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大于10时歧化成有一定氧化能力的氯酸盐和氯酸盐离子，但据报道如果浓度太高就会对身体有害。在溶液中二氧化氯是不稳定的，即使在氧化过程中二氧化氯有比导致阴离子占主导地位的亚氯酸盐和氯酸盐更强的氧化能力。根据方程（6）和（8），二氧化氯的实际浓度依靠氯、亚氯酸盐和氯酸盐的存在，他们的浓度是由溶液的pH值决定的。因此，pH值是二氧化氯、氯酸盐和氯酸盐浓度至关重要的控制因素。后两个有害离子能被很快去除,pH值在5～7时通过减少代理例如二氧化硫-亚硫酸盐离子。二价铁可用于去除水中的亚氯酸盐和在pH值5～7时使氧化还原反应更加迅速。很明显分解在酸性条件下比在碱性条件下更好，因为在碱性条件下二氧化氯的歧化数量通过反应被消耗。对于饮用水处理，有建议指出0.8毫克/升的氯气和0.5毫克/升的二氧化氯混合物将达到消毒和控制三卤甲烷的形成在优先使用纯二氧化氯。根据美国环保局用水标准，二氧化氯的残留量被限制在0.8毫克/升，它将趋于0.4毫克/升的目标。 4.消毒剂对病原体的控制
人类病原体是通过水体被传播的包括细菌、病毒和原生动物。通过水体传播的生物体通常生长在肠道内和遗弃在粪便壳体内。因此，他们易感染。假如水不妥善处理，在水被消耗时病原体进入一个新的宿主，水供应的粪便污染可能再次发生，因此，即使是只包含少量的致病微生物，它也可能会传染的。大部分水源性疾病的爆发是由于处理系统中的故障或是管道中快速污染的结果。
令人关注的微生物是那些可能引起人类不适、生病或疾病的。这些微生物是由众多致病细菌、病毒、藻类和原生动物等组成。消毒效率的测量作为囊肿灭活的精确水平。原生动物囊肿是最难以摧毁的。如果对囊肿灭火要求的条件得到满足，细菌和病菌灭活就被认为是足够的。因此对水的消毒水质量标准已经对微生物做了规定，通常采用原生动物孢囊作为指标。因此病毒将被完全控制在相同的运行条件下满足原生动物孢囊的灭活要求。普遍发现饮用水污染是由原生动物引起的，在日常的家畜中它是一个重要的肠道致病菌，这种污染爆发的来源是很可能的。
有两个最重要的原生动物-隐孢子虫和贾第虫包囊已知疾病的爆发，他们往往是在大自然和饮水池中被发现。形成保护阶段的原生动物像卵囊，允许他们在水中长期生存同时等待被主体吸收。原生动物孢囊因为他们的体积和密度小所以
不能通过水存储被有效去除。隐孢子虫卵囊有一个0.5微米/秒的速度调整。如果水箱深2米，它将采取卵囊46天解决底部。贾第虫囊肿是非常大的，它有一个5.5微米/秒的非常快的调整速度。很明显然氯和氯胺对于抵抗隐孢子虫卵囊是无效的，对氯有抵抗力这个发现是令人惊讶的，只有臭氧和二氧化氯是适合的消毒剂。调查证实，隐孢子虫具有较高的抗氯能力，甚至是耐氯性贾第虫的14倍，因此，在过去消除它的方法依靠沉淀和过滤。沃森研究原生动物消毒法，内容如下：
K = Cηt (9)
在公式中：
K —在固定的pH和温度条件下一个给定的微生物被消毒剂处理的常数； C —消毒剂的浓度（毫克/升）； η—稀释经验系数；
t —要求达到一定灭活比例的时间。
对于预氧化和有机物的减少，根据污染物在水中的特点接触时间15～30分钟推荐用量是0.5～2.0毫克/升。在后消毒的情况下，二氧化氯的安全用量为0.2?0.4毫克/升。在这些剂量下，潜在的副产品亚氯酸盐和氯酸盐不构成任何健康危害。消毒剂浓度和接触时间之间的关系可以通过使用以实验数据为基础的总碳酸盐（Ct）产物来建立。基于消毒剂的温度，pH值和接触时间消毒剂的效率能够被估计。由于在美国和英国隐孢子虫已经成为一个调整中介的焦点，控制这种病菌的前景显得更为可观。通过对贾第鞭毛虫和隐孢子虫囊肿使用臭氧，二氧化氯，氯和氯胺，Ct值得比较列于表5，对一些微生物消毒被显示在表6。
表.5CT值（毫克?分钟/ L）对于通过使用4种消毒剂隐孢子虫和贾第虫包囊的消毒
原生动物臭氧 二氧化氯氯 氯胺 贾第虫包囊(pH6. 9 , 5 ℃) 0. 34.316～47 365 隐孢子虫包囊(pH7 , 25 ℃) 5～10 7872022200平均Ct值在pH为7和温度为5°C时是11.9毫克·分钟/升，在pH为7和温度为25°C时下降至5.2毫克·分钟/升。高温通常能增强消毒的效果，但低温达到对应效果需要额外的接触时间或过量的消毒剂。ClO2的最佳性能是在pH为9和温度为25°C的条件下生成Ct产物的速率为2.8毫克·分钟/升。二
氧化氯对隐孢子虫卵囊似乎
表.6通过使用4种消毒剂对各种微生物的灭活产物Ct数值区间的比较
