Gravity-driven Membrane Filtration Research Articles

Targeting membrane fouling with low dose oxidant in drinking water treatment: Beneficial effect and biological mechanism

Membrane fouling is the principal factor that currently limits the performance of gravity-driven membrane (GDM) filtration systems in drinking water treatment. In this study, the benefits of applying a low dose (approximately 0.1 mg·L−1) of environmentally benign oxidants, both H2O2 and KMnO4, as a pretreatment to GDM filtration system has been evaluated in terms of reduced membrane fouling and treated water quality. While both oxidants improved permeate flux, the effect of KMnO4 was greater than H2O2. Both oxidants reduced the size of influent organic substances and those of large molecular weight (>20 kDa), such as biopolymers, disappeared. The thickness of the fouling layers was substantially reduced after oxidation, and the KMnO4 system had a markedly different physical structure of fouling layer, with an apparent sub-layer of δ-MnO2 nanosheets below a fouling sub-layer. The formation of the δ-MnO2 nanosheets sub-layer appeared to protect the underlying membrane pores from contamination by influent organics. Oxidation pretreatment reduced the presence of proteins and polysaccharides in the fouling layers and significantly altered the bacterial community structures (p < 0.01) and decreased biodiversity. The microbial species that secreted amounts of extracellular polymeric substances (EPS), such as Xanthobacter, were not eliminated in the H2O2 fouling layer, while for the KMnO4 system, the manganese oxidizing bacteria (MOB; e.g., Pseudoxanthomonas) and metal-resistant genus Acidovorax, dominated the community.

Dynamics of microbial community and their effects on membrane fouling in an anoxic-oxic gravity-driven membrane bioreactor under varying solid retention time: A pilot-scale study

Membrane fouling in a membrane bioreactor (MBR) is highly influenced by the characteristics of the influent, the mixed liquor microbial community and the operational parameters, all of which are environment specific. Therefore, we studied the dynamics of microbial community during the treatment of real municipal wastewater in a pilotscale anoxic-oxic (A/O) MBR equipped with a gravity-driven membrane filtration system. The MBR was operated at three different solid retention times (SRTs): 25, 40 and 10 days for a total period of 180 days in Nordic environmental conditions. Analysis of microbial community dynamics revealed a high diversity of microbial species at SRT of 40 days, whereas SRT of 25 days was superior with microbial richness. Production of soluble microbial products (SMP) and extracellular polymeric substances (EPS) was found to be intensely connected with the SRT and food to microorganism (F/M) ratio. Relatively longer operational period with the lowest rate of membrane fouling was observed at SRT of 25 days, which was resulted from the superior microbial community, lowest production of SMP and loosely bound EPS as well as the lower filtration resistance of larger sludge flocs. Abundance of quorum quenching (QQ) bacteria and granular floc forming bacterial genera at SRT of 25 days provided relatively lower membrane fouling tendency and larger floc formation, respectively. On the other hand, substantial amount of various surface colonizing and EPS producing bacteria was found at SRT of 10 days, which promoted more rapid membrane fouling compared with the fouling rate seen at other tested SRTs. To sum up, this research provides a realistic insight into the impact of SRT on microbial community dynamics and resulting characteristics of mixed liquor, floc size distribution and membrane fouling for improved MBR operation.

Presence of powdered activated carbon/zeolite layer on the performances of gravity-driven membrane (GDM) system for drinking water treatment: Ammonia removal and flux stabilization

