Promoting Hydroxyl Radical Production during Ozonation of Municipal Wastewater

Hydroxyl radical (HO^?) production during ozonation of municipal wastewater was investigated with and without liquid or solid-phase promoters. For liquid-phase promoters, an “ozone dose threshold” was observed, below which addition of H₂O₂ yielded no discernible increase in the rate of HO^? production. This threshold occurs because ozonation of bulk organics in wastewater promotes HO^? due to the presence of ambient promoters. Although solid-phase catalysts are reported to promote oxidation of contaminants, ozonation of effluent over TiO₂ or GAC was no more effective on trace organic removal than over inert surfaces.     

Oxidation of Emerging Contaminants during Pilot-Scale Ozonation of Secondary Treated Municipal Effluent

The transformation of 41 target emerging contaminants in secondary treated municipal wastewater effluent in Canada was examined at pilot-scale, at transferred ozone doses of 2.8 mg/L (0.46 O₃/mg DOC) and 4.4 mg/L (0.72 mg O₃/mg DOC). In general, transformation efficiencies of CECs either increased or were retained at the higher ozone dose. The higher ozone dose of 0.72 mg O₃/mg DOC (Z_spec = 0.6 mg O₃/mg DOC) was sufficient to transform 21 of the 31 detected CECs by over 80% as well as achieving the disinfection target of < 200 MPN E. coli per 100 mL.     

Ozone Treatment of Secondary Effluent at U.S. Municipal Wastewater Treatment Plants

Ozone is a strong oxidant used to treat a variety of constituents in potable water, wastewater, water reuse, and industrial water treatment applications. Ozone is effective at oxidizing a wide range of organic and inorganic compounds and disinfection. Well-known in potable water treatment, with about 400 US installations and 3,000 world-wide, ozone has limited application at wastewater treatments, with less than 10 operating facilities in the US. The ability of ozone to significantly reduce low level concentrations of trace organic compounds, including endocrine disrupting chemicals (EDCs), pharmaceuticals and personal care products (PPCPs), and other emerging contaminants have increased interest in applying ozone in potable water and wastewater treatment. Treating at the point source discharge rather than the water supply intake may be more effective. A recent American Water Works Research Foundation (AwwaRF) report indicated high removals of many EDCs and PPCPs at typical disinfection doses. Several wastewater utilities have installed or are in the process of installing ozone to treat secondary effluent. These utilities are using ozone in a variety of ways: as a primary disinfectant, for treatment of microconstituents, and in combination with other processes (e.g. membranes and UV) to produce high-quality water for indirect potable reuse (IPR). The different applications, treatment goals and basis of process selection are compared and contrasted. Secondary benefits of ozone treatment of secondary effluent, including the use of off-gas in biological treatment is also discussed.     

Photoredox processes in the Cr(VI)–Cr(III)–oxalate system and their environmental relevance

Irradiation of the [Cr(C₂O₄)₃]³⁻ complex or the chromate(VI)–oxalate mixture, or the ternary system composed of Cr(III), Cr(VI) and oxalate, leads to chromium photoreductions in consequence of the ligand to metal charge transfer (LMCT) excitations induced by artificial solar radiation. In the case of the Cr(III) complex, the photoreduction involves the innersphere electron transfer leading to the formation of the Cr(II) species and the C₂O₄⁻ radicals. On reacting with molecular oxygen, Cr(II) is oxidised to Cr(III) catalysing thereby the oxalate substitution reaction. Moreover, under specific conditions, Cr(II) can be also oxidised to Cr(VI). Chromate(VI) is not photoreducible, but in the presence of oxalate, or other sacrificial electron donor, the outersphere photoinduced electron transfer (PET) produces Cr(V) species and the C₂O₄⁻ radicals. This initiates a series of thermal reactions leading to the formation of Cr(III) and oxidized oxalate (CO₂). In the system composed of [Cr(C₂O₄)₃]³⁻ and chromate(VI), the acidic medium and anoxic conditions favour the Cr(VI) photoreduction, whereas alkaline oxygenated solutions assist the Cr(VI) photoproduction. When an approximately neutral solution equilibrated with the ambient air is irradiated intermittently, Cr(VI) is consumed and/or produced, accordingly to the time sequence of exposure and dark periods. The oscillations of Cr(VI) concentrations are accompanied by continuous oxidation of oxalate, playing the role of the sacrificial electron donor. The effects of solution pH, molecular oxygen, concentrations of reagents and cations on the reaction rates were investigated. The results of this paper revealed that the Cr(III)/Cr(VI) system under environmental conditions behaves as the photocatalytic one catalysing the oxidation of oxalate or other organic matter by molecular oxygen, contributing thereby to the abatement of pollution.     

