Membrane fouling remains a critical barrier to the widespread implementation of low-pressure membranes for drinking water treatment. Although hydrophilic biopolymers—high-molecular-weight components of natural organic matter—have been frequently associated with fouling, the specific contribution of different size classes remains poorly understood. In this study, we isolated biopolymers from surface waters and fractionated them into large (> 0.1 µm), medium (0.025–0.1 µm), and small (10 kDa to 0.025 µm) size classes. Bench-scale microfiltration and ultrafiltration tests revealed that large biopolymers exhibited the highest fouling potential, inducing rapid transmembrane pressure buildup and severe irreversible fouling in microfiltration. Quartz crystal microbalance with dissipation monitoring further demonstrated that while all fractions readily adsorbed onto polyvinylidene fluoride membrane surfaces, only large biopolymers continued to accumulate through intermolecular interactions and formed soft, viscoelastic layers that might facilitate pore plugging. By linking the molecular size, adsorption dynamics, and fouling mechanisms, this study establishes a mechanistic basis for biopolymer-driven fouling and offers valuable insights into the development of fouling-resistant membranes and targeted pretreatment strategies for drinking water treatment.

Municipal wastewater contains sufficient chemical energy to make wastewater treatment energy-neutral. However, conventional activated sludge (CAS) processes mineralize most of the organic matter in raw wastewater. High-rate membrane bioreactors (HR-MBRs) enable the direct recovery of organics by operating at extremely short hydraulic and solid retention times (SRT and HRT) while maintaining a permeate quality comparable to that of CAS effluent. However, severe membrane fouling prevents a stable operation. This study demonstrates that intensive membrane cleaning combining granular scouring with an air-exposed chemically enhanced backwash (AECEB) enables long-term, stable operation of HR-MBRs treating municipal wastewater. Bench-scale HR-MBRs equipped with ceramic flat-sheet membranes were operated under realistic conditions in a municipal wastewater treatment plant. It was confirmed that the HR-MBRs achieved high organic matter recovery of up to 74% of the influent chemical oxygen demand (COD). The polyurethane sponge cubes provide effective physical scouring, thereby substantially reducing both reversible and irreversible fouling. Elemental analyses revealed that Fe-associated constituents are major players in irreversible fouling. Carrying out routine AECEB using oxalic acid proved to be promising for the stable long-term (e.g., several weeks) operation of HR-MBRs at a practical membrane flux, such as the 16 L/m2/h (LMH) examined in this study, which has not achieved previously. These results demonstrate a feasible route toward stable HR-MBR operation for energy-efficient municipal wastewater treatment.

Ammonia in municipal wastewater, which is commonly treated by energy-intensive biological nitrification/denitrification systems, should be recovered in terms of energy efficiency, resource utilization and mitigation of global warming (prevention of emission of N2O). The use of a membrane contactor (MC) seems to be effective for recovery of ammonia in municipal wastewater. However, fouling of the membrane in the MC is a potential problem. The effectiveness of direct membrane filtration (DMF) as a pretreatment for controlling the fouling in an MC was investigated in this study. Although DMF can efficiently capture the carbon in municipal wastewater, it cannot remove or capture ammonia in municipal wastewater. Therefore, it was thought that the combination of the use of an MC and DMF would compensate each other. Experiments were carried out by using both synthetic wastewater and municipal wastewater under a high alkaline condition (pH of 11) and a moderate alkaline condition (pH of 9.2). For municipal wastewater, the effluent from primary sedimentation and the permeate from DMF using microfiltration (pore size: 0.1 µm) were examined. It was shown that pretreatment by DMF significantly mitigated the fouling in the MC, leading to maintenance of efficient mass transfer of ammonia in the MC. Ammonia in municipal wastewater could almost completely be recovered by the MC within 6 h regardless of pH. Thus, it was shown that the integration of DMF and an MC could recover 70-80 % of carbon and almost 100 % of ammonia from municipal wastewater. Long-term operations of the proposed system with multiple replacements of the feed wastewater demonstrated that concentration of ammonia nitrogen in the recovery solution (sulfuric acid) could be increased to >350 mg/L. In the long-term operations, due to strict removal of particles/colloids by DMF, the performance of an MC operated at pH of 9.2 was comparable to that operated at pH of 11. This justifies the operation of an MC under a moderate alkaline condition rather than a high alkaline condition, for enhancing the feasibility of the use of an MC for recovery of ammonia from municipal wastewater.

