Investigation of environmental influences on membrane biofouling in a Southern California desalination pilot plant

Side effects of antiscalants on biofouling of reverse osmosis membranes in brackish water desalination

This study elucidates the mechanisms by which extracellular polymeric substances (EPS) impact permeate water flux and salt rejection during biofouling of reverse osmosis (RO) membranes. RO fouling experiments were conducted with Pseudomonas aeruginosa PAO1, EPS extracted from PAO1 biofilms, and dead PAO1 cells fixed in formaldehyde. While a biofouling layer of dead bacterial cells decreases salt rejection and permeate flux by a biofilm-enhanced osmotic pressure mechanism, the EPS biofouling layer adversely impacts permeate flux by increasing the hydraulic resistance to permeate flow. During controlled fouling experiments with extracted EPS in a simulated wastewater solution, polysaccharides adsorbed on the RO membranes much more effectively than proteins (adsorption efficiencies of 61.2-88.7% and 11.6-12.4% for polysaccharides and proteins, respectively). Controlled fouling experiments with EPS in sodium chloride solutions supplemented with 0.5 mM calcium ions (total ionic strength of 15 mM) indicate that calcium increases the adsorption efficiency of polysaccharides and DNA by 2- and 3-fold, respectively. The increased adsorption of EPS onto the membrane resulted in a significant decrease in permeate water flux. Corroborating with these calcium effects, atomic force microscopy (AFM) measurements demonstrated that addition of calcium ions to the feed solution results in a marked increase in the adhesion forces between a carboxylated particle probe and the EPS layer. The increase in the interfacial adhesion forces is attributed to specific EPS-calcium interactions that play a major role in biofouling of RO membranes.

As potable water scarcity increases across the globe; it is imperative to identify energy and cost-effective processes for producing drinking-water from non-traditional sources. One established method is desalination of brackish and seawater via reverse osmosis (RO). However, the buildup of microorganisms at the water-membrane interface, known as biofouling, clogs RO membranes over time, increasing energy requirements and cost. To investigate biofouling mitigation methods, studies tend to focus on single-species biofilms; choice of organism is crucial to producing useful results. To determine a best-practice organism for studying antimicrobial treatment of biofilms, with specific interest in biofouling of RO membranes, we answered the following two questions, each via its own semi-systematic review: 1. Which organisms are commonly used to test antimicrobial efficacy against biofilms on RO membranes? 2. Which organisms are commonly identified via genetic analysis in biofilms on RO membranes? We then critically review the results of two semi-systematic reviews to identify pioneer organisms from the listed species. We focus on pioneer organisms because they initiate biofilm formation, therefore, inhibiting these organisms specifically may limit biofilm formation in the first place. Based on the analysis of the results, we recommend utilizing Pseudomonas aeruginosa for future single-species studies focused on biofilm treatment including, but not limited to, biofouling of RO membranes.

Application of monochloramine for wastewater reuse: Effect on biostability during transport and biofouling in RO membranes

