Investigation of severe UF membrane fouling induced by three marine algal species

Calcium sulfite oxidation activated by ferrous iron integrated with membrane filtration for removal of typical algal contaminants

Oxidation treatment of algae-laden water may cause cells rupture and emission of intracellular organics, thus restricting its further popularization. As a moderate oxidant, calcium sulfite could be slowly released in the liquid phase, thus exhibiting a potential to maintain the cells integrity. To this end, calcium sulfite oxidation activated by ferrous iron was proposed integrated with ultrafiltration (UF) for removal of Microcystis aeruginosa, Chlorella vulgaris and Scenedesmus quadricauda. The organic pollutants were significantly eliminated, and the repulsion between algal cells was obviously weakened. Through fluorescent components extraction and molecular weights distribution analyses, the degradation of fluorescent substances and the generation of micromolecular organics were verified. Moreover, the algal cells were dramatically agglomerated and formed larger flocs under the premise of maintaining high cell integrity. The terminal normalized flux was ascended from 0.048 to 0.072 to 0.711-0.956, and the fouling resistances were extraordinarily decreased. Due to the distinctive spiny structure and minimal electrostatic repulsion, Scenedesmus quadricauda was easier to form flocs, and its fouling was more readily mitigated. The fouling mechanism was remarkably altered through postponing the formation of cake filtration. The membrane interface characteristics including microstructures and functional groups firmly proved the fouling control efficiency. The reactive oxygen species (i.e., SO₄^•- and ¹O₂) generated through the principal reactions and Fe-Ca composite flocs played dominant roles in alleviating membrane fouling. Overall, the proposed pretreatment exhibits a brilliant application potential for enhancing UF in algal removal.

Evaluation of membrane fouling at elevated temperature impacted by algal organic matter

Membrane distillation (MD) is a thermally driven technology applied in desalination and water reuse with utilisation of sustainable energy. However, algal organic matter (AOM) could foul membrane critically and plague MD's long-term operational stability. In this study, the soluble extracellular polymeric substance (sEPS) and intracellular organic matter with bound extracellular polymeric substance (IOM + bEPS) of two algal species (Amphora coffeaeformis and Navicula incerta) were exposed to 60 °C, 70 °C and 80 °C for 8 h with polypropylene hydrophobic membrane, simulating heated AOMs contacted with membrane inside MD unit, to study the temperature effect on membrane fouling. The dissolved carbohydrate and protein in the sEPS and IOM + bEPS samples generally increased after being heated. Heating caused cell lysis and the release and dissolution of carbohydrate and protein from sEPS, IOM and bEPS into water. As heating temperature increased, the carbohydrate release from the AOM usually increased. The contact angle of membrane contacted with sEPS and IOM + bEPS reduced significantly after heat treatment. The reduction in IOM + bEPS was larger than sEPS, in line with SEM analysis, indicating membrane surfaces and pores with IOM + bEPS fouled more severely than sEPS. It is due to higher hydrophobicity in IOM + bEPS causing adherence to membrane and presence of amphiphiles. High protein, lipid, and saturated fats proportions also cause severe fouling. SEM-EDX analysis indicated presence of O, Na, Cl and Mg elements, pointing to carbohydrate and lipids, and salt trapped in foulants. AOM heating and composition had direct effect to the membrane integrity, dictating severity of fouling in MD operations.

Divergent accumulation of membrane biofouling by slight elevation of nitrogen and phosphorus in drinking water treatment: Performances and mechanisms

