The role of hydrodynamic conditions and pH on algal-rich water fouling of ultrafiltration

Mitigating membrane fouling in an internal loop airlift membrane photobioreactor containing Spirulina platensis: effects of riser cross-sectional area and hydrophilic baffles

Membrane photobioreactors (MPBRs) have gained significant attention due to their ability to support microalgae activities such as cultivation, harvesting, and production of beneficial products. Despite various efforts to mitigate membrane fouling, a fundamental issue in membrane processes, in these systems, a cost-effective and less energy-consuming method is still needed. This study examines the impact of the cross-sectional area of the riser and the baffle material on membrane fouling in an internal loop airlift MPBR. The use of hydrophilic polyester-polypropylene (PES-PP) baffles proves to be more effective than plexiglass baffles. Specifically, in configurations with d = 0.7 cm and d = 1.4 cm, RC/RT decreased by approximately 20% and 13%, respectively, compared to plexiglass baffles. As for the values of RP/RT at a distance of d = 0.7, nearly a 5% increase was observed, and at a distance of d = 1.4, an increase of approximately 11% was observed. This is due to the development of the cake layer on the matrix structure of the PES-PP baffles instead of the membrane itself. The most optimal outcomes were reached while working with PES-PP at a distance of 0.7 cm, as it prolonged the membrane fouling time to 46 h.

Characterization of an aerated submerged hollow fiber ultrafiltration device for efficient microalgae harvesting

The present work characterizes a submerged aerated hollow fiber polyvinylidene fluorid (PVDF) membrane (0.03 μm) device (Harvester) designed for the ultrafiltration (UF) of microalgae suspensions. Commercial baker's yeast served as model suspension to investigate the influence of the aeration rate of the hollow fibers on the critical flux (CF, J c) for different cell concentrations. An optimal aeration rate of 1.25 vvm was determined. Moreover, the CF was evaluated using two different Chlorella cultures (axenic and non-axenic) of various biomass densities (0.8-17.5 g DW/L). Comparably high CFs of 15.57 and 10.08 L/m/2/h were measured for microalgae concentrations of 4.8 and 10.0 g DW/L, respectively, applying very strict CF criteria. Furthermore, the J c-values correlated (negative) linearly with the biomass concentration (0.8-10.0 g DW/L). Concentration factors between 2.8 and 12.4 and volumetric reduction factors varying from 3.5 to 11.5 could be achieved in short-term filtration, whereat a stable filtration handling biomass concentrations up to 40.0 g DW/L was feasible. Measures for fouling control (aeration of membrane fibers, periodic backflushing) have thus been proven to be successful. Estimations on energy consumption revealed very low energy demand of 17.97 kJ/m3 treated microalgae feed suspension (4.99 × 10-3 kWh/m3) and 37.83 kJ/kg treated biomass (1.05 × 10-2 kWh/kg), respectively, for an up-concentration from 2 to 40 g DW/L of a microalgae suspension.

Management of concentrate and waste streams for membrane-based algal separation in water treatment: A review

Frequent occurrence of harmful algal blooms (HABs) and red tides in freshwater and seawater poses serious threats to water treatment and drives the application of membrane-based technologies in algal separation. Despite the high removal efficiency of algal cells and their metabolites (e.g. organic matter and toxins) by membranes, the generation of concentrate and waste streams presents a major challenge. In this paper, we review the scenarios under which membrane-based processes are integrated with algal separation, with particular attention given to (i) drinking water production and desalination at low algal concentrations and (ii) cyanobacteria-laden water treatment/desalination. The concentrate and waste streams from backwashing and membrane cleaning in each scenario are characterised with this information facilitating a better understanding of the transport of algal cells and metabolites in membrane processes. Current strategies and gaps in managing concentrate and waste streams are identified with guidance and perspectives for future studies discussed in an Eisenhower framework.

