Disinfectant efficacy of chlorite and chlorine dioxide in drinking water biofilms

The drinking water industry is closely examining options to maintain disinfection in distribution systems. In particular this research compared the relative efficiency of the chlorite ion (ClO2-) to chlorine dioxide (ClO2) for biofilm control. Chlorite levels were selected for monitoring since they are typically observed in the distribution system as a by-product whenever chlorine dioxide is applied for primary or secondary disinfection. Previous research has reported the chlorite ion to be effective in mitigating nitrification in distribution systems. Annular reactors (ARs) containing polycarbonate and cast iron coupons were used to simulate water quality conditions in a distribution system. Following a 4 week acclimation period, individual ARs operated in parallel were dosed with high (0.25mg/l) and low (0.1mg/l) chlorite concentrations and with high (0.5 mg/l) and low (0.25mg/l) chlorine dioxide concentrations, as measured in the effluent of the AR. Another set of ARs that contained cast iron and polycarbonate coupons served as controls and did not receive any disinfection. The data presented herein show that the presence of chlorite at low concentration levels was not effective at reducing heterotrophic bacteria. Log reductions of attached heterotrophic bacteria for low and high chlorite ranged between 0.20 and 0.34. Chlorine dioxide had greater log reductions for attached heterotrophic bacteria ranging from 0.52 to 1.36 at the higher dose. The greatest log reduction in suspended heterotrophic bacteria was for high dose of ClO2 on either cast iron or polycarbonate coupons (1.77 and 1.55). These data indicate that it would be necessary to maintain a chlorine dioxide residual concentration in distribution systems for control of microbiological regrowth.     

A field study evaluation for mitigating biofouling with chlorine dioxide or chlorine integrated with UV disinfection

The drinking water industry is continually seeking innovative disinfection strategies to control biofouling in transmission systems. This research, conducted in collaboration with the East Bay Municipal Utility District (EBMUD) in California, compared the efficacy of chlorine dioxide (ClO2) to free chlorine (Cl2) with and without pre-treatment with low-pressure ultraviolet (UV) light for biofilm control. An additional goal was to determine disinfection by-product (DBP) formation with each disinfection strategy. Annular reactors (ARs) containing polycarbonate coupons were used to simulate EBMUD's 90-mile aqueduct that transports surface water from a source reservoir to treatment facilities. ARs were dosed with chemical disinfectants to achieve a residual of 0.2 mg/L, which is a typical value mid-way in the aqueduct. The experiment matrix included four strategies of disinfection including UV/ClO2, ClO2, UV/Cl2 and Cl2. Two ARs acted as controls and received raw water (RW) or UV-treated water. The data presented show that the UV/ClO2 combination was most effective against suspended and attached heterotrophic (heterotrophic plate count, HPC) bacteria with 3.93 log and 2.05 log reductions, respectively. ClO2 was more effective than Cl2 at removing suspended HPC bacteria and similarly effective in biofilm bacterial removal. UV light alone was not effective in controlling suspended or biofilm bacteria compared to treatment with ClO2 or Cl2. Pre-treatment with UV was more effective overall for removal of HPC bacteria than treating with corresponding chemical disinfectants only; however, it did not lower required chemical dosages. Therefore, no significant differences were observed in DBP concentrations between ARs pre-treated with UV light and ARs not pre-treated. Disinfection with ClO2 produced fewer total trihalomethanes (TTHMs) and haloacetic acids (HAAs) than chlorination but did produce low levels of chlorite. These data indicate that replacing Cl2 with ClO2 would further control microbiological re-growth and minimize TTHM and HAA formation, but may introduce other DBPs.     

Enhanced disinfection efficiency of mechanically mixed oxidants with free chlorine

To the best of our knowledge, this study is the first investigation to be performed into the potential benefits of mechanically mixed disinfectants in controlling bacterial inactivation. The purpose of this study was to evaluate the disinfection efficiency of mechanically mixed oxidants with identical oxidant concentrations, which were made by adding small amounts of subsidiary oxidants, namely ozone (O3), chlorine dioxide (ClO2), hydrogen peroxide (H2O2) and chlorite (ClO2(-)), to free available chlorine (Cl2), using Bacillus subtilis spores as the indicator microorganisms. The mechanically mixed oxidants containing Cl2/O3, Cl2/ClO2 and Cl2/ClO2(-) showed enhanced efficiencies (of up to 52%) in comparison with Cl2 alone, whereas no significant difference was observed between the mixed oxidant, Cl2/H2O2, and Cl2 alone. This enhanced disinfection efficiency can be explained by the synergistic effect of the mixed oxidant itself and the effect of intermediates such as ClO2(-)/ClO2, which are generated from the reaction between an excess of Cl2 and a small amount of O3/ClO2(-). Overall, this study suggests that mechanically mixed oxidants incorporating excess chlorine can constitute a new and moderately efficient method of disinfection.     

