Biodegradation of microcystins during gravity-driven membrane (GDM) ultrafiltration

Global Occurrence of Cyanotoxins in Drinking Water Systems: Recent Advances, Human Health Risks, Mitigation, and Future Directions

This paper applies a semi-quantitative approach to review the diversity, environmental controls, detection methods, human health risks, and mitigation of cyanotoxins in drinking water systems (DWSs). It discusses the environmental factors controlling the occurrence of cyanotoxins, presents the merits and limitations of emerging methods of their detection (qPCR, liquid chromatography-mass spectrometry, and electrochemical biosensors), and outlines the human exposure pathways and health outcomes with identification of high-risk groups and settings. High-risk groups include (1) communities relying on untreated drinking water from unsafe, polluted water sources and (2) low-income countries where cyanotoxins are not routinely monitored in DWSs. The fate and behavior processes are discussed, including removing cyanotoxins in DWSs based on conventional and advanced treatment processes. The available methods for cyanotoxin removal presented in this paper include (1) polymer-based adsorbents, (2) coagulation/flocculation, (3) advanced oxidation processes, (4) ultra- and nanofiltration, and (5) multi-soil layer systems. Future research should address (1) detection and fate in storage and conveyance facilities and at the point of consumption, (2) degradation pathways and toxicity of by-products or metabolites, (3) interactive health effects of cyanotoxins with legacy and emerging contaminants, (4) removal by low-cost treatment techniques (e.g., solar disinfection, boiling, bio-sand filtration, and chlorination), (5) quantitative health risk profiling of high-risk groups, and (6) epidemiological studies to link the prevalence of human health outcomes (e.g., cancer) to cyanotoxins in DWSs.

Temporal and functional interrelationships between bacterioplankton communities and the development of a toxigenic Microcystis bloom in a lowland European reservoir

The cyanobacteria-associated microbiome is constantly reshaped by bloom development. However, the synergistic-antagonistic nature of the relationships between Microcystis and its microbiome still remains unclear. Therefore, temporal changes of bacterioplankton communities and their functional potential through different developing stages of a Microcystis toxigenic bloom were investigated, considering bacterioplankton assemblages as particle-attached (PAB) and free-living (FLB) bacteria. 16S rRNA sequencing revealed that PAB were represented by Proteobacteria and Cyanobacteria, while FLB by Proteobacteria and Actinobacteria. Network and ordination analyses indicated that PAB inter-relationships were more complex-numerous connections between taxa with stronger correlations, than FLB-rather influenced by physico-chemical parameters. PAB in pre-summer was diverse with Proteobacteria containing potential taxa involved in nitrogen-transforming processes. In mid-summer, PAB presented a mix-bloom dominated by Snowella, Aphanizomenon, and Microcystis, which were succeeded by toxigenic Microcystis in post-summer. Both periods were associated to potential taxa with parasitic/predatory lifestyles against cyanobacteria. In post-summer, Sutterellaceae were recognized as poor water quality indicators, and their strong association with Microcystis could have represented an increased threat for that period. Microcystis was a major factor significantly reducing PAB diversity and evenness, suggesting that it negatively influenced bacterioplankton assemblages, probably also altering the overall community functional potential.

Dynamics of the prokaryotic and eukaryotic microbial community during a cyanobacterial bloom

Toxic cyanobacterial blooms frequently develop in eutrophic freshwater bodies worldwide. Microcystis species produce microcystins (MCs) as a cyanotoxin. Certain bacteria that harbor the mlr gene cluster, especially mlrA, are capable of degrading MCs. However, MC-degrading bacteria may possess or lack mlr genes (mlr+ and mlr- genotypes, respectively). In this study, we investigated the genotype that predominantly contributes to biodegradation and cyanobacterial predator community structure with change in total MC concentration in an aquatic environment. The 2 genotypes coexisted but mlr+ predominated, as indicated by the negative correlation between mlrA gene copy abundance and total MC concentration. At the highest MC concentrations, predation pressure by Phyllopoda, Copepoda, and Monogononta (rotifers) was reduced; thus, MCs may be toxic to cyanobacterial predators. The results suggest that cooperation between MC-degrading bacteria and predators may reduce Microcystis abundance and MC concentration.