Ozonation of Trace Nitrobenzene in Water in the Presence of a TiO2/Silica-Gel Catalyst

Heterogeneous catalytic ozonation was investigated for the degradation of nitrobenzene in the presence of TiO₂ supported on Silica-gel as a solid catalyst. The conditions in preparing the catalyst are experimentally optimized. The catalytic activity of the supported TiO₂ is strongly influenced by the calcination temperature. The TiO₂/Silica-gel calcined at 500 °C showed the highest activity for the degradation of nitrobenzene. An approximate increase of 21% in removal efficiency was achieved for catalytic ozonation compared with the case of ozonation alone. Nitrobenzene degradation was significantly influenced by the presence of carbonate and t-butanol, which confirmed that TiO₂/Silica-gel catalyzed ozonation followed a radical-type mechanism. Kinetic study demonstrated that catalytic ozonation is pseudo–first-order with no respect to the initial nitrobenzene concentration. The effect of catalyst dosage and pH on the oxidation efficiency of nitrobenzene was also investigated. Catalyst dosage exerted a positive influence on nitrobenzene removal, and nitrobenzene degraded more completely under neutral or basic conditions. Finally, the catalyst stability was tested through repeated experiments.     

Synergistic Role of Pumice Surface Composition in Hydroxyl Radical Initiation in the Catalytic Ozonation Process

This study investigated the catalytic ozonation of p-chloronitrobenzene (p-CNB) in aqueous solution by using different pumice surface compositions as catalysts. The results indicate that the surface hydroxyl groups of metal oxides on pumice play an important role in ozone decomposition to generate the hydroxyl radical (?OH). An increase in the mass ratio of SiO₂/metal oxides results in the acceleration of ozone adsorption on the catalyst surface. The synergistic mechanism confirms that SiO₂ induced ozone enrichment on the pumice surface and increased the probability of reaction between surface hydroxyl groups on metal oxides and ozone molecules to initiate ?OH from ozone decomposition and to stimulate p-CNB degradation.     

Effect of Media on Biofilter Performance Following Ozonation of Secondary Treated Municipal Wastewater Effluent: Sand vs. GAC

Ozone has been shown to be effective in the transformation of several chemicals of emerging concern that escape the wastewater treatment process, but there is concern whether toxic transformation products are formed. Two parallel biofilter columns with granular activated carbon (GAC) and filter sand following a pilot-scale ozone unit to treat secondary treated municipal wastewater were studied. Results show reduced wastewater genotoxicity following ozonation and further reduction following biofiltration. The BAC biofilter outperformed the sand biofilter in terms of reduction in both organics and genotoxicity. Biofilter performance correlated with biological indicators (dissolved oxygen reduction and effluent E. coli counts) but not with ATP bioactivity measurements. Limited bacterial (E. coli) regrowth was observed in treated effluent from both biofilters.     

Understanding the Effects of Ozonation on a Combined Municipal/Industrial Secondary Effluent

An increase in the biodegradability of secondary effluent after ozonation as measured by CBOD₅ (carbonaceous BOD₅), generally has been considered to be due to the conversion of non-biodegradable organic material. Experiments on a specific combined unicipal/industrial waste containing a high percentage of malting plant wastewater showed that an increase in CBOD5 realized at the waste treatment plant was due to a shift in the oxygen uptake curve. Ozonation was found to affect both non-biode-gradable and biodegradable organic matter and resulted in improved overall effluent quality despite the observed increase in CBOD₅. As an indication of effluent quality, the CBOD₅ test is felt to be inadequate.     

