial support from Scientific Research Fund of Guangdong University of Technology (220413582).

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