A mini review of preoxidation to improve coagulation

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.

Stress response of Microcystis aeruginosa to chlorine during transportation: The significance of surface-adsorbed organic matter

The desorption of surface-adsorbed organic matter (S-AOM) without damaging algal cells was reported to be the key to destabilizing Microcystis aeruginosa (M. aeruginosa) cells while avoiding intracellular organic matter (IOM) release in our previous study. However, a temporal effect was found from spontaneous and continuous damage to algal cells even after quenching. This study aims to demonstrate the mechanism of the temporal inactivation effect and the stress response exhibited by chlorine-oxidized algal cells, and finally guide the prechlorination process for algae-laden water at water sources. Chlorine was proved to cause oxidative stress to M. aeruginosa cells, and result in a rapid increase in intracellular reactive oxygen species (ROS) levels. S-AOM appeared to have a protective effect on algal cells against oxidative damage, as evidenced by the maintenance of algal cell integrity and activated antioxidant enzymes. In addition, the activity of Caspase 3, a key protease for the execution of programmed cell death (PCD), was significantly enhanced during prechlorination. Cellular chromatin condensation and DNA fragmentation occurred in the early stages of PCD in algal cells. Therefore, the pre-treatment of algae-laden water at water sources, even with low chlorine doses, can induce a risk of significant algal cell death during the water transfer process due to activation of the PCD process, resulting in a higher health risk for drinking water. These findings indicate that both the dosage of chlorine and the duration of the transportation process should be considered during the prechlorination of algae-laden water, which can provide an important basis for avoiding increasing the risk to drinking water safety.

Research and application of active species based on high-valent iron for the degradation of pollutants: A critical review

Fe(VI), as a new green treatment agent, has two indispensable processes in water treatment: coagulation and oxidation. Fe(VI) has a strong oxidation ability. The intermediate iron species (Fe(V) and Fe(IV)) and reactive radical species (H2O2, •OH, and O2•-) produced by decomposition and reduction reaction have strong oxidation ability, in addition, the hydrolyzed product formed in situ with core (γ-Fe2O3)-shell (γ-FeOOH) structure also has good coagulation effect. Because Fe(VI) is easy to decompose and challenging to preserve, it limits the application and sometimes significantly reduces the subsequent processing effect. How to make Fe(VI) more efficient use is a hot spot in current research. This article summarizes the distribution of active substances during the hydrolysis of Fe(VI), distinguish the differences mechanisms in the similar regulation methods, reviews the current preparation methods of Fe(VI), and finally reviews the applications of Fe(VI) in the field of environmental remediation.

Simultaneous electrocoagulation and E-peroxone coupled with ultrafiltration membrane for shale gas produced water treatment

Membrane fouling caused by the organics-coated particles was the main obstacle for the highly efficient shale gas produced water (SGPW) treatment and recycling. In this study, a novel hybrid electrocoagulation (EC) and E-peroxone process coupled with UF (ECP-UF) process was proposed to examine the efficacy and elucidate the mechanism for UF fouling mitigation in assisting SGPW reuse. Compared to the TMP (transmembrane pressure) increase of -15 kPa in the EC-UF process, TMP in ECP-UF system marginally increased to -1.4 kPa for 3 filtration cycles under the current density of 15 mA/cm2. Both the total fouling index and hydraulically irreversible fouling index of the ECP-UF process were significantly lower than those of EC-UF process. According to the extended Derjaguin-Landau-Verwey-Overbeek theory, the potential barriers was the highest for ECP-UF processes due to the substantial increase of the acid-base interaction energy in ECP-UF process, which was well consistent with the TMP and SEM results. Turbidity and TOC of ECP-UF process were 63.6% and 45.8% lower than those of EC-UF process, respectively. According to the MW distribution, the variations of compounds and their relative contents were probably due to the oxidation and decomposing products of the macromolecular organics. The number of aromatic compound decreased, while the number of open-chain compounds (i.e., alkenes, alkanes and alcohols) increased in the permeate of ECP-UF process. Notably, the substantial decrease in the relative abundance of di-phthalate compounds was attributed to the high reactivity of these compounds with ·OH. Mechanism study indicated that ECP could realize the simultaneous coagulation, H2O2 generation and activation by O3, facilitating the enhancement of ·OH and Alb production and therefore beneficial for the improved water quality and UF fouling mitigation. Therefore, the ECP-UF process emerges as a high-efficient and space-saving approach, yielding a synergistic effect in mitigating UF fouling for SGPW recycling.

Enhanced removal strategy towards organic matter with low coagulability: Immediate entrapment and complexation of oxidized intermediates by the hybrid ozonation-coagulation process

The existence of dissolved organic matter (DOM) with low coagulability poses great challenges for conventional coagulation (CC) in water treatment. As a kind of typical organochlorine pesticide, 2,4-dichlorophenoxyacetic acid (2,4-D) cannot be efficiently removed by CC. To enhance the 2,4-D removal, ozonation was applied with coagulation. The hybrid ozonation-coagulation (HOC) achieved 60.61% DOC removal efficiency, which was obviously higher than pre-ozonation coagulation (POC) (45.83%). Synchronous fluorescence spectroscopy revealed stronger complexation between modified 2,4-D and coagulants during the HOC than that in subsequent coagulation of the POC process. During the HOC process, ozone promoted the formation of polymeric Al species, such as Alb. To investigate the 2,4-D removal mechanism, γ-Al2O3/O3 process with the same oxidation ability as the HOC was established. 2,4-D was oxidized step-by-step to 2,4-dichlorophenol, 4,6-dichlororesorcin, 3,5-dichlorocatechol, 2-chlorohydroquinone, 4-chlorocatechol, 1,2,4,5-tetrahydroxybenzene, pentahydroxybenzene and oxalic acid in γ-Al2O3/O3 process. However, during the HOC process, these oxidized intermediates were readily complexed by coagulants and accumulated in flocs. Especially 1,2,4,5-tetrahydroxybenzene and pentahydroxybenzene, completely complexed by AlCl3•6H2O hydrolysates as soon as being formed. Immediate entrapment and complexation between coagulant hydrolysates and 2,4-D oxidized intermediates inhibited the generation of small-molecular-weight organics such as oxalic acid, which enhanced the removal of organics with low coagulability.

Is monochloramine pre-oxidation a viable strategy for enhancing the treatment efficiency of algae-laden water with conventional drinking water treatment process?

Algal blooms worldwide pose many challenges to drinking water production. Pre-oxidation with NaClO, KMnO4, or ozone is commonly used to enhance algal removal in conventional drinking water treatment processes. However, these currently utilized oxidation methods often result in significant algal cell lysis or impede the operation of the subsequent units. Higher algal removal with pre-chlorination in algal solutions prepared with natural water, compared to those prepared with ultrapure water, has been observed. In the present studies, preliminary findings indicate that ammonium in natural water alters chlorine species to NH2Cl, leading to improved treatment efficiency. NH2Cl with 1.5-3.0 mg∙L-1 as Cl2 with an oxidation time of 3-7 h significantly enhancing algal removal by coagulation. The selective oxidation of surface-absorbed organic matter (S-AOM) by NH2Cl, followed by the subsequent peeling off of this material from the algal surface, leading to an increase in zeta potential from -20.2 mV to -3.8 mV, constitutes the primary mechanism of enhanced algal removal through coagulation. These peeled S-AOM retained their large molecular weight and acted as polymer aids. Compared with NaClO and KMnO4, NH2Cl displays the best performance in improving algal removal, avoiding cell lysis, and decreasing the potential for nitrogenous disinfection byproducts formation under the reaction conditions used in this study. Notably, in major Chinese cities, water purification plants commonly rely on suburban lakes or reservoirs as water sources, necessitating the transportation of raw water over long distances for times up to several hours. These conditions favor the implementation of NH2Cl pre-oxidation. The collective results indicate the potential of NH2Cl oxidation as a viable pretreatment strategy for algal contamination during water treatment processes.