Biogas upgrading and digester compactification are needed to expand the application scope of Anaerobic digestion (AD) technology. An anaerobic membrane bioreactor (AnMBR) is a promising reactor for in situ biogas upgrading and digester compactification. This study is the first report achieving in situ biogas upgrading by using a thermophilic AnMBR. Waste activated sludge and H2 were supplied to the AnMBR to increase the CH4 content in the biogas via biomethanation by hydrogenotrophic methanogens. The H2 addition at a 7.5:1 H2:CO2 ratio promoted biogas upgrading, increasing the CH4 content to 94%. With an organic loading rate of 0.78 g-VS/L/d, the CH4 production rate and CH4 yield reached 0.32 L/L/d and 0.38 L/g-VS, respectively. Methanothermobacter was the dominant group among methanogens. Notably, increasing the H2 partial pressure did not inhibit the AD process. In contrast to the AnMBR, a continuously stirred tank reactor did not achieve biogas upgrading, likely because of its lower biomass concentration and H2 conversion into acetic acid via homoacetogenesis. During 111 days of AnMBR operation, the transmembrane pressure remained below 20 kPa at a filtration flux of 4.6 LMH. Physical and chemical cleaning after the AnMBR operation restored 94% of the membrane's water permeability.

Biopolymers have been widely recognized as playing an important role in membrane fouling in membrane bioreactors (MBRs). In-depth characterization of biopolymers in MBRs can provide important insights into membrane fouling. In this study, ultrafiltration was used to selectively isolate biopolymers (recovery rate: 70%) from a pilot-scale MBR treating municipal wastewater. The characteristics of the isolated biopolymers were compared against those of model substances, such as sodium alginate (SA) and bovine serum albumin (BSA), which have been used in many previous studies. Lipopolysaccharides (LPS), which have been proposed as alternative model polysaccharides for MBR fouling research, were also characterized. Batch filtration tests demonstrated that the isolated biopolymers had a considerably higher fouling potential than the model substances. A modified liquid chromatography with organic carbon detection (LC-OCD) analysis showed that the constituents of the isolated biopolymers had molecular weights of >1,000,000 Da. Such macromolecular constituents were only detected in LPS among the model substances. A Fourier-transform infrared (FTIR) analysis indicated various constituents for the isolated biopolymers, such as polysaccharides, proteins and lipids. A quartz crystal microbalance (QCM) analysis demonstrated that the isolated biopolymers had a high affinity to polyvinylidene fluoride (PVDF), explaining the high fouling potential of these biopolymers.

Internal phosphorus (P) release from lake sediments is now recognized as an important P supply that maintains eutrophication, especially in lakes where stratification induces hypoxic conditions in the bottom waters. Freshwater lakes are increasingly threatened by eutrophication and harmful algal blooms. Therefore, to manage lakes, it is important to quantify the internal P release. The internal flux of P (i.e., the orthophosphate (PO4) released from the sediments) may be miscalculated by the methods used to date, such as sediment core samples because the concentrations may be affected when the sediment is disturbed, and the spatial resolution of the sampling may be low. In this study, we developed a novel in-sediment passive sampler to determine the PO4 flux from sediment and deployed it in a eutrophic lake, Lake Barato in Sapporo, Japan. We also deployed Chemcatcher passive samplers for PO4 at the same time to investigate the change in the PO4 concentrations in the water column. With these methods, we obtained the vertical and horizontal distributions of the PO4 concentrations in the sediment porewater across approximately 10 × 20 cm close to the sediment-water interface (SWI) and in the water column. We observed relatively large centimeter-scale PO4 hotspots within the shallow sediment layers (-1 to -5 cm below the SWI). These PO4 hotspots were significantly larger during the summer season than in the other seasons, when thermal stratification and hypoxia influenced the P release. The PO4 fluxes calculated with data from the in-sediment passive samplers ranged from 0.05 to 0.37 mg-P/m2/d, and were considerably lower than the estimates from the conventional sediment core sampling methods. In addition, the data from the Chemcatcher passive samplers showed that the temporal patterns in the time-weighted average PO4 concentrations (around 10 µg-P/L) in the water column were consistent with the patterns from the in-sediment sampler. The results suggest that the in-sediment sampler provided a high-resolution vertical profile of the PO4 concentrations near the SWI with minimal sediment disturbance, and that passive sampling techniques could be used to monitor the fluxes of PO4 released from sediments and the PO4 concentrations in the water column.