Membrane Filtration technique is being accepted worldwide as an environment friendly and energy efficient technique in Desalination Industry as compared to Thermal Desalination techniques. However, the performance of membranes which include permeate flux and rejection is affected by the membrane fouling. The properties of membrane and surface features such as porous structure, hydrophilicity/hydrophobicity charge, polymer characteristics, surface roughness determine the fouling potential of the membrane. The hydrophilic and smooth membrane surface is usually considered desirable in tackling membrane fouling issues. Therefore, many studies have focused on to enhance surface characteristics of membranes by surface coating with polymers and nanomaterials. Since, membrane coating is not done during fabrication of the most commercially available membranes, therefore, it is also important to determine the surface features of the commercially available membranes to investigate their membrane fouling potential. Thus, the objectives of this study were (1) to perform membrane surface characterization of commercial Reverse Osmosis (RO) and Nanofiltration (NF) membranes using techniques such as SEM, AFM, FTIR and XPS; (2) to measure hydrophilicity/hydrophobicity of commercial RO and NF membranes through water contact angle measurement using sessile drop method and (3) to measure the flux and percentage rejection of NF and RO membranes using Dead end filtration technique. Here, the characterization of membrane surface in terms of surface roughness, using SEM and AFM, showed that the commercial RO membrane had more ridge and valley structures and higher average surface roughness i.e. 71.24 nm as compared to NF membranes (6.63 nm). In addition, water contact angle measurements showed that the NF membrane was more hydrophilic as compared to RO membrane. The average contact angle found for RO membrane was 59.94°. On the other hand, it was observed that NF membrane is extremely hydrophilic in nature. Due to which, contact angle value was not obtained for most of the runs. The droplet could diffuse in less than 5 seconds. In addition, the dead-end filtration experiments showed that the RO membrane had much lower flux as compared to NF membrane. This can be associated with the pore structure of these membranes. Since, the NF membrane has porous structure, in oppose to RO membrane, the flux of the NF membrane is usually higher than the RO membranes. As the membrane surface roughness and hydrophobicity makes it more susceptible to the fouling leading to reduction in membrane flux and performance, it can be concluded from this study that there is a need for surface coating of RO membrane with suitable nanomaterials such as graphene oxide to improve its hydrophilicity and surface smoothness. This will eventually make the membrane more resistant to membrane fouling and will establish the use of membrane filtration technique in desalination industry in Qatar in the future. Microorganisms have been isolated from Gulf sea water, identified and differentiated and are being used to study the biofouling of RO and NF membranes, that would be coated to limit the fouling problems. Acknowledgement: This research was made possible by NPRP grant # [9-318-1-064] from the Qatar National Research Fund (a member of Qatar Foundation). The findings achieved herein are solely the responsibility of the author[s].

Reverse osmosis (RO) membranes have been widely applied in membrane filtration processes for water purification, since the high selective RO membranes are designed to reject all materials with particle diameter larger than 10 angstrom (A) [1]. However, this optimal selectivity leads to fouling that can greatly affect the performance and productivity of RO membranes. Biofouling remains as one of the major operational problems in RO processes and is caused by unwanted deposit and growth of microorganisms on the membrane. Numerous biofouling control strategies have been developed to restore the performance of RO membranes, but none of them are able to prevent or remove biofouling completely. A novel cleaning technique using a weak and monobasic acid (pKa=3.34, 25℃)named free nitrous acid (FNA) in combined with hydrogen dioxide (H2O2) was proposed. The effects of FNA with or without H2O2 on biofouling of RO membranes were investigated in Chapter 4, five RO membranes with different degree of biofouling were cleaned using FNA solutions (10, 35 and 47 mg HNO2-N/L) at pH 2.0, 3.0 and 4.0 under cross-flow conditions for 24 hours. The cleaning efficiency of FNA solutions was compared with conventional cleaning solution sodium hydroxide (NaOH, pH 11). The cleaning tests demonstrated that FNA cleaning solutions were more efficient than NaOH at biomass removal and inactivation. At the optimum cleaning conditions (35 mg HNO2-N/L at pH 3.0),FNA has achieved higher biomass removal than NaOH for both heavily fouled (86-96% versus 41-83%) and moderately fouled (92-95% against 89-92%) membranes, respectively.In accordance to the biomass removal, 6-32% of viable cells remained on the moderately fouled RO membranes under the impact of FNA cleaning (pH 3), whereas 38-58% of viable cells stayed on the heavily fouled RO membranes. These results revealed that FNA cleaning is more effective for moderately fouled membranes, implying that early cleaning for biofouling is preferable. Although applying FNA alone, or combining it with H2O2 have shown better efficiency at biofouling removal than NaOH, the cleaning efficiency has not been significantly improved (<1% of enhancement) by adding H2O2 to FNA cleaning solutions. The effects of FNA on scaling of RO membranes were also studied using the same cleaning protocol developed for biofouling control. The results showed that FNA solutions at pH 2.0 and 3.0 were as efficient as conventional cleaning acids (hydrochloric acid and citric acid). The scaling layers which contain 32.4±1.7 g/cm2 of calcium were completely removed by all acidic cleaning solutions. Based on the results, FNA is shown to be a promising cleaning agent for RO membrane biofouling and scaling removal. Further investigation focused on the effectiveness of FNA for biofouling prevention in RO processes (Chapter 5). The results showed that weekly FNA cleanings were unable to prevent fouling in the RO filtration systems, as the hydraulic performances (permeability and salt rejection) of RO membranes have gradually declined over two to three weeks filtration period. Although FNA cleaning was able to restore the permeability of RO membranes for one to two days, continuing declined permeability implied that the fouling rate was greater than the inhibition rate of FNA. The results of prevention tests also showed that FNA was more efficient at biomass inactivation and removal. The biomass contents and viable cells of the fouling layers formed in the experiment filtration unit (with FNA weekly cleaning) were less than half of that in the control filtration unit (without FNA weekly cleaning). Moreover, the results of live/dead cell staining revealed the abundance of viable cells in the control unit(57±5%) was four times higher than that in the experiment unit (13±2%). However, there was no significant difference in the concentration of macromolecules such as proteins and polysaccharides between control and experiment filtration units.