Alginate to simulate biofouling in submerged fluidized ceramic membrane reactor: Effect of solution pH and ionic strength

Membrane fouling was investigated experimentally by fluidizing non-adsorbed plastic scouring media on flat-tubular ceramic membrane treating a sodium alginate solution as a representative of polysaccharides in wastewater. Fouling rate increased with set-point permeate flux, but it was remarkably reduced by fluidizing the scouring agent regardless of the flux applied. Higher solution pH resulted in more reduction in membrane fouling due to electrostatic repulsion enhanced between alginate foulant and membrane surface which are both negatively charged. The addition of divalent cations such as Ca²⁺ and Cu²⁺ mitigated alginate fouling significantly due to the back transport associated with formation of larger particles away from membrane. However, the addition of monovalent cations accelerated the membrane fouling with less effectiveness of the media fluidization in fluidized bed membrane reactor. Adding monovalent ions was thought to transform rigid, compact and spherocolloidal macromolecular structure of alginate into the intramolecular charge shielding to neutralize functional groups.

A novel electro-coagulation-Fenton for energy efficient cyanobacteria and cyanotoxins removal without chemical addition

Harmful cyanobacterial bloom is a serious threat to global aquatic ecology and drinking water safety. Electro-Fenton (EF) has emerged as an efficient process for cyanobacteria and cyanotoxins removal, but high consumption of energy and chemicals remain a major bottleneck. This study presents a novel convertible three-electrodes Electro-Coagulation-Fenton process for cyanobacteria and cyanotoxins removal with low energy consumption and no chemicals addition. We for the first time demonstrated the freely alternating between Electrocoagulation (EC) and EF by switching electrodes. The optimal aerated EC was operated at pH 8 and 100 mA to remove 91 ± 2% of cyanobaterial cells and 15% of Microcystins (MCs). Coagulants generated in EC were adsorbed on cyanobacterial cells to form a protect layer against algae disruption and cyanotoxins releasing. Residual MCs and cyanobaterial cells were completely mineralized by EF at 28 mA with iron ions and H₂O₂ generated in-situ. Compare to traditional EF, the optimal Electro-Coagulation-Fenton process increased total organic carbon (TOC) removal efficiency by 30%, yet energy consumption reduced up to 92%. The novel Electro-Coagulation-Fenton process is a promising technology for the efficient treatment of the mixture of suspended solid pollutants and persistent organic pollutants in one system with low energy consumption.

The effects of surfactants (sodium dodecyl sulfate, triton X-100 and cetyl trimethyl ammonium bromide) on the dewaterability of microalgae biomass using pressure filtration

The application of pressure filtration in microalgae harvesting requires chemical pretreatment in order to reduce membrane fouling and to increase water flux. Surfactants have shown potential to enhance microalgae dewaterability by charge neutralization, bridging and releasing extracellular polymeric substances (EPS) and bound water. In this study, the effect of three surfactants including anionic sodium dodecyl sulfate (SDS), non-ionic triton X-100 and cationic cetyl trimethyl ammonium bromide (CTAB) on the dewaterability of Chlamydomonas sp. was investigated. Filtration fluxes and biomass concentrations were used to evaluate the microalgae dewaterability. Based on the results, SDS and Triton X-100 had a negative effect on the dewaterability of microalgae biomass. However, CTAB improved the dewaterability by decreasing the reversible and irreversible fouling resistance. The optimum dosage of CTAB was found to be 1500 mg/L, and resulted in 95.8% and 140% improvement on average water flux and biomass recovery efficiency, respectively.

A review of membrane fouling and its control in algal-related membrane processes

Membrane technologies have received much attention in microalgae biorefinery for nutrients removal from wastewater, carbon dioxide abatement from the air as well as the production of value-added products and biofuel in recent years. This paper provides a state-of-the-art review on membrane fouling issues and its control in membrane photobioreactors (MPBRs) and other algal-related membrane processes (harvesting, dewatering, and biofuel production). The mechanisms of membrane fouling and factors affecting membrane fouling in algal-related membrane processes are systematically reviewed. Also, strategies to control membrane fouling in algal-related membrane processes are summarized and discussed. Finally, the gaps, challenges, and opportunities in membrane fouling control in algal-related membrane technologies are identified and discussed.