Gravity-driven membrane (GDM) filtration is a promising alternative for decentralized water supply, while its widespread application was hindered by the poor removals of organics and ammonia during long-term operation. In this study, powered activated carbon (PAC) and granular zeolite were selected as typical adsorbents to investigate the impacts of pre-deposited adsorbent layers on contaminant removal and membrane fouling. Results showed that the pre-deposited PAC layers exhibited higher removal of organics than the control, while the zeolites deposited layers exhibited low removal of organics. The presence of PAC only enhanced the NH4+ removal at subsequent stable stage, while zeolites were effective in deal with sudden high NH4+ concentration due to ion exchange. The presence of mixed adsorbents layers had similar organic removal with PAC and NH4+ removal with zeolite. The pre-deposited PAC layers could effectively alleviate membrane fouling in short-term UF tests, while the stable fluxes (5.88–6.54 L/(m2·h)) in long-term GDM operation were slightly lower than the control (6.63 L/(m2·h)). The zeolites deposited layer aggravated membrane fouling in both short-term ultrafiltration and long-term GDM (5.03–3.84 L/(m2·h)), but a higher stable flux (6.10 L/(m2·h)) was observed for GDM using the mixed adsorbents. The pre-deposited adsorbent layers resulted in increased concentrations of biomass, tri-phosphate (ATP) and extracellular polymeric substances (EPS), forming cake layers with a denser structure than the control. Finally, the fouling mechanism for GDM using different adsorbent layers was proposed based on fouling analysis and characteristics of biological fouling layer. The results and conclusion in this study could provide helpful information for the application of GDM with pre-deposited adsorbent layer in treating raw water with organics and/or sudden high ammonia concentration to produce potable water.

Can pre-ozonation be combined with gravity-driven membrane filtration to treat shale gas wastewater?

Low-cost gravity-driven membrane (GDM) filtration has the potential to efficiently manage highly decentralized shale gas wastewater (SGW). In this work, the feasibility of combining low dosage pre-ozonation with the GDM process was evaluated in the treatment of SGW. The results showed that pre-ozonation significantly increased the stable flux (372%) of GDM filtration, while slightly deteriorating the quality of the effluent water in terms of organic content (−14%). These results were mainly attributed to the conversion of macromolecular organics to low-molecular weight fractions by pre-ozonation. Interestingly, pre-ozonation markedly increased the flux (198%) in the first month of operation also for a GDM process added with granular activated carbon (GGDM). Nevertheless, the flux of O3-GGDM systems dropped sharply around the 25th day of operation, which might be due to the rapid accumulation of pollutants in the high flux stage and the formation of a dense fouling layer. Pre-ozonation remarkably influenced the microbial community structure. And O3-GDM systems were characterized by distinct core microorganisms, which might degrade specific organics in SGW. Furthermore, O3-GDM outperformed simple GDM as a pretreatment for RO. These findings can provide valuable references for combining oxidation technologies with the GDM process in treating refractory wastewater.

Appropriate Technology and Field Application of Non-powered Water Purification System Using Nanofiber Membrane