ClO2消毒涉及毒性问题的探讨

对二氧化氯 Cl O2 消毒无机副产物亚氯酸盐 Cl O2 - 和氯酸盐 Cl O3- 来源进行了分析 ,高浓度的 Cl O2 、Cl O2 - 和 Cl O3- 的毒性主要与血液系统有关 ,在相当于人饮用 Cl O2 、Cl O2 - 和 Cl O3- 混合溶液 1 60倍的动物血液常规检测中未发现异常 ,Ames试验显示试验浓度的 Cl O2 、Cl O2 - 和 Cl O3-水溶液不具有致突变作用 .     

ClO2预氧化去除水源水中微量酚类研究

针对微量酚类污染物在液氯消毒工艺中会产生氯酚并导致严重嗅味的问题,采用ClO2预氧化的方法去除水源水中的微量酚类,从而控制消毒过程中氯酚的形成。先通过静态烧杯试验,再用全自动微型移动式平台对试验参数进行动态模拟。结果表明,在苯酚质量浓度为280μg／L的条件下,ClO2对苯酚的去除效果基本能满足《生活饮用水卫生标准》（GB5749-2006）要求,苯酚去除率达到90％以上;从ClO2投加量及预氧化时间方面得出了最佳工艺参数,即ClO2投加量为0．6～1．0mg／L,预氧化时间为30～60s。     

Zero-valent iron reduction of nitrate in the presence of ultraviolet light,organic matter and hydrogen peroxide

This paper describes the use of metallic iron (Fe(0)) powder for nitrate removal in a well-mixed batch reactor. Important variables explored include Fe(0) dosage (1-3g/L), UV light intensity (64-128 W), and the presence of propanol (20 mg/L as DOC) and H(2)O(2) (100-200 mg/L). Accumulation of ferrous ions released from the Fe(0) surface can be expressed by an S-curve, which involves lag growth phase, exponential phase, rate-declining phase, and saturation phase. The removal of nitrate increases with increasing Fe(0) dosage; however, the removal makes no difference as the Fe(0) dosage is greater than 2 g/L. UV irradiation retards the dissolution of ferrous ion and the removal of nitrate. The species of propanol, which has a functional group of -OH, plays a role of organic inhibitor for Fe(0) corrosion. The presence of H(2)O(2) appears to inactivate all reactions as the Fe(0) of 10 microm was used; the final H(2)O(2) remains intact throughout the entire reaction period, and there were no removal of nitrate and no dissolution of ferrous ion. Surprisingly, with the use of a larger Fe(0) particle size of 150 microm, the H(2)O(2) was seen to decompose rapidly through Fenton reaction. Nevertheless, the rate of ferrous accumulation or nitrate removal is slow.     

Reduction of ClO2 demand by ClO2 oxidation and subsequent GAC filtration

The effective removal of soluble natural organic matter (NOM) during water treatment can significantly decrease the ClO(2) demand of processed water. This can be achieved through the oxidation and/or the removal of NOM. The purpose of the study was to examine the influence of ClO(2) oxidation and subsequent GAC filtration on the final ClO(2) demand of treated water. The study showed the ClO(2) demand to be strongly correlated with the abundance of high molecular fractions of NOM in treated water. As it was shown, this part of NOM was effectively removed during the GAC filtration. Moreover, the pre-treatment of water with ClO(2) considerably increased the total capacity of GAC filters for organic and inorganic (i.e. chlorites) oxidation by-products. Therefore, the oxidation of NOM molecules in conjunction with a very efficient GAC filtration can be successfully employed to control the abundance of high molecular NOM components, and thus the ClO(2) demand of finished water.     

Removal of organic pollutants from industrial wastewater by electrogenerated Fenton's reagent

This study was performed to investigate the treatment of an industrial wastewater mainly containing naphthalene- and anthraquinone-sulphonic acids, by electrogenerated Fenton's reagent. The hydrogen peroxide was produced in situ by electrochemical reduction of oxygen on graphite-felt cathodes and the Fe2+ ions were also regenerated by cathodic reduction of Fe3+. The influence of cathode potential, Fe2+ concentration and electrode surface pre-treatment on chemical oxygen demand (COD) removal and colour fading were studied. Results indicated that the higher COD removal was obtained in the presence of 3 mM of ferrous ions working at a constant potential of -1 V vs. SCE. Moreover, it was shown that both chemical and electrochemical pre-treatments of the cathode surface resulted in a decrease of COD depletion.     

