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Evaluation of potential particulate/colloidal TEP foulants on a pilot scale SWRO desalination study
... The water has a very high concentration of TOC compared to most operating SWRO plants in the world ( Table 1). Some of the worst documented membrane biofouling issues have been documented in the Arabian Gulf in the United Arab Emirates, Qatar, and Saudi Arabia, where the TOC concentrations range from 0.5 to 3.9 mg/L [50,51]. Additional data on TOC in the Arabian Gulf in coastal Saudi Arabia documented a range of 1.5 to 2.4 mg/L [51][52][53]. ...
... Some of the worst documented membrane biofouling issues have been documented in the Arabian Gulf in the United Arab Emirates, Qatar, and Saudi Arabia, where the TOC concentrations range from 0.5 to 3.9 mg/L [50,51]. Additional data on TOC in the Arabian Gulf in coastal Saudi Arabia documented a range of 1.5 to 2.4 mg/L [51][52][53]. Most of the large capacity SWRO facilities on the Arabian Gulf have had continuing issues with membrane biofouling. ...
... TEP and the biopolymer fractions of organic matter are also major factors in the cause of the rapid biofouling of membranes [51,57] The occurrence of phosphate in the raw seawater has been suggested to have an impact on the rate of membrane biofouling [16,17]. Phosphate in the raw feedwater ranged between 11.8 and 39.8 μg-P/L (Fig. 9). ...
The Tampa Bay Water seawater reverse osmosis (SWRO) facility is the first large-capacity seawater desalination plant in the United States. The feedwater source for the facility is an estuarine system that is biologically very productive and contains naturally-occurring high concentrations of algae, marine bacteria, total organic carbon (mostly dissolved), transparent exopolymer particles (TEP), the biopolymer fraction of natural organic matter, and phosphate. The high-organic composition of the feedwater places stress on the conventional sand pretreatment system utilized at the facility resulting in high organic passage into the membrane process and flow through into the permeate. In particular, the direct passage of particulate TEP (p-TEP) into the membranes has a major impact on the biofouling rate. Based on the data collected, the pretreatment is ineffective at removing key organic components that impact the rate of membrane biofouling, particularly bacteria and p-TEP. Perhaps the pretreatment could be re-designed to use a dissolved air floatation system (DAF) followed by ultrafiltration as a remedy that would likely move the biofouling problem to the ultrafiltration process, which has an easier cleaning process. Consideration could be given to using a groundwater source of feedwater as a permanent remedy to the operational issues.
... Interest has steadily risen over the past decade in the role played by transparent exopolymer particles (TEP) in the biofouling of membranes (Bar-Zeev et al., 2009;Berman, 2005;Berman et al., 2011;de la Torre et al., 2008). This interest quickly expanded from a focus only on particulate TEP to include smaller size fractions of these extracellular polymeric substances (EPS), now termed colloidal and precursor TEP (Bar-Zeev et al., 2015;Discart et al., 2015;Li et al., 2016a;Villacorte et al., 2015, Villacorte et al., 2009a. TEP was itself identified as an important player in marine ecosystems and biogeochemical processes about a decade prior to that when a tractable means of quantitatively identifying this gel-like, natural material was developed by Passow and Alldredge (1995). ...
... Relatively easily quantified surrogates allow normalization of measurements of natural samples to some known and reproducible standard (e.g., TEP expressed as mg equivalents xanthum gum/L (Passow and Alldredge, 1995)). In addition, and particularly relevant to controlled parametric trials on membrane fouling, it allows known quantities of a foulant (albeit a surrogate) to be utilized and the response observed under a variety of conditions (Le Lan et al., 2015;Li et al., 2016a;Villacorte et al., 2009a, b). This approach gives rise to several questions, both in evaluating the 'real' world impact of study results, as well as when comparing the results between different studies. ...
... In DDI water, the XG TEP 0.4mm fraction was only 13% of the total XG mass and similarly the PN TEP 0.4mm fraction was only 12% of the total mass (Fig. 1). For XG, this was consistent with recent TOC results for seawater in which TEP 0.4mm was only 16.5% of the TOC of the initial feed seawater (Li et al., 2016a). In contrast, the particulate fraction for AA was 68%. ...
Transparent exopolymer particles (TEP) and their precursors are gel-like acidic polysaccharide particles. Both TEP precursors and TEP have been identified as causal factors in fouling of desalination and water treatment systems. For comparison between studies, it is important to accurately measure the amount and fouling capacity of both components. However, the accuracy and recovery of the currently used Alcian blue based TEP measurement of different surrogates and different size fractions are not well understood. In this study, we compared Alcian blue based TEP measurements with a total carbohydrate assay method. Three surrogates; xanthan gum, pectin and alginic acid; were evaluated at different salinities. Total carbohydrate concentrations of particulates (>0.4 μm) and their precursors (<0.4 μm, >10 kDa) varied depending on water salinity and method of recovery. As xanthan gum is the most frequently used surrogate in fouling studies, TEP concentration is expressed as xanthan gum equivalents (mg XGeq/L) in this study. At a salinity of 35 mg/L sea salt, total carbohydrate assays showed a much higher particulate TEP fraction for alginic acid (38%) compared to xanthan gum (9%) and pectin (12%). The concentrations of particulate TEP therefore may only represent ∼10% of the total mass; while precursor TEP represents ∼80% of the total TEP. This highlights the importance of reporting both particulate and precursor TEP for membrane biofouling studies. The calculated concentrations of TEP and their precursors in seawater samples are also highly dependent on type of surrogate and resulting calibration factor. A linear correlation between TEP recovery and calibration factor was demonstrated in this study for all three surrogates. The relative importance and accuracy of measurement method, particulate size, surrogate type, and recovery are described in detail in this study.
... Nevertheless, many studies nowadays have revealed that membrane and permeate flux are affected by fouling in general and bio-fouling in particular. In RO system, a positive relationship has been found between Transparent Exo-polymer Particles (TEP) levels in feed water and biofouling rate on the membrane [2][3][4][5]. Furthermore, autopsy analysis of fouled RO membranes showed clearly the existence of alcian blue substances representing TEP on the membrane surfaces. ...
... Regarding TEP removal, this study clearly revealed that UF 5 kDa membranes can remove more TEP/organics (66-80%) compared to the 50 kDa membrane (53-57%) due to their lower MWCO (Figure 4). The results from the ceramic UF membranes were better than those reported before with existing pretreatment DMF and previous reported polymeric UF, which were about 10% and 40%, respectively [3,5]. The particle size distribution results presented in ( Figure 5) showed that the smallest particle sizes were HOM, then BOM, and AOM, respectively. ...
This study investigates three types of organic matter, namely algal organic matter (AOM), bacterial organic matter (BOM), and humic organic matter (HOM). These organics are different in properties and chemical composition. AOM, BOM and HOM were compared in terms of organic content, fouling behavior, and removal efficiency in ceramic UF filtration. UF experiments were conducted at a constant flux mode using 5 kDa and 50 kDa ceramic membranes. Results showed that 5 kDa membrane removed more transparent exopolymer particles (TEP)/organics than 50 kDa membranes, but less fouling formation for all the three types of organic matters tested. Membranes exhibited the lowest trans-membrane pressure (TMP) during the filtration of HOM, most probably due to the high porosity of the HOM cake layer, contributed by big HOM aggregates under Ca bridging effect. AOM shows the highest MFI-UF (modified fouling index-ultrafiltration) and TMP (transmembrane pressure) values among the three organics and during all filtration cycles for both membranes. The AOM fouling layer is well known for having high fouling potential due to its compressibility and compactness which increase the TMP and eventually the MFI values. AOM and BOM organics exhibited a similar fouling behavior and mechanism. Furthermore, the divalent cations such as calcium showed a significant impact on membrane fouling. That is probably because calcium ions made the membranes and organic matter less negatively charged and easier to deposit on membranes, thus, enhancing the membrane fouling significantly.