ì•ˆì „í•œ 음용수 확보를 위한 ì ì •ê¸°ìˆ ë¡œì¨ í™˜ê²½ì , ê¸°ìˆ ì  요구사항을 ì¶©ì¡±í• ìˆ˜ 있는 ë‚˜ë ¸ì„¬ìœ ë©¤ë¸Œë ˆì¸ìœ¼ë¡œ 구성된 ë¬´ë™ë ¥ 막 여과 ì‹œìŠ¤í œì„ 평가하였다. 이 장치는 수두차에 의한 ì¤‘ë ¥ê³¼ ìƒë¬¼í•™ì  막 오염층 ì œì–´ë¡œ 별도의 에너지원이 필요 ì—†ê³ , 핵심 소재인 PVDF ë‚˜ë ¸ì„¬ìœ ë©¤ë¸Œë ˆì¸ 필터가 ë³‘ë ¬ë¡œ 연결, 모듈화되어 있어 물 생산성을 높이는 구조이다. 이 장치의 ì‹¤ì œ 현장 ì ìš© 가능성을 평가하기 위해 Pilot-scale (3000-5000 L/day) ë‚˜ë ¸ì„¬ìœ ë©¤ë¸Œë ˆì¸ 기반 ì •ìˆ˜ ì‹œìŠ¤í œì´ 개발도상국(키리바시, 투발루 등)에 2017ë „ 8월 설치되어 3개월간 운영되었다. 14-92 L/(m2×h)의 플럭스로 ì•ˆì •ì  물 생산성을 í™•ì¸í•˜ì˜€ê³ ì²˜ë¦¬ìˆ˜ì˜ 탁도와 ë°•í Œë¦¬ì•„ì˜ 높은 ì œê±°ìœ¨ (99.99% 이상)로 ì•ˆì „í•œ 수질을 장기간 ì œê³µí• ìˆ˜ 있음을 확인하였다. 이러한 결과는 현장 ì ìš©ì„ 통해 ë‚˜ë ¸ì„¬ìœ ë©¤ë¸Œë ˆì¸ 기반 ë¬´ë™ë ¥ ì •ìˆ˜ ì‹œìŠ¤í œì´ 장기간 ì•ˆì „í•œ 음용수를 ê³µê¸‰í• ìˆ˜ 있는 ì •ìˆ˜ìž¥ì¹˜ë¡œ í‰ê°€ë˜ì—ˆê³ , ì ì •ê¸°ìˆ ë¡œì¨ 개도국의 수처리 장치로 활용 가능성을 보여준다. Gravity-driven membrane (GDM) filtration system based on the nanofiber membrane was investigated. This system can be operated with little energy demand due to a gravitational pressure-driven filtration and biological fouling control strategy. Moreover, the optimal module configuration based on the high permeance of nanofiber membrane can provide a significantly high water productivity. In order to evaluate its applicability potential, the pilot-scale (3000-5000 L/day) systems with nanofiber membranes were operated in developing countries (Kiribati and Tuvalu). Our results showed that the 14- 92 L/(m2×h) of the permeate flux was determined indicating a stabilized water productivity. In addition, the permeate water indicated a high removal rate (more than 99.99%) of turbidity and bacteria. Consequently, the system can provide a stabilized water production with safe permeate water quality during long-term operation. These findings exemplify an effective approach to decentralized drinking water treatment for developing countries. Keywords: Nanofiber membrane, Decentralized water treatment, Gravity-driven membrane filtration, Permeate water quality

Unraveling Membrane Fouling Induced by Chlorinated Water Versus Surface Water: Biofouling Properties and Microbiological Investigation

• Chlorinated tap water has more severe biofouling potential than surface water. • Densely compact morphology was observed for tap-water biofilm than surface-water. • Xanthobacter was the dominant taxon in tap-water biofilm. • Hydrophilicity of membrane plays a minor role in biofouling formation. Chlorine is usually applied in the urban water treatment process to deactivate pathogens and prevent waterborne diseases. As a pre-treatment, it remains unclear whether chlorinated water can effectively alleviate membrane fouling during ultrafiltration (UF). In this study, tap water was investigated for its effect on biofilm formation and biofouling in a gravity-driven membrane (GDM) filtration system. For comparison, biofilm/biofouling with untreated surface (lake) water was studied in parallel. It was found that more severe membrane fouling occurred with the filtration of tap water than lake water, and larger quantities of polysaccharide and extracellular DNA (eDNA) were present in the tap-water biofilm than in the lake-water biofilm. The tap-water biofilm had a densely compact morphology. In contrast, a porous, spider-like structure was observed for the lake-water biofilm, which was assumed to be associated with the bacteria in the biofilm. This hypothesis was verified by 16S ribosomal RNA (rRNA) sequencing, which demonstrated that Xanthobacter was the dominant taxon in the tap-water biofilm. Additionally, membrane hydrophobicity/hydrophilicity played a minor role in affecting the membrane fouling properties and microbial community. This study revealed the significant role of chlorine-resistant bacteria in biofouling formation and provides a deeper understanding of membrane fouling, which can potentially aid in searching for effective ways of controlling membrane fouling.