微生物 臭氧 二氧化氯氯 氯胺 (pH6～7)(pH6～7) (pH6～7)(pH8～9)大肠杆菌 0.020.4～0.750.034～0.0595～180 脊髓灰质炎0.1～0.2 0.2～6.7 1.1～2.5768～3740 轮状病毒 0.006～0.06 0.2～2.10.01～0.05 3806～6476 囊肿G的鞭毛虫 0.5～0.6 26 47～150 2200 G的鞭虫囊肿1.8～2.0 7.2～18.5 30～630 1400 平均Ct值在pH为7和温度为5°C时是11.9毫克·分钟/升，在pH为7和温度为25°C时下降至5.2毫克·分钟/升。高温通常能增强消毒的效果，但低温达到对应效果需要额外的接触时间或过量的消毒剂。ClO2的最佳性能是在pH为9和温度为25°C的条件下生成Ct产物的速率为2.8毫克·分钟/升。二氧化氯对隐孢子虫卵囊似乎比氯或氯胺更有效。在pH为7，温度为25°C的条件下卵囊暴露一小时还原脱囊从87%到5%。基于此，统计计算Ct产物为78毫克·分钟/升。然而，对于臭氧做这个实验是检查5～10毫克·分钟/升Ct产物从观测与浓度1毫克/升的臭氧反应五分钟后脱囊从84%减少到0%。如同其他消毒剂一样，随着温度降低减少Ct值和改善杀孢囊反应。随着温度意外降低使Ct值从高达6.35毫克·分钟/升在pH为5减少到低于2.91毫克·分钟/升在pH为9。一般规则，即对原生动物臭氧时最好的杀孢囊剂，二氧化氯优于氯和碘，但总体上氯优于氯胺。
虽然臭氧的消毒效率比二氧化率高，但是这种差异可以通过接触时间来补偿。实验表明，假如时间持续足够长二氧化氯对于大肠菌群的消毒能达到和臭氧相同的效果，在数据表1中已被列出。臭氧和二氧化氯的附加浓度都是2毫克/升。
在饮用水中隐孢子虫卵囊的控制要求一种综合多样的阻隔途径。在沉淀和过滤过程中凝结物是隐孢子虫有效控制的关键。（溶气气浮能完成3个检测日的卵囊去除类似于大约1个检测日的沉淀。溶气气浮和过滤提供两种有效屏障去清除隐孢子卵囊4个累积检测日清除量，他们类似于沉淀和过滤的3到4个检测日清
除量）。
图表1臭氧和二氧化氯对大肠菌群消毒效率的比较 5.饮用水消毒的趋势
在未来，生产低细菌，低气味水的负担将被分配到一系列联合的阶段，包括水源保护、混凝—絮凝—沉淀、过滤、气浮、膜处理和吸附。使用氯、二氧化氯、臭氧、紫外线或其他媒介终端处理的一些形式也将是必须的。没有单一的步骤可以或应该被期待承担整个负担去控制某一污染物。随着技术的发展，新的化学和物理代理品将满足在水处理中使用的实用性测试，并且减少病原体。这些可能包括电磁场和其他使用光或声波的能量处理的方式。
鉴于实用性、有效性、可操作性和成本，在所有目前使用的这些消毒工艺中应优先考虑紫外线消毒方式，特别是在发展中国家。这种中低压紫外线存在的巨大潜能适应于各种规模的供水厂。研究已证实非常低的紫外线剂量可以十分有效的灭活卵囊。此外，中低压灯的比较证明没有显著差异。通过在3、6和9毫焦/平方厘米剂量下使用低压紫外线，卵囊灭活水平生成量在3.4—3.7观测日。在超过1浊度水的紫外线试验中，中压紫外线的消毒能力没有受到影响，而且卵囊的高效灭活仍然可以实现。 6.结论
对于净化饮用水，因为二氧化氯的高消毒效率、稳定性、低成本、和使用方法简单等特点，它可以被选择替代氯、臭氧和其他消毒剂。对于确保从水储存的粪便污染物产生的任何微生物被消灭，臭氧和二氧化氯都是很非常重要的。二氧化氯的使用发现它能有效抑制原生动物的生长和杀灭细菌和病毒。以原生动物包囊为指标，隐孢子虫卵囊对氯有牢固的抵抗力，但是对于毁灭包囊二氧化氯可能是合适的消毒剂。因此，在对于原生动物包囊的灭活要求相同的操作条件下，病毒将被二氧化氯完全控制。实验表明，虽然臭氧比二氧化氯的消毒效率高，但是假如时间持续足够长二氧化氯对于大肠菌群的消毒能达到和臭氧相同的效果。
很显然在水处理中对于有效去除油和苯的同源化合物二氧化氯是优先选择，同时它能限制诱导有机体突变和有毒物质的形成。降解率与输入氧化剂的数量和运行时间的增加成比例。总之，剂量输入建议范围为0.5~2.0毫克/升。反应发生的有效pH是在弱酸范围5—7条件下亚氯酸盐和氯酸盐的形成被限制。二氧化
氯气体应该被直接注入处理水体中，使ClO2在溶液中维持较高的浓度。在此条件下，对有机污染物的去除率将会更高。对于存储系统，由于处理水复杂的污染物，二氧化氯浓度的输入量应该比实验研究浓度高。
Study on Disinfection and Anti – microbial Technologies for Drinking Water
ZHU Kun, FU Xiao Yong
(Dept. of Environmental Engineering, LAN Zhou Railway University, LAN Zhou 730070, China)
Abstract: Disinfection by-products produced by the reaction between chlorine and dissolved organic compounds and other chemicals are considered as a worrying problem in the drinking water treatment process since a series of mutagenic carcinogen substances are formed including trihalomethanes (THMs). Among the tested disinfectants(chlorine , ozone , chlorine dioxide , potassium permanganate , chloramines and hydrogen peroxide etc. ) , chlorine dioxide has proved to be the most feasible and effective oxidant for drinking water treatment and removal of pathogens due to its oxidation efficiency , low cost and simple way of utilization. A series of experiments indicate that chlorine dioxide can significantly restrain production of trihalomethanes (THMs) and control bacteria growth particularly for Cryptosporidium oocysts. The experiments verified that both ozone and chlorine dioxide are absolutely vital to ensure thtion of water storage are destroyed. The paper discusses oxidation capacity of chlorine dioxide, especially for removing petroleum compounds, which is affected by reaction time, gas injection way, and pH of treated water.
Key words: disinfection; oxidants; water treatment; pathogens; chlorine dioxide CLC number: X523 Document code: A 1Introduction
Chemical and filtration processes are two main methods used in China for treating drinking water meanwhile UV radiation has been used successfully for water treatment with relatively low flow rate. On the individual family level, usually chemical treatment is a feasible alternative. The following guidelines exist for the selection of suitablal of contaminants should be done by decomposition, evaporation or precipitation etc, to eliminate or decrease the toxicity, oxidants or reaction
by-products should not be harmful to human health, and the purification processes should be practical and economical. The objective of this paper is to evaluate and discuss available disinfectants for drinking water treatment. The different disinfectants are compared regarding purification efficiencies and application approaches.
2Comparison of
O3 > ClO2 > HOCl > OCl - > NHCl2 > NH2Cl
Referring to Fiessinger′s [2] suggestion, the properties of these disinfectants are compared in Tab. 1. Chlorine is shown to be an excellent disinfectant to prevent waterborne diseases such as typhoid fever over long periods. Chlorine reacts not only within oxidation, but also by electrophilic substitution to produce a variety of chlorinated organic by - products, particularly trihalomethanes (THMs) and other mutagens.