... A similar observation was made for the Flavobacteria which appear to be major bacterial colonizers on transparent exopolymer particles (TEP) [41]. The concentration of TEP, which comprise EPS, correlate to RO membrane fouling in a pilotscale desalination plant in Saudi Arabia [42]. This suggests that both Hyphomonas and Muricauda play an important role in the biofouling, likely through exopolymer production that would contribute to increased attachment of other bacteria like Bacillus. ...
... However, perhaps concerning is that among all evaluated pretreatment, CF results in microbial regrowth, as denoted by the increase in cell numbers, of most predominant bacterial populations. In an early study [42], the authors attribute this increase in cell numbers to the way pretreatment system was operated -possibly because sodium bisulfate was added to quench residual chlorine prior CF and thus deactivated bacteria recover their activity. Alternatively, phosphate-based antiscalant typically used in pretreatment may have enhanced the bioactivity, since phosphate limitation is proposed as a way to control biofouling [43]. ...
... Increasingly membrane filtration technologies are considered for full-scale implementation due to their many advantages. Algae concentration, biopolymer concentration and transparent exopolymer particles (TEP) can be used as indicators to assess the effectiveness of pretreatment systems [57,130,153,154]. Algae removal in GMF is highly variable (48-90%) as compared to the more stable MF/UF membranes characterized by higher removal efficiencies above 99% [130]. ...
... Most water sources contain microorganisms, and although pretreatment can remove 99.99 % of bacteria, the remaining bacteria can still spread over the entire membrane surface at an exponential rate [16,17]. Normally chlorination by NaOCl is dosed in the intake feed to prevent the biofouling in downstream RO, but the problem is that some of bacteria are just deactivated (not killed), and would be reactivated after dechlorination before the RO unit [18,19]. Therefore, finding methods and strategies for mitigating microbial contamination, improving the antibacterial performance of RO membranes, and alleviating the biological contamination of RO membranes have become key issues in the application of RO membranes. ...
Membrane technologies have been extensively used in municipal or industrial wastewater reuse, desalination, and other water treatment processes. However, the inevitable membrane biofouling issue still limits the application of PA RO membranes in practical production. In this paper, the various aspects of PA RO membrane biofouling were reviewed, and the related causes, theories, and mechanisms of membrane biofouling were discussed. The development and current trends of membrane cleaning and different RO pretreatment technologies were summarized, and the necessity of adopting membrane pretreatment technology was highlighted. Furthermore, progress in research of antifouling RO membrane surface construction was reviewed, and anti-biofouling modification strategies and mechanisms, and their relative characteristics, advantages, and disadvantages were compared. The final section of the review discussed the challenges, technical issues, and future research directions, and the emphasis was providing novel ideas for mitigating RO membrane biofouling. The purpose of this review is to afford up-to-date insights into the mitigation tactics and underlying mechanisms of membrane biofouling. Given that microbial fouling is a ubiquitous issue, it is of significant reference significance to clarify the mechanism, structure, strategy, and modification technology of RO membrane anti-biofouling for most membrane materials used in urban sewage treatment, industrial wastewater reuse, and desalination.
... The high molecular weight fraction of AOM (mostly polysaccharide and protein >10 kDa) easily accumulates and deposits on RO membranes [17,63]. This means that the biopolymer fraction of organic matter is one of the most promising indicators of the organic and biofouling potential of HAB-impacted seawater [64][65][66]. Furthermore, biodegradable organic matter with low-molecular-weight acids (<1 kDa) may contribute to the biological fouling of membranes [61]. ...
Harmful algal blooms (HABs) are a major barrier to the stable and efficient operation of seawater reverse osmosis (SWRO) desalination plants. Although the intensity varies regionally, the occurrence of HABs impairs plant operation through severe particulate, organic, and biological fouling of both pretreatment and RO systems. As a result, considerable effort has been put into developing and optimizing pre-treatment techniques to mitigate these problems. This study reviews the historical changes and current status of pretreatment applications based on >110 datasets for large SWRO desalination plants. Chronological analysis underscores the increasing trend in the operation of dissolved air flotation and ultrafiltration systems since severe HABs in the Middle East from 2008 to 2009. The impact of HAB characteristics on system performance was also analyzed, and the efficiency of current pretreatment systems for the removal of HABs was evaluated, along with proposals for operational guidelines. Finally, future strategies for i) intelligent monitoring and prediction of HABs, ii) improvement of unit processes for the removal of algal organic matter, and iii) utilization of algal biomass to alleviate environmental impact were systematically delineated for smarter, safer, and greener operation of future SWRO desalination plants in response to the occurrence of HABs.
... Concerning natural organic matter, coagulation is also reported to have a low to moderate removal capacity. The reported biopolymer and humic substances removal capacity of coagulation ranges from 14% to 51%, with rather stable outlet values for the two plants concerned [87,106] with 50 to 74 μg/L for biopolymers and 440 to 596 μg/L for humic substances. The only plant that reported the performance of coagulation with regards to low molecular weight-neutrals [87] reported a 9% increase following coagulation. ...
Many desalination plants still struggle to control biological fouling in seawater reverse osmosis (SWRO) systems as there are no standard methods to monitor this type of fouling. Strategies to control biofouling in SWRO systems have been proposed such as antifouling coating and lowering biofouling potential in SWRO feedwater through pretreatment processes. Measuring biofouling potential in the pretreatment and SWRO feedwater has gained increased interest due to its direct link to biofouling. Moreover, this approach can be used as an early warning system allowing for taking corrective actions in the pretreatment processes to meet the required SWRO feedwater quality. This article presents the biofouling potential methods/tools developed for seawater, their applications to monitor and assess raw seawater, SWRO pretreatment and SWRO feedwater, and how these methods are employed to control SWRO biofouling membrane systems. The reported removal efficiency of biofouling potential during SWRO pretreatment processes was found to be low to moderate. Threshold values for biofouling limitation were then proposed based on several lab and plant studies. Research on biofouling potential has provided insight into SWRO pretreatment performance optimisation and biofouling control. Future research is anticipated to determine better pretreatment processes and to identify robust threshold values for mitigating biofouling in SWRO membranes.
... Concerning natural organic matter, coagulation is also reported to have a low to moderate removal capacity. The reported biopolymer and humic substances removal capacity of coagulation ranges from 14% to 51%, with rather stable outlet values for the two plants concerned [87,106] with 50 to 74 μg/L for biopolymers and 440 to 596 μg/L for humic substances. The only plant that reported the performance of coagulation with regards to low molecular weight-neutrals [87] reported a 9% increase following coagulation. ...
... Normally, the recovery of water from the diluted draw solution needs to be done by another membrane separation process such as reverse osmosis (RO), which is a highly energyconsuming [20]. However, the fertilizer-driven FO (FDFO) process has been reported in the past decade [21]. ...
Insufficient removal of microplastics (MPs) and nanoplastics (NPs) may exert negative effects on the environment and human health during wastewater reclamation. The fertilizer-driven forward osmosis (FDFO) is an emerging potential technology to generate high-quality water for irrigation of hydroponic systems. In this study, the removal of MPs/NPs by the FDFO process together with their impact on FDFO membrane fouling was investigated, due to FDFO’s low molecular weight cut-off and energy requirement by using fertilizer as draw solution. Plastic particles with two different sizes (100 nm and 1 μm) and extracellular polymers released by real wastewater bacteria were utilized as model compounds for FDFO performance comparison. Results show that FDFO membrane system could generate high-quality irrigation water with only fertilizer, completely removing extracellular polymers, MPs and NPs from wastewater. It was found that the MPs and NPs themselves do not cause a significant membrane fouling. Moreover, it could help to reduce the membrane fouling caused by extracellular substances. That is probably because MPs and NPs helped to form a loose and porous fouling layer. Therefore, the FDFO process could be a long-term stable (low fouling) process for the reclamation of wastewater with high-quality requirements.
... Previous studies have shown the presence of TEP accumulation on fouled SWRO membranes, although it remains unclear as to the whether it is solely the result of the presence of TEP in the feed water or biofouling organisms contribute to this accumulation. The presence of TEP within the desalination system provides bacteria with a nutrient source, allowing for their proliferation on the membrane [80]. ...