Simultaneous modulation of CHO cell cytotoxicity, turbidity, and DOC by coagulation with or without pre-oxidation in water from the Pearl River Delta region, China

Coagulation with or without pre-oxidation are important drinking water treatment processes. However, the efficacy of these processes in mitigating water toxicity remains unknown. To further improve drinking water safety, we employed water from the Pearl River Delta region of southern China to investigate a treatment approach consisting of coagulation with or without pre-oxidation to simultaneously modulate health-relevant cytotoxicity to CHO cells, on top of the conventional foci of turbidity and dissolved organic carbon (DOC) during water treatment. Three coagulants (two aluminum-based and one iron-based salts) and three pre-oxidants (ozone, permanganate, and peroxymonosulfate) were studied. For coagulation without pre-oxidation, intermediate coagulant doses and pH reached optimum cytotoxicity to CHO cells, turbidity, and DOC control simultaneously. Introducing oxidants reduced cytotoxicity to CHO cells significantly, enhanced by increasing oxidant concentrations and pre-oxidation duration. The cytotoxicity to CHO cells mitigation capabilities of three pre-oxidants were: ozone > peroxymonosulfate > potassium permanganate. Modulation of water cytotoxicity to CHO cells was mostly attributable to controlling DOC (specifically humic-acid like substances, tyrosine, tryptophan). However, the addition of pre-oxidants led to significant shifts in water cytotoxicity to CHO cells forcing drivers, rendering humic-acid like substances the sole decisive cytotoxicity-inducing fluorophores. For the first time, 'sweet spots' were identified to simultaneously monitor cytotoxicity to CHO cells alongside turbidity and DOC. These methods better modulate water cytotoxicity to CHO cells without sacrificing conventional water treatment goals while shedding light onto the mechanisms behind.

Hydrogen peroxide preoxidation as a strategy for enhanced antimicrobial photodynamic action against methicillin-resistant Staphylococcus aureus

Antimicrobial photodynamic treatment (aPDT) is a photooxidative process based on the excitation of a photosensitizer (PS) in the presence of molecular oxygen, under specific wavelengths of light. It is a promising method for advanced treatment of water and wastewater, particularly targeting disinfection challenges, such as antibiotic-resistant bacteria (ARB). Research in improved aPDT has been exploring new PS materials, and additives in general. Hydrogen peroxide (H2O2) a widely applied disinfectant, mostly in the food industry and clinical settings, present environmentally negligible residuals at the usually applied concentrations, making it friendly for the water and wastewater sectors. Here, we explored the effects of preoxidation with H2O2 followed by blue light-mediated (450 nm) aPDT using curcumin (a natural-based PS) against methicillin-resistant Staphylococcus aureus (MRSA). Results of the sequential treatment pointed to a slight hampering in aPDT efficiency at very low H2O2 concentrations, followed by an increasing cooperative effect up to a deleterious point (≥7 log10 inactivation in CFU mL-1), suggesting a synergistic interaction of preoxidation and aPDT. The increased performance in H2O2-pretreated aPDT encourages studies of optimal operational conditions for the assisted technology and describes potentials for using the described strategy to tackle the issue of ARB spread.

Synergistic effect of potassium ferrate and ferrous iron for improving ultrafiltration performance in algae-laden water treatment

The application of ultrafiltration (UF) technology in algae-laden water is limited due to the serious membrane fouling caused by algal foulants. Herein, a Ferrate/FeSO4(Fe(VI)/Fe(II)) pretreatment was proposed aiming to improve the performance of UF. The results showed that the synergistic of Fe(VI) and Fe(II) significantly increased the zeta potential of Microcystis aeruginosa, which enhanced the agglomerative tendency of algal foulants, and the particle size of flocs remarkably increased due to the in-situ generated Fe(III). Results from dissolved organic carbon (DOC), UV254, K+, and fluorescent spectra indicated that the introduction of Fe(II) avoided the excessive oxidation of Fe(VI) to algal cells and reduced the production of intracellular organic matter (IOM), while the strong coagulation efficiency of in-situ Fe(III) further enhanced the removal effect of algal organics. Meanwhile, the molecular weight distribution showed that macromolecular organics were decomposed into low molecular matters under Fe(VI) oxidation, while the Fe(VI)/Fe(II) process reduced the formation of small molecular matters compared with single Fe(VI) pretreatment. The algal-source fouling was efficaciously mitigated under the optimal experimental condition, the terminal membrane flux could be increased from 0.16 to 0.62, while reversible and irreversible fouling decreased by 67.1% and 64.1%, respectively. Modeling analysis demonstrated that the Fe(VI)/Fe(II) process altered the fouling mechanism by delaying the formation of cake filtration. Membrane interface characterization further indicated that large size algal flocs form a loose cake layer and reduce the deposition of algal pollutants on the membrane surface. The Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory confirmed that the hydrophobic adsorption between the algal foulant and the membrane was weakened, thus relieving the membrane fouling. Overall, this strategy can be considered for application in improving the UF performance and mitigating algal-source membrane fouling.

Multiple roles of UV/KMnO4 in cyanobacteria containing water treatment: Cell inactivation & removal, and microcystin degradation

Cyanobacterial blooms present great challenges to drinking water treatment and human health. The novel combination of potassium permanganate (KMnO₄) and ultraviolet (UV) radiation is engaged as a promising advanced oxidation process in water purification. This study investigated the treatment of a typical cyanobacteria, Microcystis aeruginosa by UV/KMnO₄. Cell inactivation was significantly improved by UV/KMnO₄ treatment, compared to UV alone or KMnO₄ alone, and cells were completely inactivated within 35 min by UV/KMnO₄ in natural water. Moreover, effective degradation of associated microcystins was simultaneously achieved at UV fluence rate of 0.88 mW cm^-2 and KMnO₄ dosages of 3-5 mg L^-1. The significant synergistic effect is possibly attributable to the highly oxidative species produced during UV photolysis of KMnO₄. In addition, the cell removal efficiency via self-settling reached 87.9 % after UV/KMnO₄ treatment, without additional coagulants. The fast in situ generated manganese dioxide was responsible for the enhancement of M. aeruginosa cell removal. This study firstly reports multiple roles of UV/KMnO₄ process in cyanobacterial cell inactivation and removal, as well as simultaneous microcystin degradation under practical conditions.

Polymer-flooding produced water treatment using an electro-hybrid ozonation-coagulation system with novel cathode membranes targeting alternating filtration and in situ self-cleaning

Polymer-flooding produced water is more difficult to treat for reinjection compared with normal produced water because of the presence of residual hydrolyzed polyacrylamide (HPAM). A novel cathode membrane integrated electro-hybrid ozonation-coagulation (CM-E-HOC) process was proposed for the treatment of polymer-flooding produced water. This process achieved in situ self-cleaning by generated microbubbles in the cathode membrane. The CM-E-HOC process achieved a higher suspended solid (SS), turbidity and PAM removal efficiency than the CM-EC process. The SS in the CM-E-HOC effluent was ≤ 20 mg/L SS, which met the reinjection requirements of Longdong, Changqing Oilfield, China (Q/SYCQ 08,011-2019) at different current densities (3, 5 and 10 mA/cm²). The CM-E-HOC process greatly mitigated both reversible and irreversible membrane fouling. Therefore, excellent flux recovery was obtained at different in situ self-cleaning intervals during the CM-E-HOC process. Furthermore, alternating filtration achieved continuous water production during the CM-E-HOC process. On one hand, the effective removal of aromatic protein-like substances and an increase in oxygen-containing functional groups were achieved due to the enhanced oxidation ability of the CM-E-HOC process, which decreased membrane fouling. On the other hand, the CM-E-HOC process showed improved coagulation performance because of the increased oxygen-containing functional groups and polymeric Fe species. Therefore, larger flocs with higher fractal dimensions were generated, and a looser and more porous cake layer was formed on the membrane surface during the CM-E-HOC process. Consequently, the CM-E-HOC process exhibited better in situ self-cleaning performance and lower filtration resistance than the CM-EC process.