Cr(III) oxidation with lead dioxide-based anodes

A procedure for preparing PbO₂-based electrodes with a titanium substrate is proposed. A platinum underlayer is first deposited on Ti by metal organic chemical vapor deposition (MOCVD), followed by the electrodeposition of the PbO₂ layer. The prepared Ti/Pt/PbO₂ anodes were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD) before being used for oxidation of Cr(III) in sulphuric acid. The current efficiency was determined for that electrodes and the results were compared with those obtained with Pb/PbO₂ and Ebonex^®/PbO₂ electrodes with different pH conditions. The Ti/Pt/PbO₂ were found to have a very good electrochemical behaviour (current efficiency: φ=0.93 for pH 2), and may be used as dimensionally stable anodes for the oxidation of Cr(III).     

Kinetics of MIB and Geosmin Oxidation during Ozonation

Ozonation is an effective means for oxidation of two common earthy/musty odorants (MIB and geosmin) in drinking waters. Second order constants were experimentally determined between the two odorants with ozone and hydroxyl radicals (HO^?). Geosmin was oxidized faster than MIB. Under most surface water treatment conditions, hydroxyl radical mediated reactions dominate over ozone reactions during MIB or geosmin oxidation. MIB and geosmin oxidation increases with greater ozone dose, higher pH, higher temperature or addition of H₂O₂.     

Ozonation of Phenol-Containing Wastewater Using O3/Ca(OH)2 System in a Micro Bubble Gas-Liquid Reactor

The degradation of phenol in aqueous solution was investigated in an integrated process consisting of O₃/Ca(OH)₂ system and a newly developed micro bubble gas-liquid reactor. The effects of operating parameters such as Ca(OH)₂ dosage, reactor pressure, liquid phase temperature, initial phenol concentration and inlet ozone concentration on degradation and mineralization (TOC removal) were studied in order to know the ozonation performance of this new integrated process. It is demonstrated that the degradation and TOC removal efficiency increased with increasing inlet ozone concentration and increasing Ca(OH)₂ dosage before 2 g/L, as well as decreasing initial phenol concentration. The optimum Ca(OH)₂ dosage should exceed Ca(OH)₂ solubility in liquid phase. The reactor pressure and liquid phase temperature have little effects on the removal and TOC removal efficiency. When Ca(OH)₂ dosage exceeded 3 g/L, the degradation and TOC removal of phenol almost reached 100% at 30 and 55 min, respectively. The intensification mechanism of Ca(OH)₂ assisted ozonation was explored through analysis of the precipitated substances. The mechanism for Ca(OH)₂ intensified mineralization of phenol solution is the simultaneous removal of CO₃^2- ions, as hydroxyl radical scavengers, due to the presence of Ca²⁺ ions. Results indicated that the proposed new integrated process is a highly efficient ozonation process for persistent organic wastewater treatment.     

Porous Spherical Cellulose Carrier Modified with Polyethyleneimine and Its Adsorption for Cr(III) and Fe(III) from Aqueous Solutions

abstract An efficient porous spherical polyethyleneimine-cellulose (PEI-cell) absorbent was synthesized and char-acterized. The main influencing factors and adsorption mechanism for two typical metal i...     

Separation and concentration processes of solutions of Co(II), Ni(II), Cr(III), Fe(III) and Mn(II) by extraction with toluene-dissolved rosin

The extraction and stripping of Co(II), Ni(II), Cr(III) and Fe(III) from aqueous solutions by rosin dissolved in toluene has been investigated. Results obtained show that rosin is better extractant than abietic or n-lauric acids under comparable conditions. From these results, and the data of Mn(II) solvent extraction studied previously under the same conditions, a separation and concentration process for these five cations in aqueous solutions has been designed. Saturated solutions of Fe(III), Cr(III), Mn(II) and finally Co(II) and Ni(II) have been obtained successively by extraction and stripping, by addition of ammonium hydroxide to obtain the appropriate pH value, and by modifying adequately the organic phase/aqueous phase volume ratio.     