Water scarcity is known to affect 40% of the global population. By 2030, it is estimated that 700 million people will potentially become displaced due to drought. Seawater reverse osmosis (SWRO) desalination is recognised as one of the most cost effective and efficient methods to produce freshwater. However, biofouling of the SWRO membranes is detrimental to the efficacy of the desalination plants. In this study, the bacterial community composition within the pre-treatment system at Penneshaw SWRO desalination plant (Kangaroo Island, Australia) and the biological fouling parameters on the SWRO membranes were characterised. Membrane autopsies were undertaken on stages 1 and 2 positioned membranes that were operational for two and four years. Results showed that the pre-treatment system allowed for the removal of microorganisms from the water, however niche communities were able to establish and proliferate within the plant due to environmental adaption. The communities associated with the SWRO membranes were stable and had the ability to flourish on the membranes within biofilms. This study provides insights into the community structure within the pre-treatment system of the desalination plant, as well as on the SWRO membranes and examines how they impact on the performance of the plant.
... Fouling is classified in four categories: organic fouling, inorganic scaling, colloidal fouling, and biofouling. Different types of fouling can occur in desalination plants depending on the seawater quality and on the pretreatment efficiency [4][5][6][7]. The development of a biofilm on RO membranes results in an increase in membrane resistance and a reduction in permeability and solute rejection [8,9]. ...
Harmful Algal Blooms (HABs) are considered a major contributor to membrane biofouling in Seawater Reverse Osmosis (SWRO) desalination plants. The presence of HABs in the raw feed water leads to an increase in membrane fouling rate, increase of chemical consumption, and can cause temporary shutdown of plants. Effective pretreatment can reduce the amount of organic foulants reaching the RO membrane and alleviate the problem of flux decline during RO operation and frequent membrane cleaning using chemicals. This study compared the effect of in-situ generated liquid ferrate and ferric chloride in combination with dissolved air flotation (DAF) as a pretreatment strategy to remove algal cells and algal organic matter (AOM) during algal bloom events. Experiments were performed using a bench-scale DAF unit. HABs conditions were simulated by harvesting AOM from cultivating Chaetoceros affinis (CA) in raw seawater to a concentration of around 10 mg C/L of dissolved organic carbon (DOC). The liquid ferrate was generated in-situ by wet oxidation of ferric chloride in an alkaline media. The best performance was achieved with the combined use of liquid ferrate and DAF, removing up to 100% of algal cells, 99.99% of adenosine tri-phosphate (ATP), and up to 92% of AOM.
... Pre-treatment of the feed water is often used to reduce membrane fouling (Kumar et al., 2006). Examples of pretreatments commonly used for desalination systems include coagulation and flocculation, activated carbon adsorption, ion exchange, rapid sand filtration, or membrane separation such as microfiltration or ultrafiltration (Azmi and Yunos, 2014;Gur-Reznik et al., 2008;Kim et al., 2002;Li et al., 2016;Shon et al., 2004;Villacorte et al., 2009). These treatments usually target the removal of hardness-forming ions, organic compounds, or suspended solids, which enhance the membrane separation stage by reducing membrane fouling and scaling. ...
Reverse Osmosis (RO) desalination is an important step of wastewater reuse as it can remove salts and trace contaminants. However, RO also generates high salinity brines that need to be dealt with. Membrane distillation (MD), a process largely unaffected by salinity, provides a way to treat desalination brines to high water recovery and has been proposed as a solution for RO brine management. However, pore wetting of membranes in MD is one of the major hurdles that prevents its implementation in wastewater treatment systems, as amphiphilic organic compounds present in wastewater can lead to pore wetting and loss of selectivity over time. The objective of this study was to identify a pre-treatment strategy to prevent wetting in MD treatment of municipal wastewater RO brines. We compared three pre-treatments with different separation or removal mechanisms: foam fractionation, advanced oxidation, and ultrafiltration. We evaluated membrane wetting by measuring the change in conductivity in the distillate and identified the most effective pre-treatment to prevent wetting in MD. The results show that wetting is prevented by pre-treating the brine with foam fractionation. The effectiveness of foam fractionation as a wetting control strategy was confirmed for a high wetting propensity synthetic water using sodium dodecyl sulfate as a model wetting compound. Finally, the effect of the pre-treatments on the desalination brine was evaluated to understand the nature of the compounds removed by each treatment. The results of this study will help implement MD as a treatment process for desalination brines in municipal wastewater reuse systems.
... Various indicators such as algae concentration, biopolymer concentration, and transparent exopolymer particles (TEP) could be monitored to assess the magnitude of the bloom and effectiveness of pretreatment systems [53,54,69,70]. According to Villacorte et al., algae removal in granular media filters (GMF) is highly variable (48-90%) as compared to the more stable MF/UF membranes characterized by higher removal efficiencies by (> 99%) [69]. ...
Seawater desalination using reverse osmosis (RO) process has increased substantially in the recent past and is expected to grow at an increasingly rapid pace in the future. Successful operation of a seawater reverse osmosis (SWRO) plant depends on the ability of the pretreatment system to consistently produce adequately treated filtered water for the subsequent RO process. Both conventional (e.g., conventional/lamella sedimentation, dissolved air flotation, granular media gravity/pressure filtration) and membrane-based pretreatment processes (e.g., microfiltration, ultrafiltration) have found practical application worldwide. Although most of the currently operational pretreatment systems are conventional, low-pressure membrane based pretreatment systems are increasingly being considered for future plants. Thus, selection of conventional versus membrane based pre-treatment is increasingly becoming difficult. Both water quality perspectives and non-water quality based criteria (ease of operation, facility footprint, construction costs, operating costs, economy of scale, design specifications , contractual agreements, etc.) need to be critically reviewed to make a prudent decision. This paper provides a critical review of both conventional and membrane-based pretreatment technologies by presenting water quality issues impacting their performances, critical design characteristics and their impacts on pre-treatment selection, non-water quality based selection criteria, and a conceptual decision matrix for selection of pretreatment technologies for site specific conditions.
... There is a growing interest in recent years to study the relationship between TEPs and bacterial deposition/biofilm formation on the surface of RO membranes [62,[66][67][68]. The accumulation of TEPs together with other types of EPSs can significantly reduce the turbulence effect on the membrane surface in the presence of microorganisms, leading to increased concentration polarisation and drop in transmembrane pressure. ...
Water scarcity at global level has called for attentions to establish new and innovative technologies that can be tapped to provide sustainable solutions to water crisis. Membrane-based desalination has been acknowledged as one of the promising approaches to resolve the global challenges. Currently, different membrane-based technologies have been deployed worldwide for clean water production. However, despite the great advances made in terms of the permeate flux and rejection, the practical application of membrane for desalination is still limited by the inevitable membrane fouling issue. Membrane fouling is known to be the major culprit to the elevated operating costs due to the deterioration of permeate flux, increasing transmembrane pressure, and frequent chemical cleaning which shorten the membrane's lifespan. This review provides insights into the recent advancement in mitigating membrane desalination fouling. The fouling control strategies which encompass the efforts made in the novel membrane development, feed water pretreatment, and membrane cleaning are highlighted. The advantages and limitations of these techniques are discussed and reviewed based on a substantial number of up-to-date literatures.
... The effect of AOM in the feed water on the operation of capillary/ tubular UF membranes [20,24,33,34] and polyamide thin film composite membranes [35][36][37][38] have already been demonstrated in previous studies. However, these studies mainly focused on the short-term direct effect of AOM on membrane fouling (organic or particulate fouling) rather than their role in biofouling development. ...