Modifications to microplastics by potassium ferrate(VI): impacts on sorption and sinking capability in water treatment

Pre-treatment (oxidation) may induce potential modifications to microplastics (MPs), further affecting their behaviors and removal efficiency in drinking water treatment plants. Herein, potassium ferrate(VI) oxidation was tested as a pre-treatment for MPs with four polymer types and three sizes each. Surface oxidation occurred with morphology destruction and oxidized bond generation, which were prosperous under low acid conditions (pH 3). As pH increased, the generation and attachment of nascent state ferric oxides (Fe_xO_x) gradually became dominant, making MP-Fe_xO_x complexes. These Fe_xO_x were identified as Fe(III) compounds, including Fe₂O₃ and FeOOH, firmly attaching to the MP surface. Using ciprofloxacin as the targeted organic contaminant, the presence of Fe_xO_x enhanced MP sorption dramatically, e.g., the kinetic constant K_f of ciprofloxacin raised from 0.206 (6.5 μm polystyrene) to 1.062 L g^-1 (polystyrene-Fe_xO_x) after oxidation at pH 6. The sinking performance of MPs was enhanced, especially for small MPs (< 10 μm), which could be attributed to the increasing density and hydrophilicity. For instance, the sinking ratio of 6.5 μm polystyrene increased by 70% after pH 6 oxidation. In general, ferrate pre-oxidation possesses multiple enhanced removals of MPs and organic contaminants through adsorption and sinking, reducing the potential risk of MPs.

Unraveling the over-oxidation inhibition mechanism during the hybrid ozonation-coagulation process: Immediate entrapment and complexation between intermediate organic matter and coagulants

Pre-ozonation coagulation process had a very low and narrow range of ozone dosages for enhancing the dissolved organic matter (DOC) removal efficiency, in which over-oxidation may occur if the ozone dosage was not strictly controlled. In contrast, the proposed hybrid ozonation-coagulation (HOC) process with higher oxidation ability notably inhibited over-oxidation in this study, and exhibited improved DOC removal efficiency compared with coagulation at a much wider range of ozone dosages at different initial pH for the treatment of WWTP effluent. The HOC process also had a higher DOC removal efficiency than pre-ozonation coagulation. According to zeta potential analysis, a rising trend indicated that complexation between organic matter and metal coagulants persisted throughout the HOC process. However, the zeta potential remained almost unchanged during subsequent coagulation after pre-ozonation at high ozone dosages. Synchronous fluorescence spectroscopy analysis revealed that immediate entrapment and complexation between hydrolysed coagulants and oxidized intermediate organic matter occurred in the HOC process. Furthermore, FT-IR analysis showed that more oxygen-containing functional groups were generated, which were effectively trapped by metal coagulants and readily flocculated. To further prove the immediate entrapment and complexation during the HOC process, UPLC-Q-TOF-MS was applied to analyze the intermediate organic matter in the supernatant and flocs. The results implied that C21- organic matter was oxidized and decomposed into C11-C20, and C11-C20 intermediate organic matter was trapped and complexed by metal coagulants once formed, which led to the increase of C11-C20 in the flocs. Nevertheless, the catalytic ozonation process (γ-Al₂O₃/O₃) with the same oxidation ability as the HOC process decomposed the organic matter into C1-C10. XPS analysis further confirmed the immediate entrapment and removal of aliphatic/aromatic carbon and oxygen-containing functional groups during the HOC process. Therefore, over-oxidation can be effectively inhibited, and wide range of ozone dosages was obtained during the HOC process, which facilitate the application of the HOC process.

Permanganate preoxidation affects the formation of disinfection byproducts from algal organic matter

During harmful algal blooms (HABs), permanganate may be used as a preoxidant to improve drinking water quality by removing algal cells and degrading algal toxins. However, permanganate also lyses algal cells, releasing intracellular algal organic matter (AOM). AOM further reacts with permanganate to alter the abundance of disinfection byproduct (DBP) precursors, which in turn affects DBP formation during disinfection. In this study, we evaluated the impacts of preoxidation by permanganate applied at commonly used doses (i.e., 1-5 mg/L) on DBP generation during chlorination and chloramination of AOM. We found that permanganate preoxidation increased trichloronitromethane (TCNM) formation by up to 3-fold and decreased dichloroacetonitrile (DCAN) formation by up to 40% during chlorination, indicating that permanganate oxidized organic amines in AOM to organic nitro compounds rather than organic nitrile compounds. To test this proposed mechanism, we demonstrated that permanganate oxidized organic amines in known DBP precursors (i.e., tyrosine, tryptophan) to favor the production of TCNM over DCAN during chlorination. Compared to the decreased formation of DCAN during chlorination, permanganate increased DCAN formation by 30-50% during chloramination of AOM. This difference likely arose from monochloramine's ability to react with non-nitrogenous precursors (e.g., organic aldehydes) that formed during permanganate preoxidation of AOM to generate nitrogen-containing intermediates that go on to form DCAN. Our results also showed that permanganate preoxidation favored the formation of dichlorobromomethane (DCBM) over trichloromethane (TCM) during chlorination and chloramination. The increased formation of DBPs, especially nitrogenous DBPs that are more toxic than carbonaceous DBPs, may increase the overall toxicity in finished drinking water when permanganate preoxidation is implemented.

Comparison of peracetic acid and sodium hypochlorite enhanced Fe(Ⅱ) coagulation on algae-laden water treatment

In this study, Fe(Ⅱ)/peracetic acid (PAA) and Fe(Ⅱ)/sodium hypochlorite (NaClO) systems were applied as the combined preoxidation and coagulation process to enhance algae removal. A high removal rate of algae and turbidity could be achieved, with most algal cells keeping intact when adding reasonable concentrations of PAA and NaClO to enhance Fe(Ⅱ) coagulation. The variations of chlorophyll a, malondialdehyde, and intracellular reactive oxygen species suggested that moderate oxidation with only destroying surface-adsorbed organic matter rather than cell integrity was realized. The generated organic radicals, Fe(Ⅳ), and hydroxy radical played the major roles in the Fe(Ⅱ)/PAA system for the moderate oxidation of algal cells, but direct oxidation by NaClO rather than producing reactive species in the Fe(Ⅱ)/NaClO process contributed to the preoxidation. Concurrently, the in-situ formed Fe(Ⅲ) greatly promoted the agglomerating and settling of algae. The analysis of cell integrity, biochemical compositions, and fluorescence excitation-emission matrices spectra demonstrated that excess NaClO but not PAA would seriously damage the algal cells. This might be because that NaClO would directly oxidize the cell wall/membrane, while PAA mainly permeates into the cell to inactivate algae. These results suggest that Fe(Ⅱ)/PAA is an efficient strategy for algae-laden water treatment without serious algae lysis.

Removal mechanism of Microcystis aeruginosa in Fe2+/sodium percarbonate and Fe2+/sodium persulfate advanced oxidation-flocculation system

Advanced oxidation process (AOPs) can be used for the treatment of harmful algal blooms (HABs). In this study, two systems of Fe²⁺/sodium percarbonate (Fe²⁺/SPC system) and Fe²⁺/sodium persulfate (Fe²⁺/PS system) were established to explore the removal mechanism of Microcystis aeruginosa (M. aeruginosa). The results indicated that the Fe²⁺/SPC system catalyzed H₂O₂ to generate a large amount of [Formula: see text] for oxidation by Fe²⁺ and formed Fe³⁺ to promote the flocculation of M. aeruginosa. The persulfate was activated by Fe²⁺ to generate [Formula: see text] with super-oxidizing properties, and Fe³⁺ was generated to realize the oxidation and flocculation of M. aeruginosa in the Fe²⁺/PS system. Compared with the traditional method in which the pre-oxidation and flocculation processes are carried out separately, the method in this study effectively improves the utilization rate of the flocculant and the removal effect of M. aeruginosa. The absolute value of zeta potential of Fe²⁺/PS system (|ζ|= 0.808 mV) was significantly lower than that of Fe²⁺/SPC system (|ζ|= 21.4 mV) (P < 0.05), which indicated that Fe²⁺/PS system was more favorable for the flocculation of M. aeruginosa cells than the Fe²⁺/SPC system.

