Mechanism of the hybrid ozonation-coagulation (HOC) process: Comparison of preformed Al13 polymer and in situ formed Al species

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.

Treatment of landfill leachate by coagulation: A review

Landfill leachate is a seriously polluted and hazardous liquid, which contains a high concentration of refractory organics, ammonia nitrogen, heavy metals, inorganic salts, and various suspended solids. The favorable disposal of landfill leachate has always been a hot and challenging issue in wastewater treatment. As one of the best available technologies for landfill leachate disposal, coagulation has been studied extensively. However, there is an absence of a systematic review regarding coagulation in landfill leachate treatment. In this paper, a review focusing on the characteristics, mechanisms, and application of coagulation in landfill leachate treatment was provided. Different coagulants and factors influencing the coagulation effect were synthetically summarized. The performance of coagulation coupled with other processes and their complementary advantages were elucidated. Additionally, the economic analysis conducted in this study suggests the cost-effectiveness of the coagulation process. Based on previous studies, challenges and perspectives met by landfill leachate coagulation treatment were also put forward. Overall, this review will provide a reference for the coagulation treatment of landfill leachate and promote the development of efficient and eco-friendly leachate treatment technology.

Design and application of gel coagulation-spontaneous flotation integrated process in water treatment: "Clouds in water"

The gel coagulation-spontaneous flotation (GCSF) process designed in this paper mainly rely on dissolved gas in water rather than auxiliary gas equipment to achieve spontaneous flotation. Compared with the traditional coagulation-dissolved air flotation method, GCSF has more stable flotation efficiency and shorter operation cycle under conventional hydraulic conditions. In this study, the GCSF scheme was applied for surface water treatment, and its operating efficiency, mechanism of action, and environmental implications were explored systematically. The results illustrate that the dosage ratio of sodium alginate (SA) to aluminum sulfate (AS) should be controlled in the range of approximately 1.5:1-2.5:1, and SA should be added 15∼120 s before AS during the coagulation process. Under these conditions, the adsorption cross-linking between SA and Al3+ promoted the generation of gel flocs and effectively encapsulated the dissolved gasses, thereby achieving a stable spontaneous flotation process and 80%-95% removal of pollutants. The purification efficiency of GCSF was positively correlated with pH 4-9, which was attributed to the enhanced hydrophobicity of the chains of organic polymer groups. The residual SA and aluminum concentration in effluent were lower than 1 and 0.05 mg/L, respectively, which guarantee the ecological security of GCSF application. In addition, the results of density functional theory calculations revealed that -OH and -AlO6 in cross-linked flocs could adsorb dissolved oxygen synergistically, while -OH combined with oxygen had a stronger binding energy and stable adsorption.

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.

Characteristics of dissolved ozone flotation for the enhanced treatment of bio-treated drilling wastewater from a gas field

Working fluid preparation using treated drilling wastewater is of great potential for drilling wastewater reuse, especially in water-deficient and ecologically fragile areas, which require low levels of organic matter and suspended solids (SS). This study established the dissolved ozone flotation (DOF) process as the advanced treatment process to replace the original electrocatalytic system (ECS) which exhibited low organic and suspended solids removal efficiency. Higher and more stable organic matter, suspended solids and turbidity removal efficiency were obtained for the DOF process. Consequently, the lower fouling potential and higher water production rate of treated water from DOF process was observed for the following reverse osmosis (RO) system. In addition, brine drilling fluids can be successfully prepared using DOF effluent directly, which exhibited promising practical implications in the advanced drilling wastewater treatment. Based on organic matter fractionation and redundancy analysis (RDA), the hydrophobic acid (HOA), hydrophobic neutral (HON) and hydrophilic fraction (HI) contents significantly impacted brine drilling fluid preparation. Based on X-ray photoelectron spectroscopy (XPS) analysis, the aromatic carbon species in the HOA, HON and HI fractions were found to be the critical factors deteriorating the brine drilling fluid preparation. However, oxygen-containing groups played a positive role. The favorable brine drilling fluid preparation performance using DOF effluent directly can be ascribed to the removal of HOA, HON and HI fractions and enhanced generation of oxygen-containing groups in ozone flotation zone.