Algal-derived organic matter (AOM), particularly transparent exopolymer particles, has been suspected to facilitate biofilm development in membrane systems (e.g., seawater reverse osmosis). This study demonstrates the possible role of AOM on biofouling in membrane systems affected by marine algal blooms. The tendency of AOM from bloom-forming marine algae to adhere to membranes and its ability to enhance biofilm growth were measured using atomic force microscopy, flow cytometry, liquid chromatography and accelerated membrane biofouling experiments. Adhesion force measurements indicate that AOM tends to adhere to clean membranes and even more strongly to AOM-fouled membranes. Batch growth tests illustrate that the capacity of seawater to support bacterial growth can significantly increase with AOM concentration. Biofouling experiments with spiral wound and capillary membranes illustrate that when nutrients availability are not limited in the feed water, a high concentration of AOM – whether in suspension or attached to the membrane – can substantially accelerates biofouling. A significantly lower biofouling rate was observed on membranes exposed to feed water spiked only with AOM or easily biodegradable nutrients. The abovementioned findings indicate that AOM facilitates the onset of membrane biofouling primarily as a conditioning platform and to some extent as a nutrient source for biofilm-forming bacteria.
Iron fouling induced by residual Fe(III) was frequently reported in reverse osmosis operation. However, previous studies mainly focused on inorganic precipitation or Fe-organics coupling, yet ignoring its potential effects on microbes. In this study, by using ultrafiltration effluent (~2.3 mg/L Fe(III)) as the feed, a long-term fouling experiment was conducted to investigate how excessive Fe influenced biofouling. Results showed, under continuous stresses of Fe-deposits (eventually >40 μg/cm2), fouling development clearly exhibited a three-stage process, where Fe-metabolism, extracellular polymeric substance synthesis and quorum sensing (QS) were intricately intertwined. Compared to Early (3 h–2 d) and Developed (4 d–10 d) Stage, ecological interaction network in Mature Stage (12 d–30 d) was shifted and highly modularized, with iron-depositing genera (such as Leptothrix) in emerging Fe-metabolic modules as the core nodes. Foulant structure was also more porous. Moreover, by detecting signal molecules, the upregulation of QS was unraveled. Functional gene co-occurrence analysis further revealed the profound involvement of QS in enhancing polysaccharides (PS) secretion and ferric-reduction, which were verified by the constantly rising PS proportion and Fe(II)/Fe(III) ratio on surface. Therefore, more attentions should be paid to residual Fe-issues, as it not only affected inorganic fouling, but also significantly altered biofouling behaviors.
Although reverse osmosis (RO) currently dominates the global desalination market, membrane fouling remains a major operational obstacle, which penalizes sustainable plant operation. This study explores a new membrane cleaning technique that uses a saturated CO2 solution to alleviate membrane fouling caused by organic matter, without any additional chemicals. When the CO2 saturated solution is injected into the membrane module at a given pressure, CO2 bubbles start nucleating throughout the membrane surface. This phenomenon is intensified underneath the deposited foulants. The porous structure of the foulants presents cavities, which are considered as imperfection sites that act as a substrate for CO2 bubbles nucleation, leading to an effective membrane cleaning. In this study, sodium alginate, a model polysaccharide, was mixed with different concentrations of Ca²⁺ to evaluate the cleaning efficiency of the CO2 technique under severe operating conditions when formed Ca²⁺/alginate fouling layers significantly impend the RO process performance. Furthermore, the effect of hydrodymamic conditions and CO2 saturation pressure on efficiency of permeate flux recovery and membrane morphology is also evaluated and the results are compared to those achieved with Milli-Q water and acidic solution at pH 4 cleanings. Better permeate flux recoveries were observed at higher Ca²⁺ concentrations comparing to fouling expriments at lower concentrations. The observed effect was attributed to a transition from the gel layer to a looser cake layer which makes CO2 bubble nucleation and subsequent permeate flux recovery more effective due to the presence of a larger number of CO2 nucleation sites as a result of a formation of more porous fouling structures. Permeate flux recovery increased with the increase in cleaning time, cross-flow velocity and CO2 saturation pressure.
Marine organic colloids (MOCs) cause serious membrane fouling in the process of seawater desalination. Hence, this study was designed to efficiently eliminate MOCs by biomineralization of forming calcium carbonate/phosphate precipitate. Results showed that crystal formed by calcium solution and carbonate dosage was calcite, while the precipitate of calcium solution with phosphate was similar to hydroxyapatite. MOCs in the solution were throughout involved in forming the calcite and hydroxyapatite precipitates. The removal rates of protein, acid polysaccharide and humic acid in the MOCs by forming hydroxyapatite approached 56.56%, 86.07% and 95.34%, extremely higher than that of calcite. Finally, MOCs removal mechanism by the biomineralization was proposed, where calcium ions were firstly absorbed by functional groups of organic macromolecules in the solution, then the added carbonate or phosphate anions were attracted by calcium ion, and finally the crystal growth occurred with macromolecules as templates. The present results might provide a new idea for the engineers to solve the problem of membrane fouling and thus present promising application in desalination process.
Membrane fouling is an intrinsic deficiency common for all membrane processes. Fouling mitigation is therefore required to achieve sustainable membrane performance. Our study suggests a novel backwash concept which utilizes network of ultrafiltration interconnected membrane pores as a substrate for CO2 bubbles nucleation from a saturated CO2 solution which enters membrane pores from the support side. As a result, enhanced fouling removal was achieved due to additional hydrodynamic forces caused by expanded and lifted CO2 bubbles. An investigation of CO2 nucleation kinetics using a high speed camera revealed that initial CO2 nucleation rate is strongly determined by the module height and feed water type. A saturated CO2 solution backwash effectively removed bovine serum albumin (BSA) which caused both internal and external membrane fouling. A fouling reduction was also observed in BSA/seawater matrix opposite to cake layer buildup observed experiments with Milli-Q backwash. CO2 nucleation allowed to remove hydraulically irreversible fouling which was caused by transparent exopolymer particles (TEP) at pH 4 and 8. This is a promising result as TEP is biofouling precursor which tends to adsorb to a membrane surface making conventional cleaning practices inefficient. Complete transmembrane pressure recovery was achieved with a feed water containing sodium alginate and SiO2 nanoparticles with sizes compatible with membrane pores. The observed results emphasized the importance of the specific interactions in membrane/foulant/CO2 bubble triangle for a successful membrane recovery.
Transparent exopolymer particles (TEP) as gel-like particulate acidic polysaccharide have been commonly found in marine, surface water and wastewater. Currently, increasing interest has been devoted to TEP-associated membrane fouling in different membrane systems for water and wastewater treatment, thus this review attempts to provide a holistic view and critical analysis with regard to the definition, formation, detection and properties of TEP, which could ultimately determine its fouling potential. It appears that there is not a common consensus on the actual role of TEP in membrane fouling development due to the subjective definition and highly debatable detection method of TEP. It was clearly demonstrated in this review that the formation of TEP was largely related to cations in water and wastewater which indeed determined the cross-linking degree of precursor materials (e.g. polysaccharides) via intermolecular interactions, and subsequently the quantity of TEP formed. The binding between cations ions (e.g. monovalent, divalent and trivalent cations) and polysaccharide not only depends on the functional groups of polysaccharide, but also its spatial configuration. These in turn suggest that the formation, property and ultimate fouling potential of TEP would be closely related to the type and concentration of cations, while well explaining the controversial reports on TEP-associated fouling in the literature. In addition, the fouling mechanisms of TEP are also elucidated with details in this review, including (i) the formation of TEP-associated gel layer on membrane surface; (ii) carrying microorganisms to membrane surface via protobiofilm and (iii) trapping of deformable TEP in membrane pores. Consequently, it is apparent that TEP is an ignored determinant of membrane fouling, which has not yet been seriously addressed in the design and operation of membrane systems for water and wastewater treatment.