We previously proposed a novel approach to mitigate membrane fouling in membrane bioreactors: nanobubble water (NB water) was used for backwashing of membranes used in MBRs. It was demonstrated in our previous work that nanobubble-assisted backwashing (NBB) was more effective than conventional hydraulic backwashing for fouling mitigation. However, there remains room for further improvement in the cleaning efficiency of NBB. In this work, an attempt was made to improve the cleaning efficiency of NBB by adding a small amount of sodium hypochlorite (NaClO) to NB water used for backwashing. The cleaning efficiency of NBB was improved by the synergetic effect of NaClO and NBs working in different ways when they were co-present in the backwash solution. It was suggested that NBs made the structures of the fouling layer looser and more porous, whereas NaClO chemically degraded the components of the fouling layer. The importance of pH in NBB combined with NaClO was shown in this study. The cleaning efficiency was found to be high at pH 4, followed by 8.5 and 12. This difference was mainly attributed to differences in the sizes of NBs under different pH. Thus, the synergetic effects of NBB combined with NaClO were mostly enhanced under acidic conditions. The application of NBB has still been rarely investigated and this study showed for the first time that the efficiency of NBB could be further improved by mixing NB with chemical reagents. This can lead to a new method for alleviating membrane fouling in MBRs.

Municipal wastewater should be regarded as a resource for the establishment of a sustainable society. A combination of high-rate activated sludge and membrane separation (high-rate membrane bioreactors (HR-MBRs)) can be a viable option for efficient recovery of organic matter from municipal wastewater, although severe membrane fouling is expected. The use of ceramic flat-sheet membranes enabled the application of intensive membrane cleaning: granular scouring and chemically enhanced backwash (CEB) with high concentrations of chemical reagents were carried out. To facilitate a long-term operation (e.g., 1 month) of the HR-MBR under the condition of a practical net flux of 16 LMH, a novel in-situ CEB, termed air-exposed CEB (AECEB), was developed in this study. Optimization of the AECEB parameters including choice and concentration of chemicals, duration, and flux were investigated. The optimized AECEB achieved almost complete cancellation of the fouling developed in long-term experiments. However, transmembrane pressure surged when the operation of the HR-MBR was resumed after implementing the AECEB. It was found that scouring agents made from polyethylene glycol released foulants during the AECEB. Based on this finding, alternative scouring agents made from polyurethane were used. The combination of AECEB and granular scouring using the polyurethane agents worked exceptionally well. With the optimized cleaning protocol, the fouling of the HR-MBR was controlled well and long-term operation was possible without implementing off-line cleaning, indicating the feasibility of HR-MBRs to recover organic matter in municipal wastewater treatment (about 70 %) and sufficiently reduce the concentration of organic matter in the effluent (<20 mg-COD/L).

