Detailed algal extracellular carbohydrate-protein characterisation lends insight into algal solid-liquid separation process outcomes

Interference of extracellular soluble algal organic matter on flocculation-sedimentation harvesting of Chlorella sp

Extracellular soluble algal organic matter (AOM) significantly interferes with microalgae flocculation. This study investigated the effects of various AOM fractions on Chlorella sp. flocculation using ferric chloride (FeCl3), sodium hydroxide (NaOH), and chitosan. All flocculants achieved high separation efficiency (87-99 %), but higher dosages were required in the presence of AOM. High molecular weight (>50 kDa) AOM fraction was identified as the primary inhibitor of flocculation across different pH levels, whereas low/medium molecular weight (<3 and <50 kDa) AOM had minimal impact. Compositional analysis revealed that the inhibitory AOM fraction is a glycoprotein rich in carbohydrates, including neutral, amino, and acidic sugars. The significance of this study is in identifying carboxyl groups (-COOH) from acidic monomers in >50 kDa AOM that inhibit flocculation. Understanding AOM composition and the interaction dynamics between AOM, cells, and flocculants is crucial for enhancing the techno-economics and sustainability of flocculation-based microalgae harvesting.

Algal carbohydrates: Sources, biosynthetic pathway, production, and applications

Algae play a significant role in the global carbon cycle by utilizing photosynthesis to efficiently convert solar energy and atmospheric carbon dioxide into various chemical compounds, notably carbohydrates, pigments, lipids, and released oxygen, making them a unique sustainable cellular factory. Algae mostly consist of carbohydrates, which include a broad variety of structures that contribute to their distinct physical and chemical properties such as degree of polymerization, side chain, branching, degree of sulfation, hydrogen bond etc., these features play a crucial role in regulating many biological activity, nutritional and pharmaceutical properties. Algal carbohydrates have not received enough attention in spite of their distinctive structural traits linked to certain biological and physicochemical properties. Nevertheless, it is anticipated that there will be a significant increase in the near future due to increasing demand, sustainable source, biofuel generation and their bioactivity. This is facilitated by the abundance of easily accessible information on the structural data and distinctive characteristics of these biopolymers. This review delves into the different types of saccharides such as agar, alginate, fucoidan, carrageenan, ulvan, EPS and glucans synthesized by various macroalgal and microalgal systems, which include intracellular, extracellular and cell wall saccharides. Their structure, biosynthetic pathway, sources, production strategies and their applications in various field such as nutraceuticals, pharmaceuticals, biomedicine, food and feed, cosmetics, and bioenergy are also elaborately discussed. Algal polysaccharide has huge a scope for exploitation in future due to their application in food and pharmaceutical industry and it can become a huge source of capital and income.

Moderate oxidation of algae-laden water: Principals and challenges

The occurrence of seasonal algae blooms represents a huge dilemma for water resource management and has garnered widespread attention. Therefore, finding methods to control algae pollution and improve water quality is urgently needed. Moderate oxidation has emerged as a feasible way of algae-laden water treatment and is an economical and prospective strategy for controlling algae and endogenous and exogenous pollutants. Despite this, a comprehensive understanding of algae-laden water treatment by moderate oxidation, particularly principles and summary of advanced strategies, as well as challenges in moderate oxidation application, is still lacking. This review outlines the properties and characterization of algae-laden water, which serve as a prerequisite for assessing the treatment efficiency of moderate oxidation. Biomass, cell viability, and organic matter are key components to assessing moderate oxidation performance. More importantly, the recent advancements in employing moderate oxidation as a treatment or pretreatment procedure were examined, and the suitability of different techniques was evaluated. Generally, moderate oxidation is more promising for improving the solid-liquid separation process by the reduction of cell surface charge (stability) and removal/degradation of the soluble algae secretions. Furthermore, this review presents an outlook on future research directions aimed at overcoming the challenges encountered by existing moderate oxidation technologies. This comprehensive examination aims to provide new and valuable insights into the moderate oxidation process.