Biofouling has been referred to as “the Achilles heel” of reverse osmosis (RO) membrane technology; the main cause being polyamide RO membranes lack of chlorine tolerance. Biofouling increases the operating cost of water treatment by increasing RO system feed pressure (i.e., energy demand) and increasing membrane cleaning frequency, which increases downtime and reduces membrane useful life. For waters with known high biofouling potential, plant designs also may require more extensive pretreatment, which increases capital and operating costs as well as the footprint of a desalination plant. It is known from the literature that the three keys to fending off biofouling in RO systems and/or recovering from biofouling once it takes root include (1) understanding site-specific processes governing biofilm formation, (2) implementing effective biofouling pretreatment ahead of RO membranes, and (3) monitoring biofouling to enable more proactive and effective RO membrane cleaning. Herein, we present four case studies of RO membrane biofouling in seawater, municipal wastewater, brackish groundwater and industrial wastewater. Next, we describe what is known about the causes and consequences of bacterial biofilm formation and growth through a process level RO membrane biofouling model. Finally, we review common biofouling control methods including pre-treatment, chemical cleaning and the most common strategies for monitoring biofouling in RO membrane systems.

Membrane biofouling is the accumulation of microorganisms onto the membrane surface and into the membrane pores, and is deliberated as an Achilles heel of membrane processes. This chapter discusses in detail biofouling in reverse osmosis membranes in seawater desalination. At the beginning of the chapter, biofouling mechanisms and their adverse effects on the membrane have been elucidated. The chapter also describes biofouling detection and remediation methods. It also highlights the key issues related to the use of conventional and advanced pretreatment schemes for biofouling mitigation. The potential foulants present in seawater and their analysis methods such as assimilable organic carbon are briefed.

