Role of feed water biodegradable substrate concentration on biofouling: Biofilm characteristics, membrane performance and cleanability

Elucidating biofouling over thermal and spatial gradients in seawater membrane distillation in hot climatic conditions

Biofouling is a hurdle of seawater desalination that increases water costs and energy consumption. In membrane distillation (MD), biofouling development is complicated due to the temperature effect that adversely affects microbial growth. Given the high relevance of MD to regions with abundant warm seawater, it is essential to explore the biofouling propensity of microbial communities with higher tolerance to elevated temperature conditions. This study presents a comprehensive analysis of the spatial and temporal biofilm distribution and associated membrane fouling during direct contact MD (DCMD) of the Red Sea water. We found that structure and composition of the biofilm layer played a significant role in the extent of permeate flux decline, and biofilms that built up at 45°C had lower bacterial concentration but higher extracellular polymeric substances (EPS) content as compared to biofilms that formed at 55 °C and 65°C. Pore wetting and bacterial passage to the permeate side were initially observed but slowed down as operating time increased. Intact cells in biofilms dominated over the damaged cells at any tested condition emphasizing the high adaptivity of the Red Sea microbial communities to elevated feed temperatures. A comparison of microbial abundance revealed a difference in bacterial distribution between the feed and biofilm samples. A shift in the biofilm microbial community and colonization of the membrane surface with thermophilic bacteria with the feed temperature increase was observed. The results of this study improve our understanding of biofouling propensity in MD that utilizes temperature-resilient feed waters.

The effect of different carbon sources on biofouling in membrane fouling simulators: microbial community and implications

Biofouling is a problem affecting the operation of nanofiltration systems due to the complexity of the carbon matrix affecting bacteria and biofilm growth. This study used membrane fouling simulators to investigate the effects of five different carbon sources on the biofouling of nanofiltration membranes. For all the carbon sources analyzed, the increase in pressure drop was most accelerated for acetate. The use of acetate as the single carbon source produced less adenosine triphosphate but more extracellular polymers than glucose. The microbial community was analyzed using 16 s rRNA. The use of more than a single carbon source produced an increase in bacteria diversity even at similar concentrations. The relative abundance of proteobacteria was the highest at the phylum level (95%) when a single carbon source was added. Additionally, it was found that the use of different carbon sources produced a shift in the microbial community, affecting the biofouling and pressure drop on membranes.

Permeation Increases Biofilm Development in Nanofiltration Membranes Operated with Varying Feed Water Phosphorous Concentrations

Nutrient limitation has been proposed as a biofouling control strategy for membrane systems. However, the impact of permeation on biofilm development under phosphorus-limited and enriched conditions is poorly understood. This study analyzed biofilm development in membrane fouling simulators (MFSs) with and without permeation supplied with water varying dosed phosphorus concentrations (0 and 25 μg P·L−1). The MFSs operated under permeation conditions were run at a constant flux of 15.6 L·m2·h−1 for 4.7 days. Feed channel pressure drop, transmembrane pressure, and flux were used as performance indicators. Optical coherence tomography (OCT) images and biomass quantification were used to analyze the developed biofilms. The total phosphorus concentration that accumulated on the membrane and spacer was quantified by using microwave digestion and inductively coupled plasma atomic emission spectroscopy (ICP-OES). Results show that permeation impacts biofilm development depending on nutrient condition with a stronger impact at low P concentration (pressure drop increase: 282%; flux decline: 11%) compared to a higher P condition (pressure drop increase: 206%; flux decline: 2%). The biofilm that developed at 0 μg P·L−1 under permeation conditions resulted in a higher performance decline due to biofilm localization and spread in the MFS. A thicker biofilm developed on the membrane for biofilms grown at 0 μg P·L−1 under permeation conditions, causing a stronger effect on flux decline (11%) compared to non-permeation conditions (5%). The difference in the biofilm thickness on the membrane was attributed to a higher phosphorus concentration in the membrane biofilm under permeation conditions. Permeation has an impact on biofilm development and, therefore, should not be excluded in biofouling studies.

Phosphorus Concentration in Water Affects the Biofilm Community and the Produced Amount of Extracellular Polymeric Substances in Reverse Osmosis Membrane Systems

Biofouling is a problem that hinders sustainable membrane-based desalination and the stratification of bacterial populations over the biofilm's height is suggested to compromise the efficiency of cleaning strategies. Some studies reported a base biofilm layer attached to the membrane that is harder to remove. Previous research suggested limiting the concentration of phosphorus in the feed water as a biofouling control strategy. However, the existence of bacterial communities growing under phosphorus-limiting conditions and communities remaining after cleaning is unknown. This study analyzes the bacterial communities developed in biofilms grown in membrane fouling simulators (MFSs) supplied with water with three dosed phosphorus conditions at a constant biodegradable carbon concentration. After biofilm development, biofilm was removed using forward flushing (an easy-to-implement and environmentally friendly method) by increasing the crossflow velocity for one hour. We demonstrate that small changes in phosphorus concentration in the feed water led to (i) different microbial compositions and (ii) different bacterial-cells-to-EPS ratios, while (iii) similar bacterial biofilm populations remained after forward flushing, suggesting a homogenous bacterial community composition along the biofilm height. This study represents an exciting advance towards greener desalination by applying non-expensive physical cleaning methods while manipulating feed water nutrient conditions to prolong membrane system performance and enhance membrane cleanability.