Molecular insight into the binding properties of marine algogenic dissolved organic matter for polybrominated diphenyl ethers and their combined effect on marine zooplankton

Polybrominated diphenyl ethers (PBDEs) are widespread in marine ecosystems, despite the limits placed on several congeners, and pose a threat to marine organisms. Many coexisting factors, especially dissolved organic matter (DOM), affect the environmental behavior and ecological risk of PBDEs. Since blooms frequently occur in coastal waters, we used algogenic DOM (A-DOM) from the diatom Skeletonem costatum and examined the interaction of A-DOM with 2,2',4,4'-tetrabromodiphenyl ether (BDE-47). Moreover, their combined effect on the rotifer Brachionus plicatilis was analyzed. During the stationary period, A-DOM had more proteins than polysaccharides, and 7 extracellular proteins were identified. A-DOM fluorescence was statically quenched by BDE-47, and amide, carbonyl, and hydroxyl groups in A-DOM were involved. Molecular docking analysis showed that all 5 selected proteins of A-DOM could spontaneously bind with BDE-47 and that hydrophobic interactions, van der Waals forces and pi-bond interactions existed. The reproductive damage, oxidative stress and inhibition of mitochondrial activity induced by BDE-47 in rotifers were relieved by A-DOM addition. Transcriptomic analysis further showed that A-DOM could activate energy metabolic pathways in rotifers and upregulate genes encoding metabolic detoxification proteins and DNA repair. Moreover, A-DOM alleviated the interference effect of BDE-47 on lysosomes, the extracellular matrix pathway and the calcium signaling system. Alcian blue staining and scanning electron microscopy showed that A-DOM aggregates were mainly stuck to the corona and cuticular surface of the rotifers; this mechanism, rather than a real increase in uptake, was the reason for enhanced bioconcentration. This study reveals the complex role of marine A-DOM in PBDEs bioavailability and enhances the knowledge related to risk assessments of PBDE-like contaminants in marine environments.

Influence of algal organic matter in the in-situ flotation removal of Microcystis using positively charged bubbles

Positively charged bubbles efficiently capture and remove negatively charged algal cells without relying on coagulation-flocculation. However, the efficiency is notably influenced by the presence of algal organic matter (AOM). This study investigated the impact of AOM composition on flotation performance by analyzing AOM from various growth phases of Microcystis flos-aquae. The results indicated that low-concentration AOM (<5 mg C L-1), particularly the high molecular weight (>30 kDa) fractions containing high percentages of protein during the exponential growth phase, significantly improved the flotation efficiency by >18%. A high-speed camera system illustrates the pivotal role of low-concentration protein-containing AOM in forming network structures that enhance cell capture. These protein-driven network structures, which enhance the flotation efficiency, provide valuable insights into the development of effective in-situ algal bloom prevention techniques.

Different responses of Sinorhizobium sp. upon Pb and Zn exposure: Mineralization versus complexation

Lead (Pb) and zinc (Zn) have been discharged into environment and may negatively impact ecological security. Rhizobia has gained attention due to their involvement in the restoration of metal polluted soils. However, little is known about the responses of rhizobia under Pb and Zn stress, especially the roles played by extracellular polymeric substances (EPS) in the resistance of these two metals. Here, Sinorhizobium sp. C10 was isolated from soil around a mining area and was exposed to a series of Pb/Zn treatments. The cell morphology and surface mineral crystals, EPS content and fluorescent substances were determined. In addition, the extracellular polysaccharides and proteins were characterized by attenuated total reflection infrared spectroscopy (ATR-IR) and X-ray photoelectron spectroscopy (XPS). The results showed that Zn stress induced the synthesis of EPS by C10 cells. Functional groups of polysaccharides (CO) and proteins (C-O/C-N) were involved in complexation with Zn. In contrast, C10 resisted Pb stress by forming lead phosphate (Pb3(PO4)2) on the cell surface. Galactose (Gal) and tyrosine played key roles in resistance to the Zn toxicity, whereas glucosamine (N-Glc) was converted to glucose in large amounts during extracellular Pb precipitation. Together, this study demonstrated that C10 possessed different strategies to detoxify the two metals, and could provide basis for bioremediation of Pb and Zn polluted sites.