Using easy-to-biodegrade co-substrate to eliminate microcystin toxic on electrochemically active bacteria and enhance bioelectricity generation from cyanobacteria biomass

Cyanobacterial biomass is a promising natural resource for power generation, through the reactions bio-catalyzed by electrochemically active bacteria (EAB). However, the major limitation is the involvement of Microcystin-LR (MC-LR) in inhibiting EAB activation. In this work, toxic M. aeruginosa biomass was employed as analyte of a microbial fuel cell (MFC), and sodium acetate was applied as easy-to-biodegrade co-substrate to alleviate the MC-LR stress on EAB survival. The running stability was continuously enhanced with the increment of co-substrate concentration. The sufficient co-substrate supply (6.0 mM) eliminated the negative effects of MC-LR on the cyanobacteria biomass fed-MFC performance; it contributed 12.7% extension on the electric cyclic terms and caused the productions of the power density which was comparable and even 3.8% higher than its corresponding control (MFC treated with acetate alone). The co-substrate addition also increased coulombic efficiency by 60.1%, microcystin-LR removal efficiency increased by 64.7%, and diversified the microbial community with more species able to biodegrade the MC-LR, bio-transforming the metabolites and EAB. Microcystin-degrading bacteria, such as Sphingopyxis sp., Burkholderia-Paraburkholderia, and Bacillus sp., were remarkably increased, and EAB, including Shewanella sp., Desulfovibrio desulfuricans, Aeromonas hydrophila, were also much more enriched in co-substrate use protocol. Therefore, this study verified a co-substrate strategy for simultaneously eliminating MC-LR toxin and enhancing bioelectricity generation from cyanobacterial biomass via an MFC.

Tailoring the distribution of microbial communities and gene expressions to achieve integrating nitrogen transformation in a gravity-driven submerged membrane bioreactor

A pilot-scale upgraded gravity-driven submerged membrane (GDSM) reactor was constructed to enhance nitrogen removal. It was artificially formed multiple stratified environments (dissolved oxygen (DO) and substrate supply (TOC, TN, COD, NH₄⁺-N, NO₂^--N, and NO₃^--N)) by embedding moving water baffles to control water-flow process in bulk liquid with slow-flowing liquid state. Significant diversity and relative abundance of microorganisms associated with nitrogen transformation paths (i.e., ammonia-oxidizing archaea, ammonia-oxidizing bacteria, nitrite oxidizing bacteria, and denitrifying bacteria) were tailored to distribute on different spatial and temporal regions, and performed their dominant functions. The process simultaneously integrated diverse and effective nitrogen transformation paths (i.e., nitrification, partial nitrification, denitrification, anammox, and dissimilatory nitrate reduction) to achieve high nitrogen removal, with NH₄⁺-N, TN, and COD eliminated by 94.68 ± 2.55%, 55.16 ± 5.53%, and 80.17 ± 6.75%, respectively. Gene expressions involved in the nitrogen transformations were estimated by qPCR to explore the shifts of dominant nitrogen transforming bioreactions in multiple stratified environments. Pearson correlation coefficients supported that the functional genes had more stable and active ability by complementing each other. As a result, an endogenous integration of diverse nitrogen transformation paths was achieved in a single system by artificially tailoring the distributions of microbial communities and gene expressions with enhanced nitrogen removal.

Exploring microcystin-degrading bacteria thriving on recycled membranes during a cyanobacterial bloom

