Extracellular Organic Matter Research Articles

Biofouling metallic membranes in algae-laden water treatment: Performance, mechanism, and the role of the pre-coated layer

Biofouling is the primary factor limiting the performance of ultrafiltration membrane systems in drinking water treatment. This study systematically investigated the application effect and biofouling behavior of metallic membranes in the treatment of algae-laden water and evaluated the mitigation effect and mechanism of the pre-coated layer on biofouling. The findings demonstrate that the metallic membrane can almost completely remove algal cells (>99.99 %) and macromolecular organics in EOM (Extracellular Organic Matter) but has limited effects on micromolecules, such as humic acid and fulvic acid, in EOM (47.29 % and 17.72 % for TOC (Total Organic Carbon) and UV254, respectively). During direct filtration, algal cells can form a cake layer in front of the metallic membrane to reduce the membrane flux, whereas the presence of EOM makes the cake layer denser and hastens the decline in flux. In addition, irreversible membrane fouling is exasperated in acidic or Ca2+-rich solutions, whereas an increase in algal cell particles exacerbates reversible membrane fouling, causing a faster decline in flux. According to the membrane mechanism analysis, the fouling mechanism ranges from complete pore blocking to standard and intermediate blocking throughout the direct filtration process. In contrast, the application of a pre-coated kaolin layer significantly decreases membrane fouling compared with direct filtration. The pre-coated layer can prevent direct contact between the foulants and the membrane surface during the early stages of filtration, resulting in a delayed complete pore blockage stage. On the other hand, XDLVO (Derjaguin-Landau-Verwey-Overbeek) data reveals that the force between kaolinite and algae (−102.323 mJ/m2) is greater than that between kaolinite and metallic membranes (−61.424 mJ/m2), resulting in better backwashing efficacy. These findings support the use of metallic membranes in drinking water treatment.

Novel calcium hypochlorite/ferrous iron as an ultrafiltration membrane pretreatment process for purifying algae-laden water

Algal fouling has become one of the most critical factors hindering the large-scale development of membrane processes in algae-laden water treatment. Herein, novel calcium hypochlorite (Ca(ClO)2)/ferrous iron (Fe(II)) process was proposed as an ultrafiltration (UF) membrane pretreatment technology, and its effects on membrane fouling and water properties were systematically studied. Results showed that the terminal specific fluxes were significantly elevated to 0.925 and 0.933, with the maximum removal ratios of reversible resistance reaching 99.65% and 96.99% for algae-laden water and extracellular organic matter (EOM), respectively. The formation of cake filtration was dramatically delayed, accompanied by a significant reduction of the adhesion free energy, and the contaminants attached to the membrane surface were effectively decomposed. With respect to water quality, the removal ratios of OD685 and turbidity achieved 81.25–95.31% and 90.16–97.72%, individually. The maximum removal rates of DOC, UV254 and fluorescent organics in influent water reached 46.14%, 55.17% and 75.77%, respectively. Furthermore, the generated reactive species (e.g., •OH, Cl•, Cl2•- and ClO•) could efficiently degrade EOM, which appreciably reduced the electrostatic repulsion between the algal foulants while ensuring the integrity of algal cells. At the Ca(ClO)2/Fe(II) dosage of 0.04/0.24 mM, the zeta potential changed from −32.9 mV to −10.8 mV, and a large range of aggregates was formed. The macromolecules in the algal solution were significantly removed, and the proportion of micromolecular organics was increased to some extent. Coagulation of in-situ formed Fe(III) dominated the membrane fouling mitigation, and the reactive species also contributed to the improvement of filtration performance. Overall, Ca(ClO)2/Fe(II) pretreatment has an exceptional prospect for efficient degradation of algal pollutants and enhancement of UF capability.