Enhancement of sewer sediment control and disruption of adhesive gelatinous sediment structure using low-dose calcium peroxide

Large quantities of sediments in urban sewer systems pose significant risk of pipe clogging and corrosion. Owing to their gel-like structure, sewer sediments have strong resistance to hydraulic shear stress. This study proposed a novel approach to weaken the erosion resistance of sewer sediments by destroying viscous gel-like biopolymers in sediments with low doses of calcium peroxide (CaO2). After treatment with 10-50 mg g-1 TS of CaO2, the critical erosion shear stress was significantly reduced by 25.7%-59.9%. The sediment aggregates gradually disintegrated into small diameter particles with increasing CaO2 dosage. Further analysis showed that the strong oxidizing and alkaline environment induced by CaO2 treatment led to cell lysis and changes in the composition and property of extracellular polymeric substances (EPS). After CaO2 treatment, aromatic proteins and humic acid-like substances associated with adhesion translocated from the inner EPS layers to outer layers while being disintegrated into small organic molecules. Concomitantly, CaO2 treatment disrupted the main functional groups (-OH, COO-, C-N, CO, and CN) in inner EPS layers, thus weakening EPS adhesion. Analysis of protein secondary structure and zeta potential reflected the reduced aggregation capacity of sediment microorganisms and loosening of sediment structure after CaO2 treatment. Thus, CaO2 treatment facilitated fragmentation and disaggregation of the gelatinous structure of sewer sediments. Such green strategy decreased the cost of sewer sediment disposal by 42.10-68.95% when compared to water flushing, and it would improve the self-cleaning capacity of sewer system and efficiency of dredging equipment.

Relating algal-derived extracellular and intracellular dissolved organic nitrogen with nitrogenous disinfection by-product formation

The dissolved organic nitrogen (DON) pool from algal-derived extracellular and intracellular organic matter (EOM and IOM) comprises proteins, peptides, free amino acids and carbohydrates, of which, proteins can contribute up to 100% of the DON. Previous reports of algal-derived DON character have focused on bulk properties including concentration, molecular weight and hydrophobicity. However, these can be similar between algal species and between the EOM and IOM even when the inherent molecular structures vary. A focus on bulk character presents challenges to the research on algal-derived nitrogenous-disinfection by-product (N-DBP) formation as N-DBP formation is sensitive to the changes in molecular structure. Hence, the main aim of this study was to characterize algal EOM and IOM-derived DON, specifically proteinaceous-DON, using a combination of bulk and molecular characterization techniques to enable a more detailed exploration of the relationship between the character of algal-derived proteins and the N-DBP formation potential. DON from the EOM and IOM of four commonly found algae and cyanobacteria in natural waters were evaluated, namely Chlorella vulgaris, Microcystis aeruginosa, Dolichospermum circinale, and Cylindrospermopsis raciborskii. It was observed that 77-96% of total DON in all EOM and IOM samples was of proteinaceous origin. In the proteins, DON concentrations were highest in the high molecular weight fraction of IOM-derived bulk proteins (0.13-0.75 mg N L^-1) and low to medium molecular weight fraction of EOM-derived bulk proteins (0.15-0.63 mg N L^-1) in all species. Similar observations were also made via sodium dodecyl sulphate polyacrylamide gel electrophoresis and liquid chromatography-high resolution mass spectrometry. Solid-state ¹⁵N nuclear magnetic resonance (NMR) spectroscopy of the EOM and IOM revealed the existence of common aliphatic and heterocyclic N-groups in all samples, including a dominant 2° amide peak. Species dependent variability was also observed in the spectra, particularly in the EOM; e.g. nitro signals were found only in the Cylindrospermopsis raciborskii EOM. Dichloroacetonitrile (DCAN) and N-nitrosamine concentrations from the EOM of the species evaluated in this study were lower than the guideline limits set by regulatory agencies. It is proposed that the dominant 2° amide in all samples decreased N-DBP formation upon chlorination. For chloramination, the presence of nitro groups and aliphatic and heterocyclic N-DBP precursors could cause variable N-nitrosamine formation. Compared to non-algal impacted waters, algae-laden waters are characterised by low organic carbon: organic nitrogen ratios of ∼7-14 and elevated DON and protein concentrations. Hence, relying only on bulk characterization increases the perceived risk of N-DBP formation from algae-laden waters.