Microcystins (MC) are highly toxic secondary metabolites produced by cyanobacterial blooms in many freshwater ecosystems used for recreational and drinking water purposes. So far, biological processes remain to be optimized for an efficient cyanotoxin removal, and new approaches are necessary to compete with physical-chemical treatments. In previous studies we provided a new concept of membrane biofilm reactor made of recycled material, in which a single MC-degrading bacterial strain was inoculated. The present study evaluates the capacity of bacterial consortia associated with freshwater cyanobacterial blooms to form biofilms on recycled membranes and remove MC. Three different discarded reverse osmosis (RO) membranes, previously used in desalination plants after treating brackish water (BWd), seawater (SWd) and brackish water but transformed into nanofiltration (BWt-NF), were exposed to a cyanobacterial bloom in San Juan reservoir (central Spain). Results showed that the three recycled membranes developed a bacterial community with MC removal capacity. Little differences in bacterial coverage and MC removal efficiency between membranes were observed after their exposure in the reservoir. High-throughput sequencing of 16S rRNA gene analysis showed similar bacterial community composition at the phylum level but dissimilar at the order level between the three membranes. This suggests possible surface selectivity on the attached bacterial community. The mlr^- candidates such as Burkholderiales and Methylophilales were highly abundant in BWt-NF and BWd, respectively, while mlr⁺ candidates (e.g. Sphingomonadales) were low abundant in all membranes. Analysis of mlrA and mlrB genes used as markers for MC degradation following mlr-pathway confirmed the presence of this pathway in all membranes. These results suggest the co-existence of both genotypes in membrane-attached native biofilms. Therefore, this study confirms that recycled membranes are suitable support for many MC-degrading bacteria, thus giving value to discarded membranes for eco-friendly and low-cost biological filters.

Biomass addition alters community assembly in ultrafiltration membrane biofilms

Freshwater biofilms assemble from a pool of rare water column genotypes. Random density fluctuations and temporal species turnover of functionally equivalent potential colonizers result in compositional variability of newly formed biofilm communities. We hypothesized that stronger environmental filtering as induced by enhanced substrate levels might reduce the impact of a locally variable pool of colonizers and instead select for more universal habitat specialists. Our model were heterotrophic biofilms that form on membranes during gravity-driven ultrafiltration of lake water. In four separate experiments, biomass of the cyanobacterium Microcystis was added to the feed water of one set of treatments (BM) and the resulting biofilm communities were compared to unamended controls (CTRL). Biomass addition led to a significant shift of community assembly processes: Replicate BM biofilms were more similar to each other than by chance in 3 of 4 experiments, whereas the opposite was the case for CTRL communities. Moreover, BM communities were more stochastically assembled across experiments from a common 'regional' pool of biofilm colonizers, whereas the composition of CTRL communities was mainly determined by experiment-specific 'local' genotypes. Interestingly, community assembly processes were also related to both, physiology (aerobic vs. anaerobic lifestyle) and the phylogenetic affiliation of biofilm bacteria.

Role of pre-coagulation in ultralow pressure membrane system for Microcystis aeruginosa-laden water treatment: Membrane fouling potential and mechanism

This work systematically studied the role of pre-coagulation in the performance of ultralow pressure membrane system for algae-laden water treatment. The membrane performance with/without pre-coagulation was compared in terms of membrane permeate flux, water quality and membrane fouling. Ultralow pressure membrane system can effectively reduce TOC of Microcystis aeruginosa-laden water from 5.8 to 2.1 mg/L, and pre-coagulation removed most large inorganic particles but few small organic particles. Interestingly, pre-coagulation aggravated the fouling of ultralow membrane system which is generally acknowledged method to alleviate the ultrafiltration membrane fouling. According to Extended Derjaguin-Landau-Verwey-Overbeek theory (XDLVO), the interaction energy of membrane-foulants (ΔG_fm^TOT = - 41.95mJ/m²), and foulant-foulant (ΔG_ff^TOT = - 30.15mJ/m²) with coagulation were higher than those without coagulation (ΔG_fm^TOT = - 36.54mJ/m²) and (ΔG_ff^TOT = - 15.73mJ/m²) suggesting greater adherence between membrane and foulants & foulant and foulant after coagulation, which well agreed with SEM results. Membrane fouling models were also applied to analyze the fouling mechanism of ultralow-pressure membrane filtration. Based on above analysis, the possible fouling mechanisms for membrane filtration with/without precoagulation were proposed and then confirmed by pre-filtration experiment, where large inorganic particles played important roles. Our study could be indicative for membrane fouling control of ultralow-pressure membrane filtration for the treatment of algae-laden water.