Beyond filterability: Understanding the complexities of sludge dewatering through typical coagulation and advanced oxidation

Coagulation and advanced oxidation process (AOP) are the two most significant conditioning methods in sludge dewatering. However, previous studies mainly focus on their differences and overlook their similarities in dewatering mechanisms. In this study, PAC and Fe3+ were selected as typical coagulants, and Fenton and Fenton-like reactions as typical AOP. By regulating the reagent dosage to achieve basically the CST-based filterability (CST = 22.9 ± 1.5 s), AOP had superior dewatering efficiency (59.54–63.04 wt%) compared to coagulation (68.84–69.16 wt%). Obviously, the filterability of sludge could not accurately reflect its dewatering performance. The mechanism revealed that AOP had both “extracellular oxidation” and “intracellular oxidation”, whereas coagulation mainly affected the properties of extracellular organic matter. Both processes improved the sludge's relative hydrophobicity and reduced its interfacial free energy. AOP was superior to coagulation in improving sludge fluidity, hydrophobicity, permeability, cell disintegration and reducing bound water content. While coagulation had a better effect on electric neutralization, floc particle size growth and colloidal structure strength. Moreover, this study emphasized the significance of considering the vector-based relationship between sludge dewatering performance and various influencing factors for analyzing the dewatering mechanism. These findings are expected to deepen our understanding of the similarities and disparities in dewatering efficacy and the related mechanisms between the these two pretreatments.

Flocculation with intermittent dosing for enhanced microalgae harvesting

One of the bottlenecks to the industrialization of algal biofuel is the high cost of harvesting. Flocculation is an efficient technique for algae harvesting, but flocculant consumption results in additional expenses. To lower the overall cost of algae harvesting, it is essential to explore approaches to reduce flocculant usage. In this study, a novel flocculant dosing strategy of intermittent dosing was investigated for enhanced algae harvesting efficiency, with comparisons to traditional one-time dosing, continuous dosing and dual dosing. The impacts of critical operational parameters were examined, and the mechanism was investigated based on floc analysis. The results demonstrated that intermittent dosing can significantly enhance harvesting efficiency. At the optimal operational conditions of 5 s dosing interval, 10 mg/L min−1 dosing speed and 5 min stirring time, the algae harvesting efficiency increased from 71.1 % to 91.3 % compared to one-time dosing when 15 mg/L CPAM was used as a flocculant. According to floc analysis, intermittent dosing tended to form larger and loosely connected flocs, while traditional one-time dosing tended to produce smaller and denser flocs. The different floc growth mechanisms can help to explain how intermittent dosing improved algae flocculation. In addition, intermittent dosing resulted in a higher removal efficiency of dissolved extracellular organic matter and improved algae recultivation. Finally, the cost estimation was performed, and the operational cost was lowered from 32.65 USD ton−1 algae for one-time dosing to 20.64 USD ton−1 algae for intermittent dosing. Through the investigation of this study, the technical, mechanical and economic feasibility of intermittent dosing for enhanced algae harvesting were established.

Effects of the nutrient inhibition on the yield of DBPFPs by Microcystis aeruginosa.

The yield of three disinfection byproduct formation potentials (DBPFPs), including trichloromethane, dichloroacetic acid and trichloroacetic acid formation potential (TCMFP, DCAAFP and TCAAFP), by Microcystis aeruginosa under the nitrate and phosphate inhibition conditions was investigated. The results showed that excessive nitrate could inhibit the growth of M. aeruginosa, but the concentration of DBPFPs in the five fractions of algal metabolites, including hydrophilic extracellular organic matter (EOM), hydrophobic EOM, hydrophilic intracellular organic matter, hydrophobic intracellular organic matter and cell debris, only decreased slightly. Accordingly, the productivity of DBPFPs by M. aeruginosa increased by approximately 40% under the nitrate inhibition condition and the increased productivity of DBPFPs mainly came from EOM. The phosphate inhibition also performed a similar pattern with a lesser extent. The nutrient inhibition did not change the proportion of these three DPBFPs, and TCMFP accounted for approximately 87% of the total DBPFPs. The inhibition could promote M. aeruginosa to secrete more metabolites. However, the cyanobacteria tended to secrete more DBPFPs under the nitrate inhibition condition, which resulted in an increased specific DBPFP, while they tended to secrete more non-DBPFPs under the phosphate inhibition condition, which resulted in a decreased specific DBPFP.