Fouling evolution of extracellular polymeric substances in forward osmosis based microalgae dewatering

Membrane fouling was still a challenge for the potential application of forward osmosis (FO) in algae dewatering. In this study, the fouling behaviors of Chlorella vulgaris and Scenedesmus obliquus were compared in the FO membrane filtration process, and the roles of their soluble-extracellular polymeric substances (sEPS) and bound-EPS (bEPS) in fouling performance were investigated. The results showed that fouling behaviors could be divided into two stages including a quickly dropped and later a stable process. The bEPS of both species presented the highest flux decline (about 40.0%) by comparison with their sEPS, cells and broth. This performance was consistent with the largest dissolved organic carbon losses in feed solutions, and the highest interfacial free energy analyzed by the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. The chemical characterizations of algal foulants further showed that the severe fouling performance was also consistent with a proper ratio of carbohydrates and proteins contents in the cake layer, as well as the higher low molecular weight (LMW) components. Compared with the bEPS, the sEPS was crucial for the membrane fouling of S. obliquus, and an evolution of the membrane fouling structure was found in both species at the later filtration stage. This work clearly revealed the fundamental mechanism of FO membrane fouling caused by real microalgal suspension, and it will improve our understanding of the evolutionary fouling performances of algal EPS.

Changes of the physicochemical properties of extracellular polymeric substances (EPS) from Microcystis aeruginosa in response to microplastics

Microplastics (MPs) are ubiquitous in aquatic ecosystems and can significantly influence the growth, aggregation and functions of phytoplankton biomass. However, variations in the extracellular polymeric substances (EPS) of phytoplankton in terms of compositions and structures in response to MPs were still not reported. In this study, EPS matrix of Microsystis aeruginosa was applied and fractionated into loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS) fractions, with the time-dependent changes in response to different concentrations (10, 100 and 500 mg/L) of MPs being explored via using the fluorescence excitation emission matrix coupled with parallel factor (EEM-PARAFAC) and two-dimensional Fourier transform infrared correlation spectroscopy (2D-FTIR-COS) analysis. Results showed that 500 mg/L of MP concentration significantly inhibited Microcystis growth by 30.5% but enhanced EPS secretion. In addition, organic composition in LB-EPS and TB-EPS varied differently in response to increased MP exposure, as the ratio of polysaccharide/protein increased in the TB-EPS but decreased in LB-EPS. Further analysis revealed obvious heterogeneities in organic component variations in response to MPs, as the C-O functional groups and glycosidic bonds in the TB-EPS preferentially responded, which lead to the domination of polysaccharides and humus substances; while the carbonyl, carboxyl and amino functional groups in the LB-EPS exhibited a preferential response, which caused the enhanced percentage of the tryptophan-like proteins. In addition to organic compositions, the aromaticity, hydrophobicity and humification in the LB-EPS fraction increased with enhanced MP exposure, which, as a result, may influence the ecotoxicological risk of MPs. Therefore, Microcystis can dynamically adjust not only the EPS contents but also the compositions in response to MPs exposure. The results can improve our understanding on the eco-physiological impact of phytoplankton-MP interaction in aquatic environment, and indicate that the dose-dependent and long-term effects of MPs on phytoplankton should be considered in future study.