In the present work, a convenient and direct technique which enables to characterize the intrinsic structure and the mechanical properties of the biofilm without altering its chemical and physical properties is proposed. By utilizing the Optical Coherence Tomography (OCT)as a structural imaging tool coupled with an advance mathematical framework, thickness, micro-porosity, normal stress-strain curve, bulk modulus and total permeability of the biofilm structures are determined. The accuracy of this mathematical technique for the in situ characterization is validated by analyzing two different membrane structures for porosity and permeability values against the mercury intrusion porosimetry method. Three-dimensional images of biofouling were obtained with high resolution aided to numerically analyze the intrinsic biofilm structure at microscale. Growth of biofilm in a dead-end filtration experimental setup was investigated by varying the feed flow rate which allowed uniform compression and decompression to compute normal stress-strain relation of the evolving biofilm structure. At early development of biofilm (day 3), the thickness and normal stress/strain curve showed that the biofilm structure behave similar to elastic material. However, hysteresis-like trend starts to appear with the growth of biofilm suggesting the deviation of biofilm properties to viscoelastic nature at day 8. The microstructure porosity increased from 0.214 (day 3)to 0.482 (day 8)at a feed flow rate of 15 mL/min. The total membrane/biofilm permeability decreased with biofilm age to reach 5.19 × 10 ⁻¹⁵ m ² at day 8 at the same flow rate, leading to a reduction of permeate flux over time. All the structural properties were found to be time dependent as the biofilm continuously evolved.
Biofouling in environmental systems employs bacterial quorum sensing signals (autoinducers) and extracellular polymeric substances to onset the event. The present review has highlighted on the fundamental mechanisms behind biofilm formation over broad spectrum environmental niches especially membrane biofouling in water systems and consequent chances of pathogenic contamination leading to global economic loss. It has broadly discussed on bioelectrical signal (via, potassium gradient) and molecular signal (via, AHLs) mediated quorum sensing which help to propagate biofilm formation. The review has illustrated the potential of genomic intervention towards biofouled membrane microbial community and has uncovered possible features of biofilm microenvironment like quorum quenching bacteria, bioelectrical waves capture, siderophores arrest and surface modifications. Based on information, the concept of interception of quorum signals (AHLs) and bioelectrical signals (K+) by employing electro-modified (negative charges) membrane surface have been hypothesized in the present review to favour anti-biofouling.
This study investigated the impact of coagulation on the transformation between colloidal and particulate transparent exopolymer particles (TEP) in seawater; and the effectiveness of a combined pretreatment consisting of coagulation and UF on minimizing TEP fouling of seawater reverse osmosis (SWRO) membranes. Coagulation with ferric chloride at pH 5 substantially transformed colloidal TEP (0.1–0.4) into particulate TEP (>0.4) leading to a better membrane fouling control. Both 50 and 100 kDa molecular weight cut-off (MWCO) UF membranes removed most of particulate and colloidal TEP without the assistance of coagulation, but coagulation is still necessary for better UF fouling control. The improvement of combined SWRO pretreatment with coagulation and 50 kDa UF membranes was not that much significant compared to UF pretreatment with 50 KDa alone. Therefore, the minimal coagulant dosage for seawater containing TEP should be based on the UF fouling control requirements rather than removal efficiency.
In this study, we investigated the organic matter responsible for reverse osmosis (RO) membrane fouling in seawater desalination. In addition, pretreatments by which fouling organic fractions causing severe membrane fouling can be removed effectively were investigated. The results from continuous operation of a pilot-scale sand filter indicated that the changes in the concentrations of specific organic fractions, such as transparent exopolymer particles (TEP) and biopolymers determined by liquid chromatography with organic carbon detection (LC-OCD), cannot be evaluated by comprehensive water quality indices (e.g., total organic carbon (TOC) or dissolved organic carbon (DOC) concentrations). In addition, the changes in the degree of membrane fouling cannot be explained by the comprehensive TOC and DOC concentrations. Among the water quality indices examined in this study, the concentration of TEP with relatively large particle size (i.e., >. 1.0 μm) and content of protein-like organic matter that can be detected by fluorescence excitation-emission matrix (EEM) spectral analysis were well correlated to RO membrane fouling levels. The results obtained in this study strongly suggest that developing a pretreatment method that can effectively eliminate TEP and proteinaceous compounds contained in seawater is important for the stable operation of a seawater desalination plant utilizing an RO membrane.
In order to clarify the fouling mechanism during silt density index (SDI) measurements of seawater in the seawater reverse osmosis (SWRO) desalination process, 11 runs were conducted under constant-pressure (207 kPa) dead-end filtration mode according to the standard protocol for SDI measurement, in which two kinds of 0.45 μm membranes of different material and seawater samples from the Mediterranean including raw seawater and seawater pre-treated by coagulation followed by sand filtration (CSF) and coagulation followed by microfiltration (CMF) technologies were tested. Fouling mechanisms based on the constant-pressure filtration equation were fully analyzed. For all runs, only t/(V/A) ∼ t showed very good linearity (correlation coefficient R2 > 0.99) since the first moment of the filtration, indicating that standard blocking rather than cake filtration was the dominant fouling mechanism during the entire filtration process. The very low concentration of suspended solids rejected by MF of 0.45 μm in seawater was the main reason why a cake layer was not formed. High turbidity removal during filtration indicated that organic colloids retained on and/or adsorbed in membrane pores governed the filtration process (i.e., standard blocking) due to the important contribution of organic substances to seawater turbidity in this study. Therefore the standard blocking coefficient ks, i.e., the slope of t/(V/A) ∼ t, could be used as a good fouling index for seawater because it showed good linearity with feed seawater turbidity. The correlation of SDI with ks and feed seawater quality indicated that SDI could be reliably used for seawater with low fouling potential (SDI15 min < 5) like pre-treated seawater in this study. From both ks and SDI, the order of fouling potential was raw seawater > seawater pre-treated by CSF > seawater pre-treated by CMF, indicating the better performance of CMF than CSF.
Reverse osmosis (RO) membrane systems are widely used in the desalination of water. However, flux decline due to fouling phenomena in RO remains a challenge. To minimize fouling, a reliable index is necessary to predict the fouling potential of the RO feed water. The ASTM introduced the silt density index (SDI) as a standard fouling index to measure the fouling potential due to colloidal and suspended particles. For decades, the SDI is worldwide accepted and applied. There are growing doubts about the predictive value of this parameter. In addition there are several deficiencies observed, affecting the accuracy and reproducibility e.g. no correction factor for temperature, nor for variations in membrane resistance, and no linear correlation with the concentration of colloidal/suspended particles. This paper gives an overview of our work on limitations, improvements and alternatives for the SDI. Firstly, the influence of the applied 0.45 μm test membrane on the SDI will be investigated. Variations in SDI values can be attributed to differences in properties of these membranes. In order to quantify the influence of pressure, temperature and membrane resistance on the SDI a mathematical relation was developed between the SDI and the MFI0.45, assuming cake filtration. In addition, also other fouling mechanisms were incorporated in the model using the well-known blocking laws. Based on a cake filtration fouling mechanism and assuming 100% particle retention, the models were used to normalize the experimental SDI values for temperature, pressure and membrane resistance to the SDI. By applying this normalization, the results of SDI tests carried out under different conditions and/or with different membranes can be compared easily as was proven experimentally in the lab and at a seawater desalination plant. Finally, an alternative filtration index will be introduced, the volume-based SDI_v. The SDI_v compares the initial flow rate to the flow rate after filtering a standard volume of feed water using MF membranes with an average pore size of 0.45 μm. Our experimental results show that SDI_v is independent of the membrane resistance. In that way, it eliminates most of the disadvantages of the SDI and has great potential to replace the SDI in the field.
Transparent exopolymer particles (TEP) exist abundantly in oceans and lakes and have been found to play an important role in sedimentation and biochemical cycling of matter. However, the origin of these particles and the factors regulating their formation are not well understood. This study examined several strains of algae and bacteria with respect to their production of TEP or TEP precursors. The formation rate of TEP in batch cultures of algae varied widely between species, and interspecies variability among diatoms was as large as that among species belonging to different classes or even divisions. Species, growth phase and environmental factors acted in concert in determining the accumulation of TEP in algal cultures and no general rules or patterns could be derived. The concentration of TEP during the growth phase of algal batch cultures, mesocosm or natural phytoplankton blooms was a significant function of chlorophyll a, confirming the significance of phytoplankton for the formation of TEP. Experiments with 3 bacterial strains and a natural bacteria population indicated that bacteria are also able to generate TEP, but the role of bacterial derived TEP for in situ TEP concentrations remains unclear.