Unveiling the enhancement mechanisms of algogenic extracellular organic matters on chlortetracycline photodegradation: Constitutive relationships of compound components and reactive oxygen species generation

Algogenic extracellular organic matters (EOMs) have been found to play a crucial role in the photodegradation of antibiotics. However, the specific molecular structure compositions of EOMs have not been fully characterized, and the intrinsic association between the structure and the production of ROS remains unclear. In this study, EOMs from Chlorella Vulgaris were characterized using FT-ICR-MS. Based on the FT-ICR-MS results, nine representative model compounds (MCs, i.e., benzene, naphthalene, anthracene, phenanthrene, glucose, l-glutamic, triglyceride, tannic and lignin) were applied to investigate the physicochemical properties of EOMs and the ROS changes induced by the photoreaction of chlortetracycline (CTC). With the help of quenching assays, nine MCs were classified into prone-ROS and non-prone-ROS fractions. Prone-ROS compounds generate O2− upon electron transfer to 3O2, which then produces ·OH after disproportionation to generate hydrogen peroxide. The formation of 1O2 is attributed to energy transfer from prone-ROS to 3O2. Density functional theory revealed that prone-ROS exhibited higher reactivity compared to non-prone-ROS, this finding is as well supported by the result of steady-state photolysis measurement. Our study gives a new insight into photochemical fate of CTC in aquatic environments, providing theoretical basis for assessing antibiotics' ecological risk accurately.

Photobleaching-induced changes in the optical and photochemical properties of algal organic matter.

Algal organic matter (AOM), a significant source of endogenous dissolved organic matter (DOM) is released in high concentrations during cyanobacterial blooms, along with cyanotoxins. Subsequent photobleaching of AOM is an important phenomenon to investigate. In this study, intracellular organic matter (IOM) and extracellular organic matter (EOM) were extracted from cultured cyanobacteria taken from Taihu Lake in China. The formation of photochemically produced reactive intermediates in different stages of IOM and EOM photobleaching was compared to Suwannee River DOM (SRDOM, reference standard DOM). Results revealed notable differences influenced by the pigment component among IOM, EOM, and SRDOM. The pigment in IOM contributed to a triplet state pool with strong energy-transfer but limited electron-transfer capabilities. Notably, IOM exhibited the highest triplets state quantum yield value in the visible region, suggesting its potential significance in pollutant degradation in deeper water layers. For EOM, one of the pools exhibits photolability and remarkable electron-transfer capability, indicating it as a high-energy triplet state component. Moreover, three cyanotoxins (MC-LR, ACA, and ATX-a) were detected in the extracted AOM, and their photodegradation was monitored during the AOM photobleaching process. This highlights the potential role of AOM as a photosensitizer in the natural self-cleaning mechanisms of water bodies, facilitating the degradation of organic pollutants through photochemical reactions. The findings of this study contribute to understanding the dynamic nature of AOM and its implications in environmental processes.

Molecular complexation properties of Cd2+ by algal organic matter from Scenedesmus obliquus