Biodegradability of organic by-products after natural organic matter oxidation with ClO2--case study

Apart from well-known chlorites and chlorates, chlorine dioxide also generates easily biodegradable carbonyl compounds and short chain carboxylic acids during water disinfection. The main goal of the presented study was to examine the influence of natural organic matter (NOM) oxidation with chlorine dioxide, on the quantity as well as the quality of formed biodegradable by-products. In the experiments conducted at the pilot plant the sand filtered water (MWI) and ozonated/biofiltrated water (BAF) were oxidised with ClO2. The amount of BDOC formed as a result of the oxidation of both waters with ClO2 was compared. The results showed considerable differences in formation of ClO2 oxidation by-products between non-ozonated and ozonated/biofiltered waters. The disinfection of ozonated/biofiltrated water with ClO2 generated comparable amounts of aldehydes and much higher amounts of carboxylic acids than ClO2 oxidation of sand filtered water. These findings are essential for waterworks with ozonation/biofiltration units and ClO2 disinfection implemented.     

Mechanisms of virus control during iron electrocoagulation--microfiltration of surface water

Results from a laboratory-scale study evaluating virus control by a hybrid iron electrocoagulation - microfiltration process revealed only 1.0-1.5 log MS2 bacteriophage reduction even at relatively high iron dosages (∼13 mg/L as Fe) for natural surface water containing moderate natural organic matter (NOM) concentrations (4.5 mg/L dissolved organic carbon, DOC). In contrast, much greater reductions were measured (6.5-log at pH 6.4 and 4-log at pH 7.5) at similar iron dosages for synthetic water that was devoid of NOM. Quantitative agreement with Faraday’s law with 2-electron transfer and speciation with phenanthroline demonstrated electrochemical generation of soluble ferrous iron. Near quantitative extraction of viruses by dissolving flocs formed in synthetic water provided direct evidence of their removal by sorption and enmeshment onto iron hydroxide flocs. In contrast, only approximately 1% of the viruses were associated with the flocs formed in natural water consistent with the measured poor removals. 1-2 logs of virus inactivation were also observed in the electrochemical cell for synthetic water (no NOM) but not for surface water (4.5 mg/L DOC). Sweep flocculation was the dominant destabilization mechanism since the ζ potential did not reach zero even when 6-log virus reductions were achieved. Charge neutralization only played a secondary role since ζ potential → 0 with increasing iron electrocoagulant dosage. Importantly, virus removal from synthetic water decreased when Suwanee River Humic Acid was added. Therefore, NOM present in natural waters appears to reduce the effectiveness of iron electrocoagulation pretreatment to microfiltration for virus control by complexing ferrous ions. This inhibits (i) Fe²⁺ oxidation, precipitation, and virus destabilization and (ii) virus inactivation through reactive oxygen species intermediates or by direct interactions with Fe²⁺ ions.     

Microbial reduction of perchlorate in pure and mixed culture packed-bed bioreactors

Perchlorate (ClO4-) has been detected in a large number of surface and ground waters in the US. Due to health concerns of perchlorate in drinking water, the California Department of Health Services has established a provisional action level of 18 microg/L. Several microbial isolates have been obtained capable of microbiological perchlorate reduction through cell respiration, but few of these have been tested for perchlorate removals to these low levels. The feasibility of using one isolate (KJ) for water treatment was tested in a packed-bed bioreactor by comparing minimum detention times necessary to achieve complete removal of perchlorate. Perchlorate was reduced approximately from 20 mg/L to non-detectable (< 4 microg/L) levels in acetate-fed columns inoculated with KJ or mixed cultures. The complete conversion of perchlorate to chloride was demonstrated by a stoichiometric ratio of perchlorate to chloride of 1.0 +/- 0.14. Perchlorate removal to non-detectable levels required a minimum empty bed contact time (EBCT) of only 2.1 min for the column inoculated with KJ, vs. 31 min for the mixed culture column. Acetate was used at a molar ratio of C2H3O2-/ClO4- of 2.9 (n = 6) for the mixed culture, while more than twice as much acetate was consumed on average (6.6 +/- 2.0, n = 156) by the pure culture. These results demonstrate that detention times of packed-bed bioreactors can be substantially reduced using isolate KJ, but that larger concentrations of acetate will be necessary to reduce perchlorate to low levels necessary for drinking water.     