Analysis of effect of particles on cake layer compressibility during ultrafiltration of upflow biological activated carbon effluent

Three different hollow-fibre ultrafiltration (UF) membranes were applied to treat upflow biological activated carbon (UBAC) effluent to determine the characteristics of membrane biofouling by microorganisms and particles. At the beginning of filtration, the cake layer formed on the membrane was loose and highly compressible, and the trans-membrane pressure (TMP) rapidly increased. When compressed to a certain extent, cake layer with low compressibility was formed by the accumulated particles and resulted in slower TMP increment. Thus, the decreased compressibility of the cake layer formed on the UF membrane during filtration of UBAC effluent led to the rapid increase in TMP at the beginning and slow increment in subsequently. The results were confirmed by filtering Escherichia coli, Staphylococcus aureus and kaolinite mixed suspensions with flat-sheet UF membrane. Our findings provide a new insight into membrane biofouling control and may facilitate better membrane application in drinking water treatment.

An application of cellular organic matter to coagulation of cyanobacterial cells (Merismopedia tenuissima)

Algae affect the performance of drinking water treatment significantly when they decay and release considerable amounts of cellular organic matter (COM). The study describes the cyanobacterium Merismopedia tenuissima and its COM and investigates the effect of their simultaneous coagulation. As COM is highly complex mixture, we characterised it in terms of hydrophobicity, protein content and molecular weights (MWs). To describe the coagulation mechanisms and molecular interactions in the system, we determined both COM and cell surface charge by means of potentiometric titration and zeta potential analysis, respectively, and performed the jar tests with single components and their mixtures with and without a coagulant (ferric sulphate). The coagulation tests performed with the individual components or with their mixtures proved efficient cell removals (up to 99%) but relatively low COM removals (37 ÷ 57%). This disproportion can be attributed to the prevalence of hydrophilic compounds and to the high portion of low-MW organics in COM. Coagulation of COM/cell mixtures achieved comparable efficacy with single component tests, using even lower coagulant doses. Furthermore, COM presence substantially deviated the pH optimum for cell removal and thus altered coagulation mechanisms. While single cells interacted prevailingly through adsorption onto Fe-oxide-hydroxides at about neutral pH (6.0-7.7), the COM/cell mixtures succumbed to charge neutralisation by Fe-hydroxopolymers within moderately acidic pH range (5.0-6.5). Moreover, COM initiated cell flocculation also at acidic pH in both the presence (pH 3.4-3.9) and the absence of a coagulant (pH 3.6-4.6). This supportive effect is ascribed to relatively high-MW COM (>10 kDa), serving as a natural flocculant through inter-particle bridging mechanism and exhibiting nearly the same COM/cell removals as ferric sulphate.

Unique characteristics of algal dissolved organic matter and their association with membrane fouling behavior: a review

Over the last several decades, the frequent occurrence of algal bloom in drinking water supplies, driven by increasing anthropogenic input and climate change, has posed serious problems for membrane filtration processes, resulting in reduced membrane permeability and increased energy consumption. It is essential to comprehensively understand the characteristics of algal dissolved organic matter (DOM) and the subsequent effects on the filtration processes for better insight into membrane fouling mitigation. Many studies have revealed that algal DOM has displayed unique characteristics distinguished from other sources of DOM with respect to the chemical composition, the structures, and the molecular weight distributions. Algal DOM is considered to be a major obstacle in understanding membrane fouling due to its complicated interactions among dissimilar algal DOM constituents as well as between algal DOM and membrane material matrices. The present review article summarizes (1) recent characterizing methods for algal DOM, (2) environmental factors affecting the characteristics of algal DOM, (3) the discrepancies between algal DOM and other sources of aquatic DOM, particularly terrestrial sources, and (4) potential fouling effects of algal DOM on membrane filtration processes and their associations with algal DOM characteristics. A broad understanding of algal DOM-driven membrane fouling can lead to breakthroughs in efficient membrane filtration processes to treat algal bloom water sources.

Membrane fouling in ultrafiltration of natural water after pretreatment to different extents

The combined fouling during ultrafiltration (UF) of surface water pretreated to different extents was investigated to disclose the roles of polysaccharides, proteins, and inorganic particles in UF membrane fouling. Both reversible and irreversible fouling decreased with enhanced pretreatment (biologically active carbon (BAC) treatment and sand filtration). The sand filter effluent fouled the membrane very slowly. The UF membrane removed turbidity to less than 0.1 nephelometric turbidity unit (NTU), reduced polysaccharides by 25.4%-29.9%, but rejected few proteins. Both polysaccharides and inorganic particles were detected on the fouled membranes, but inorganic particles could be effectively removed by backwashing. The increase of turbidity in the sand filter effluent to 3.05 NTU did not significantly increase the fouling rate, but an increase in the turbidity in the BAC effluent to 6.11 NTU increased the fouling rate by more than 100%. The results demonstrated that the polysaccharide, not the protein, constituents of biopolymers were responsible for membrane fouling. Membrane fouling was closely associated with a small fraction of polysaccharides in the feed water. Inorganic particles exacerbated membrane fouling only when the concentration of fouling-inducing polysaccharides in the feed water was relatively high. The combined fouling was largely reversible, and polysaccharides were the predominant substances responsible for irreversible fouling.