A detailed understanding the metals binding with algal organic matter (AOM) is essential to gain a deeper insight into the toxicity and migration of metals in algae cell. However, the molecular complexation mechanism of the metals binding with AOM remains unclear. In this study, cadmium ion (Cd2+) binding properties of AOMs from Scenedesmus obliquus, which included extracellular organic matter (EOM) and intracellular organic matter (IOM), were screened. When Cd2+ < 0.5 mg/L, the accumulation of Cd2+ could reach 40%, while Cd2+ > 0.5 mg/L, the accumulation of Cd2+ was only about 10%. EOM decreased gradually (from 8.51 to 3.98 mg/L), while IOM increased gradually (from 9.62 to 21.00 mg/L). The spectral characteristics revealed that IOM was richer in peptides/proteins and had more hydrophilic than EOM. Both EOM and IOM contained three protein-like components (containing tryptophan and tyrosine) and one humic-like component, and their contents in IOM were higher than that in EOM. The tryptophan protein-like substances changed greatly during Cd2+ binding, and that the tryptophan protein-like substances complexed to Cd2+ before tyrosine protein-like substances in IOM was identified. Moreover, the functional groups of N-H, O-H, and CO in AOM played an important role, and the N-H group was priority to interacts with Cd2+ in the complexing process. More functional groups (such as C-O and C-N) were involved in the metals complexing in EOM than in IOM. It could be concluded that Cd2+ stress promoted the secretion of AOM in Scenedesmus obliquus, and proteins in AOM could complex Cd2+ and alleviate its toxicity to algal cell. These findings provided deep insights into the interaction mechanism of AOM with Cd2+ in aquatic environments.

Oxidation of Sb(III) by Shewanella species with the assistance of extracellular organic matter

Antimony (Sb) is a toxic substance that poses a serious ecological threat when released into the environment. The species and redox state of Sb determine its environmental toxicity and fate. Understanding the redox transformations and biogeochemical cycling of Sb is crucial for analyzing and predicting its environmental behavior. Dissolved organic matter (DOM) in the environment greatly affects the fate of Sb. Microbially produced DOM is a vital component of environmental DOM; however, its specific role in Sb(III) oxidation has not been experimentally confirmed. In this work, the oxidation capacity of several Shewanella strains and their derived DOM to Sb(III) was confirmed. The oxidation rate of Sb(III) shows a positive correlation with DOM concentration, with higher rates observed under neutral and weak alkaline conditions, regardless of the presence of light. Incubation experiments indicated that extracellular enzymes and common reactive oxygen species were not involved in the oxidation of Sb(III). Characteristics of DOM suggests that microbial humic acid-like and fulvic acid-like substances are the potential contributors to Sb(III) oxidation. These findings not only experimentally validate the role of bacterial-derived DOM in Sb(III) oxidation but also reveal the significance of Shewanella and biogenic DOM in the biogeochemical cycling of Sb.

Removal of Microcystis aeruginosa and its extracellular organic matters by using covalently bonded coagulant: An alternative choice in enhanced coagulation for algae-polluted water treatment

High coagulants dosage, Al or Fe residual, unstable flocs, and extracellular organic materials (EOM) released from cells cannot be avoided by using traditional Al or Fe based coagulants in algae removal. Thus, a novel covalently bonded coagulant poly aluminum ferrous chloride/cationic polyacrylamide (CAFM) was developed. The algae and EOM removal performance and mechanisms were investigated in current research. Al and Fe residual, cell integrity, and compactness of flocs were analyzed after coagulation. Furthermore, the disinfection by-products formation potential (DBPFP) and economics was also figured out to confirm the advantage of using CAFM. Results showed that 95.01% of turbidity and 99.3% of OD686 were effectively removed by using CAFM at 6 mg/L. Moreover, lower breakage probability of algae cells and denser flocs were realized by using CAFM compared to common used Al or Fe coagulants. In addition, lower Al and Fe residual and lower DBPFP were confimed in CAFM coagulation. DFT (Density functional theory) and ESP (Electrostatic Potential) results showed CAFM could combine bEOM (bonded extracellular organic matter) and dEOM (dissolved extracellular organic matter) through H-bond from Al atom in the covalent bond. Due to the introduction of various functional groups of different coagulants, charge neutralization and adsorption ability contributed to treating algae-containing water.