Removal of selenocyanate from water using elemental iron

Batch experiments were conducted to investigate the removal of selenocyanate (SeCN-) from oil refinery wastewater and artificial wastewater with elemental iron [Fe(0)]. The chemical forms of selenium in the reacted solids were determined with X-ray photoelectron spectroscopy (XPS) and a sulfite extraction procedure. SeCN- was effectively removed from the wastewater with Fe(0) filings when the water pH was controlled at approximately 6. SeCN- was removed by Fe(0) through the formation of elemental selenium [Se(0)] and ferrous selenide. The possible chemical reactions between SeCN- and Fe(0) included deselenation of SeCN- and electrochemical reduction of Se(0) to selenide. A cost-effective process may be developed for the treatment of SeCN- in wastewater using Fe(0).     

Photodegradation of 2,4-dichlorophenoxyacetic acid in various iron-mediated oxidation systems

The oxidation of herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) by different iron-mediated processes, with or without the presence of ultraviolet (at 253.7 nm) and oxalate, was investigated and compared. The initial decay rate and the overall removal percentage were used as the performance indexes. To extensively explore the associated processes, the following combinations or blank systems were investigated: UV radiation only, Fe(2+)/H(2)O(2), Fe(2+)/H(2)O(2)/UV, ferrous oxalate/H(2)O(2), ferrous oxalate/H(2)O(2)/UV, Fe(3+)/H(2)O(2), Fe(3+)/H(2)O(2)/UV, ferrioxalate/H(2)O(2), and ferrioxalate/H(2)O(2)/UV. This study showed that the degradation of 2,4-D by sole UV or dark processes (without UV) is generally slow, except by the conventional Fenton's process (Fe(2+)/H(2)O(2)). However, these slow reactions can be accelerated by exposure to UV irradiation, which can increase the initial 2,4-D decay rate from ten to more than one hundred times. Furthermore, if the reaction is initiated by ferrous oxalate or ferrioxalate instead of Fe(2+) or Fe(3+) ions, the rates can be further improved, because of the higher light sensitivity of the organometallic complexes. These reactions were also found sensitive to the initial hydrogen peroxide concentration. The competition of hydroxyl free radicals by the primary intermediate, 2,4-dichlorophenol, was also observed.     

新生态铁锰氧化物的混凝及强化混凝效能分析

采用FeSO4与KMnO4反应制备成新生态铁锰氧化物,并对松花江水进行处理。试验结果表明：新生态铁锰氧化物较新生态二氧化锰的除污效果好,但新生态铁锰氧化物的除污能力会随着放置时间的延长而逐渐下降;在不同温度和浊度的松花江水的混凝试验中,浊度低时新生态铁锰氧化物对水中有机物的去除效果较好,温度对其混凝效果影响较小;利用新生态铁锰氧化物强化硫酸铝的混凝过程,能够明显提高对低温、低浊水中有机物的去除效果,对UV254的去除率较单独投加硫酸铝时几乎提高了1倍,对TOC的强化去除作用在混凝剂投量低时更为明显。     

Removal of arsenic using hardened paste of Portland cement: batch adsorption and column study

Hardened paste of Portland cement (HPPC) has been used as a low-cost adsorbent for the removal of arsenic from water environment. Results from the batch experiments, conducted at an initial concentration of 0.2 ppm of arsenate, suggest arsenate removal up to 95%. Kinetic profiles were developed for various conditions. Effects of adsorbent dose, common ions such as Ca(2+), Mg(2+), Fe(3+), Fe(2+), Cl(-), SO(4)(2-), NO(3)(-), PO(4)(3-) and of pH were studied in detail. Adsorption isotherm studies revealed that the Freundlich isotherm was followed with a better correlation than the Langmuir isotherm. Arsenite could also be removed up to approximately 88% using the same material, HPPC. Finally, column studies were undertaken involving the new HPPC to check the suitability of the material for the removal of total arsenic content from water body. Kinetic experiments for the removal of arsenic by column studies revealed a film diffusion mechanism.     