Natural deep eutectic solvents as biofilm structural breakers

Natural Deep Eutectic Solvents (NADES) are composed of supramolecular interactions of two or more natural compounds, such as organic acids, sugars, and amino acids, and they are being used as a new media alternative to conventional solvents. In this study, a new application of NADES is presented as a possible technology for biofilm structural breaker in complex systems since the current solvents used for biofilm cleaning and extraction of biofilm components use hazardous solutions. The NADES (betaine:urea:water and lactic acid:glucose:water) were analyzed before and after the biofilm treatment by attenuated total reflection Fourier-transform infrared spectroscopy and fluorescence excitation-emission matrix spectroscopy. Our results indicate that the green solvents could solubilize up to ≈70 percent of the main components of the biofilms extracellular matrix. The solubilization of the biomolecules weakened the biofilm structure, which could enhance the biofilm solubilization and removal. The NADES have the potential to be an environment-friendly, green solvent to extract valuable compounds and break the main structure from the biofilm, leading to a greener method for extracellular polymeric substance (EPS) extraction and biofilm treatment in various water treatment systems.

Enhanced hydraulic cleanability of biofilms developed under a low phosphorus concentration in reverse osmosis membrane systems

A critical problem in seawater reverse osmosis (RO) filtration processes is biofilm accumulation, which reduces system performance and increases energy requirements. As a result, membrane systems need to be periodically cleaned by combining chemical and physical protocols. Nutrient limitation in the feed water is a strategy to control biofilm formation, lengthening stable membrane system performance. However, the cleanability of biofilms developed under various feed water nutrient conditions is not well understood. This study analyzes the removal efficiency of biofilms grown in membrane fouling simulators (MFSs) supplied with water varying in phosphorus concentrations (3 and 6 μg P·L^-1 and with constant biodegradable carbon concentration) by applying hydraulic cleaning after a defined 140% increase in the feed channel pressure drop, through increasing the cross-flow velocity from 0.18 m s^-1 to 0.35 m s^-1 for 1 h. The two phosphorus concentrations (3 and 6 μg P·L^-1) simulate the RO feed water without and with the addition of a phosphorus-based antiscalant, respectively, and were chosen based on measurements at a full-scale seawater RO desalination plant. Biomass quantification parameters performed after membrane autopsies such as total cell count, adenosine triphosphate, total organic carbon, and extracellular polymeric substances were used along with feed channel pressure drop measurements to evaluate biofilm removal efficiency. The outlet water during hydraulic cleaning (1 h) was collected and characterized as well. Optical coherence tomography images were taken before and after hydraulic cleaning for visualization of biofilm morphology. Biofilms grown at 3 μg P·L^-1 had an enhanced hydraulic cleanability compared to biofilms grown at 6 μg P·L^-1. The higher detachment for biofilms grown at a lower phosphorus concentration was explained by more soluble polymers in the EPS, resulting in a lower biofilm cohesive and adhesive strength. This study confirms that manipulating the feed water nutrient composition can engineer a biofilm that is easier to remove, shifting research focus towards biofilm engineering and more sustainable cleaning strategies.

Biofouling control by phosphorus limitation strongly depends on the assimilable organic carbon concentration

Nutrient limitation is a biofouling control strategy in reverse osmosis (RO) membrane systems. In seawater, the assimilable organic carbon content available for bacterial growth ranges from about 50 to 400 μg C·L^-1, while the phosphorus concentration ranges from 3 to 11 μg P·L^-1. Several studies monitored biofouling development, limiting either carbon or phosphorus. The effect of carbon to phosphorus ratio and the restriction of both nutrients on membrane system performance have not yet been investigated. This study examines the impact of reduced phosphorus concentration (from 25 μg P·L^-1 and 3 μg P·L^-1, to a low concentration of ≤0.3 μg P·L^-1), combined with two different carbon concentrations (250 C L^-1 and 30 μg C·L^-1), on biofilm development in an RO system. Feed channel pressure drop was measured to determine the effect of the developed biofilm on system performance. The morphology of the accumulated biomass for both carbon concentrations was characterized by optical coherence tomography (OCT) and the biomass amount and composition was quantified by measuring total organic carbon (TOC), adenosine triphosphate (ATP), total cell counts (TCC), and extracellular polymeric substances (EPS) concentration for the developed biofilms under phosphorus restricted (P-restricted) and dosed (P-dosed) conditions. For both carbon concentrations, P-restricted conditions (≤0.3 μg P·L^-1) limited bacterial growth (lower values of ATP, TCC). A faster pressure drop increase was observed for P-restricted conditions compared to P-dosed conditions when 250 μg C·L^-1 was dosed. This faster pressure drop increase can be explained by a higher area covered by biofilm in the flow channel and a higher amount of produced EPS. Conversely, a slower pressure drop increase was observed for P-restricted conditions compared to P-dosed conditions when 30 μg C·L^-1 was dosed. Results of this study demonstrate that P-limitation delayed biofilm formation effectively when combined with low assimilable organic carbon concentration and thereby, lengthening the overall membrane system performance.