In situ extracellular algal organic matter beneficial to the coagulation removal of Microcystis aeruginosa at the stationary growth phase

Cyanobacterial blooms in source water deteriorate drinking water quality. Coagulation process is the first barrier to treat cyanobacteria-laden source water in drinking water treatment plants (DWTPs). The changed growth phase of exponential-stationary of cyanobacteria might affect their coagulation efficiency. However, previous studies mainly conducted coagulation tests using one growth phase of exponential or stationary, and the comparative study is limited. Herein, coagulation tests to remove M. aeruginosa at the exponential and stationary phase were compared using a coagulant of polyaluminum chloride, and factors and mechanisms to differ the coagulation efficiency of M. aeruginosa at the two phases were also investigated. Results showed that the removal efficiency of M. aeruginosa at the stationary phase became higher (3.5–98.4 %) than that at the exponential phase (1.2–94.5 %) with in situ bEOM (binding extracellular organic matter)/dEOM (dissolved extracellular organic matter). The larger and looser flocs were formed at the stationary phase than that at the exponential phase. It could be ascribed to their stronger adsorption inter-bridge and sweeping flocculation via more aromatic/N-containing functional groups in bEOM and molecular-weight of dEOM (>30 kDa) interacted with elevated Al(OH)3(am). These findings provide a deep insight into the coagulation removal of cyanobacterial cells at the two phases. Moreover, this study reveals that these in situ bEOM/dEOM biosynthesized by M. aeruginosa at the stationary phase might be promising bio-flocculants to assist cyanobacterial control at a high efficiency in DWTPs.

Enhancement of KMnO4 treatment on cyanobacteria laden-water via 1000 kHz ultrasound at a moderate intensity

1000 kHz high-frequency ultrasound at 0.12 and 0.39 W/mL intensity was used to enhance the inactivation of suspensions of Microcystis aeruginosa cells using KMnO4. With 10 mg/L of KMnO4, ultrasound at 0.12 W/mL intensity was found to be effective in inactivating the cyanobacteria within 10 min. A Weibull model was found to describes the inactivation well. Its concave shape shows that some cells have a certain resistance to this treatment. Cytometry and microscopic analysis confirm that the treatment damages cell integrity. Despite that the extracellular organic matter in the water was not significantly increased. The concentration of extracellular cyanobacterial toxins even decreased. The filtered suspension of inactivated cyanobacteria was used to cultivate mung beans, and the suspension did not hinder their germination. This provides a new idea for using cyanobacteria-laden wastewater. These findings suggest a technique for speeding up the oxidation of Microcystis cells using KMnO4 with ultrasound at moderate intensity, which provide new insights into the biological effects of ultrasound.

Effect of Electrochemical Pre-Oxidation for Mitigating Ultrafiltration Membrane Fouling Caused by Extracellular Organic Matter

Algal extracellular organic matter (EOM) will cause grievous membrane fouling during the filtration of algae-laden water; hence, boron-doped diamond (BDD) anodizing was selected as the pretreatment process before the ultrafiltration, and the EOM fouling mitigation mechanism and the purification efficiency were systematically investigated. The results showed that BDD oxidation could significantly alleviate the decline of membrane flux and reduce membrane fouling, and the effect was more notable with an increase in oxidation time. Less than 10% flux loss happened when oxidation duration was 100 min. The dominant fouling model was gradually replaced by standard blocking. BDD anodizing preferentially oxidizes hydrophobic organic matter and significantly reduces the DOC concentration in EOM. The effluent DOC was reduced to less than 1 mg/L when 100 min of BDD anodizing was applied. After the pre-oxidation of BDD, the zeta potential and interfacial free energy, including the cohesive and adhesive free energy, were all constantly increasing, which implied that the pollutants would agglomerate and deposit, and the repulsion between foulants and the ultrafiltration membrane was augmented with the extensive oxidation time. This further confirms the control of BDD on membrane fouling. In addition, the BDD anodizing coupled ultrafiltration process also showed excellent performance in removing disinfection by-product precursors.