The effect of UV pre-treatment on biofouling of BWRO membranes: A field study

Effect of N-acetylcysteine against biofouling of reverse osmosis membrane

During the last decade, water reuse has been widely recognized in many regions of the world. Fouling of ceramic membranes, especially hydraulically irreversible fouling, is a critical aspect affecting the operational cost and energy consumption in water treatment plants. In addition, the reverse osmosis (RO) membranes, that are often used for water reuse plant, frequently face the problem of bio-fouling. The main objective of this thesis is to develop innovative applications of the ceramic ultra- and nanofiltration membranes for water reuse purpose. Improving RO pre-treatment using tight ceramic UF or ceramic NF To prevent the biofouling often relies on pre-treatment technology, since frequent chemical cleaning does not only increase the operational cost but also destructs the polyamide skin layer of RO membrane. A good RO pre-treatment should aim at both particulate removal to release clogging in the module, and organic carbon or nutrient limitation to release biofouling in the RO. Intense pre-treatment has been applied to alleviate the biofouling in RO membranes during wastewater reclamation. Whereas, current filtration-based pre-treatment processes cannot adequately prevent biofouling due to their poor removal of nutrients and organic carbon from feed water. The tight ceramic UF and ceramic NF membranes can potentially be deployed for RO pre-treatment in order to constrain the biofouling in RO by removing the organic carbon and phosphate (as nutrient). Theoretical research on the phosphate rejection by the charged tight ceramic UF was conducted, and the results have emphasized the importance of electrostatic interactions between the negatively charged membrane and the phosphate. The results indicate that the rejection of phosphate is dependent on the pH of the solutions and the results can be interpreted by Donnan exclusion and formation of an electrical double layer in the membrane pores. The greater phosphate rejection due to electrostatic repulsion results from a stronger overlapping of the electrical double layers in membrane pores. A Debye ratio (ratio of the Debye length to the pore radius) can be used to evaluate double layer overlapping in tight UF membranes. However, the membrane fouling caused by organic matter and cations can potentially influence the phosphate rejection by the tight ceramic UF. The phosphate rejection appeared to be linearly correlated to the surface charge of the organics in the feed waters. In addition, the biopolymers in WWTP effluent water organic matter (EfOM) can adsorb phosphate with the bridging of multivalent cations, which leads to higher phosphate rejection by the EfOM-fouled membranes than humics-fouled membranes. Sewer mining using ceramic nanofiltration Ceramic NF can be used for direct municipal sewage filtration aiming at energy, fresh water and nutrient recovery, so called sewer mining. Over 80% of organic carbon substrates and 90% of dissolved phosphate were rejected by a ceramic NF membrane (450 Da), but only 10% for ammonia. Concentration of the municipal sewage using ceramic NF increases the efficiency in the energy and nutrient recovery during the anaerobic digestion. The fouling on the ceramic NF membranes during sewage filtration can be suppressed by chemical cleaning with NaClO (0.1%) and HCl (0.1 mol L-1), while the cleaning of polymeric NF membranes is reportedly far more difficult. As such, sewer mining with ceramic NF is believed as an innovative and viable technology for energy, fresh water and nutrient recovery.

High pressure membrane filtration processes such as nanofiltration (NF) and reverse osmosis (RO) are capable to produce high quality water, for industrial applications and human consumption, from virtually any feed water source. Biofouling, the unwanted biofilm formation causing negative effects on membrane performance, contributes significantly to the operational costs and downtime of membrane filtration installations. In this thesis, the microbiological aspects and process technological aspects of NF and RO membrane biofouling have been extensively investigated in full-scale industrial installations and in simplified laboratory systems. An operational definition of NF and RO biofouling is given and generally applied preventative and corrective measures to manage membrane biofouling are reviewed. Anoxic groundwater treating NF installations (including their microbial communities), have been studied and compared to oxic NF systems and RO systems. The anoxic groundwater treating NF installations were shown to be much more resistant towards biofouling development, when compared to their oxic feed water treating counterparts. Membrane cleaning, the first-choice remedy against biofouling development, was studied in full-scale installations and in a laboratory setup. It was shown that aged foulants could not be removed by chemical cleaning without compromising membrane integrity. It was concluded that the prevention of biofilm formation, rather than the control of biofilm formation (cleaning), should be the main focus of a successful anti-biofouling approach. Biofouling microorganisms from the Sphingomondaceae family, which have been isolated from the fouled membranes obtained from the full-scale studies, were then physiologically characterized. It was shown that the Sphingomonadaceae membrane isolates share many features that are uncommon for other members of the Sphingomonadaceae family. Those ‘uncommon’ features were then linked to the specific physiological traits that are required for successful membrane colonization. In a proof of principle biofouling experiment with one of the Sphingomonadaceae isolates, it could be confirmed that Sphingomonadaceae may not only play a crucial role in membrane colonization, but also in subsequent biofilm development and maturation. The results presented were then brought into perspective regarding full-scale membrane biofouling prevention and control. Finally, suggestions for future research are given.

Impact of microfiltration treatment of secondary wastewater effluent on biofouling of reverse osmosis membranes

Quorum quenching bacteria can be used to inhibit the biofouling of reverse osmosis membranes

The application of electromagnetic fields to the control of the scaling and biofouling of reverse osmosis membranes - A review