Widespread application of membrane bioreactors (MBRs) is still difficult because of the problem of membrane fouling, which increases operational and maintenance costs. A novel approach to mitigate fouling in MBRs, nanobubble-assisted backwashing (NBB), is presented in this paper. Nanobubbles generated in this study had an average diameter of about 150 nm, and some of the nanobubbles could pass through the pores of microfiltration (MF) membranes used for MBRs. Therefore, nanobubbles introduced from the permeate side during backwashing might exhibit cleaning effects for membranes since applications of nanobubbles for cleaning of various materials such as protein-coated gold electrodes and stainless steel have been reported. The effectiveness of NBB was compared with that of tap water backwashing (TWB) and chemically enhanced backwashing (CEB) using 50 ppm NaClO. Experiments were carried out by using three bench-scale MBRs installed at a local wastewater treatment plant. The three MBRs were operated in parallel with identical operating conditions except for backwashing conditions. Polyethylene glycol (PEG) granular materials were inserted in each membrane tank to scour the membrane surface. Regardless of the variation of the feed, NBB always exhibited better cleaning performance than TWB, and the cleaning efficiency of NBB was sometimes comparable to that of CEB. The degree of reversible fouling observed with TWB was 424 % higher than that with NBB, indicating that NBB is much more effective than TWB for the control of reversible fouling. It was also found that NBB could mitigate irreversible fouling even in the absence of a thick protective layer on the surface of the membrane. It was attempted to reveal the mechanism by which NBB enhances the cleaning effect by carrying out comprehensive analysis of the distribution of filtration resistance, mixed liquor suspension and the fouling layer. Significant changes in microbial activity of the mixed liquor suspension and sludge properties in the MBRs were not observed when NBB was carried out. The results of the analysis suggested that NBB made the structure of the fouling layer more porous, possibly facilitating the removal of the fouling layer from the membrane.

Passive sampling is a technique for monitoring orthophosphate (PO4-P) in the water environment. Compared with traditional grab sampling followed by PO4-P quantification, kinetic-type passive samplers such as Chem catcher & REG; express representative concentrations of PO4-P as time-weighted average concentrations (CTWA). They can also potentially evaluate much lower PO4-P concentrations, but the available receiving phases of Chem catcher & REG; used for PO4-P were extremely limited. We developed a new receiving phase, the PSfZS sheet, comprising a zirconium sulfate-surfactant micelle mesostructure and polysulfone matrix. We examined its performance in terms of PO4-P sorption characteristics, PO4-P selectivity, and PO4-P sampling rate (Rs). Its capacity was adequate (12.0 & mu;g-P/cm2) and selectivity for PO4-P uptake was good. The Rs for PO4-P increased with increasing water temperature (8.1-29.1 degrees C) and decreasing pH (4.1-9.7) in a laboratory calibration, and ranged from 5.27 x 10-2 L/d to 1.66 x 10-1 L/d. We placed the samplers in a municipal wastewater treatment plant, a shallow eutrophic lake, and an oligotrophic caldera lake. The Rs in the deployment sites was calibrated by monitored water temperature and pH. The estimated CTWA of PO4-P in the municipal wastewater treatment plant was similar to the averaged concentration of soluble reactive phosphorus determined by multiple grab samplings. In the lake deployments, we found that the new sampler can quantify CTWA values of PO4-P below 10 & mu;g/L, and thus it provides more technical monitoring options and contributes to the conservation and management of the water environment.

高速膜分離活性汚泥法(高速MBR)は下水中有機物の分解を抑制して嫌気性消化で活用できる有機物量を増加させる(有機物回収率>80%)のと同時に,膜分離によって処理水質の高度化(有機物除去率>90%)を達成する処理技術である.高速MBRでは極短SRTを設定することに伴う深刻な膜の目詰まり(膜ファウリング)が問題となる.本研究では,高速MBRに担体による高強度物理洗浄と薬品添加逆洗(CEB)を適用し,担体の種類やCEB条件が膜ファウリング特性に与える影響を調査した.スポンジ担体は高い膜洗浄効果を発揮し,高速MBRは30日間に渡って安定した連続運転が可能であった.高頻度のCEBは膜ファウリングの発生を加速させる傾向が観察された.