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

Microbial 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.

Potential of biological approaches for cyanotoxin removal from drinking water: A review

Biological treatment of cyanotoxins has gained much importance in recent decades and holds a promise to work in coordination with various physicochemical treatments. In drinking water treatment plants (DWTPs), effective removal of cyanotoxins with reduced toxicity is a primary concern. Commonly used treatments, such as ozonation, chlorination or activated carbon, undergo significant changes in their operating conditions (mainly dosage) to counter the variation in different environmental parameters, such as pH, temperature, and high cyanotoxin concentration. Presence of metal ions, natural organic matter (NOM), and other chemicals demand higher dosage and hence affect the activation energy to efficiently break down the cyanotoxin molecule. Due to these higher dose requirements, the treatment leads to the formation of toxic metabolites at a concentration high enough to break the guideline values. Biological methods of cyanotoxin removal proceed via enzymatic pathway where the protein-encoding genes are often responsible for the compound breakdown into non-toxic metabolites. However, in contrast to the chemical treatment, the biological processes advance at a much slower kinetic rate, predominantly due to a longer onset period (high lag phase). In fact, more than 90% of the studies reported on the biological degradation of the cyanotoxins attribute the biodegradation to the bacterial suspension. This suspended growth limits the mass transfer kinetics due to the presence of metal ions, NOMs and, other oxidizable matter, which further prolongs the lag phase and makes biological process toxic-free, albeit less efficient. In this context, this review attempts to bring out the importance of the attached growth mechanism, in particular, the biofilm-based treatment approaches which can enhance the biodegradation rate.

Source Community and Assembly Processes Affect the Efficiency of Microbial Microcystin Degradation on Drinking Water Filtration Membranes

Microbial biofilms in gravity-driven membrane (GDM) filtration systems can efficiently degrade the cyanotoxin microcystin (MC), but it is unclear if this function depends on the presence of MC-producing cyanobacteria in the source water habitat. We assessed the removal of MC from added Microcystis aeruginosa biomass in GDMs fed with water from a lake with regular blooms of toxic cyanobacteria (ExpL) or from a stream without such background (ExpS). While initial MC removal was exclusively due to abiotic processes, significantly higher biological MC removal was observed in ExpL. By contrast, there was no difference in MC degradation capacity between lake and stream bacteria in separately conducted liquid enrichments on pure MC. Co-occurrence network analysis revealed a pronounced modularity of the biofilm communities, with a clear hierarchic distinction according to feed water origin and treatment type. Genotypes in the network modules associated with ExpS had significantly more links to each other, indicating that these biofilms had assembled from a more coherent source community. In turn, signals for stochastic community assembly were stronger in ExpL biofilms. We propose that the less "tightly knit" ExpL biofilm assemblages allowed for the better establishment of facultatively MC degrading bacteria, and thus for higher overall functional efficiency.

Microcystis aeruginosa and microcystin-LR removal by household slow sand filters operating in continuous and intermittent flows

A household slow sand filter (HSSF) is a widely used water treatment technology recognized as one of the most effective and sustainable in reducing waterborne diseases. However, there is a lack of knowledge concerning its behaviour in the presence of cyanobacteria and cyanotoxins. In this context, the study aimed to evaluate HSSF ability to remove Microcystis aeruginosa cells (stain BB005) and microcystin-LR from water, among other parameters, when operated under continuous (C-HSSF) and intermittent (I-HSSF) flows. CHSSF was operated at a constant filtration rate (1.22 m³ m^-2 day^-1), while I-HSSF was operated at a variable filtration rate (starting at 2.95 m³ m^-2 day^-1 and finishing at zero). Each filter produced 60 L day^-1. The influence of the pause period was also tested in the I-HSSF. The water from the study was prepared by inoculating M. aeruginosa culture in water from a well to a final cell density of ± 1 × 10⁵ cells mL^-1. M. aeruginosa removal rates were 2.39 ± 0.34 log and 2.01 ± 0.43 log by CHSSF and I-HSSF, respectively. Microcystin-LR concentration in studied water was 5.55 μg L^-1, and both filters produced filtered water with microcystin concentrations below 1.0 μg L^-1, the maximum value recommended by the World Health Organization (WHO), for most of the samples. Turbidity and apparent colour were also within WHO guidelines. Filters operating with different flow regimes and distinct residence times did not statistically influence treatment efficiencies. Both filters showed promising results in the M. aeruginosa and microcystin-LR removals from water; nevertheless, more research is needed to understand the mechanisms involved in the reduction of both cyanobacteria and cyanotoxin through household slow sand filtration.