An abundant form of extracellular polymeric substances (EPS) called transparent exopolymer particles (TEP) was recently regarded by Berman and Holenberg (T. Berman and M. Holenberg, Don't fall foul of biofilm through high TEP levels, Filtrat. Separat., 42 (2005), 30–32) as a major initiator of biofilm formation on membrane surfaces, which can eventually lead to biofouling. The TEP method applied here was an adapted version of the spectrophotometric technique developed by Passow and Alldredge (U. Passow and A.L. Alldredge, A dye-binding assay for the spectrophotometric measurement of transparent exopolymer particles (TEP), Limnol. Oceanogr., 40(7) (1995) 1326–1335). The main modifications were to the calibration procedure. TEP can be visualized by staining with alcian blue, a dye specific for acidic polysaccharides. The amount of TEP can be semi-quantified by measuring the absorbance of the dye that complexed with polysaccharides in water samples. Since TEP is a very complex polysaccharide, a commercially available polysaccharide: Gum Xanthan was used to standardize the amount of alcian blue dye bound to TEP and therefore express concentrations of TEP in terms of Xanthan equivalents per liter (µg Xeq/ L). For the calibration, TOC measurements were made in order to relate the TOC removed by filtration to the amount of Xanthan (µg) retained in the filter. Biopolymer LC-OCD analyses were also employed for some of the samples in order to support the TEP results. Using the modified TEP method, the presence of TEP was assessed in the feed water and at various points along the treatment lines of two integrated membrane systems (IMSs) treating surface water and secondary wastewater effluent. Results showed that significant amounts of TEP were present in surface water (˜990 µg GX/L) and secondary wastewater effluent (˜270 µg GX/L). TEP removal efficiencies of 100% were measured for ultrafiltration (UF). TEP (>0.4 mm) removal efficiencies of ca. 70% were measured with in-line coagulation employing a high coagulant dose (10 µg Al/ L) in surface water. Significantly lower TEP removal efficiencies (ca. 27%) were observed with in-line coagulation employing a low coagulant dose (1.5 mg Al/L) in secondary treated effluent. Biopolymer LC-OCD analyses also revealed removal of high molecular weight biopolymers by UF and in-line coagulation in both IMS.
Seasonal abundance, volume, size distribution and carbon content of transparent ex- opolymeric particles (TEP) were examined in 2 hydrologically distinct sites in the NW Mediterranean (NWM) Sea: a coastal (Point B, Villefranche Bay) and an offshore (DYFAMED, France, JGOFS) site. TEP concentration varied from 0.2 to 2.2 × 10 5 particles ml -1 , and was higher offshore. The TEP pool was low during the winter mesotrophic period and increased after the spring bloom, remaining rela- tively high throughout summer at both sites. The increase in TEP abundance during the oligotrophic period was relatable to nitrate limitation and a decline in primary production. TEP formation in spring was associated to a nanoflagellate bloom, while the build-up of a large pool of TEP in summer oc- curred in the presence of a phytoplankton community dominated by picoplankters and during strong thermal stratification, limiting vertical sedimentation. In the NWM Sea, when the TEP carbon pool (TEP-C) is high, it may represent up to 22% of the total organic carbon, and reach down to 1% when it is low, suggesting that the particles play a significant role in the carbon cycle. In the NWM Sea, the trophic status of the system and the composition of primary producers control TEP formation. Hydrological processes appear to be of primary importance in governing seasonal TEP distribution.
A large pool of organic carbon resides in the world's oceans in the form of dissolved organic matter (DOM) *RF 1,2*.DOM is operationally defined as the fraction of organic matter that passes through a filter with a given pore size (which can range from less than 0.1 micro m to 0.46 micro m). This fraction has a longer oceanic residence time-and is generally less biodegradable-than particulate organic matter (POM) [1-4]. Processes transforming DOM into POM are therefore crucial for our understanding of the cycling of organic material in the oceans [1-4]. The aggregation of marine colloids, which constitute 10-40% of DOM [2,3,5], is thought to be an important step in the transformation of DOM into POM [3]. It has been suggested that colloids, as well as transparent exopolymer particles and large aggregates ('marine snow') can be viewed as polymer gels [6-8]. Whether free DOM polymers can indeed spontaneously assemble to form polymer gels has, however, not yet been shown. Here we present experimental obse
This paper assesses the performance of a UF/RO demonstration plant located in the Oosterschelde estuary in the south-western part of the Netherlands. Spring blooms in the seawater pose a challenge to the plant because of the resulting increased fouling potential of the water. Determinations of the fouling indices SDI, SDI+ and MFI0.45 were carried out at the plant with different operational conditions, such as of coagulant addition and pH correction. Eight different membranes were used in the tests. In general, the UF performance was found to be good as the SDI values were around 1, provided standard membranes were used, and the MFI0.45 values lower than 1 s/L2. The MFI0.45 showed the same tendency as the SDI in most cases. As expected, whereas the SDI showed marked sensitivity to used membrane type and operational conditions, the SDI+ did not display this dependency and hence appear to be a more reliable fouling index than the SDI. Storing the RO feed overnight in the feed tank increased the fouling potential of the RO feed, likely caused by continued coagulation.
Harmful algal blooms (HABs) caused by the marine ichthyotoxic dinoflagellate Cochlodinium polykrikoides Margalef are responsible for mass mortalities of wild and farmed fish worldwide, with catastrophic impacts to aquaculture and local economies. Here we report on the Cochlodinium species responsible for a severe and widespread HAB in the Arabian Gulf and Gulf of Oman that has lasted for more than eight months at this writing, killing thousands of tons of fish and limiting traditional fishery operations, damaging coral reefs, impacting coastal tourism, and forcing the closure of desalination plants in the region. To identify the causative organism, cultures were established from cells isolated along the Arabian Gulf shore of the United Arab Emirates. Taxonomic analyses using scanning and light microscopy, and partial analysis of the large subunit (LSU) ribosomal RNA (rRNA) gene confirmed the C. polykrikoides classification. rRNA gene sequences of C. polykrikoides isolates from the Arabian Gulf were identical to isolates from the northeastern USA, Puerto Rico, Mexico, and Malaysia, known as the “American/Malaysian” ribotype. To our knowledge, this is the first HAB event associated with C. polykrikoides in the Arabian Gulf or the Gulf of Oman. The sudden emergence of C. polykrikoides in these Gulfs coincides with an apparent global expansion of this taxon, as well as a recent increase in HAB impacts observed in this region. The mechanisms underlying this expansion require further investigation, and may include increased nutrient enrichment of coastal waters in the Arabian Gulf and Gulf of Oman from domestic and industrial inputs, natural meteorological and oceanographic forcings, and the recent introduction of this species through ballast water discharge. A pattern of subsequent recurrence of C. polykrikoides blooms following an initial outbreak has been observed in other parts of the world, suggesting that this species may become a persistent HAB problem in this region. As Arabian Gulf countries rely on desalination plants as the primary source of freshwater, the disruption of plant operations by recurring Cochlodinium blooms poses a serious threat to the drinking water supply in the region, and represents an unprecedented HAB impact.
Size-exclusion chromatography in combination with organic carbon detection (SEC-OCD) is an established method to separate the pool of NOM into major fractions of different sizes and chemical functions and to quantify these on the basis of organic carbon. One specific approach, also known as LC-OCD-OND, is based on the Gräntzel thin-film UV-reactor. This approach is described with recent improvements in fraction assignation (humic substances, biopolymers, building blocks, low molecular weight organic acids and neutrals, hydrophobic organic carbon), the coupling of a novel organic nitrogen detector (OND), and an improved diagram for the characterisation of aquatic humic substances (HS-diagram). The diagram replaces the operational distinction between humic and fulvic acids by a continuum ranging from aquagenic fulvic acids to pedogenic humic acids.