Enhancing ultrafiltration of algal-rich water using ferrate activated with sodium percarbonate: Foulants variation, membrane fouling alleviation, and collaborative mechanism

Ultrafiltration (UF) is a reliable method to treat algal-rich water, whereas severe membrane fouling has impeded its actual application. To improve UF performance and alleviate membrane fouling resulted by algal foulants, a novel strategy coupling ferrate (Fe(VI)) and sodium percarbonate (SPC) was proposed. During the coupling process, Fe(VI) was activated by SPC to generate high-valent Fe intermediates (Fe(V) and Fe(IV)), which played a crucial role in high-efficiency oxidation for algal foulants, and the in-situ formed Fe(III) particles decomposed by Fe(VI) also enhanced the coagulation and adsorption capacity to the coupling system. Under the triple effects of coagulation, adsorption and oxidation, the algal foulants were efficiently eliminated. The zeta potential increased from -32.70 mV to -6.56 mV at most, the particle size was significantly enlarged, and the generated flocs possessed a great settleability. The morphology, viability, and integrity of algae cells were effectively maintained. The dissolved organic matters and fluorescent organics were efficiently removed, as well as macromolecular organics were reduced into lower molecular weight components. With the collaborative effect of Fe(VI) and SPC, the terminal specific flux was increased from 0.29 to 0.92, and the reversible and irreversible fouling resistances were reduced by 98.5% and 69.4%, individually. The surface functional groups were changed, and the dominant mechanisms were also converted to pore blocking from cake layer filtration. Overall, the experimental results would provide some new thoughts in actual production for algal-rich water treatment and UF membrane fouling alleviation.

Management of biogenic taste and odour: From source water, through treatment processes and distribution systems, to consumers

Biogenic taste and odour (T&O) have become a global concern for water utilities, due to the increasing frequency of algal blooms and other microbial events arising from the combined effects of climate change and eutrophication. Microbially-produced T&O compounds impact source waters, drinking water treatment plants, and drinking water distribution systems. It is important to manage across the entire biogenic T&O pathway to identify key risk factors and devise strategies that will safeguard the quality of drinking water in a changing world, since the presence of T&O impacts consumer confidence in drinking water safety. This study provides a critical review of current knowledge on T&O-causing microbes and compounds for proactive management, including the identification of abiotic risk factors in source waters, a discussion on the effectiveness of existing T&O barriers in drinking water treatment plants, an analysis of risk factors for biofilm growth in water distribution systems, and an assessment of the impacts of T&O on consumers. The fate of biogenic T&O in drinking water systems is tracked from microbial production pathways, through the release of intracellular T&O by cell lysis, to the treatment of microbial cells and dissolved T&O. Based on current knowledge, five impactful research and management directions across the T&O pathway are recommended.

Enhanced coagulation and oxidation by the Mn(VII)-Fe(III)/peroxymonosulfate process: Performance and mechanisms

To improve the performance of the conventional coagulation process, a permanganate (Mn(VII)) pre-oxidation combined with Fe(III)/peroxymonosulfate (PMS) coagulation process (Mn(VII)-Fe(III)/PMS) that can significantly improve the removal of dissolved organic carbon (DOC), turbidity, and micropollutants is proposed in this study. Compared with conventional Fe(III) coagulation, the Mn(VII)-Fe(III)/PMS process can also significantly enhance the removal of iohexol and sulfamethoxazole in raw water. During this process, the primary reduction product, Mn(IV), after Mn(VII) pre-oxidation was adsorbed on the floc surfaces and involved in the Fe(III)/PMS process. The natural organic matter (NOM) in raw water mediated the redox cycle of iron. The synergistic effect of NOM, Fe, and Mn facilitated the redox cycle of Mn(III)/Mn(IV) and Fe(III)/Fe(II) to promote the activation of PMS. The sulfate radical (SO₄^•-) played an important role in the degradation of micropollutants. The formation potential of the detected volatile disinfection by-product (DBP) during the subsequent chlorination was reduced by 21.9% after the Mn(VII)-Fe(III)/PMS process. This study demonstrated the promising application of the Mn(VII)-Fe(III)/PMS process for coagulation and micropollutant control and illustrated the reaction mechanism. This study provides guidance for improving conventional drinking water treatment processes.

Kinetics and mechanism of ozonation to treat Microcystis-laden source waters affected by cell-viability

Toxic cyanobacteria are challenging drinking water safety globally, and their cell-viability declines at decay stage of a succussive bloom. Ozone might be a more effective oxidant to treat both high- and low-viability cyanobacteria than other common oxidants (e.g., chlorine, potassium permanganate). However, previous studies only conducted ozonation experiments using high-viability cyanobacteria, and potential influences of cell-viability on ozonation process, remains unknown. In this study, kinetics of ozone decay, cell inactivation, membrane destruction, and cyanotoxin fate of high- and low-viability Microcystis (the most common genus), was investigated, and associated mechanism was discussed. Results showed that low-viability Microcystis exhibited a higher rate constant of membrane destruction (665-744 M^-1 s^-1) than high-viability Microcystis (364-600 M^-1 s^-1) by equal concentrations of ozone, ascribed to loosely gelatinous sheath comprised with fewer organic matters as oxidant scavengers. Meanwhile, a higher rate constant of photosynthetic inactivation induced by ozonation, was observed for low-viability Microcystis (312-364 M^-1 s^-1) than that for high-viability Microcystis (168-294 M^-1 s^-1). However, elevated aromatic organics competitively inhibited microcystin ozonation for low-viability Microcystis, and hydroxyl radicals for microcystin oxidation could be reduced by elevated organic loads and alkalinity. Moreover, elevated ozone exposure (>51 mg min L^-1) did not totally oxidize microcystin with a residual of 30 μg L^-1 for low-viability Microcystis. These findings suggested that elevated microcystin risk would be the great barrier to limit ozonation application for low-viability Microcystis, even with benefits of higher cell inactivation compared to high-viability Microcystis.

A innovative stepwise strategy using magnetic Fe3O4-co-graft tannin/polyethyleneimine composites in a coupled process of sulfate radical-advanced oxidation processes to control harmful algal blooms

A novel co-graft tannin and polyethyleneimine co-coating magnetic composite (TP@Fe₃O₄) was prepared in the study. On this premise, an unique stepwise efficient strategy based on magnetic flocculation and Sulfate radical (SO₄^•-)-advanced oxidation processes (S-AOPs) for eliminating Microcystis aeruginosa (M. aeruginosa) and algal organic matters (AOMs) was presented. Due to the high positive charge of TP@Fe₃O₄, a > 99 % high algae removal rate was obtained at a modest TP@Fe₃O₄ dosage of 100 mg/L at pH = 8.0 with a short separation time of 5 min. Further, peroxymonosulfate (PMS) treatment was employed as a pre-oxidation method to lower cell stability and promote M. aeruginosa removal by subsequent TP@Fe₃O₄ flocculation. The PMS/TP@Fe₃O₄ system successfully cuts the optimum dose of TP@Fe₃O₄ in half (50 mg/L) without causing obvious cell damage. Following algal fast magnetic separation, ultraviolet (UV) was introduced to activate PMS to totally degrade AOM and microcystin. Response surface methodology (RSM) demonstrated that UV/PMS oxidation removed > 80 % of DOC and > 94 % of microcystin under optimal conditions. SO₄^•- was the main radical species that aided in the elimination of AOM. This is the first study to use magnetic flocculation in conjunction with AOPs to mitigate harmful algal blooms, which can enable the non-destructive eradication of M. aeruginosa while also efficiently degrading AOMs.

Novel insights into formation mechanism of organic chloramines from pre-oxidized algae-laden water: Multiple roles of dissolved organic nitrogen

Organic chloramines posed significant risks to drinking water safety. However, the formation mechanism of algae-derived organic chloramines remained unclear. In this study, it was observed that pre-oxidation of algal suspensions increased organic chloramine formation during chlorination. Compared to KMnO₄ pre-oxidation, O₃ significantly increased the organic chloramine formation potential of algal suspensions. Characterization was performed with size exclusion chromatography-multiple detectors (SEC-MDs) to better understand the organic chloramine formation mechanism. The results revealed that low molecular weight proteins (AMW ≤ 0.64 kDa) were the main precursors of organic chloramines after conventional water treatment processes. We then focused on 14 essential amino acids involved in protein formation. Their concentrations and organic chloramine formation potentials were determined, based on which the theoretical organic chloramine formation potentials of the studied samples were evaluated. However, dramatic gaps between theoretical and experimental organic chloramine formations were observed, which suggested that not all organic nitrogen could react with chlorine to form organic chloramine. The condensed dual descriptor (CDD) was calculated to predict the electrophilic substitution reaction sites on peptides. Furthermore, the activation barrier of each proposed reaction was computed to confirm that the reaction sites for chlorine were located on amino groups. This study clarified the formation mechanism of algal-derived organic chloramines, which could provide a powerful theoretical foundation for controlling organic chloramine formation in drinking water processes.