Gravity-driven membrane filtration for water and wastewater treatment: A review

Gravity-driven membrane (GDM) filtration has been investigated for almost 10 years. The technology is characterized not only by relatively lower transmembrane pressures which can be achieved by gravity (extremely low energy consumption), but also by the phenomenon of flux stabilization: A biofilm is allowed to form on the membrane and a stabilization of flux occurs which is related to biological processes within the biofilm layer on the membrane. This enables stable operation during a year or longer without any cleaning or flushing. Initially, the technology was developed mainly for household drinking water treatment, but in the meantime, the research and application has expanded to the treatment of greywater, rainwater, and wastewater as well as the pretreatment of seawater for desalination. This review covers the field from the rather fundamental research on biofilm morphology and microbial community analysis to the impact of feedwater composition, process parameters and organic removal performance. Not only household applications, but also for community-scale treatment and full-scale applications are discussed. In addition, the application potential is highlighted in comparison to conventional ultrafiltration. Finally, an overall assessment is illustrated and the research and development needs are identified.

Effects of water temperature and light intensity on the performance of gravity-driven membrane system

The selection of favorable environmental conditions for gravity-driven membrane (GDM) systems is crucial to their widespread application. In this study, GDM systems operated under different light intensities (illuminance levels of 0, 200, and 3000 Lux) and water temperatures (10, 20, and 30 °C) were investigated for their performance and fouling layer characteristics. The results showed that indoor light (200 Lux) had limited effects on the performance of the GDM system. However, full daylight (3000 Lux) led to algal growth; these algae increased fouling resistance and deteriorated permeate water by releasing algogenic organic matter, although they could also enhance the heterogeneity of the biofouling layer by increasing the microbial activity. Water temperature rarely influenced the total organic matter removal. The fouling layers had different thicknesses and heterogeneity, but the same level of EPS; therefore, the hydraulic resistances of these fouling layer were almost the same at different water temperatures. These findings suggest that GDM system could be operated at low water temperature and indoor light conditions, and that strong light should be avoided during the operation of GDM systems.

Biofouling in ultrafiltration process for drinking water treatment and its control by chlorinated-water and pure water backwashing

We investigated biofouling in ultrafiltration (UF) for drinking water treatment and its control by backwashing with chlorinated-water or pure water. By using sodium azide to suppress biological growth, the relative contribution of biofouling to total fouling was estimated, and its value (5.3-56.0%) varied with the feed water, and increased with the increases of filtration time and membrane flux. The biofouling layer could partially remove biodegradable organic matter and ammonia (32.9-74.2%). Backwashing using chlorinated-water partly inactivated the microorganisms (23.8%) but increased the content of extracellular polymeric substances (7.7%) in the biofouling layer. In contrast, backwashing using pure water led to a looser and more porous fouling layer according to optical coherence tomography observation. Consequently, the latter was more effective in reducing fouling resistance (33.41% reduction) compared to backwashing by chlorinated-water (8.6%). These findings reveal the critical roles of biofouling in pollutants removal in addition to membrane permeability, which has important implications for addressing seasonal ammonia pollution.