Assessment of low-cost, non-electrically powered chlorination devices for gravity-driven membrane water kiosks in eastern Uganda

Recontamination during transport and storage is a common challenge of water supply in low-income settings, especially if water is collected manually. Chlorination is a strategy to reduce recontamination. We assessed seven low-cost, non-electrically powered chlorination devices in gravity-driven membrane filtration (GDM) kiosks in eastern Uganda: one floater, two in-line dosers, three end-line dosers (tap-attached), and one manual dispenser. The evaluation criteria were dosing consistency, user-friendliness, ease of maintenance, local supply chain, and cost. Achieving an adequate chlorine dosage (∼2 mg/L at the tap and ≥ 0.2 mg/L after 24 h of storage in a container) was challenging. The T-chlorinator was the most promising option for GDM kiosks: it achieved correct dosage (CD, 1.5–2.5 mg/L) with a probability of 90 per cent, was easy to use and maintain, economical, and can be made from locally available materials. The other in-line option, the chlorine-dosing bucket (40 per cent CD) still needs design improvements. The end-line options AkvoTur (67 per cent CD) and AquatabsFlo® (57 per cent CD) are easy to install and operate at the tap, but can be easily damaged in the GDM set-up. The Venturi doser (52 per cent CD) did not perform satisfactorily with flow rates &gt; 6 L/min. The chlorine dispenser (52 per cent CD) was robust and user-friendly, but can only be recommended if users comply with chlorinating the water themselves. Establishing a sustainable supply chain for chlorine products was challenging. Where solid chlorine tablets were locally rarely available, the costs of liquid chlorine options were high (27–162 per cent of the water price).

Effect of coagulation pretreatment on the performance of gravity-driven membrane filtration with Yangtze River water

Gravity-driven membrane (GDM) filtration is promising for decentralized drinking water treatments. To improve the permeability and permeate quality of GDM process, three GDM systems (in-line coagulation system, pre-coagulation system, and system without coagulation as control) were operated in parallel at 65 mbar for 63 days to evaluate the effects of coagulation on flux development, permeate quality, composition and structure of the biofouling layer. The optimal dose of coagulant (polyaluminium chloride) was determined as 4 mg/L. The stable flux increased by 140% and 210% in the in-line coagulation and pre-coagulation systems compared to that of the control system, respectively. Dissolved organic compounds were removed by 31.9% and 29.4% in the in-line coagulation and pre-coagulation systems, while the control system showed a poor removal efficiency (5%). The effective removal of organics from raw water by coagulation resulted in less substrates available in the biofouling layers, thus affecting the production of extracellular polymeric substances (EPS) by limiting the biological processes. The EPS content in the biofouling layer of pre-coagulation system was the lowest, consistent with the highest flux. And such nutrient-poor biofouling layers modified the microbial community, changing the dominant bacteria from Firmicutes to Actinobacteria, thereby affecting the flux. Loose and heterogeneous biofouling layers with numerous holes and cracks were observed under coagulation conditions, which was conducive to the passage of water. All systems exhibited excellent ammonia removal efficiency (96%), but coagulation extended the start-up time for nitrification from 3 days to 5 days. Overall, these results demonstrate that coagulation can improve the GDM filtration performance, and the pre-coagulation system is globally optimal.

Effects of predator movement patterns on the biofouling layer during gravity-driven membrane filtration in treating surface water