Molecular transformation of dissolved organic matter and the formation of disinfection byproducts in full-scale surface water treatment processes

Dissolved organic matters (DOM) have important effects on the performance of surface water treatment processes and may convert into disinfection by-products (DBPs) during disinfection. In this work, the transformation of DOM and the chlorinated DBPs (Cl-DBPs) formation in two different full-scale surface water treatment processes (process 1: prechlorination-coagulation-precipitation-filtration; process 2: coagulation-precipitation-post-disinfection-filtration) were comparatively investigated at molecular scale. The results showed that coagulation preferentially removed unsaturated (H/C < 1.0 and DBE > 17) and oxidized (O/C > 0.5) compounds containing more carboxyl groups. Therefore, prechlorination produced more Cl-DBPs with H/C < 1.0 and O/C > 0.5 than post-disinfection. However, the algal in the influent produced many reduced molecules (O/C < 0.5) without prechlorination, and these compounds were more reactive with disinfectants. Sand filtration was ineffective in DOM removal, while microorganisms in the filter produced high molecular weight (MW) substances that were involved in the Cl-DBPs formation, causing the generation of higher MW Cl-DBPs under post-disinfection. Furthermore, the CHO molecules with high O atom number and the CHON molecules containing one N atom were the main Cl-DBPs precursors in both surface water treatment processes. In consideration of the putative Cl-DBPs precursors and their reaction pathways, the precursors with higher unsaturation degree and aromaticity were prone to produce Cl-DBPs through addition reactions, while that with higher saturation degree tended to form Cl-DBPs through substitution reactions. These findings are useful to optimize the treatment processes to ensure the safety of water quality.

Evidence-Based Framework to Manage Cyanobacteria and Cyanotoxins in Water and Sludge from Drinking Water Treatment Plants

Freshwater bodies and, consequently, drinking water treatment plants (DWTPs) sources are increasingly facing toxic cyanobacterial blooms. Even though conventional treatment processes including coagulation, flocculation, sedimentation, and filtration can control cyanobacteria and cell-bound cyanotoxins, these processes may encounter challenges such as inefficient removal of dissolved metabolites and cyanobacterial cell breakthrough. Furthermore, conventional treatment processes may lead to the accumulation of cyanobacteria cells and cyanotoxins in sludge. Pre-oxidation can enhance coagulation efficiency as it provides the first barrier against cyanobacteria and cyanotoxins and it decreases cell accumulation in DWTP sludge. This critical review aims to: (i) evaluate the state of the science of cyanobacteria and cyanotoxin management throughout DWTPs, as well as their associated sludge, and (ii) develop a decision framework to manage cyanobacteria and cyanotoxins in DWTPs and sludge. The review identified that lab-cultured-based pre-oxidation studies may not represent the real bloom pre-oxidation efficacy. Moreover, the application of a common exposure unit CT (residual concentration × contact time) provides a proper understanding of cyanobacteria pre-oxidation efficiency. Recently, reported challenges on cyanobacterial survival and growth in sludge alongside the cell lysis and cyanotoxin release raised health and technical concerns with regards to sludge storage and sludge supernatant recycling to the head of DWTPs. According to the review, oxidation has not been identified as a feasible option to handle cyanobacterial-laden sludge due to low cell and cyanotoxin removal efficacy. Based on the reviewed literature, a decision framework is proposed to manage cyanobacteria and cyanotoxins and their associated sludge in DWTPs.

Effect of Al hydrates on minimization of disinfection-by-products precursors by coagulation with intensified pre-oxidation towards cyanobacteria-laden water

Pre-oxidation is warranted to improve cyanobacteria removal and minimize disinfection by-products (DBPs) precursors for subsequent coagulation with polyaluminum chloride (PACl) in drinking water treatment. However, the reduction in DBP precursors strongly depends on the Al hydrates for PACl coagulation. This study aimed to investigate the effects of intensified NaOCl and ClO₂ pre-oxidation on the removal of Microcystis aeruginosa (MA) and the corresponding halogenated DBP precursors by PACl coagulation with different Al hydrates. Two PACl coagulants, namely PACl-W with 51% monomeric Al and PACl-H with 71% polymeric Al, were used for FlocCAM jar test. The results have shown that the reductions in MA cell and algogenic organic matter (AOM) are more pronounced by sweep flocculation in PACl-W coagulation coupled with NaOCl pre-oxidation. In contrast, ClO₂ pre-oxidation with PACl-H coagulation outperforms the floc formation and the reduction in each fluorescent DOM substance, especially for humic acid-like (HAL) substances reduction in response to charge neutralization. Regardless of pre-oxidation approach, PACl-H coagulation exhibits a superior reduction in carbonaceous DBP formation potential (C-DBPFP) comparative PACl-W coagulation, especially for intensified pre-oxidation (Cl₂:DOC = 3:1). Intensified NaOCl pre-oxidation is effective to enhance DBPFP reduction in a similar way to ClO₂ oxidation by coagulation with both PACl coagulants. In addition, it clearly demonstrates that the halogenated DBP precursors are well-correlated with UV₂₅₄ absorbance on the basis of principal component analysis (PCA) inference. It is concluded that intensified NaOCl pre-oxidation is an alternative approach to ClO₂ pre-oxidation for the minimization of DBP precursors in oxidation-coagulation processes for cyanobacteria-laden water treatment.

Moderate pre-ozonation coupled with a post-peroxone process remove filamentous cyanobacteria and 2-MIB efficiently: From bench to pilot-scale study

The increasing frequency and intensity of taste- and odour-producing cyanobacteria in water sources is a growing global issue. Odour events caused by 2-methylisoborneol (2-MIB) mainly arising from filamentous cyanobacteria have been a very common problem in water supply. Removal rates of filamentous cyanobacteria and 2-MIB by conventional water treatment, such as coagulation, and disinfection treatment processes is low. Hence, a moderate pre-ozonation of cyanobacteria (with little cell damage) was proposed in this study as an enhanced coagulation step to remove filamentous cyanobacteria and intracellular 2-MIB effectively, while avoiding the release of intracellular 2-MIB. A post-peroxone (O₃/H₂O₂) process was applied after sand filtration to degrade the residual dissolved 2-MIB. Results show that moderate pre-ozonation (0.2 mg/L O₃ oxidation for 20 min) can substantially enhance the coagulation efficiency for algae, with low cell lysis and high cell viability. Furthermore, 2.0 mg/L O₃ combined with 2.0 mg/L H₂O₂ can degrade the residual dissolved 2-MIB nearly 100% after 20 min reaction. Based on the optimal dosages, a 0.6 m³/h pilot system, including pre-ozonation, coagulation and sedimentation, sand filtration, and post-peroxone processes, was continuously run for 14 days, and it was found that the proposed process can effectively and stably remove filamentous cyanobacteria and 2-MIB.

Adsorption isotherms and kinetics for the removal of algal organic matter by granular activated carbon

Seasonal algal blooms in surface water release a significant amount of algal organic matter (AOM), which alters the composition of dissolved organic matter (DOM). AOM affects the drinking water treatment processes and finished water quality. In this study, the relative removal efficiency of AOM and humic acid by granular activated carbon (GAC) adsorption was determined. Batch experiments were conducted to evaluate the adsorption capacity of GAC, which varied from 4.235-31.45 mg/g for AOM originated from different algae. Freundlich isotherm models fitted the adsorption equilibrium data, and the adsorption kinetics data were fitted well using a pseudo-second order kinetic model. The calculated thermodynamics parameters (∆G⁰, ∆H⁰ and ∆S⁰) indicated that GAC adsorption for DOM removal was endothermic and spontaneous in nature.