膜ろ過を用いた浄水処理・下水処理において,高分子量親水性有機物であるバイオポリマー画分が膜ファウリングの発生に強く関与することが示されている.これまでに浄水処理と下水処理におけるバイオポリマーの詳細な比較は行われておらず,その違いはほとんど明らかになっていない.本研究では異なる時期に水道原水とMBR槽内水からバイオポリマーを回収・精製し,それぞれのバイオポリマー試料の膜ファウリングポテンシャルと物理・化学的特性の差異を検討した.MBR槽内水から回収したバイオポリマーの膜ファウリングポテンシャルは明らかに水道原水中バイオポリマーのそれよりも高かった.QCM分析によるバイオポリマー試料と膜材質との親和性評価結果は膜ファウリングポテンシャルの大小と非常によく一致し,膜ファウリングポテンシャルが膜材質との親和性に大きく影響されることが示された.LC-OCD/UVD/OND分析とFT-IR分析により,本研究で検討した二種類のバイオポリマーには明確な差異があることが明らかになった.MBR槽内水中バイオポリマーにはアミノ糖やリポ多糖様成分が含まれており,このことが膜ファウリングポテンシャルの上昇に関係していた可能性がある.

High-rate processes have been investigated for the recovery of organic matter from municipal wastewater. High-rate membrane bioreactors (HR-MBRs) may simultaneously achieve the increased recovery of carbon and high effluent quality, although control of membrane fouling is extremely difficult. To address the severe fouling in HR-MBRs, the combination of granular scouring and frequent chemically enhanced backwashing was examined. The use of robust flat-sheet ceramic membranes enabled the application of those cleaning strategies. Experiments were carried out at an existing wastewater treatment plant. To operate as a high-rate system, the bioreactor solid residence time and hydraulic residence time were set at 0.5 days and 1.6 h, respectively. Although a relatively high flux of 20 L m−2 h−1 was applied, the proposed HR-MBR exhibited a very low fouling rate of 1.3 kPa/day. The system could recover >70% of the carbon from raw wastewater, whereas the concentration of chemical oxygen demand in the effluent was lowered to <20 mg/L. The performance of the proposed HR-MBR observed in this study was clearly superior to those reported in previous related studies.

In this study, the fouling potential of mixed liquor suspension samples collected from a pilot-scale membrane bioreactor (MBR) that treated municipal wastewater was monitored for more than 3 years. The fouling potential was assessed by batch filtration experiments using the same type of membrane as equipped in the MBR. The fouling potential increased when the temperature of the mixed liquor suspension in the MBR decreased. However, the polysaccharide and protein concentrations in the mixed liquor suspension, which have been focused on many previous studies, did not correlate with the fouling potential (R2 = 0.15 and 0.39, respectively). In contrast, the concentration of biopolymers, quantified by liquid chromatography-organic carbon detection (LC-OCD), exhibited a marked correlation with the fouling potential (R2 = 0.89). A high concentration of biopolymers with large molecular weight (>1 million Da) was likely responsible for the high fouling potential. Fourier transform infrared (FTIR) analysis of the dissolved organic matter in the mixed liquor suspension indicated that the chemical properties of the biopolymers considerably varied with the seasonal temperature variation, which has rarely been reported and gives insights into fouling in MBRs. The effect of temperature on the biopolymer concentration and molecular weight of biopolymers was also investigated in a separate bench-scale experiment in which temperature was controlled. It was clearly shown that a low temperature induced an increase in the biopolymer concentration and an associated increase in the fouling potential of the mixed liquor suspension.