Biological predation has a significant effect on biofouling layers in gravity-driven membrane (GDM) filtration systems. However, the detailed process of predatory activities is still not well known. This study explored the effects of predator movement patterns on the biofouling layer at different temperatures and the factors affecting the stable flux level. The results indicated that Demospongiae, Spirotrichea and Saccharomycetes were the main species, with the body contracting or rotating in one position at 5 °C, and Litostomatea accounted for 55.1% at 10 °C. The weak agility of these species resulted in a less porous biofouling layer with a high extracellular polymeric substance (EPS) concentration, which was responsible for the low permeate flux and the time to reach flux stability. Bdelloidea was dominant at 20 and 30 °C, and the more heterogeneous biofouling layer with a lower EPS concentration was related to their intense creeping and swimming movements and their ability to create current in the water. The grazing of spongy flocs by predators affected the GDM system performance, and a high stable flux was obtained with large and loose flocs. In addition, the diversity of the eukaryotic community decreased after the flux stabilized due to the particular predominance of Bdelloidea at high temperatures, corresponding to a high stable flux. Pollutant removal was less affected by eukaryotes, and decreased ammonia nitrogen removal rates were related to the lower activity of nitrifying bacteria. Moreover, the reliable linear correlation between the temperature and the stable flux implied that the stable flux could be well predicted in the GDM system. The findings are beneficial for developing new strategies for regulating flocs and the biofouling layer to improve the performance of GDM systems.

Effect of Microplastics on the Physical Structure of Cake Layer for Pre-Coagulated Gravity-Driven Membrane Filtration

Effect of Microplastics on the Physical Structure of Cake Layer for Pre-Coagulated Gravity-Driven Membrane Filtration

Integrating granular activated carbon (GAC) to gravity-driven membrane (GDM) to improve its flux stabilization: Respective roles of adsorption and biodegradation by GAC

As a low-maintenance and cost-effective process, gravity-driven membrane (GDM) filtration is a promising alternative for decentralized drinking water supply, while the low flux impedes its extensive application. In order to address such issue, an integrated process consisting of granular activated carbon (GAC) layer and GDM was developed. The performance of virgin (fresh GAC) or preloaded GAC (saturated GAC) was compared. Flux stabilization was observed both in the fresh and saturated GAC/GDM process during long-term filtration and their stable fluxes were both improved by approximately 50% relative to the GDM control. Moreover, integrating GAC with GDM contributed to efficient removals for dissolved organic compounds (DOC), assimilable organic carbon (AOC) and low molecular weight substances both in fresh and saturated GAC/GDM filtration. Compared to GDM control, coupling GAC to GDM could significantly reduce the concentrations of extracellular polymeric substances (EPS) and total cell counts (TCC) within the biofouling layer, and engineer highly heterogeneous structures of biofouling layer on the membrane surface. In the fresh GAC/GDM process, the improved flux obtained was mainly related to less coverage of biofouling layer and lower EPS concentrations due to efficient removals of membrane foulants by GAC adsorption. The achieved higher stable flux can be maintained during long-term filtration (after GAC saturation) owing to the combined effects of EPS reduction and formation of highly heterogeneous structures of biofouling layer in the saturated GAC/GDM system. Overall, the integrated GAC/GDM process can hopefully facilitate improvements both in the stabilized flux and permeate quality, with practical relevance for GDM applications in decentralized drinking water supply.

Can ultrafiltration singly treat the iron- and manganese-containing groundwater?

The presence of Fe2+ and Mn2+ would cause severe ultrafiltration (UF) membrane fouling and limited its extensive application in treating the groundwater. A pilot-scale gravity-driven membrane (GDM) process which coupled the dual roles of biocake layer and UF membrane was introduced to treat the groundwater under high Mn2+concentrations and low temperature conditions. The results indicated that flux stabilization was observed during long-term GDM filtration with average stabilized fluxes of 3.6–5.7 L m−2 h−1. GDM process conferred efficient removals of Fe2+ and Mn2+ with both average removals > 95%. Pre-adding manganese oxides (MnOx) could effectively shorten the ripening period of manganese removal from 50 to 30 days, and simultaneously contribute to the Mn2+ removal and flux improvements. The presence of Mn2+ facilitated the formation of heterogeneous structures of biocake layer to primarily determine the flux stabilization of GDM, while the influence of extracellular polymeric substances (EPS) concentrations was nearly negligible. Besides, the Mn2+ removal was primarily attributed to the biocake layer other than UF membrane itself, and the chemically auto-catalytic oxidation by MnOx particles played the pivotal role. Therefore, these findings provide relevance for establishing new strategies in treating the iron-and manganese-containing groundwater.