Inactivation of Microcystis Aeruginosa by peracetic acid combined with ultraviolet: Performance and characteristics

The inactivation of algae by a combined process of peracetic acid and ultraviolet irradiation (UV/PAA) was systematically investigated by choosing Microcystis aeruginosa as the reference algal species. Both hydroxyl (HO^•) and organic radicals (RO^•) contributed to the cell integrity loss and RO^• played the dominant roles. The algae inactivation kinetics can be well fitted by the typical Hom model, showing that the inactivation kinetic curves followed a type of shoulder and exponential reduction. The initial shoulder might be induced by the protection from the cell wall. Although the results from the cell morphology, UV-vis spectra and fluorescence excitation-emission matrices analysis suggested the cell lysis and the release of algal organic matter (AOM) in the UV/PAA process, the AOM could be subsequently degraded. Humic acid (1 - 5 mg/L) inhibited the algal cell inactivation, and the presence of chloride (0.5 - 2 mM) had little effect on the cell viability reduction. However, the addition of bicarbonate (1 - 5 mM) promoted cell integrity loss. The UV/PAA process displayed better performance under the natural water background, demonstrating the extensive potential for the practical application of this approach. This study suggests that the UV/PAA process is an effective strategy for algae inactivation.

Enhanced complexation of humic acids: Homogenization of protonated groups in the hybrid ozonation-coagulation process

In this study, we compared dissolved organic carbon (DOC) and UV₂₅₄ removal efficiencies of the hybrid ozonation-coagulation (HOC) and pre-ozonation-coagulation (POC) processes for humic acid (HA) at pH 5 with AlCl₃•6H₂O as the coagulant. The DOC and UV₂₅₄ removal efficiencies of the HOC process were higher than those of the POC process at ozone dosages less than 2.0 mg O₃ (mg DOC)^-1. The ozone dosage was optimized at 0.3 and 0.1 mg O₃ (mg DOC)^-1 for the HOC and POC processes, respectively, implying a more rigorous ozone dosage for the POC process. During the POC process, pre-ozonation was observed to increase the binding sites of HA (e.g., hydroxyl and carboxyl groups), improving the complexation of dissolved organic matter. For the HOC process, in addition to its role in the oxidation of organic matter, ozone also reacted with coagulants. The reaction between ozone and coagulants can facilitate the formation of Al₁₃. Moreover, the oxidation of •OH and ozone can increase the charge density of the HA binding sites, homogenizing the binding sites of HA and enhancing the complexation with Al₁₃.

Enhanced coagulation for mitigation of disinfection by-product precursors: A review

The unintended formation of disinfection by-products (DBPs) has received considerable attention as it may pose risks to human health. Coagulation is the most common process for removing particulates as well as dissolved organic matter (DOM) (i.e., DBP precursors) during drinking water and wastewater treatments. With the improvement of water quality standards and the increased fluctuation in source water quality, conventional coagulation becomes challenging. Thus, significant efforts have been made to enhance coagulation to promote the removal of DOM in source water and mitigate the formation of DBPs in drinking water. This review provides a brief summary of the properties of DBP precursors and summarizes the effectiveness of enhanced coagulation involving three types of coagulants (metal-based coagulants, organic polymers, and organic-inorganic hybrid coagulants) in controlling the formation of DBPs during chlor(am)ination disinfection. Metal-based coagulants can achieve a reduction in DBP formation potential of approximately 20%-60% in natural water under enhanced coagulation conditions. Both the organic polymers (used as coagulant aids) and novel hybrid coagulants increase the removal of DOM and exhibit high potential for mitigating DBP formation. In addition, integrated treatments combining coagulation with other treatment processes (e.g., oxidation, membrane filtration, ion exchange, and adsorption) to enhance DBP precursor removal are evaluated in terms of performance, mechanisms, and features. Advanced treatments, such as membrane filtration and activated carbon adsorption, are effective coagulation-assisted processes, and can further control chlorinated DBPs; however, the elevated formation of bromate or highly brominated DBPs is of particular concern.

Simultaneous mitigation of disinfection by-product formation and odor compounds by peroxide/Fe(II)-based process: Combination of oxidation and coagulation

To remove disinfection by-product (DBP) precursors and mitigate odor compounds, peroxide (peroxymonosulfate and persulfate)/Fe(II)-based process was applied as a combination of coagulation and oxidation. Compared with traditional Fe-based salt coagulation (FeSO₄ and FeCl₃), peroxide/Fe(II)-based process was more efficient in dissolved organic carbon, UV₂₅₄ and turbidity removal, and peroxymonosulfate showed better performance than persulfate. The better coagulation performance arose from a combination of enhanced neutralization and different characteristics of flocs. Even though the combined process would increase the bromine substitution factor of DBPs, DBP formation and DBP-associated toxicity after peroxide/Fe(II)-based process were 9.2-38.8% and 5.2-27.2% lower than that after conventional Fe(III) coagulation. Both enhanced dissolved organic matter removal and oxidation of DBP precursors played vital roles in DBP control. Conventional Fe-based salt coagulation could hardly remove odor compounds (less than 10%, generally), whereas 28.2-84.9% of odor compounds were degraded during peroxide/Fe(II)-based process, due to free radical formation. This study demonstrated that PMS/Fe(II)-based process might be a promising treatment process for simultaneous DBP control and odor removal in source water.

Microcystis aeruginosa removal by the combination of ultrasound and TiO2/biochar

Harmful cyanobacteria blooms are increasing. They call for novel removal technology, since the required doses of algaecides may cause further environmental pollution or damage treatment facilities. Undesirable intracellular compounds can be released in the water when cyanobacterial cells are damaged. For the first time, ultrasound irradiation was combined with TiO₂/biochar (TiO₂/BC) at relatively low dosage and tested as an alternative for promoting the coagulation of Microcystis aeruginosa in water treatment. This pre-oxidation process removed 92% of cyanobacterial cells after coagulation. With the combination of ultrasound and TiO₂/BC treatment, the dissolved organic carbon and microcystins levels did not increase significantly. The oxidative treatments enhanced the permeability of the cyanobacterial cell membranes, which may be due to the various active species generated from the ultrasound and TiO₂/BC process. The results showed that the TiO₂/BC hybrid catalyst could be a potential candidate for cyanobacterial cells removal in water.

Application of potassium ferrate combined with poly-aluminum chloride for mitigating ultrafiltration (UF) membrane fouling in secondary effluent: Comparison of oxidant dosing strategies

Coagulation has been widely applied as a pretreatment for ultrafiltration (UF) membrane in wastewater reclamation, however, it is unable to effectively ensure the removal of organic micropollutants (OMPs) and genotoxicity. To solve this problem, oxidant ferrate (VI) (Fe^VIO₄^2-, Fe^VI) was combined with coagulant poly-aluminum chloride (PAC) as the pretreatment of UF to treat secondary effluent, and three oxidant dosing strategies (namely oxidation followed by coagulation (O-Fe^VI-PAC), simultaneous oxidation and coagulation (S-Fe^VI-PAC), and coagulation followed by oxidation (C-PAC-Fe^VI)) were compared at two oxidant doses. The results showed that C-PAC-Fe^VI pretreatment exhibited the best performance for the removal of DOC (35.9%), UV₂₅₄ (33.7%), protein (71.8%), and polysaccharide (22.1%). Molecular weight and fluorescence analysis revealed that the removed organics were mainly humic substances. Both the direct UF process and PAC pretreatment showed limited removal of OMPs and genotoxicity, however, the combined pretreatments of Fe^VI and PAC dramatically removed them. The maximum removal efficiency of the fourteen selected OMPs and genotoxicity was obtained under S-Fe^VI-PAC (59.6% on average) and C-PAC-Fe^VI (84.1%), respectively. With respect to membrane fouling control, the normalized flux reduction showed an apparent regularity of C-PAC-Fe^VI > O-Fe^VI-PAC > S-Fe^VI-PAC, however, Fe^VI dose should be carefully determined. The addition of Fe^VI delayed the transition of membrane fouling mechanism from pore blockage to cake filtration, especially in C-PAC-Fe^VI pretreatment, which was confirmed by the fluorescence characterization of hydraulic reversible and hydraulic irreversible foulants. To sum up, C-PAC-Fe^VI dosing strategy seems to have more potential in membrane fouling alleviation and effluent quality improvement.