Here
THMs
mainly
refer
to
chloroform,
bromoform,
dibromochloromathane and bromodichloromathane etc. Since the 1970`s, the usage of Cl2 in drinking water disinfection has been questioned with ozone being substituted as the preferred disinfectant in the water supply plants. But , ozone could not be introduced to the rural farmer community due to its high costs and short half - life (15～20 min. ) . As with other disinfectants, ozonation also leads to formation of organic by - product s such as aldehyde, ketones, and carboxylic acids, and also mutagenicity may be induced if bromic anion exists. Tab. 1 Comparison of various oxidants
Comparison Cl2 ClO2 O3KMnO4 NH2Cl H2O2 THM formation + + + - - -- - Disinfection effects++ + ++ + ++ - + -+ Enhanced biodegradability++ + + + + - + Taste removal- ++ + + - + Iron and manganese++ + ++ + ++ - +
Ammonia+ + + - -- -- Formation of mutagens or toxic substances+ ++ - + - + -+ -+ - - no effect ;+ little effect ;+ + effect ;+ + + largest effect
Many studies have pointed out that disinfection is absolutely vital to ensure that any microorganisms arising from fecal contamination of water storage are destroyed. The selection of the available disinfectant s must concern to reduce risk from microbial contamination of drinking water and the potential increase in risk from chemical contamination that result from using any of the disinfectant s. The biocidal efficiency of commonly used disinfectants - ozone, chlorine dioxide, chlorine and chloramines are ranked almost with the same order as the oxidizing capacity, but the stability of those are following the order as [3]:
Chloramines > Chlorine dioxide > Chlorine > Ozone 3Purification of organic pollutants by chlorine dioxide
According to WHO guideline for drinking water quality, much consideration should be paid to benzene homologous compounds; therefore, the study on purification effect s of chlorine dioxide is focused on petrochemical pollutants. A series of experiment s were carried out to simulate the oxidation processes of contaminated water. The polluted solutions were prepared in a dark barrel (10L capacity) of seven kinds of benzene homologous compounds-Benzene , toluene , ethyl benzene , p-phenylmethane, o-phenylmethane, m-phenylmethane and styrene. Samples were taken to determine the initial concentration of the compounds prior to the test s. Standard chlorine dioxide solution was produced from sodium chlorite reacted with HCl solution of 10% [4]. The GR - 16A Gas - chromatograph with FID detector Shenyang LZ-2022 was used for measurement of Cl2, ClO2, ClO-2 and ClO-3[5]. Oil concentrations were determined with an UV -120-20 spectrophotometer (Shimadzu) following the procedure described by APHA [4]. Organic compounds in the water samples were measured with a GC-MS (QP-1000A). ClO2 and O3 were standardized by iodimetric titration at pH7.
For the purpose of chemical disinfection for drinking water, chlorine was instantaneously ignored due to the formation of THMs and other mutagenic substances. The results indicated that potassium permanganate and hydrogen peroxide did not have enough oxidation capability to decompose petroleum contaminant s achieving only 46 %, and 5.7% decomposition of styrene, respectively. Ozone could not be selected due to it s high cost, complex operation and short half-life although it is an excellent oxidant for water treatment. Chlorine dioxide was the next most successful alternative for disinfection. The benefit s include-effective oxidation capacity, algicidal effect and negligible formation of halogenated by-products. Based on economic and operational requirement, the mixing gas method is easily used. The results obtained suggest that disinfection of drinking water with ozone and or chlorine dioxide seems to be a suitable alternatives to the use of NaClO for cont rolling the formation of non-volatile mutagens[6].
In the laboratory experiments, the oxidants ozone, chlorine dioxide, potassium permanganate and the mixing gas (mainly contained ClO2 and a certain amount of Cl2, O3 and H2O2) were tested for removal of the petroleum compounds, and results are shown in Tab. 2.
Tab. 2 Comparison of oxidation capacity for the various oxidants
Organic Compounds Initial conc.O3ClO2H2O2Mixing Gas KMnO4 / mg·L – 1Oil11. 34 67. 245. 8061. 8 0 Benzene3. 61 78. 3 71. 40 82. 30 Toluene 5. 23 91. 883. 0095. 2 0 Ethyl benzene8. 37 95. 191. 1094. 5 0 p–phenylmethane 7. 86 95. 890. 50100 o-phenylmethane 8. 36 95. 990. 301000 m–phenylmethane9. 29 95. 487. 3 0 1000 Styrene 9. 36 96. 284. 7 5. 7 100 46. 1 A study was conducted to elucidate the decay pathway of monochloramine in the
presence and absence of natural organic matter (NOM) [7]. It was found that natural organic matter acted primarily as a reductant rather than catalyst. This conclusion was verified using a redox balance, and much of oxidizing capacity of monochloramine goes towards NOM oxidation. Cleaning agents and disinfectants from house keeping, hospitals, kitchens are sources of absorbable halogenated organic compounds (AOX) in municipal wastewater. The amount of AOX generated strongly depends on the nature and concentrations of dissolved and solid organic compounds, the concentration of active substances, temperature, pH and reaction time [8] When the mixing gases react with water molecules and organic micro-pollutants, hypochlorous acid is formed by chlorine, chlorite and chlorate ions are produced from chlorine dioxide in a series of redox reactions. The principal reactions are summarized as follows:
ClO2 + organic →ClO -2 + oxidized organic (1)
2ClO -2 + Cl2 = 2ClO2 + 2Cl - (2) 2ClO -2+ HOCl = 2ClO2 + 2Cl - + OH- (3) 2ClO2 + HOCl + H2O = 2ClO - 3 + HCl + 2H+ (4)
The rate of chlorate yield can be described by Equation (5):
d [ClO3]/ d t = 2 k [ClO2] [HOCl] (5)
in which k = 1.28 M/ min at 25 ℃ [9].