Relationships among Permeability, Membrane Roughness, and Eukaryote Inhabitation during Submerged Gravity-Driven Membrane (GDM) Filtration

Gravity-driven membrane (GDM) filtration is one of the promising technologies for decentralized water treatment systems due to its low cost, simple operation, and convenient maintenance. The objective of this study was to evaluate the permeability of submerged GDM filtration with three different membranes, i.e., polyethersulfone and polyvinylidene difluoride ultrafiltration (PES-UF and PVDF-UF) and polytetrafluoroethylene microfiltration membrane (PTFE-MF). The GDM system was operated using lake water for about one year. The determined average permeability values were high for PVDF-UF (192.9 L/m2/h/bar (LMH/bar)) and PTFE-MF (80.6 LMH/bar) and relatively lower for PES-UF (46.1 LMH/bar). The observed higher permeability for PVDF-UF and PTFE-MF was thought to be related to the rougher surface of these two membranes compared to PES-UF. The fouling layers of PVDF-UF and PTFE-MF were characterized by high biomass and the presence of a number of nematodes, while PES-UF showed a thin fouling layer with no nematode. The relatively high and fluctuated permeability of PVDF-UF and PTFE-MF could thus be attributed to the high biological activity of nematodes making the fouling layer more loose and porous. This was supported by a good linear relationship among the permeability, biomass concentration, and the number of nematodes in the fouling layers. These results provide important insights into membrane selection as a critical factor affecting the flux performance of the GDM filtration system for a decentralized drinking water supply.

Bio-cake layer based ultrafiltration in treating iron-and manganese-containing groundwater: Fast ripening and shock loading

Groundwater was a desired alternative for decentralized water supply. However, the presence of iron, manganese and ammonia significantly limited its extensive adoptions. In this study, an innovative gravity-driven membrane (GDM) process has been developed to address such problems. The results indicated that GDM process can efficiently diminish the concentrations of iron, manganese and ammonia, with average removal efficiencies of 97%, 95% and 70%, respectively, since the bio-cake layer on the membrane surface can serve as a dynamic barrier for the foulants rejection. In GDM filtration, the manganese removal was mainly attributed to the synergistic effects between the chemically auto-catalytic oxidation by manganese oxides (MnOx) and biological activity by manganese-oxidizing bacteria (MnOB). Pre-addition of MnOx particles into GDM system could significantly enhance the manganese removal and shorten its ripening time by approximately 50%. During long-term filtration, the fluxes of GDM remained stabilized (4–5 L m−2 h−1), and MnOx particles pre-additions could improve the stable fluxes by 23%–37%. The flux stabilization of GDM process was mainly determined by the heterogeneous structures of bio-cake layer, and the generated iron and manganese oxides would improve its heterogeneities. Furthermore, MnOx assisted GDM process conferred robust capacities in resisting the shock loading of manganese and ammonia in the feed water, and the highest concentrations of manganese and ammonia were suggested to be less than 2.96 mg/L and 0.9 mg/L, respectively. Therefore, these findings are full of relevance to develop new strategies to treat the iron- and manganese-containing groundwater and promote the extensive application of UF technology for decentralized water supply.

Algae-Laden Fouling Control by Gravity-Driven Membrane Ultrafiltration with Aluminum Sulfate-Chitosan: The Property of Floc and Cake Layer