Fe2+ activating sodium percarbonate (SPC) to enhance removal of Microcystis aeruginosa and microcystins with pre-oxidation and in situ coagulation

The frequent occurrence of cyanobacterial blooms has become a concern for drinking water safety. Common pre-oxidation, which was widely considered to enhance the followed coagulation, can cause the rupture of algae cell, leading to the undesirable release of intracellular organic matter. In this study, the Fe²⁺ activating sodium percarbonate (SPC/Fe²⁺) process for pre-oxidation and in situ coagulation was proved to effectively remove Microcystis aeruginosa without damaging cell integrity at optimal combined doses of SPC (0.2 mM) and Fe²⁺ (0.2 mM). Moreover, the SPC/Fe²⁺ process can not only control the release of MCs, but also reduce extracellular MCs from 5.22 μg/L to 1.50 μg/L, due to their moderate oxidation. Meanwhile, the SPC/Fe²⁺ treatment produces low levels of residual Fe after settling. During sludge ageing, owing to oxidation damage on cells arising from the SPC/Fe²⁺ treatment, cells continually suffered severe damage and lysed on day 4, leading to large release of intracellular organic matter and MCs, correspondingly. As a result, it is worth noting that the M. aeruginosa cells in stored sludge should be treated or disposed of early. These findings support the development of a green and cost-effective technology to handle cyanobacteria-containing water based on SPC/Fe²⁺ for ensuring water quality.

Hazardous cyanobacteria integrity response to velocity gradient and powdered activated carbon in water treatment plants

Although some studies have investigated the impact caused by chemicals used on water treatment (coagulants and oxidants) on cyanobacteria integrity, the isolated effect of shear stress during coagulation is still not fully understood. This study evaluated the impact of different velocity gradients, mixing times, and the addition of powdered activated carbon (PAC) on the integrity of Microcystis aeruginosa, Raphidiopsis raciborskii, and Dolichospermum circinale, known producers of toxin and taste and odor (T&O) compounds. No association was found between R. raciborskii cell lysis and velocity gradient, with or without PAC, demonstrating the high resilience of this taxon to shear stress. In contrast, an association was found for M. aeruginosa at the highest velocity gradient evaluated (1000 s^-1) and for D. circinale above the lowest velocity gradient studied (600 s^-1). After PAC addition, there was a reduction in the chances of finding M. aeruginosa intact cells above velocity gradient 800 s^-1 at 45 s, while D. circinale show cell lysis in all the scenarios expect at 600 s^-1 and 10 s of agitation. The additional impact of PAC on cell lysis may lead to more release of metabolites and shows the need to adjust the hydraulic conditions in the rapid mixing stage, especially when more "fragile" cyanobacteria are present. Neither cyanobacterial cell size nor morphology was shown to be relevant to shear stress sensitivity, indicating that cell wall composition might have been an important factor in controlling cell lysis.

Comparison between permanganate pre-oxidation and persulfate/iron(II) enhanced coagulation as pretreatment for ceramic membrane ultrafiltration of surface water contaminated with manganese and algae

Concurrent presence of algae and manganese (Mn) in water poses a significant challenge for water treatment. This study compared the treatment efficiency of Mn-containing and algae-laden water using either permanganate pre-oxidation (KMnO₄) or persulfate/iron(II) (PMS/Fe²⁺) enhanced coagulation as pretreatment for ceramic membrane ultrafiltration. The results showed that KMnO₄ pre-oxidation achieved a slightly more effective Mn removal, and was almost unaffected by the initial dissolved organic carbon (DOC) concentrations. PMS/Fe²⁺ removed UV₂₅₄ more efficiently (above 90% at a dose of 0.25 mmol/L), compared with KMnO₄ (less than 60% UV₂₅₄ removal). According to X-ray photoelectron spectroscopy (XPS) analysis of aggregates, both KMnO₄ and Fe²⁺/PMS oxidation resulted in the formation of MnO₂ precipitate. Electron paramagnetic resonance(EPR) analysis demonstrated that only the reactors dosed with PMS/Fe²⁺ were able to generate the highly reactive hydroxyl radical(·OH). The production of ·OH caused significant rupture of algal cells and thus higher algal removal compared to the treatment with KMnO₄ (whereby insignificant cell breakage was observed). The cell rupture resulted in higher amounts of organic matter released in the systems containing PMS/Fe²⁺, as demonstrated by excitation-emission matrix (EEM) and protein analysis. Despite the elevated level of organic matter, adding PMS/Fe²⁺ was found to notably mitigate membrane fouling due to the formation of large flocs (311-533 μm) as well as the elimination of major ceramic membrane foulants, i.e. humic substances.

Effects of pre-oxidation on residual dissolved aluminum in coagulated water: A pilot-scale study

Residual dissolved aluminum (Al) in drinking water is becoming a serious concern due to its high potential risks to human health. However, the mechanism by which residual dissolved Al forms is yet to be elucidated in detail. In this study, the effects of pre-oxidation by ozonation and chlorination on the properties of dissolved organic matter (DOM) and residual Al concentrations remaining in solution after coagulation were explored in a pilot-scale test. Changes in the DOM properties caused by the water treatment process were characterized by ultraviolet-visible absorbance spectroscopy. Theprotonation-active sites, carboxylic- and phenolic-type groupsof DOM were quantified by spectral parameter DlnA400 (differential log-transformed spectra at wavelength 400 nm) in combination with the revised non-ideal competitive adsorption model. The results show that ozonation and chlorination significantly affect the properties of DOM and the amount of residual dissolved Al in coagulated drinking water. This effect was associated with the changes in the carboxylic- and phenolic-type groups in DOM. Results of the study show that residual dissolved Al in coagulated water can be controlled by affecting theAl binding sites in DOM by pre-oxidation before coagulation. The nature of pre-oxidation agent and its dosage should be selected depending on the quality of the raw water to be treated. Ozonation was concluded to be preferable pre-oxidation agent for the water in examined this study.

Impacts of pre-oxidation on the formation of disinfection byproducts from algal organic matter in subsequent chlor(am)ination: A review

Algae cells and algal organic matter (AOM) present in algae impacted source water pose a serious threat to the safety of drinking water. Conventional water treatment processes poorly remove AOM that can transform to harmful disinfection byproducts (DBPs) during ensuing disinfection. This article offers a comprehensive review on the impacts of pre-oxidation on the formation of DBPs from AOM in subsequent chlor(am)ination. Various characterization techniques for algal cells and AOM are first overviewed with an effort to better understanding of correlation between the AOM properties and downstream DBP formation. Then, the present work reviews recent studies on application of different pre-oxidation technologies, such as chlor(am) ination, UV irradiation, ozonation, ferrate (VI), permanganate oxidation and UV-based advanced oxidation processes (AOPs), to remove algal cells and degrade AOM. Pre-oxidation can reduce the stability of algal cells and inactivate algal cells for promoting cell aggregation and thus favoring coagulation. Meanwhile, pre-oxidation can mitigate and degrade AOM into small molecular weight organic compounds to reduce DBP formation potential during subsequent chlor(am)ination. Finally, this review provides an overall evaluation on the applicability of different pre-oxidation processes, and identifies future research demands.