Priming of microbial microcystin degradation in biomass-fed gravity driven membrane filtration biofilms

Gravity-driven membrane (GDM) filtration is a promising tool for low-cost decentralized drinking water production. The biofilms in GDM systems are able of removing harmful chemical components, particularly toxic cyanobacterial metabolites such as microcystins (MCs). This is relevant for the application of GDM filtration because anthropogenic nutrient input and climate change have led to an increase of toxic cyanobacterial blooms. However, removal of MCs in newly developing GDM biofilms is only established after a prolonged period of time. Since cyanobacterial blooms are transient phenomena, it is important to understand MC removal in mature biofilms with or without prior toxin exposure. In this study, the microbial community composition of GDM biofilms was investigated in systems fed with water from a lake with periodic blooms of MC-producing cyanobacteria. Two out of three experimental treatments were supplemented with dead biomass of a MC-containing cyanobacterial strain, or of a non-toxic mutant, respectively. Analysis of bacterial rRNA genes revealed that both biomass-amended treatments were significantly more similar to each other than to a non-supplemented control. Therefore, it was hypothesized that biofilms could potentially be 'primed' for rapid MC removal by prior addition of non-toxic biomass. A subsequent experiment showed that MC removal developed significantly faster in mature biofilms that were pre-fed with biomass from the mutant strain than in unamended controls, indicating that MC degradation was a facultative trait of bacterial populations in GDM biofilms. The significant enrichment of bacteria related to both aerobic and anaerobic MC degraders suggested that this process might have occurred in parallel in different microniches.

Operation of passive membrane systems for drinking water treatment

The widespread adoption of submerged hollow fibre ultrafiltration (UF) for drinking water treatment is currently hindered by the complexity and cost of these membrane systems, especially in small/remote communities. Most of the complexity is associated with auxiliary fouling control measures, which include backwashing, air sparging and chemical cleaning. Recent studies have demonstrated that sustained operation without fouling control measures is possible, but little is known regarding the conditions under which extended operation can be sustained with minimal to no fouling control measures. The present study investigated the contribution of different auxiliary fouling control measures to the permeability that can be sustained, with the intent of minimizing the mechanical and operational complexity of submerged hollow fiber UF membrane systems while maximizing their throughput capacity. Sustained conditions could be achieved without backwashing, air sparging or chemical cleaning (i.e. passive operation), indicating that these fouling control measures can be eliminated, substantially simplifying the mechanical and operational complexity of submerged hollow fiber UF systems. The adoption of hydrostatic pressure (i.e. gravity) to provide the driving force for permeation further reduced the system complexity. Approximately 50% of the organic material in the raw water was removed during treatment. The sustained passive operation and effective removal of organic material was likely due to the microbial community that established itself on the membrane surface. The permeability that could be sustained was however only approximately 20% of that which can be maintained with fouling control measures. Retaining a small amount of air sparging (i.e. a few minutes daily) and incorporating a daily 1-h relaxation (i.e. permeate flux interruption) period prior to sparging more than doubled the permeability that could be sustained. Neither the approach used to interrupt the permeate flux nor that developed to draw air into the system for sparging using gravity add substantial mechanical or operational complexity to the system. The high throughput capacity that can be sustained by eliminating all but a couple of simple fouling control measures make passive membrane systems ideally suited to provide high quality water especially where access to financial resources, technical expertise and/or electrical power is limited.

Presence of an adsorbent cake layer improves the performance of gravity-driven membrane (GDM) filtration system

Gravity-driven membrane (GDM) filtration is a promising decentralized drinking water treatment process. To improve the performance of GDM system, a thin layer of adsorbent was pre-deposited on the membrane surface prior to filtration (adsorbent-laden GDM system). The tested adsorbents include powdered activated carbon (PAC) and anion exchange resin (AER), and an unmodified GDM system and a SiO₂-laden GDM system were used as controls. In the adsorbent-laden GDM systems, the adsorption of the PAC and AER increased the removal efficiency of natural organic matter by 7.2-43.5% and microcystin-LR, atrazine, and bisphenol A by 7.9-81.2%. The presence of adsorbent particles increased the amount of microorganisms in the cake layer and therefore increased the removal efficiency of assimilable organic matter (AOC) by 20.1-34.4%. In the adsorbent-laden GDM systems, the physically irrecoverable fouling decreased because of the reduction in membrane foulants by the adsorbent layer. However, the presence of adsorbent particles in the cake layer counteracted this effect and increased the physically recoverable fouling. Consequently, the pre-deposited adsorbent layers had only a limited effect on the stabilized flux (2.26-2.65 L/m² h). A bilayer structure was found in the cake layer of the adsorbent-laden GDM systems via scanning electron microscopy (SEM), and the cake layer was looser in the presence of adsorbent particles. These results demonstrate that pre-depositing a thin layer of adsorbents on the membrane surface of the GDM system can significantly improve the quality of the permeate without decreasing the stabilized flux.