Gravity-driven membrane (GDM) ultrafiltration is a promising water treatment method due to its low energy consumption and low maintenance. However, the low stable permeability in algae-laden water treatment is currently limiting its wider application. With the ultimate goal of increasing permeability, the aim of this study was to evaluate the effect of a composite coagulant of aluminum sulfate-chitosan (AS-CS) on the GDM filtration performance. In parallel tests with a single AS coagulant and without pre-coagulation, the analysis of membrane fouling resistance and the membrane fouling mechanism were evaluated. The results indicated that the AS-CS/GDM system can alleviate 23.74% and 58.80% membrane fouling, respectively, compared with AS/GDM and the GDM system. The AS-CS/GDM system can effectively remove humic-like substances having a molecular weight (MW) of 3–100 kDa, resulting in removal of 98.32% of algae cells and removal of 66.25% of dissolved organic carbon; the AS-CS/GDM system thereby improved the concentration of attached biomass on the membrane surface with the stronger biodegradability of organic matters. The application of AS-CS pre-coagulation in the GDM process could enhance the proliferation of microorganisms and the removal of low molecular weight humic-like substances. Therefore, the AS-CS/GDM system is a potentially important approach for algae-laden water treatment.

Contribution of biofilm layer to virus removal in gravity-driven membrane systems with passive fouling control

Passive gravity-driven membrane (PGM) filtration is a type of gravity-driven membrane (GDM) filtration operated with passive physical fouling control measures that add limited to no complexity to the system (e.g. permeate flux interruptions, gravity-driven air scouring, system draining), allowing a greater permeate flux to be sustained. A key aspect of PGM/GDM systems is the development of a structurally loose and permeable biofilm layer on the membrane that enables a sustainable flux to be achieved and has also been associated with improved removal of humic acids, polysaccharides, proteins, assimilable organic carbon and microcystins. The present study investigated if the biofilm layer of PGM systems also contributes to virus removal, for both intact and breached PGM systems. Breach sizes considered ranged from 20 to 180 µm. Challenge tests (CTs) identified an increase of 2.0+ in the log removal value (LRV) for viruses in both intact and breached PGM systems when a biofilm layer was present, suggesting that the biofilm layer is capable of bridging the gap over integrity breaches, acting as a secondary barrier to contaminants that would otherwise bypass treatment by flowing through the breach. Pressure decay tests (PDTs), however, did not identify the same increase in LRVs as that of CTs, suggesting that the standard PDT approach cannot consider the contribution of the biofilm layer to the removal of small material such as viruses. An alternative integrity testing protocol for PGM systems was developed using a modified PDT approach that takes into account the additional removal provided by the biofilm layer. This alternative protocol is also simpler and requires less frequent testing, contributing to the simplification of overall operation of PGM systems in small/remote communities and decentralized applications.

Gravity-driven membrane filtration treating manganese-contaminated surface water: Flux stabilization and removal performance

Severe membrane fouling caused by iron and manganese limited the intensive application of ultrafiltration (UF) technologies. In the present study, an innovative biofilm based UF process, gravity driven membrane (GDM) filtration, was employed to evaluate its possibility to directly treat manganese-contaminated surface water at high concentration. Surprisingly, an extremely short and stable ripening period (less than 10 d) of iron and manganese removal was achieved in GDM filtration, with average removals of 90% and 58%, respectively, attributing to the effective rejection of active catalytic manganese oxides and fast colonization of iron- and manganese-oxidizing bacteria (e.g Acinetobacter and Lysobacter) within the biofilm due to the efficient rejection by UF membrane. Importantly, the presence of iron and manganese did not influence the occurrence of flux stabilization, and fluxes stabilized at 7–9 L m−2 h−1 during long-term filtration. Furthermore, a highly rough, porous and heterogeneous biofilm was observed on the membrane surface, which was potentially responsible for the flux stabilization. Additionally, pre-coating manganese oxides on the membrane surface can efficiently enhance the removals of manganese and iron, and engineer even highly heterogeneous structures of biofilm to improve the stable flux. Therefore, the findings are relevant to develop new strategies for the treatments of manganese-contaminated water resource at high concentration, and to encourage extensive applications of membrane technologies for decentralized drinking water supply.