The synergistic effect of electrocoagulation coupled with E-peroxone process for shale gas fracturing flowback water treatment

Electrocoagulation (EC) coupled with E-peroxone process (ECP) was a promising and cost-effective integrated technology for shale gas fracturing flowback water (SGFFW) treatment. In this study, the ECP process was comprehensively compared with two sequential processes (EC followed by E-peroxone (EC-E-peroxone) and E-peroxone followed by EC (E-peroxone-EC)) to elucidate the synergistic effect of this coupled process. In EC-E-peroxone process, COD decreased by 89.2%, while COD decreased by 82.5% for 180 min in E-peroxone-EC process. However, COD removal efficiency was 82.4% in ECP for only 90 min. Average current efficiency of the ECP process was 29.9%, which was twice than that of the sequential processes. The enhancement factor was calculated to be 1.63, demonstrating the substantial significant synergistic effects for ECP. Only low MW components could be observed for the EC-E-peroxone (average MW = 533 Da with PD ≈ 1) and ECP process (MW = 538 Da with PD ≈ 1). These results suggested that EC-E-peroxone and ECP process had much stronger oxidation ability, demonstrating the enhancement of OH production induced by the Al-based coagulants might be responsible for the significant enhancement of COD removal. These results indicated there could be a synergistic effect between EC and ozone in addition to EC and E-peroxone reactions. Compared to the two sequential processes, ECP was a high efficiency and space-saving electrochemical system with simultaneous coagulation and enhanced OH generation by the products of anode and the cathode.

Simultaneous Removal of Microcystis aeruginosa and 2,4,6-Trichlorophenol by UV/Persulfate Process

UV/persulfate (UV/PS) could effectively degrade algal cells and micro-organic pollutants. This process was firstly applied to remove Microcystis aeruginosa (M. aeruginosa) and 2,4,6-trichlorophenol (TCP) simultaneously in bench scale. Algal cells can be efficiently removed after 120 min reaction accompanied with far quicker removal of the coexisted TCP, which could be totally removed within 5 min in the UV/PS process. Both SO4•- and HO^• were responsible for algal cells and TCP degradation, while SO4•- and HO^• separately dominated TCP degradation and algal cells removal. Apart from the role of radicals (SO4•- and HO^•) for algal cells and TCP degradation, UV also played a role to some extent. Increased PS dose (0–4.5 mM) or UV intensity (2.71–7.82 mW/cm²) could enhance the performance of the UV/PS process in both TCP and algae removal. Although some intracellular organic matters can be released to the outside of algal cells due to the cell lysis, they can be further degraded by UV/PS process, which was inhibited by the presence of TCP. This study suggested the good potential of the UV/PS process in the simultaneous removal of algal cells and micro-organic pollutants.

Characterization and reusability suggestions of the sludge generated from a synthetic acid mine drainage treatment using sodium ferrate (VI)

Mining activities are the main cause of generation of the voluminous sludge waste, loaded with metals precipitated from the treatment of acid mine drainage (AMD) and this is always disposed to the landfill. This study aimed at characterizing and suggesting the reusability potential of AMD sludge to reduce the environmental problem caused by its accumulation so that it could become a valuable material. The sludge was obtained after treating a synthetic AMD with a green oxidant sodium ferrate (VI) (Na₂FeO₄) that was prepared by a wet oxidation method. Chemical and physical characterization of a dried sludge generated after treatment was then performed using the Fourier Transform-Infrared and X-Ray powder Diffraction spectroscopy. Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy also served to identify the surface morphology of the sludge. The sludge presented a high weight percentage of Fe and O and lower concentrations of other metals such as Al, Mn, Si, and Na. Nitrogen adsorption/desorption isotherms or Brunauer-Emmett-Teller (BET) was used to assess the surface area, pore volume and diameter of the sludge. The BET results showed that the surface area of the sludge obtained after treating the synthetic AMD using Na₂FeO₄ was 31.50 ± 0.03 m²/g with pore diameter and volume of 52.50 nm and 0.41 cm³/g, respectively. However, the produced sludge could serve as an adsorbent to remove pollutants from water or to synthesize different magnetic nanocomposites due to its high surface area (>natural zeolite) and high composition of Fe and O.

The role of hydraulic conditions of coagulation and flocculation on the damage of cyanobacteria

Limited information exists on the damage of harmful cyanobacteria cells, such as Raphidiopsis raciborskii and Dolichospermum circinale, caused by the hydraulic conditions at water treatment plants especially when it comes to the mechanical stresses imposed by coagulation and flocculation. To close this gap, this study evaluated the impacts of rapid and slow-mixing on R. raciborskii and D. circinale cells and trichomes. The hydraulic conditions used during the experiment were selected based on AWWA, which are widely applied in the absence of specific treatability tests. Cellular integrity was evaluated by the Erythrosine B staining method and logistic regression was used to study the association between organism integrity and hydraulic conditions (i.e., velocity gradient and mixing time). Wilcoxon rank-sum test was used to verify if there was a significant reduction of the trichome length and cell integrity. Rapid-mixing (velocity gradient of 750 s^-1 for 60 s) reduced the odds of finding intact D. circinale to <50%, whereas the odds of finding intact R. raciborskii cells did not significantly decrease. The odds of finding intact cells of R. raciborskii were 124 times greater than D. circinale. Rapid-mixing also reduced the length of D. circinale trichomes by approximately 50% but did not significantly decrease R. raciborskii trichomes. Slow-mixing did not significantly affect organisms or trichomes of either species. The results indicate that AWWA recommendations for coagulation may cause damage to D. circinale but not to R. raciborskii, suggesting that the operation of water treatment plants could be adjusted according to the dominant cyanobacterium present in the reservoir to avoid cell rupture and metabolite release.

Ferrous-activated sodium percarbonate pre-oxidation for membrane fouling control during ultrafiltration of algae-laden water

Membrane technology has been shown to be promising for the treatment of algae-laden water, but membrane fouling is still an obstacle influencing the purification efficiency and effluent quality. To mitigate ultrafiltration membrane fouling during Microcystis aeruginosa-laden water treatment, a strategy of sodium percarbonate pre-oxidation activated with ferrous ion (Fe²⁺/SPC) was put forward in this study. Due to the synergistic effect of Fe²⁺ and SPC, this process was significantly more efficient with the terminal specific flux increased from 0.097 to 0.397, and the reversible fouling resistance reduced by approximately 80%. It was also found that subsequent sedimentation followed by Fe²⁺/SPC could further improve the fouling control efficiency. The model fitting results indicated that Fe²⁺/SPC pre-oxidation delayed the transition from standard blocking to cake filtration. Extracellular organic matter and algal cells were extracted from algal foulants to explore the contribution of each component, and the fouling control efficiencies were systematically studied. The characteristics of the algal foulants were determined with fluorescence excitation-emission matrix spectrum, and the results suggested that macromolecular proteinaceous substances were more efficiently removed by Fe²⁺/SPC, in comparison with humic-like matters. The alleviation of membrane fouling was also verified by the characterization methods of scanning electron microscopy and attenuated total reflection-Fourier infrared spectroscopy. Overall, the proposed strategy of Fe²⁺/SPC has an application prospect for membrane fouling control in algal-laden water treatment.

Exploring the relative changes in dissolved organic matter for assessing the water quality of full-scale drinking water treatment plants using a fluorescence ratio approach

This study aims to extend and demonstrate the application of fluorescence spectroscopy for monitoring the water quality of three differently operated full-scale drinking water treatment plants located in the Shenzhen city (China). A ratio of fluorescent dissolved organic matter (FDOM), which describes relative changes in humic-like to protein-like fluorescence, was used to explain mechanisms behind the physicochemical processes. The fluorescence components obtained through individual and combined parallel factor analysis (PARAFAC) modeling revealed the presence of humic-like (C1) and protein-like (C2) structures in the DOM. The C1/C2 ratio provided a direct relationship between the seasonal variations and DOM composition. Wet season generated DOM enriched with humic-like fluorescence, while dry season caused a higher release of protein-like fluorescence. The fluorescence ratio presented unique patterns of DOM in treatment trains. The chemical pretreatment and disinfection unit processes showed a higher tendency to remove the humic-like fluorescence. However, the C1/C2 ratio increased during physical treatment processes such as coagulation-precipitation and sand filtration, indicating preferential removal of protein-like fluorescence. The DOM composition in influent directly (R² = 0.77) influenced the relative intensities of fluorescence components in the treated water. Compared to the dry season, the wet season caused significant changes in DOM composition and produced treated water enriched with humic-like fluorescence. This fluorescence ratio offers an approach to explore the role of different treatment units and determine the factors affecting the composition of DOM in the surface water and drinking water treatment plants.