Algal organic matter impact on 2-methylisopropanol and geosmin adsorption by activated carbon: Variation with algae growth and decay, prediction by fluorescence surrogates

Algae-producing odorants have negative effects on the aesthetic quality of drinking water, and activated carbon adsorption is commonly used to abate the odorants. However, in eutrophic waters, algal organic matter (AOM) produced by the dominant cyanobacteria (e.g., Microcystis aeruginosa) strongly interferes with the activated carbon adsorption of trace odorants from source waters. In this study, the applicability of using spectrometric surrogates to evaluate the effect of AOM from Microcystis aeruginosa on odorant adsorption by activated carbon was investigated. During the stationary phase, the DOC concentration of extracellular organic matter (EOM) increased due to algal metabolism (from 3.52 mg/L to 7.81 mg/L) and further accumulated (to 12.40 mg/L) during the decline phase due to the release of intracellular organic matter (IOM), resulting in increased competition against odorants. The adsorption of chromogenic organics in AOM was first evaluated to screen possible spectrometric surrogates for predicting odorant adsorption. The fluorescence component (Ex: 260 nm/Em: 400–460 nm) was the best candidate for predicting odorant adsorption in both AOM solutions and reservoir water. In contrast, poor performance was found for the commonly used UV absorbance at 254 nm and protein-like fluorescence components, due to the weak adsorbability of corresponding chromogenic materials. Our results indicate that fluorescence spectroscopy is promising in predicting odorant adsorption with AOM interference in source waters.

Variations in trihalomethane formation potential of Microcystis aeruginosa under different growth conditions: Phenomenon and mechanism

Cyanobacteria and their metabolites are one of the primary precursors of disinfection by-products (DBPs) in natural water environments. However, few studies have investigated whether the production of DBPs by cyanobacteria changes under complex environmental conditions and possible mechanisms underlying these changes. Therefore, we investigated the effects of algal growth phase, water temperature, pH, illumination and nutrients on the production of trihalomethane formation potential (THMFPs) by Microcystis aeruginosa in four algal metabolic fractions, that is, hydrophilic extracellular organic matter (HPI-EOM), hydrophobic EOM (HPO-EOM), hydrophilic intracellular organic matter (HPI-IOM) and hydrophobic IOM (HPO-IOM). Additionally, correlations between THMFPs and some typical algal metabolite surrogates were analyzed. The results showed that the productivity of THMFPs by M. aeruginosa in EOM could be affected significantly by the algal growth phase and incubation conditions, while the IOM productivity varied insignificantly. M. aeruginosa in the death phase could secrete more EOM and have a higher THMFP productivity than those in the exponential or stationary phases. Cyanobacteria grown under harsh conditions could have increased THMFP productivity in EOM by increasing the reactivity of algal metabolites with chlorine, for example, under low pH conditions, and secreting more metabolites in EOM, for example, under low temperature or nutrient limitation conditions. Polysaccharides were responsible for the enhanced THMFP productivity in HPI-EOM fraction, and a significant linear correlation was found between the concentration of polysaccharides and THMFPs (r = 0.8307). However, THMFPs in HPO-EOM did not correlate with dissolved organic carbon (DOC), ultraviolet absorbance at 254 nm (UV254), specific UV absorbance (SUVA) and cell density. Thus, we could not specify the kind of algal metabolites that contribute to the increased THMFPs in the HPO-EOM fraction under harsh growth conditions. Compared with the case in EOM, the THMFPs in IOM were more stable and correlated with the cell density and total amount of IOM. The results implied that the THMFPs in the EOM were sensitive to growth conditions and were independent of algal density. Considering the fact that traditional water treatment plants cannot remove dissolved organics as efficiently as algal cells, the increased THMFP productivity in EOM by M. aeruginosa under harsh growth conditions could be a potentially serious threat to the safety of the water supply.