Experimental studies and statistical analysis of membrane fouling behavior and performance in microfiltration of microalgae by a gas sparging assisted process

Response surface methodology (RSM) and central composite design (CCD) were applied for modeling and optimization of cross-flow microfiltration of Chlorella sp. suspension. The effects of operating conditions, namely transmembrane pressure (TMP), feed flow rate (Qf) and optical density of feed suspension (ODf), on the permeate flux and their interactions were determined. Analysis of variance (ANOVA) was performed to test the significance of response surface model. The effect of gas sparging technique and different gas-liquid two phase flow regimes on the permeate flux was also investigated. Maximum flux enhancement was 61% and 15% for Chlorella sp. with optical densities of 1.0 and 3.0, respectively. These results indicated that gas sparging technique was more efficient in low concentration microalgae microfiltration in which up to 60% enhancement was achieved in slug flow pattern. Additionally, variations in the transmission of exopolysaccharides (EPS) and its effects on the fouling phenomenon were evaluated.

Characterization of algal organic matters of Microcystis aeruginosa: biodegradability, DBP formation and membrane fouling potential

Algal organic matters (AOM), including extracellular organic matters (EOM) and intracellular organic matters (IOM), were comprehensively studied in terms of their biodegradability, disinfection byproduct (DBP) formation potentials and membrane fouling. EOM and IOM were fractionated into hydrophobic (HP), transphilic (TP) and hydrophilic (HL) constituents. The HP, TP and HL fractions of EOM and IOM were highly biodegradable with BDOC/DOC ranging from 52.5% to 67.4% and the DBP formation potentials followed the order of HP > TP > HL, except of IOM-HL. Biodegradable process proved very effective in removing the DBP formation potentials. Moreover, the AOM characteristics were also evaluated during ultrafiltration (UF) treatment. Results demonstrated that UF favourably remove DOC and DBP formation potential of IOM than those of EOM. And the HL constituents played a more important role in membrane fouling than HP and TP. The UF foulants exhibited higher BDOC/DOC than AOM, suggesting EOM and IOM might enhance biofouling because more biodegradable proteins and polysaccharides were found in membrane foulants. Therefore, appropriate biological treatment, ultrafiltration, or combination of the both are potential options to address these algae-caused water quality issues.

Influence of hydrophobic/hydrophilic fractions of extracellular organic matters of Microcystis aeruginosa on ultrafiltration membrane fouling

Fouling is a major obstacle to maintain the efficiency of ultrafiltration-based drinking water treatment process. Algal extracellular organic matters (EOMs) are currently considered as one of the major sources of membrane fouling. The objective of this study was to investigate the influence of different hydrophobic/hydrophilic fractions of EOM extracted from Microcystis aeruginosa on ultrafiltration membrane fouling at lab scale. The experimental data indicated that EOM exhibited similar flux decline trends on polyethersulfone (PES) and regenerated cellulose (RC) membranes but caused greater irreversible fouling on PES membrane than RC membrane due to its hydrophobic property. It was also observed that charged hydrophilic (CHPI) and neutral hydrophilic (NHPI) fractions caused greater flux decline over hydrophobic (HPO) and transphilic (TPI) fractions. For PES membrane, the order of the irreversible fouling potentials for the four fractions was HPO>TPI>CHPI>NHPI, while the irreversible fouling potentials of RC membrane were tiny and could be ignored. Fluorescence excitation-emission matrix (EEM) spectra and Fourier transform infrared (FTIR) spectra suggested that protein-like, polysaccharide-like and humic-like substances were the major components responsible for membrane fouling. The results also indicated that the irreversible fouling increased as the pH decreased. The addition of calcium to feed solutions led to more severe flux decline and irreversible fouling.