Membrane biofouling has been intensively investigated over many years; however, little attention has been paid to the role of slightly elevated nutrients in the fouling formation, especially during drinking water treatment. In this study, we evaluated the respective contribution of slightly elevated nitrogen (ca. 0.5 mg·L^-1) and phosphorus (ca. 0.1 mg·L^-1) concentrations to membrane biofouling formation and deciphered the associated mechanisms. The results demonstrated that the slight concentration elevation of nitrogen did not substantially decrease the permeate flux, but approximately 50% of the permeate flux was reduced by the elevated phosphorus. The fouling layer was thickened about 4-fold due to the slight elevation of phosphorus and this resulted in the best removal performance of fluorescent organics. In contrast, the fouling layer structure was not markedly changed by the elevated nitrogen. The concentrations of proteins and polysaccharides in the fouling layers increased to different degrees, with phosphorus playing a more pronounced role than nitrogen. The molecular experiments revealed that the filamentous Arthronema dominated the nitrogen-enriched fouling layer, but they decreased to nearly zero in the phosphorus-enriched fouling layer, and the Xanthobacter, which was capable of secreting large amounts of EPS, was enhanced. The growth of algae Chlorellales was stimulated by the elevated nitrogen, while Sphaeropleales was dominant with elevated phosphorus, and for protozoa, Vannella was increased by nitrogen, whereas members of Hartmannella were enhanced by phosphorus. Our findings indicate that the impact of slight increases in concentration of nutrients, especially phosphorus, should be taken into account in drinking water treatment.

Mitigation of algal organic matter released from Chaetoceros affinis and Hymenomonas by in situ generated ferrate

This study demonstrates the application of in situ ferrate (Fe(VI)) for the efficient removal of dissolved algal organic matter (AOM) from seawater. Sodium hypochlorite (NaOCl) and ferric (Fe(III)) were used to produce in situ Fe(VI) by wet chemical oxidation. First, the removal efficiencies of two model AOM compounds, humic acid (HA) and sodium alginate (SA), were evaluated in the presence of sodium chloride with an initial influent dissolved organic carbon (DOC) concentration of 5.0 mg C L^-1 at different pH levels to establish the optimal doses for in situ Fe(VI) generation. The concentration of Fe(VI) was determined by the 2,2-Azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) ultraviolet-visible spectrophotometry method. In the case of HA, 72% DOC removal was recorded when applied with 1.5 mg L^-1 of Fe(III) and 1.5 mg L^-1 of NaOCl (in situ Fe(VI) concentration of 1.46 mg L^-1) while 42% DOC removal was observed for SA. Subsequently, the removal of AOM extracted from two bloom-forming algal species, Chaetoceros affinis (CA) and Hymenomonas (Hym), cultivated in seawater from the Red Sea, were tested with in situ generated Fe(VI) at the established optimum condition. In situ Fe(VI) recorded superior performance in removing AOM extracted from CA and Hym, showing 83% and 92% DOC removal when the influent DOC concentrations were 2.48 and 2.63 mg L^-1, respectively. A detailed AOM characterization was conducted using liquid chromatography-organic carbon detection.

Interfacial interactions between Skeletonema costatum extracellular organic matter and metal oxides: Implications for ceramic membrane filtration

In the current study, the interfacial interactions between the high molecular weight (HMW) compounds of Skeletonema costatum (SKC) extracellular organic matter (EOM) and ZrO₂ or Al₂O₃, were investigated by atomic force microscopy (AFM). HMW SKC-EOM was rigorously characterized and described as a hydrophilic organic compound mainly comprised of polysaccharide-like structures. Lipids and proteins were also observed, although in lower abundance. HMW SKC-EOM displayed attractive forces during approaching (i.e., leading to jump-to-contact events) and adhesion forces during retracting regime to both metal oxides at all solution conditions tested, where electrostatics and hydrogen bonding were suggested as dominant interacting mechanisms. However, the magnitude of these forces was significantly higher on ZrO₂ surfaces, irrespective of cation type (Na⁺ or Ca²⁺) or concentration. Interestingly, while HMW SKC-EOM interacting forces to Al₂O₃ were practically insensitive to solution chemistry, the interactions between ZrO₂ and HMW SKC-EOM increased with increasing cation concentration in solution. The structure, and lower charge, hydrophilicity, and density of hydroxyl groups on ZrO₂ surface would play a key role on favoring zirconia associations with HMW SKC-EOM. The current results contribute to advance our fundamental understanding of Algogenic Organic Matter (AOM) interfacial interactions with metal oxides (i.e., AOM membrane fouling), and would highly assist in the proper selection of membrane material during episodic algal blooms.