Algal organic matter accelerates the photodegradation of tetracycline: Mechanisms, degradation pathways and product toxicity

Algae are ubiquitous in eutrophic surface waters and they play important roles in the sunlight-driven transformation of environmental contaminants. The present study aims to investigate the role of algal organic matter (AOM) separated from Microcystis aeruginosa at different growth stages in tetracycline photodegradation along with the related photosensitive mechanisms. The results showed that AOM, both extracellular organic matter (EOM) and intracellular organic matter (IOM) at different growth phases all promote tetracycline photodegradation, but in different ways and performances. EOM exerts greater photolysis effectiveness than IOM with logarithmic phase (LP) organic matter more effective than that from the algae’s declining growth phase (DP). The promoting efficiencies of tetracycline photodegradation by four AOM types follow the order of LP-EOM > DP-EOM > LP-IOM > DP-IOM, accounting for 1.0420 h−1, 0.8281 h−1, 0.6527 h−1 and 0.4974 h−1, respectively. Such discrepant effectiveness is related to the different chemical constituents and structures of EOM and IOM. Scavenger experiments further demonstrated that triplet-state AOM (3AOM*) contributes the most to tetracycline degradation (30.01%∼48.51%), followed by OH (1.68%∼10.66%) and 1O2 (1.34%∼10.08%). The steady-state concentrations of 3AOM*, OH and 1O2 were measured with probe compounds in the range of 13.950 ∼ 155.234 L/mol, 2.494 × 10−17 ∼ 27.636 × 10−17 M and 1.410 × 10−14 ∼ 26.493 × 10−14 M, respectively. Interestingly, the photolytic intermediates show increasing toxicity to Photobacterium phosphoreum at first which then decreases with the extension of irradiation. These results provide comprehensive insight into the transport and environmental behavior of antibiotics in a eutrophic aqueous environment.

Visible light photocatalysis alleviates ceramic membrane fouling caused by intracellular organic matter of Microcystis aeruginosa

Ultrafiltration (UF) is an effective technology for treating algae-containing water. The algal intracellular organic matter (IOM) released by dead algae during an excessive algae growth phase causes more serious membrane fouling than viable algal cells and algal extracellular organic matter (EOM), which seriously restricts stable operation of UF. This paper studied the removal of IOM by a Bi2O3–TiO2/PAC (Bi-doped TiO2 nano-composites supported by powdered activated carbon) photocatalytic oxidation-ceramic ultrafiltration membrane, the inhibitory effect of membrane fouling at different photocatalytic oxidation times, and the fouling mechanism. The results show that both the adsorption and photocatalytic oxidation of Bi2O3–TiO2/PAC can significantly remove proteinoids and humic acid-like fluorescent components present in IOM, with removals of UV254 and DOC reaching up to 81.8% and 44.0%. The generated superoxide radicals and hydroxyl radicals positively affect the degradation of organic compounds of various molecular weights, and holes mainly degrade acidic low-molecular weight organic compounds. When the photocatalytic oxidation time was extended to 60 min, the membrane flux increased by 70.7% and the reversible resistance and irreversible resistance decreased by 83.4% and 90.1%, respectively. Under these conditions, the fouling mechanism changed from complete blockage to intermediate blockage. In the late stage of photocatalytic oxidation (90–120min), part of the high and medium molecular weight organic components were oxidized to produce low-molecular weight organic compounds (LMW acidic and neutral organic compounds), and blockage of filter cake became the main fouling mechanism. After photocatalysis, the zeta potential decreases significantly and the interaction forces between pollutants and pollutants are transformed into repulsive forces. Application of an appropriate photocatalytic oxidation time can alleviate the ultrafiltration membrane fouling caused by algal-derived organics.

Impact of sonication on the algal organic matter characteristics of Anabaena circinalis, Microcystis aeruginosa, and Chlorella vulgaris

The frequent occurrence of harmful algal blooms (HABs) in drinking water reservoirs causes water treatment and supply problems. Ultrasonication is a physicochemical treatment method used to treat HABs. During the ultrasonication of HABs, algal organic matter (AOM), including both extracellular organic matter (EOM) and intracellular organic matter (IOM), is released and changes the dissolved organic matter characteristics. In this study, Anabaena circinalis (A. circinalis), Microcystis aeruginosa (M. aeruginosa), and Chlorella vulgaris (C. vulgaris) were used to investigate changes in AOM characteristics during ultrasonication using a fluorescence excitation-emission matrix and assimilable organic carbon. We found that ultrasonication increased the dissolved AOM content, resulting in increased dissolved organic carbon from IOM released from algal cells. A. circinalis, M. aeruginosa, and C. vulgaris had dissolved organic carbon concentrations of 22.5, 3.8, and 6.7 mg/L before sonication, which increased to 29.1, 10.8, and 7.7 mg/L after ultrasonication, respectively. Additionally, the IOM released during ultrasonication held more protein-like substances and was mostly associated with biodegradable organic matter.