In summary, periodic direct chlorination of the thin-film composite membrane RO plant may be a viable option for those enduring intense biofouling conditions. It may require the use of carbon beds in the pretreatment to knock out all of the utility's chlorine/chloramine, as required to allow fine control of the dosing. It may require use of SO2 as the reducing agent. It may require lowering the pH to turn the NaOCl into the more potent HOCl form. It will require dedicated operator attention. Without this, the results are apt to fall short of expectation. If the point of reduction can be moved very close to the arrays, biofouling will be less severe and may even obviate the necessity or at least reduce the amount of direct chlorination required. If the temperature can be allowed to drop somewhat, biofouling may be suppressed. If periodic direct chlorination can be restricted to the warm months, biofouling suppression and long module service life may coincide.
Transparent-exopolymer-particles (TEP) have been recently identified as a significant contributor to surface biofouling, such as on reverse osmosis (RO) membranes. TEP research has mainly focused on algal TEP/TEP precursors while limited investigations have been conducted on those released by bacteria. In this study, TEP/TEP precursors derived from both algae and bacteria were isolated and then characterized to investigate their similarities and/or differences using various advanced analytical techniques, thus providing a better understanding of their potential effect on biofouling. Bacterial TEP/TEP precursors were isolated from two species of marine bacteria (Pseudidiomarina homiensis and Pseudoalteromonas atlantica) while algal TEP/TEP precursors were isolated from two marine algae species (Alexandrium tamarense and Chaetoceros affinis).
Biofouling is the major problem of reverse osmosis (RO) membranes used for desalting seawater (SW). The use of chlorine is a conventional and common practice to control/prevent biofouling. Unlike polyamide RO membranes, cellulose triacetate (CTA) RO membranes display a high chlorine tolerance. Due to this characteristic, CTA membranes are used in most of the RO plants located in the Middle East region where the elevated seawater temperature and water quality promote the risk of membrane biofouling. However, there is no detailed study on the investigation/characterization of CTA-RO membrane fouling. In this investigation, the fouling profile of a full-scale SWRO desalination plant operating with not only continuous chlorination of raw seawater but also intermittent chlorination of CTA-RO membranes was studied. Detailed water quality and membrane fouling analyses were conducted. Profiles of microbiological, inorganic, and organic constituents of analysed fouling layers were extensively discussed. Our results clearly identified biofilm development on these membranes. The incapability of chlorination on preventing biofilm formation on SWRO membranes could be assigned to its failure in effectively reaching throughout the different regions of the permeators. This failure could have occurred due to three main factors: plugging of membrane fibers, chlorine consumption by organics accumulated on the front side fibers, or chlorine adaptation of certain bacterial populations.
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Desalination capacity has rapidly increased in the last decade because of the increase in water demand and a significant reduction in desalination cost as a result of significant technological advances, especially in the reverse osmosis process. The cost of desalinated seawater has fallen below US$0.50/m3 for a large scale seawater reverse osmosis plant at a specific location and conditions while in other locations the cost is 50% higher (US$1.00/m3) for a similar facility. In addition to capital and operating costs, other parameters such as local incentives or subsidies may also contribute to the large difference in desalted water cost between regions and facilities. Plant suppliers and consultants have their own cost calculation methodologies, but they are confidential and provide water costs with different accuracies. The few existing costing methodologies and software packages such as WTCost© and DEEP provide an estimated cost with different accuracies and their applications are limited to specific conditions. Most of the available cost estimation tools are of the black box type, which provide few details concerning the parameters and methodologies applied for local conditions. Many desalination plants built recently have greater desalinated water delivery costs caused by special circumstances, such as plant remediation or upgrades, local variation in energy costs, and site-specific issues in raw materials costs (e.g., tariffs and transportation). Therefore, the availability of a more transparent and unique methodology for estimating the cost will help in selecting an appropriate desalination technology suitable for specific locations with consideration of all the parameters influencing the cost. A techno-economic evaluation and review of the costing aspects and the main parameters influencing the total water cost produced by different desalination technologies are herein presented in detail. Some recent developments, such as the increase of unit capacity, improvements in process design and materials, and the use of hybrid systems have contributed to cost reduction as well as reduction in energy consumption. The development of new and emerging low-energy desalination technologies, such as adsorption desalination, will have an impact on cost variation estimation in the future.
Large transparent exopolymer particles (TEP) are found abundantly in the ocean and play an important role in many fields of marine ecology. Quantification of TEP by light microscopy, however, is labor-intensive and slow. Here we introduce a simple, semiquantitative method to determine the concentration of TEP colorimetrically. In this method TEP are first stained with alcian blue. The dye complexed with TEP is then redissolved and measured spectrophotometrically. Several independent tests of the method show that the concentration of TEP measured spectrophotometrically compares well with parallel light microscope counts. Fractionation experiments confirm that TEP are not generated as an artifact of filtration. Field data show that the concentration of TEP in different oceanic environments ranges from 10 to 500 μg liter -1 xanthan equivalent depending on season, depth, and plankton community composition.
Biofouling is a major and very expensive problem for the water industry. Biofilm resulting from the growth of microorganisms on reverse osmosis and ultrafiltration membranes is a serious issue in desalination and water treatment plants, requiring a variety of pretreatment strategies to minimise the problem. Tom Berman of the Kinneret Limnological Laboratory describes the latest thinking on the subject.
Dissolved organic matter (DOM) production was examined in axenic batch cultures of five coastal diatom species. For "Chaetoceros decipiens", dissolved organic carbon (DOC) accumulated beginning in late exponential growth as a result of increased cell density. For "Cylindrotheca closterium", DOC actually decreased in late exponential growth and reached zero by the end of the experiment. This coincided with continued particulate organic carbon (POC) production and a threefold increase in the per-cell concentration of transparent exopolymer particles after nutrients were depleted. DOC release rates varied between species but were significantly higher for all five species in exponential or transition growth than during stationary growth. On average, 5% of the total fixed C was released as DOC for four of the diatoms, whereas C. "decipiens" released ~21% of its fixed C as DOC. The percentage of fixed C released as DOC varied little with nutrient availability or diatom growth stage. The DOM produced by some diatom species adheres to filters and is measured in the particulate organic matter (POM) fraction when cells are separated from the medium by filtration. This may be an important problem when diatom species with known benthic life histories are prevalent. In contrast, for species like "Chaetoceros" that have no benthic life history, DOM release rates estimated by bulk measurements or $^{14} {\rm{C}}$ appear to be accurate. Overall, these results indicate that the species composition of phytoplankton blooms has the potential to influence the relative importance of POM and DOM production and can complicate interpretation of those measurements.
Most marine and fresh waters are full of microscopic Transparent Exopolymer Particles (TEP). Aquatic microbiologist Tom Berman and filtration specialist Marina Holenberg argue that TEP in source waters can lead to biofilm growth on membrane surfaces. TEP concentrations should therefore be measured upstream from membranes to determine the efficiency of pre-filtration arrays.
Transparent exopolymer particles (TEP) form from polysaccharides released by many phytoplankton species, but this process by which dissolved organic matter becomes particulate is poorly understood. Here, the abiotic formation of TEP from precursors <0.2 mu m and the minimum molecular weight (MW) of TEP-precursors were studied. In most samples TEP formed from material <0.2 mu m (polycarbonate membrane filters, Poretics) when exposed to laminar shear in Couette flocculators. This result was unexpected as no TEP formed from material <0.45 mu m (polycap capsules, Whatman) due to surface coagulation onto bubbles (Zhou et al. 1998; Limnol Oceanogr 43:1860-1871). Some TEP-precursors were able to pass through dialysis bags with a nominal pore size of 8 kDa (natural cellulose, Spektrum), although their MW is presumably 2 orders of magnitude larger, suggesting that TEP-precursors can be fibrillar. It is suggested that freshly released precursors are fibrillar and that these fibrillar precursors form larger colloids and eventually TEP within hours to days after their release.