Assessing the performance of coral reef-like floc towards the removal of low molecular weight organic contaminant

The removal of low molecular weight (MW) organics by coagulation is always a challenge in water treatment. In this study, we proposed a novel coagulation strategy: continuous dosing coagulation (CDC). The metallic coagulant and alkali were continuously dosed into water that was pre-acidized, rather than adding all the coagulant and alkali at once as in conventional coagulation (CC). The CDC process promoted the removal of different low MW organics, performing 15% better than the CC process. The best performance occurred at initial pH 6 and the coagulant dosing rate was 2 mg/(L·min). Under optimal conditions, the continuously dosed coagulant formed medium polymer Al in the early stages, which bound low MW organics to form complexes. Then, the subsequently dosed coagulant could adhere to the primary complexes and form coral reef-like surfaces with higher zeta potential and specific surface area. Each freshly formed surface bound contaminants and covered the previous surface. As a result, more dissolved low MW organic contaminants were included in the interior of flocs. However, in the CC process, all the coagulant was dosed at once, resulting in the rapid formation of aluminum hydroxide clusters, which had cotton-like surfaces with fewer binding sites. To achieve similar organic removal in treating secondary effluent, the CDC dosage was half of the CC dosage, indicating the potential economic benefits. The CDC process is a promising technology and the application in various water treatments should be further investigated.

Behaviour of ozone in the hybrid ozonation-coagulation (HOC) process for ibuprofen removal: Reaction selectivity and effects on coagulant hydrolysis

Simultaneous ozonation and coagulation can be realized in one unit in the developed hybrid ozonation-coagulation (HOC) process. To reveal the reaction sequence within the HOC process, the ibuprofen (IBP) removal efficiency of the ozonation only, HOC and HOC-PO₄^3- (inhibition of the reactions between ozone and metal coagulant) processes at pH 5 and different ozone dosages were investigated. The removal efficiency is almost the same for the three processes at a low ozone dosage (4.8 mg/L), and higher removal performance can be achieved by the HOC process with increasing ozone dosage. It can be implied that ozone preferentially reacts with OH^- to generate OH which react with IBP in the HOC process, and subsequently reacts with the surface hydroxyl groups of hydrolysed Al species to enhance OH generation. Moreover, based on the kinetics, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) analyses, the synergistic reactions between ozone and the metal coagulants (SOC) started to take effect from ozone dosage of 9.6 mg/L, which further verified that ozone will be involved in the IBP ozonation prior to the SOC reactions. The subsequent SOC reactions also resulted in the increased generation of polymeric Al species and more abundant intermediates in the HOC process.

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.

Insights into the electro-hybrid ozonation-coagulation process-Significance of connection configurations and electrode types

The electro-hybrid ozonation-coagulation process (E-HOC) integrates electrocoagulation (EC) and ozonation simultaneously in a single unit. Nevertheless, the performance of the EC process is highly dependent on the polar connection configuration (monopolar vs. bipolar connection) and the type of generated coagulants (single-coagulant vs. dual-coagulants). In this study, the removal efficiency of the E-HOC process with different connection configurations and types of coagulants was assessed. The E-HOC process with bipolar connection (BE-HOC) exhibited higher removal efficiency for wastewater treatment plant (WWTP) effluent organic matter and ibuprofen (IBP) compared with the E-HOC process with monopolar connection (ME-HOC). Furthermore, dual-coagulant generation (released from both Al and Fe electrodes) in the BE-HOC process greatly improved the WWTP effluent organic matter and IBP removal efficiency. Lower energy consumption was observed for the BE-HOC process compared with the ME-HOC process. It was found that ozonation promoted the polymerization reactions during coagulant hydrolyzis in the E-HOC process. Compared with the ME-HOC process, the BE-HOC configuration and dual-coagulant mode further facilitated polymeric hydrolyzed coagulant species formation, thereby improving ozone catalytic and coagulation performance. According to trapping experiments and EPR analysis, •OH formation was enhanced in the BE-HOC process and dual-coagulant mode. In addition, more active reaction sites of generated hydrolyzed coagulant species were observed with bipolar connection and in the dual-coagulant generation mode based on X-ray photoelectron spectroscopy (XPS) analysis.

Influences of coagulation pretreatment on the characteristics of crude oil electric desalting wastewaters

Highly polluted crude oil electric desalting wastewaters (EDWs) severely affect the efficiency of refinery wastewater treatment plants (WWTPs). Coagulation is an efficient pretreatment to reduce the impacts of EDWs. In the present study, the influences of coagulation pretreatment on the characteristics of EDWs of three typical Chinese crude oils, Liaohe heavy oil (LHO), Karamay heavy oil (KHO) and Daqing light oil (DLO), were investigated. The stability of three raw EDWs was broken and the contents of organic pollutants were significantly reduced by aluminum sulfate coagulation. More soluble COD and polar oils were removed from LHO-EDW (1241 and 98 mg L^-1) and KHO-EDW (779 and 57 mg L^-1) compared to DLO-EDW (417 and 11 mg L^-1). Coagulation significantly changed the compositions of the organic pollutants of two heavy oil EDWs; however, slightly influenced DLO-EDW, particularly the polar organic pollutants. Most types of aromatic compounds, aliphatic acids and O_x polar compounds were removed from two heavy oil EDWs, but mainly alkanes were removed from DLO-EDW. As such, the differences in the types of dominant polar compounds became insignificant among treated heavy oil and light oil EDWs. Coagulation notably decreased the acute biotoxicity and improved the biodegradability of all treated EDWs. The residual organic nitrogen compounds in treated KHO-EDW contributed to a higher residual biotoxicity compared to treated LHO-EDW. The results demonstrate that coagulation can effectively improve the qualities of heavy oil EDWs by lowering the contents of organic pollutants and removing recalcitrant compounds, thus guaranteeing the efficiency of refinery WWTPs.