Gravity-driven membrane (GDM) filtration of algae-polluted surface water

This study compared the membrane performances and water quality of gravity-driven membrane (GDM) systems in treating algae-polluted lake water under different operation conditions (microfiltration (MF) vs. ultrafiltration (UF); Chlorella vulgaris (green algae) vs. Phaeodactylum tricornutum (diatom); different algal amounts in the lake water). The results showed the cake layer fouling was predominant in the UF-GDM systems, while irreversible fouling contributed majorly to the MF-GDM fouling. As a result, the UF-GDM systems achieved >1.5 time higher permeate flux compared to the MF-GDM systems in treating algae-polluted lake water. Compared to the green algae, the presence of the diatom cells in the feed water had more negative impacts on the UF permeate flux (increasing the cake layer resistance) and water quality (containing more low molecule weight neutrals). The analysis of cake layer foulants revealed that more aromatic protein-based biopolymers were accumulated on the membranes during filtration of algae-polluted lake water and the biopolymer amounts were almost linearly associated with membrane fouling potential of the GDM systems.

A combination of membrane relaxation and shear stress significantly improve the flux of gravity-driven membrane system

Gravity-driven membrane (GDM) filtration system is a promising process for decentralized drinking water treatment. During the operation, membrane relaxation and shear stress could be simply achieved by intermittent filtration and water disturbance (created by occasionally shaking membrane model or stirring water in membrane tank), respectively. To better understand the impact of membrane relaxation and shear stress on the biofouling layer and stable flux in GDM system, action of daily 60-min intermission, daily flushing (cross-flow velocity = 10 cm s−1, 1 min), and the combination of the two (flushed right after the 60-min intermission) were compared. The results showed that membrane relaxation and shear stress lonely was ineffective in improving the stable flux, while their combination enhanced the stable flux by 70%. A more open and spatially heterogeneous biofouling layer with a low extracellular polymeric substance (EPS) content and a high microbial activity was formed under the combination of membrane relaxation and shear stress. In-situ optical coherence tomography (OCT) observation revealed that, during intermission, the absence of pushing force by water flow induced a reversible expansion of biofouling layer, and the biofouling layer restored to its initial state soon after resuming filtration. Shear stress caused abrasion and erosion on the biofouling surface, but it exerted little effect on the interior of biofouling layer. Under the combination, however, both the surface and interior of biofouling layer were disturbed because of 1) the water vortexes caused by rough biofouling layer surface, and 2) the porous structure after 60-min intermission. This disturbance, in turn, helped the biofouling layer maintain its roughness and porosity, thereby improving the stable flux of GDM system.

Priming of microcystin degradation in carbon-amended membrane biofilm communities is promoted by oxygen-limited conditions.

ABSTRACTMicrobial biofilms are an important element of gravity-driven membrane (GDM) filtration systems for decentralized drinking water production. Mature biofilms fed with biomass from the toxic cyanobacterium Microcystis aeruginosa efficiently remove the cyanotoxin microcystin (MC). MC degradation can be ‘primed’ by prior addition of biomass from a non-toxic M. aeruginosa strain. Increased proportions of bacteria with an anaerobic metabolism in M. aeruginosa-fed biofilms suggest that this ‘priming’ could be due to higher productivity and the resulting changes in habitat conditions. We, therefore, investigated GDM systems amended with the biomass of toxic (WT) or non-toxic (MUT) M. aeruginosa strains, of diatoms (DT), or with starch solution (ST). After 25 days, these treatments were changed to receiving toxic cyanobacterial biomass. MC degradation established significantly more rapidly in MUT and ST than in DT. Oxygen measurements suggested that this was due to oxygen-limited conditions in MUT and ST already prevailing before addition of MC-containing biomass. Moreover, the microbial communities in the initial ST biofilms featured high proportions of facultative anaerobic taxa, whereas aerobes dominated in DT biofilms. Thus, the ‘priming’ of MC degradation in mature GDM biofilms seems to be related to the prior establishment of oxygen-limited conditions mediated by higher productivity.