The abundance of Transparent Exopolymer Particles (TEP) in surface waters has been unnoticed for many years until recently as a potential foulant in reverse osmosis systems. Recent studies indicate that TEP may cause organic and biological fouling and may enhance particulate/colloidal fouling in reverse osmosis membranes. The presence of TEP was measured in the raw water, the pre-treatment processes and reverse osmosis (RO) systems of 6 integrated membrane installations. A spectrophotometric method was used to measure TEP in the particulate size range (>0.40 um) and was extended to measure TEP in the colloidal size range (0.05–0.40 um). Ultrafiltration pre-treatment applied in 4 plants, totally removed particulate TEP while microfiltration systems (2 plants) and coagulation/ sedimentation/rapid sand filtration systems (3 plants) partially removed this fraction. None of the pre-treatment systems investigated totally removed colloidal TEP. Biopolymer analysis using LC–OCD showed consistency between colloidal TEP and polysaccharide removal by UF pre-treatment and further verified the presence of TEP in the RO feedwater. TEP deposition in the RO system was determined after measuring total TEP concentrations in the RO feed and concentrate. The TEP deposition factors and specific deposition rates indicate that TEP accumulation had occurred in all plants investigated. This observation was verified by an autopsy of RO modules from two RO plants. Further improvement and verification of the (modified) TEP method, in particular the calibration, is necessary so that it can be employed to investigate the role of TEP in the fouling of RO systems.
Transparent exopolymer particles (TEP) are sticky organic microgels, ubiquitous in natural waters, which have been implicated as a potentially important factor in the development of aquatic biofilm. An experimental cross-flow membrane array was used to investigate the relationship between the rate of membrane clogging and levels of TEP and other water quality variables in a lake water source. In three experimental series, feedwater TEP concentrations correlated significantly with membrane fouling rate. To check whether feedwater TEP could be a source for extracellular polymeric substances (EPS) of early biofilm, 5 experiments were run with either untreated (active bacteria) or chlorinated (inactivated bacteria) feedwater. Confocal scanning laser microscopy and image analysis of biofilm on membranes after 50 h showed similar EPS in the biofilm, irrespective of whether the bacteria were ~ 98% inactivated or fully active. This would indicate that most of the EPS appearing at early stages of biofilm on membranes originated from TEP in the feedwater rather than from metabolizing bacteria adhering to the surface. Taken together, these experiments support the premise that TEP in source waters play a significant role in the early stages of aquatic biofilm formation and are an important causative factor in membrane fouling.Research Highlights► Are Transparent exopolymer particles (TEP) important in aquatic biofilm formation? ► Experimental cross-flow membrane array used to study TEP role in biofilm formation. ► Rate of membrane fouling correlated significantly with TEP levels in feedwater. ► EPS in early biofilm derives mainly from TEP, not from adhering bacteria.
Polysaccharide-specific staining techniques reveal the existence and high abundance of a class of large, discrete, transparent particles in seawater and diatom cultures formed from dissolved exopolymers exuded by phytoplankton and bacteria. Transparent exopolymer particles (TEP), ranged from 28 to 5000 particles ml−1 and 3 to 100s μm in longest dimension at five coastal stations off California. A high percentage of seemingly free-living bacteria (28–68%) were attached to these transparent sheets and films, suggesting that they may alter the distributions and microenvironments of marine microbes in nature. Preliminary coagulation experiments demonstrated that TEP are major agents in the aggregation of diatoms and in the formation of marine snow. The existence of microbial exudates acting as large, discrete particles, rather than as dissolved molecules or as coating on other particles, suggests that the transformation of dissolved organic matter into particulate form in the sea can occur via a rapid abiotic pathway as well as through conventional microbial uptake. The existence of these particles has far reaching implications for food web structure, microbial processes, carbon cycling and particulate flux in the ocean.
Phosphate limitation as a method to control biofouling of spiral wound reverse osmosis (RO) membranes was studied at a full-scale installation fed with extensively pretreated water. The RO installation is characterized by (i) a low feed channel pressure drop increase and (ii) low biomass concentrations in membrane elements at the installation feed side. This installation contrasted sharply with installations fed with less extensively pretreated feed water (and therefore higher phosphate concentrations) experiencing a high-pressure drop increase and high biomass concentrations in lead elements. Membrane fouling simulator (MFS) studies showed that low phosphate concentrations (∼0.3 μg P L−1) in the feed water restricted the pressure drop increase and biomass accumulation, even at high substrate (organic carbon) concentrations. In the MFS under ortho-phosphate limiting conditions, dosing phosphonate based antiscalants caused biofouling while no biofouling was observed when acids or phosphonate-free antiscalants were used. Antiscalant dosage could increase both phosphate and substrate concentrations of the water. Therefore, antiscalant selection may be critical for biofouling control. Since no biofouling was observed at low phosphate concentrations, restricting biomass growth by phosphate limitation may be a feasible approach to control biofouling, even in the presence of high organic carbon levels.
On-site greywater reclamation is thought to enhance water usage efficiency and decrease urban water demand. Direct membrane filtration is an attractive approach for treatment of greywater in residential areas due to its small footprint and high reliability, as product quality is insensitive to fluctuations in inflow quality. The research evaluated coagulation or chlorination as possible pretreatments for ultrafiltration followed by RO desalination. The system was fed with “light” greywater (avg. TOC, TN and P — 24.9, 4.6 and 0.7 mg L
−1; turbidity — 34 NTU). Direct filtration, without pretreatment, resulted in sharp decrease of the UF permeability, due to organic fouling and biofouling. The RO membrane was suspected for phosphate salt scaling. To overcome these drawbacks, coagulation and chlorination were evaluated as pretreatment options prior to UF. Chlorine demand of the greywater was 10–20 mg L
−1. Applying this dose and controlling residual chlorine in the UF feed, lowered the fouling rate by 33%. Mass balances calculations indicated that chlorination inhibited microbial activity in the UF system. The partial oxidation of the organic matter, achieved by the chlorination pretreatment, may have changed the properties of the organic matter and thus its reactivity with the UF and RO membranes. Next, ferric chloride was tested as a coagulant. Dosing 50 mg L
−1 ferric chloride reduced the UF flux decline rate by 43%, mainly due to a 38% decrease of the organic load. After coagulation, larger particles with narrower size distribution were observed in the feed (average size ~0.5 μm). This upward shift resulted in a more porous filtration cake buildup on the UF membrane, a finding which supports the enhanced performance. Although a positive synergistic effect could be anticipated, the combined coagulation-chlorination process increased fouling rate in the RO membrane probably due to the increase in the concentration polarization phenomenon, as a result of a rise in the concentrations of counter ions associated with the pretreatment reactants and possible surplus of the latter. The results indicate that greywater pretreatment is a prerequisite to hamper UF membrane biofouling. Coagulation was found to be superior to chlorination for the UF. Effluents produced by each of the membranal steps were of excellent quality that can be used for various purposes, being low in organic content, suspended matter and colloids (UF), or desalinated (RO).
Effect of coagulation on aggregating colloidal transparent exopolymer particles (TEP) released from marine bacteria
- S Li
- S.-T Lee
- S Sinha
- N Ghaffour
S. Li, S.-T. Lee, S. Sinha, N. Ghaffour, Effect of coagulation on aggregating colloidal
transparent exopolymer particles (TEP) released from marine bacteria, EuroMed
2015 Desalination for Clean Water and Energy, Palermo, Italy, 2015.
- A Alhadidi
- B Blankert
- A J B Kemperman
- R Schurer
- J C Schippers
- M Wessling
- W G J Van Der Meer
A. Alhadidi, B. Blankert, A.J.B. Kemperman, R. Schurer, J.C. Schippers, M. Wessling,
W.G.J. van der Meer, Limitations, improvements and alternatives of the silt density
index, Desalin. Water Treat. 51 (2013) 1104-1113.
The abundance and significance of a class of large, transparent organic particles in the ocean
- Alldredge
Effect of coagulation on aggregating colloidal transparent exopolymer particles (TEP) released from marine bacteria
- Li