Towards a comparison between the hybrid ozonation-coagulation (HOC) process using Al- and Fe-based coagulants: Performance and mechanism

In this study, the removal performance of a hybrid ozonation-coagulation (HOC) process using AlCl₃6H₂O (Al-HOC) and FeCl₃6H₂O (Fe-HOC) as coagulants for the treatment of wastewater treatment plant (WWTP) effluent and ibuprofen (IBP) was investigated. Compared with the conventional coagulation process and pre-ozonation-coagulation process, much better organic matter removal performance can be achieved for both the Al-HOC and Fe-HOC processes. The Fe-HOC process showed an obviously higher dissolved organic carbon (DOC) removal efficiency than that of the Al-HOC process. Surface hydroxyl groups were determined to be the active sites in generating OH in the HOC process, and the hydrolysed Fe species possessed a higher content of surface hydroxyl groups than the hydrolysed Al species according to fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectra (XPS) analyses. In addition, the hydrolysed Fe species contained a higher portion of tetrahedral sites that were more likely to be stronger Lewis acid sites to react with ozone to generate OH. Furthermore, peroxone reactions in the HOC process were other possible way to enhance the OH generation, and higher H₂O₂ generation was observed in the Fe-HOC process due to higher O₂^- generation. Therefore, better removal performance of the Fe-HOC process can be obtained due to the increased OH generation in the Fe-HOC process.

The role of synergistic effects between ozone and coagulants (SOC) in the electro-hybrid ozonation-coagulation process

In order to improve the dissolved organic matter removal efficiency, an electro-hybrid ozonation-coagulation (E-HOC) system was developed in this study, in which the electro-coagulation (EC) and ozonation occurred simultaneously in one integrated unit. Higher removal efficiency was observed for the E-HOC process compared with those of EC, ozonation and pre-ozonation-EC process for the treatment of wastewater treatment plant (WWTP) effluent and ibuprofen (IBP). 58.6% dissolved organic carbon (DOC) removal efficiency was achieved in the E-HOC process for the treatment of WWTP effluent at optimal operational condition (current density 15 mA/cm², initial pH 5 and ozone dosage 1.5 mg O₃/mg DOC). Based on the reactive oxygen species (ROS) detection and reactions on the electrodes, the synergistic effects between ozone and coagulants (SOC) were found to be involved in the E-HOC process. According to pseudo-first-order rate constant analyses, the contribution of five possible organic removal pathways was quantified. It was revealed that the peroxone and SOC effects exhibited almost equal contribution to IBP removal at initial pH 5 under different current densities, both of which played the dominant role in the E-HOC process. However, the contribution of the SOC effects decreased significantly when the initial pH increased to 7 and 9. As an important pathway for organic removal in the E-HOC process at initial pH 5, the mechanism of the SOC effects was analysed at initial pH 5. It was revealed the SOC effects can further improve hydroxyl radicals (•OH) generation, and the surface hydroxyl groups of the hydrolysed Al species generated from anode electrolysis were determined to be the active sites to generate ROS in the SOC effects.

Study on mitigating membrane fouling based on precursor and flocculant Alb matching in EC/O-UF system

One of the effective ways to remove halogenated disinfection by-products (DBPs) from drinking water is the application of ultrafiltration technology. However, membrane fouling is an important factor affecting the service life and treatment effect. In this study, the electrocoagulation/oxidation-ultrafiltration (EC/O-UF) process was used to remove the precursor substance that produced DBPs, i.e. dissolved organic matters (DOMs). Operating parameters were optimized from the matching of different flocculant morphology to low concentration DOM. The degree of membrane fouling was characterized by analyzing DOMs concentration and membrane flux. The results showed that the optimal conditions for the production of Al_b were: current density 10 A/m², hydraulic retention time 10 min, and initial pH 5.0-7.0. Under these conditions, the production of flocculant Al_b could reach 58-61%, 94-97% DOMs were removed by EC/O-UF.