Morphologically-different cells and colonies cause distinctive performance of coagulative colloidal ozone microbubbles in simultaneously removing bloom-forming cyanobacteria and microcystin-LR

Morphology, the important feature of bloom-forming cyanobacteria, was studied for its impacts on the harmful cyanobacterial bloom (HCB) treatment by coagulative colloidal ozone microbubbles (CCOMBs). The globally-appeared HCB species - Microcystis aeruginosa (spherical cells, block mass colonies), Microcystis panniformis (spherical cells, flat penniform-like colonies) and Anabaena flos-aquae (filamentous morphology) were chosen as representative species. CCOMBs were generated by modifying the bubble surface and the gas core with coagulant and ozone, respectively. The removal of spherical cells and filaments was > 99.5% and ≤ 34.6%, individually, and the latter was ascribed to chain breakage. CCOMBs collected Microcystis panniformis via complexing with the fluorescent and non-fluorescent functional groups of cell colonies but captured Anabaena flos-aquae through the fluorescent ones. More Microcystis aeruginosa got membrane-damaged than Microcystis panniformis; nevertheless, the microcystin-LR (MC-LR) removal was guaranteed through efficiently oxidizing the released MC-LR. Although the outer peptidoglycan sheet of Anabaena flos-aquae was destroyed, the inner cyte membrane remained intact, preventing intracellular MC-LR from releasing. The HCBs dominated by single species with spherical cells were more readily treated than those with co-occurred species. The toxicological tests imply that, as a robust tool for HCB treatment, the CCOMB technology could be eco-environmentally friendly to the aquatic environment.

Harvesting of Microcystis flos-aquae using dissolved air flotation: The inhibitory effect of carboxyl groups in uronic acid-containing carbohydrates

Harvesting algal biomass reduces nutrient loading in eutrophicated lakes and the protein-rich microalgal biomass could be recycled as feedstocks of feed and fertilizer. Due to the complexity of algogenic organic matter (AOM), the key components and functional groups in AOM that inhibit coagulation-based microalgal harvesting have not been disclosed thus far. This study quantitatively analysed the responsive compositions and functional groups of AOM involved in the dissolved air flotation (DAF) harvesting of M. flos-aquae with 1 × 10⁹ cell L^-1 density at coagulation pH 6.2. The results showed that harvesting efficiency dropped drastically from 95.5 ± 0.7% to 43 ± 0.7% in the presence of AOM (26.77 mg L^-1) at the coagulant dosage of 0.75 mg L^-1 and further deteriorated with increasing AOM concentration. Carbohydrates contributed 81% of the total composition of substances involved in the DAF, while the contribution of protein and humic-like substances were only 18% and 1%, respectively. Stoichiometric analysis of functional groups in carbohydrates, proteins, and humic-like substances using model components revealed that carboxyl groups in uronic acid-containing carbohydrates accounted for 76% of the total reduction in carboxyl groups, which was much higher than that in proteins (23%) and humic-like substances (1%), indicating that carboxyl groups in uronic acids containing carbohydrates were the major inhibitors. A conceptual model of charge competition was proposed to explain the inhibition mechanism of carboxyl functional groups in uronic acid-containing carbohydrates on microalgal DAF. Strategies such as preventing carboxyl deprotonation by pH reduction and employment of sweeping/bridging polymeric coagulants/flocculants were proposed for the to reduce the inhibitory effect of carboxyl functional groups.

Mineralization, characteristics variation, and removal mechanism of algal extracellular organic matter during vacuum ultraviolet/ozone process

Extracellular organic matter (EOM) produced by algal blooms in source water is detrimental to drinking water treatment processes and supplied water quality. Ozonation has been used to treat algal EOM, but it could not mineralize EOM effectively. In this study, mineralization and characteristics variation of EOM by vacuum ultraviolet/ozone (VUV/O₃) and its sub-processes were comprehensively investigated. Results showed that EOM removal in different processes followed the order of VUV/O₃ > UV/O₃ > O₃ > VUV > UV. For VUV/O₃ process, removal efficiencies of dissolved organic carbon (DOC), UV₂₅₄, protein, and polysaccharide at 50 min were 75.6%, 80.8%, 80.1%, and 78.0%, respectively, and fluorescence components received very high removal rates (≥92.8%, at 10 min). The yield of trichloromethane dropped from 102.0 to 30.1 μg/L after treating for 50 min by VUV/O₃. Besides, effects of O₃ dosage, initial pH, and water matrices on EOM removal in VUV/O₃ process were investigated. Moreover, fluorescent molecular probe experiments confirmed that hydroxyl radical and superoxide radical were the main reactive oxygen species (ROS) in VUV/O₃ process, and the transformation of ROS was proposed. The mechanism of EOM removal by VUV/O₃ included VUV photolysis, direct O₃ oxidation, and ROS oxidation. Furthermore, the removal of EOM in filtered water by VUV/O₃ was satisfactory. All results indicated that VUV/O₃ process had great application potential in treating EOM-rich filtered water.