UV/Fe(II)/S(IV) Pretreatment for Ultrafiltration of Microcystis aeruginosa-Laden Water: Fe(II)/Fe(III) Triggered Synergistic Oxidation and Coagulation

Ultrafiltration (UF) has been proven effective in removing algae during seasonal algal blooms, but the algal cells and the metabolites can induce severe membrane fouling, which undermines the performance and stability of the UF. Ultraviolet-activated sulfite with iron (UV/Fe(II)/S(IV)) could enable an oxidation-reduction coupling circulation and exert synergistic effects of moderate oxidation and coagulation, which would be highly preferred in fouling control. For the first time, the UV/Fe(II)/S(IV) was systematically investigated as a pretreatment of UF for treating Microcystis aeruginosa-laden water. The results showed that the UV/Fe(II)/S(IV) pretreatment significantly improved the removal of organic matter and alleviated membrane fouling. Specifically, the organic matter removal increased by 32.1% and 66.6% with UV/Fe(II)/S(IV) pretreatment for UF of extracellular organic matter (EOM) solution and algae-laden water, respectively, while the final normalized flux increased by 12.0-29.0%, and reversible fouling was mitigated by 35.3-72.5%. The oxysulfur radicals generated in the UV/S(IV) degraded the organic matter and ruptured the algal cells, and the low-molecular-weight organic matter generated in the oxidation penetrated the UF and deteriorated the effluent. The over-oxidation did not happen in the UV/Fe(II)/S(IV) pretreatment, which may be attributed to the cyclic redox Fe(II)/Fe(III) coagulation triggered by the Fe(II). The UV-activated sulfate radicals in the UV/Fe(II)/S(IV) enabled satisfactory organic removal and fouling control without over-oxidation and effluent deterioration. The UV/Fe(II)/S(IV) promoted the aggregation of algal foulants and postponed the shift of the fouling mechanisms from standard pore blocking to cake filtration. The UV/Fe(II)/S(IV) pretreatment proved effective in enhancing the UF for algae-laden water treatment.

CO2-Inorganic Carbon Auto-Buffering System for Efficient Ammonium Reclamation Coupled with Valuable Biomass Production in a Euryhaline Microalga Tetraselmis subcordiformis

The performance of microalgae-based wastewater treatment processes for ammonium-N (NH4+-N) removal depends on the maintenance of a favorable pH that is critical for minimizing nitrogen escape in the form of free ammonia (NH3) and preventing high-NH3 or extreme-pH stress. This study developed a CO2-inorganic carbon (CO2-IC) buffering system that automatically stabilized pH with the supply of a carbon source for efficient photosynthetic reclamation of NH4+-N by a euryhaline microalga Tetraselmis subcordiformis. The soluble (NaHCO3) and insoluble (CaCO3 and MgCO3) ICs were compared for this purpose. The pH was well controlled in the range of 6.5~8.5 in the CO2-IC system, which was suitable for the photosynthetic growth of T. subcordiformis. The NH4+-N (100 mg/L) was almost completely removed in three days, with the maximum removal rate of 60.13 mg N/L/day and minimal N escape of 19.65% obtained in the CO2-NaHCO3 system. The CO2-IC system also restricted the release of extracellular organic matter by preventing stress conditions. The CO2-NaHCO3 system enabled the highest “normal” starch production suitable for fermentation, while the CO2-CaCO3/MgCO3 system facilitated high-amylose starch accumulation that was conducive to producing bio-based materials and health-promoting ingredients. The proteins accumulated in T. subcordiformis were of good quality for animal feeds.