Ceramic membranes are physically and chemically robust, and intensive membrane cleaning can therefore be used. The use of ceramic membranes may alleviate the problem of membrane fouling. In this study, fouling in flat-sheet ceramic membranes operated with intensive mechanical cleaning (scouring with granular materials) was investigated in the context of drinking water production. Samples collected from multiple drinking water treatment plants were used as the feed. Bench-scale experiments were carried out in a realistic style: with a constant flow rate and periodical hydraulic backwashing. Granular materials for the purpose of scraping off the fouling layer from the membrane surface were placed in the membrane tank (10 % v/v) and they moved freely by the aid of periodical aeration provided during the routine backwashing. Operation under the condition of a high membrane flux of 125 LMH was possible with little fouling when granular scouring was carried out with pretreatment using PACl coagulation. As for irreversible fouling, in-line chemical membrane cleaning at a low frequency (1-hour cleaning/120 h of filtration) using oxalic acid followed by NaClO exhibited a very high cleaning efficiency (permeability recovery of > 99 %). Analysis of the foulants extracted from the fouled ceramic membrane revealed that the properties and compositions of the foulants causing the irreversible fouling in ceramic membranes were different from those in polymeric membranes. Analysis using liquid chromatography with organic carbon detection (LC-OCD) suggested that humics were dominant (35 % of the total organic carbon) in the foulant extracted from the fouled membranes, instead of biopolymers (11 % of the total organic carbon) that have been shown in recent studies to be the major foulants for polymeric membranes. However, spectral analysis of the foulant using infra-red and fluorescence and filtration data obtained with pretreatments suggested the feature of biopolymers, particularly polysaccharides. These discrepancies indicate the necessity for new analytical methods to distinguish overlaps between humics and biopolymers.

High molecular weight biopolymers quantified by a liquid chromatography with organic carbon detection (LC-OCD) system have been recognized as major players in fouling that occurs in microfiltratrion (MF) and ultrafiltration (UF) of surface water. Although detailed investigation of biopolymers is important, there have been few studies in which real biopolymers isolated with sufficient purities and high recoveries were used. In this study, cross-flow UF using membranes with a molecular weight cut-off (MWCO) of 13,000 was performed to isolate biopolymers from surface water. Merely carrying out UF, however, did not result in a high purity of biopolymers in the isolated product: significant contamination by humics became a problem. Adjustments of electric conductivity and calcium concentration during UF were found to be effective. With the optimized conditions, humics could be eliminated almost completely and the isolated biopolymers were obtained with a high recovery rate (67–85%). Isolated biopolymers were compared with model biopolymers such as alginate. Fouling potentials and physical-chemical properties of the isolated biopolymers were considerably different from those of model compounds. The results obtained in this study give rise to an important question of whether common biopolymers such as alginate should be blindly used for fouling studies: they might not represent fouling caused by real biopolymers.

Nanofiltration (NF) is utilized in water treatment for controlling disinfection by-products formation potential (DBPFP) and disinfection by-products (DBPs). Attention regarding NF-based technology has been paid on membrane fouling of NF and the rejection efficiency of contaminants by NF membranes. Natural organic matter (NOM) presenting in surface waters is one main removal target in drinking water treatment by NF-based technology, and is thereby a contributor to the membrane fouling of NF. In application, pretreatments of other membrane filtration (e.g., microfiltration (MF) and ultrafiltration (UF)) has been taken prior to NF, resulting in the separation of NOM of specific molecular weight. Meanwhile, it is well known that NOM is composed of organic compounds of different molecular weights. However, the effect of NOM of specific molecular weight has been seldom investigated from the aspects of membrane fouling and the resulting DBPFP after membrane filtration. By using combinations of MF and UF (molecular weight cut-off of 100K or 20K) as pretreatment prior to NF, the NOM of various molecular weight on DBPFP and DBPs in the NF-treated water were investigated. The experiments were conducted with two real-world surface water samples and one tap water sample. It was found that medium molecular weight NOM, defined as NOM that passed UF100K but did not pass UF20K in this study, reduced fouling of the NF membrane. This is supported by the excitation and emission matrix (EEM) fluorescence spectra, size exclusion chromatography (SEC) and flux analysis. In addition, the medium molecular weight NOM also reduced the DBPFP in the NF treated water and eventually the DBPs by participating in forming a protective layer on the NF surface, blocking the transfer of small molecular weight NOM into the NF filtrate, thereby reducing the DBPFP of the NF filtrate since small molecular weight NOM was the major contributor to DBPFP in this study.