Toxic cyanobacteria and drinking water: Impacts, detection, and treatment

Blooms of toxic cyanobacteria in water supply systems are a global issue affecting water supplies on every major continent except Antarctica. The occurrence of toxic cyanobacteria in freshwater is increasing in both frequency and distribution. The protection of water supplies has therefore become increasingly more challenging. To reduce the risk from toxic cyanobacterial blooms in drinking water, a multi-barrier approach is needed, consisting of prevention, source control, treatment optimization, and monitoring. In this paper, current research on some of the critical elements of this multi-barrier approach are reviewed and synthesized, with an emphasis on the effectiveness of water treatment technologies for removing cyanobacteria and related toxic compounds. This paper synthesizes and updates a number of previous review articles on various aspects of this multi-barrier approach in order to provide a holistic resource for researchers, water managers and engineers, as well as water treatment plant operators.

Biofilm increases permeate quality by organic carbon degradation in low pressure ultrafiltration

We investigated the influence of biofouling of ultrafiltration membranes on the removal of organic model foulants and ultimately on the quality of permeate. Gravity Driven Membrane ultrafiltration (GDM) membrane systems were operated with modified river water during five weeks without control of the biofilm formation. Three GDM systems were studied: two systems with biofilms exposed to (A) variable or (B) constant load of organic foulants, and (C) one system operated without biofilm and exposed to constant foulant loading. Biodegradable dextran or non-biodegradable polystyrene sulfonate model foulants were tested. Substrate biodegradability was confirmed by Size Exclusion Chromatography (SEC) and by degradation batch tests (D). The GDM systems (A) and (B) were fed with pre-filtered river water supplemented with dextran (Dex) of 1, 150 or 2000 kDa, or polystyrene sulfonate (PSS) of 1 or 80 kDa at concentrations of 2-3.5 mgC L(-1). In exp. (C) the feed water consisted of deionized water with 25 mgC L(-1) of either PSS 1, 80 kDa or Dex 2000 kDa. The biofilm formation on UF membrane surfaces controlled the foulant permeation and thus the permeate quality. Biofilms exposed to continuous foulant loading (exp. B) degraded low molecular weight (LMW) biodegradable foulants (1 kDa Dex), which improved the permeate quality. For high molecular weight (HMW) substrates (150, 2000 kDa Dex), the improvement of the permeate quality was observed after 7 days of biofilm formation, and resulted from the foulant hydrolysis followed by degradation. For non-biodegradable foulants, an improvement of 20% of the retention was observed for the polystyrene (1, 80 kDa PSS) due to the presence of biofilms on membrane surfaces. For variable foulant loading (exp. A) the biofilms hydrolysed the large biodegradable foulants but did not degraded them fully, which resulted a deterioration of the permeate quality (except for the LMW dextran (1 kDa) that was fully degraded). Overall, the "biofilm + membrane" composite retained a larger amount of biodegradable foulant than the membrane alone, due to the activity of the biofilm. However, this resulted in an increased biofilm accumulation and reduced flux. In presence of the biofilm, the highest fluxes were observed for control (no foulant) and for small non-biodegradable foulants (PSS 1 kDa). Low fluxes were observed for the accumulating on membrane surface or degradable foulants (exp. B). But, the lowest fluxes were observed in absence of the biofilm (exp. C) due to physical accumulation of the foulants (PSS 80 kDa and Dextran 2000 kDa). Overall our study demonstrates that the presence of biofilms on membrane surfaces has some benefits: (i) biofilm helps to increase the permeate quality and (ii) biofilms protect the membrane from further fouling. Permeate flux stabilizes in the case of biofilm-membrane composite, while it continuously declines in the case of the membrane only.