The role of extracellular organic matter on the cyanobacteria ultrafiltration process

The membrane fouling caused by extracellular organic matter (EOM) and algal cells and organic matter removal of two typical cyanobacteria (M. aeruginosa and Pseudoanabaena sp.) during ultrafiltration (UF) process were studied in this work. The results showed that EOM had a broad molecular weight (Mw) distribution and the irreversible membrane fouling was basically caused by EOM. Moreover, humic acid and microbial metabolites were major components of EOM of two typical cyanobacteria. Since EOM could fill the voids of cake layers formed by the algal cells, EOM and algal cells played synergistic roles in membrane fouling. Fourier transform infrared spectroscopy analysis indicated that the CH₂ and CH₃ chemical bonds may play an important role in membrane fouling caused by EOM. Interestingly, the cake layer formed by the algal cells could trap the organic matter produced by algae and alleviate some irreversible membrane fouling. The results also showed that although the cake layer formed by the algal cells cause severe permeate flux decline, it could play a double interception role with UF membrane and increase organic matter removal efficiency. Therefore, when using UF to treat algae-laden water, the balance of membrane fouling and organic matter removal should be considered to meet the needs of practical applications.

Enhanced Secretions of Algal Cell-Adhesion Molecules and Metal Ion-Binding Exoproteins Promote Self-Flocculation of Chlorella sp. Cultivated in Municipal Wastewater

The mechanism of self-flocculation remains unclear, partially impeding its efficiency enhancement and commercial application of microalgae-based municipal wastewater (MW) bioremediation technology. This study revealed the contributions of exoproteins [PN, proteins in extracellular polymeric substances (EPS)] to the separation of indigenous microalgae from treated MW. Compared to the low light intensity group, the high light intensity (HL) group produced Chlorella sp. with 4.3-fold higher self-flocculation efficiencies (SE). This was attributed to the enriched biological functions and positional rearrangement of increased PN within 2.9-fold higher EPS. Specifically, a total of 75 PN was over-expressed in the HL group among the 129 PN identified through label-free proteomics. The algal cell-adhesion molecules (Algal-CAMs) and metal-ion-binding PN were demonstrated as two dominant contributors promoting cell adhesion and bridging, through function prediction based on the contained domains. The modeled 3D structure showed that Algal-CAMs presented less hydrophilic α-helix abundance and were distributed in the outermost position of the EPS matrix, further facilitating microalgal separation. Moreover, the 10.1% lower hydrophily degree value, negative interfacial free energy (-19.5 mJ/m²), and 6.8-fold lower energy barrier between cells also supported the observed higher SE. This finding is expected to further fill the knowledge gap of the role of PN in microalgal self-flocculation and promote the development of biomass recovery from the microalgae-wastewater system.

Diatomite-enhanced coagulation for algal removal in polluted raw water: performance optimization and pilot-scale study

Algae blooms have seriously threatened the health of aquatic ecosystems and the safety of drinking water. In this study, diatomite-enhanced coagulation technology was developed to improve the removal of algae and other pollutants. The dosage and ratio of diatomite and aluminum salts were optimized to 40mg/L and 1:1 which achieved algal removal efficiency of 98.8±0.65%. The effect of environmental factors was studied and it shows that cell density, pH, and temperature had a significant impact on algal removal. The mechanism of diatomite-enhanced coagulation was speculated to be adsorption bridging and sweep flocculation. Pilot-scale equipment was set up to verify the performance of diatomite-enhanced coagulation in engineering applications on algae polluted raw water. Results showed a better removal efficiency of algae, NH₄⁺-N, NO₂^--N, and COD_Mn and lower operation cost than the actual operation in the Waterwork Corporation were achieved with good application prospects and promotion value.