Removal of phosphate by layered double hydroxides containing iron

Iron-based layered double hydroxides (M2+(a)Fe3+(b) (OH)2(a+b) CO3(2-) b/2mH2O) were synthesized. Removal of phosphate by the compounds was studied from the viewpoint of buffering pH effect of the compounds and buffering capacity of solution. The compounds released metal cations (Mg2+, Ca2 , Fe3+) and/or their hydroxides responding to various water environments due to their buffering pH function. The released cations and/or hydroxides worked effectively as coagulants for the phosphate removal. The removal of phosphate depended on the buffering capacity of the solution that is the function of the solution pH and the concentration of phosphate. The removal of phosphate from the solution with small buffering capacity followed a Langmuir-type isotherm. The removal of phosphate from the solution with larger buffering capacity was largely increased. The removal of phosphate by the compounds was analyzed based on the model describing the buffering pH effect of the compounds from which the amount of released cations (CB) can be determined. The removal was well correlated with the amount of dissolution of the compounds, CB. The mechanism of phosphate removal was examined based on the removal efficiency (mol of removed phosphate/mol of released alkali). The efficiencies showed below one in the solution with large buffering capacity and above one in the solution with small buffering capacity. The efficiency below one showed the removal of phosphate through coagulation by the released metal cations and hydroxides. The successful removal of dilute phosphate (0.2mg P/l) from the drain water was also demonstrated.     

生物滤层同时去除地下水中铁、锰离子研究

迄今为止的除铁、除锰工艺大多是采用传统的处理方法,即：一级除铁、二级除锰。在生物固锰、除锰技术确立之后,通过长期的试验研究,发现了铁、锰离子可以在同一生物滤层中去除的规律,从而可以采用一级曝气、过滤的简缩流程以更新传统的二级曝气、过滤的长流程设计。     

Evaluation of an MBR-RO system to produce high quality reuse water: microbial control, DBP formation and nitrate

A membrane bioreactor and reverse osmosis (MBR-RO) system was developed to assess potential reuse applications of municipal wastewater. The objective of the study was to examine the water quality throughout the system with a focus on waterborne pathogens, disinfection by-products (DBPs) and nitrate. This paper will discuss the presence of these contaminants in MBR effluent and focus on their subsequent removal by RO. This study has shown that high quality reuse water can be produced from municipal wastewater through the use of an MBR-RO system. The water meets California Title 22 reuse regulations for non-potable applications and US EPA drinking water limits for trihalomethanes (THM) (80 microg/L), haloacetic acids (HAA) (60 microg/L), chlorite (1.0 mg/L), total coliform (not detectable), viruses (not detectable), and nitrate/nitrite (10 mg N/L). However, THM formation (182-689 microg/L) attributed to cleaning of the MBR with chlorine and incomplete removal by subsequent RO treatment resulted in reuse water with THM levels (40.2+/-19.9 microg/L) high enough to present a potential concern when considering drinking water applications. Nitrate levels of up to 3.6 mg N/L, which resulted from incomplete removal by the RO membrane, are also a potential concern. A denitrification step in the MBR should be considered in potable water applications.     

Electro-Fenton degradation of synthetic dyes

The electrochemical removal of a synthetic solution containing 120 mg L⁻¹ of alizarin red has been studied by electro-Fenton process using a gas-diffusion cathode to produce in situ hydrogen peroxide by oxygen reduction. The effect of operating conditions such as Fe²⁺ concentration, applied current, solution pH and temperature on the efficacy of the process was investigated. It is shown that alizarin red and its products may be effectively degraded by the OH radicals produced by the reaction between the Fe²⁺ ions and the electrogenerated H₂O₂.After 4 h of electrolysis COD removal was only 45% when no ferrous ions were added to the solutions, while the presence of ferrous ion greatly improved COD removal up to more than 90%. In particular 1.0 mM was the optimal concentration of ferrous ions and the single step or the stepwise addition of Fe²⁺ ions enables the same COD removal. The oxidation rate increased with increasing of current density and temperature and with decreasing of pH. The UV-vis analysis indicated that the discoloration of the solution occurs simultaneous with the destruction of aromatic rings and alizarin red is oxidised firstly to colourless intermediates (mainly phthalic acid, small carbonyl species) and then to carbon dioxide.     

二级氧化工艺预处理对硝基苯甲酸废水的研究

以对硝基苯甲酸废水为处理对象,分别考察了O3／GAC、ClO2／GAC工艺以及二者的组合工艺对有机物的去除效率和改善废水可生化性的效果。结果表明,O3／GAC工艺的最佳O3投量为400mg／L,ClO2／GAC工艺的最佳ClO2投量为300mg／L;单级氧化工艺处理出水的有机物浓度仍较高,不能满足后续生化处理对进水水质的要求;O3／CAC-ClO2／GAC组合工艺的处理效果优于ClO2／GAC-O3／GAC组合工艺,其对COD的去除率可达75％左右,并使BOD5／COD值由原水的0．10升高到0．46,提高了废水的可生化性,减轻了后续生化处理的负荷,是对硝基苯甲酸废水的有效预处理方法。