Highly selective solvent extraction of Zn(II) and Cr(III) with trioctylmethylammonium chloride ionic liquid

This study investigates the recovery of Zn(II) and Cr(III) from aqueous solutions based on solvent extraction with trioctylmethylammonium chloride [TOMA⁺][Cl-], commercialy named Aliquat 336. Single metal solutions and binary mixtures of both metals were considered. The effect of relevant operating conditions such as pH, contact time, initial concentration, O/A phase volumetric ratio, and temperature were evaluated. Additionally, loading capacity and stripping studies were performed. Results showed that [TOMA⁺][Cl^?] is an effective extracting agent for Zn(II), reaching maximum removal capacity at pH 1.8 and demonstrating fast extraction kinetics. Extraction efficiencies above 99% were achieved at 0.5, 0.75, and 1.00 O/A volumetric phase ratios for 0.1 g/L initial Zn(II) concentration. At 1 g/L and 10 g/L concentration, for the same O/A ratios, approximately 88% of the initial Zn(II) was extracted. In contrast, it was found that negligible amounts of Cr(III) were transferred to the [TOMA⁺][Cl^?] phase at the 1-5 pH range. Selectivity studies showed that Zn(II) removal is boosted in the presence of Cr(III), although no Cr(III) is extracted. [TOMA⁺][Cl^?] exhibited a high Zn(II) storage capacity, since after 25 loading cycles with 1 g/L, the loading capacity reached approximately 13.5 g/L, and after five loading cycles with 5 g/L, the capacity reached 19.4 g/L. Stripping tests revealed that NaOH is an efficient agent for the removal of Zn(II) from the ionic liquids, reaching 98.5% removal after two cycles, whereas HNO₃ is not a suitable agent, reaching less than 40% removal after three cycles. [TOMA⁺][Cl^?] revealed high potential for separating Zn(II) from Cr(III).     

Computer-Assisted Determination of Reaction Parameters for the Simulation of Micropollutant Abatement in Wastewater Ozonation

For the purpose of the simulation of the kinetics of the reaction of ozone with wastewater in a bubble column, the kinetics can be adequately described by two components and their respective rate constants. Ozone uptake and ozone steady states along a bubble column have been determined. A modeling procedure has been developed that allows us to describe this behavior and to simulate the degradation of micropollutants. For fast (diclofenac) and moderately fast (bezafibrate) reacting micropollutants this model describes their measured elimination well.     

混凝法深度处理城市二级出水技术研究

试验通过采用硫酸铝、聚合硫酸铝和聚合氯化铝铁等3种混凝药剂深度处理城市二级出水,经试验研究得到硫酸铝、聚合硫酸铝和聚合氯化铝铁的最佳投药量,并在这三种药剂各自的最佳投药量条件下进行深度处理效果试验比较,为混凝法在深度处理城市二级出水中的应用提供实验基础。     

Effective Ozonation of Secondary Effluents with Initial Ozone Demand

It is clarified that the ozone consumption during the initial stage of ozonation before dissolved ozone appears is the same value as that during the initial 20 seconds in batch mode experiment. This initial ozone consumption was defined as initial ozone demand (IOD) in this study. IOD of secondary effluent was shown to be 0.3–0.5 mgO₃/mgC₀. More than 90% of 17β-estradiol (E2) was reduced, and formation of ozonation by-products was controlled within this ozone demand. This state can be also characterized by reduction of the values of UV₂₅₄, fluorescence intensity at 345 nm (Excitation)/435 nm (Emission) and that at 240 nm/435 nm to 0.06 /cm, 35 and 15, respectively.     

Titanium (IV)-Improved H2O2/O3 Process for Acetic Acid Degradation under Acid Conditions

The effect of Ti(IV) on the degradation efficiency of acetic acid by O₃/H₂O₂ was investigated. The removal rate of acetic acid by O₃/H₂O₂ increased from 8.0% to 62.9% after 30 min when Ti(IV) was added to acetic acid solution at pH 2.8. The optimized parameters were as follows: the pH of acetic acid solution less than 5.0; the mass concentration ratios of H₂O₂ to Ti(IV), and to acetic acid about 40:1, and 3:50, respectively. The analysis of the kinetics of acetic acid degradation using the relative method showed that Ti(IV)/H₂O₂/O₃ produced more hydroxyl radicals than did H₂O₂/O₃.     

乳化液膜法处理含铬废水的研究

采用span-80-煤油－NaOH液膜体系对含Cr(VI)废水的处理进行了研究，讨论了不同操作条件对Cr(VI)去除率的影响，得出了提取Cr(VI)的最佳实验条件，实验结果表明：含Cr(VI)废水经液膜处理后，Cr(VI)的去除率可达98％。