Pre-coagulation with cationic flocculant-composited titanium xerogel coagulant for alleviating subsequent ultrafiltration membrane fouling by algae-related pollutants

In-line coagulation-ultrafiltration is reliable to achieve the safe disposal of algae-laden water with alleviated membrane fouling. Poly(diallyl dimethyl ammonium chloride) (PDADMAC)-composited titanium xerogel (TXC) coagulant (abbreviated as P-T) was reported to possess better resistance to organic matter loads, and its mitigation effect on subsequent ultrafiltration efficiency towards algae-related pollutants was investigated in this study. Results showed that P-T coagulation effectively mitigated membrane fouling over pH 5.0-9.0, whereas TXC only worked better under acidic condition. Acidic environment facilitated algae and organic matter removal by pre-coagulation, thus greatly improving ultrafiltration efficiency. Under neutral and alkaline conditions, PDADMAC portion in P-T enhanced the coagulation removal towards algae and protein constituents, and simultaneously promoted the formation of flocs with unique porous structure, which jointly contributed to its high-efficient alleviation ability. Nevertheless, PDADMAC increased adhesion force between P-T coagulated flocs and membrane surface, thus slightly reducing the recovery rate of membrane flux at pH 5.0. Pearson correlation analyses implied that removing algae cells would prevent reversible fouling-induced flux decline, whereas eliminating organic matter could greatly promote ultrafiltration efficiency via mitigating irreversible fouling. Therefore, elevating removal efficiency of organic matters is still the major objective for ultrafiltration pretreatment technologies and the optimization direction towards TXC-based coagulants.

Understanding variability in algal solid-liquid separation process outcomes by manipulating extracellular protein-carbohydrate interactions

Coagulation-flocculation followed by sedimentation or dissolved air flotation (DAF) are processes routinely used for separating microalgae from water; however, during algae separation then can exhibit inconsistent separation, high coagulant demand, and high operating cost. To circumvent these problems, previous studies reported the development of a novel DAF process in which bubbles were modified instead of particles. While this process was shown to be sustainable and inexpensive, the problem of inconsistent algal separation across species remained. Recent research has suggested that this could be due to the varying concentration and character of algal-derived proteins and carbohydrates within the extracellular organic matter (EOM) and their associated interactions. This hypothesis is tested in the current study using the novel modified-bubble DAF process, which has been highly susceptible to EOM protein and carbohydrate concentrations and character. Biomolecular additives (commercially available proteins and carbohydrates, and algal-extracted proteins) of widely differing molecular weight (MW) and charge were dosed in varying proportions into samples containing either Chlorella vulgaris CS-42/7, Microcystis aeruginosa CS-564/01, or Microcystis aeruginosa CS-555/1 after removing the intrinsic EOM. These cell-rich suspensions were then subject to flotation using cationic bubbles modified with poly(diallyldimethylammonium chloride) (PDADMAC). When additives were dosed independently, separation increased from <5% to up to 62%. The maximum separation was obtained when the dose was double the respective biopolymer concentration measured in the intrinsic EOM for the equivalent species, and, in the case of protein additives, when MW and charge were >50 kDa, and >0.5 meq·g^-1, respectively, irrespective of the species tested. When evaluating steric- and charge-based protein-carbohydrate interactions on cell separation by simultaneously dosing high MW and high charge protein- and carbohydrate-additives, enhanced separation of up to 79% was achieved. It is suggested that enhanced cell separation is achieved due to proteins and carbohydrates bridging with cells and forming protein-carbohydrate-cell suprastructures in the presence of a flocculant, e.g. PDADMAC, and this only occurs when the intrinsic EOM comprises proteins and carbohydrates that have high MW (>25 kDa) and charge (>0.2 meq·g^-1), and interactions with each other and with the cell surface.