The stoichiometry of the undesirable reactions that form chlorate in low concentration of chlorite or presents of excess chlorine is given as:
ClO -2 + Cl2 + H2O = ClO - 3 + 2Cl - + 2H+ (6) ClO - 2 + HOCl = ClO - 3 + Cl - + H+ (7)
At alkaline conditions:
ClO -2 + HOCl + OH- = ClO - 3 + Cl - + H2O (8)
Typically, chlorine dioxide is used in drinking water treatment and the concentrations are ranging from 0.1 to 2.0 mg/L [10]. However, the relevant by - products of chlorine dioxide treatment-chlorite and chlorate have been found to induce methemoglobinemia in the human body when concentrations are more than 100 mg/L [11]. The oxidation results of the organic contaminants were affected by
reaction time. The initial concentrations and removal rate at different times are listed in Tab. 3. It is shown that chlorine dioxide has a very strong oxidation capability including the break down of the benzene ring. There are no other commonly used oxidants to do like this except for ozone.
Tab. 3 Removal rate of tested organic compounds at different operating time (at pH7) CompoundsInitial concern Removal rate/ % / mg·L – 1 2 min 10 min 15 min Oil 18. 12 45. 846. 947. 3 Benzene 41. 25 14. 141. 260. 7 Toluene 31. 75 17. 554. 986. 8 Ethylbenzene 16. 15 24. 763. 589. 8 p-phenylmethane10. 7525. 9 84. 9100 o-phenylmethane30. 2520. 9 79. 1100 m-phenylmethane33. 2022. 6100100Styrene62. 40100100100 The injecting method for chlorine dioxide gas into the solution also has an apparent influence on the removal rate. With the indirect method, the gas firstly was dissolved in a certain amount of distilled water, and then added to the tested organic solutions, as a result, removal rates appear lower than for the direct blowing method. The main reason for the difference is due to the conversion and decomposition of chlorine dioxide in the dissolving process before the reaction. It is confirmed from Tab. 3 that the removal rate was proportional to operating time. Since chlorine dioxide showed very strong oxidation capability for organic chemicals but was reduced to chlorite anion according to Equation (4), and the removal rate initially appeared quite high. Then, chlorite keeps the oxidation capacity at a level, which allows decomposition of the organic compounds to continue even though the oxidation reaction gradually became weaker with reaction time. The experiment indicated that pH values significantly influenced the removal rate of the organic compounds. The differences of degradation rates in a variety of pH through indirect input way are
shown in Tab. 4.
Tab. 4 Degradation rate of benzene homologous compounds with indirect method at different pH (after 15 min)
CompoundsInitial concern Removal rate/ %/ mg·L – 1pH5pH7pH10 Benzene 5 54. 0 48. 2 24. 1 Toluene 571. 7 63. 5 52. 6 Ethylbenzene 5 84. 2 78. 7 73. 5 p–phenylmethane 584. 477. 673. 0 o-phenylmethane 584. 1 80. 3 73. 7 m–phenylmethane584. 09. 870. 4 Styrene 5 100. 098. 790. 5 There are, however, some disadvantages with ClO2, such as easy loss from solution due to volatilization, and disproportionation above pH 10 into chlorate and chlorite ions that are of certain oxidation capacity, but reported to be harmful to health if the concentration is too high. Chlorine dioxide was unstable in the solution even though it has a stronger oxidation capability than chlorite and chlorate as the two resulted in anions being dominant in the oxidation processes. The actual concentration of chlorine dioxide depended on the existence of chlorine, chlorite and chlorate whose concentrations were determined by pH values of the solution according to Equations (6) and (8) respectively. Consequently, the pH is the critical controlling factor in the concentrations of chlorine dioxide, chlorite and chlorate. The latter two harmful ions can be removed quite quickly by treatment with a reducing agent such as sulfur dioxide - sulfite ion at pH values of 5～7[10 ,12]. Fe (II) can be used to eliminate chlorite from the water , and the redox reaction is kinetically more rapid at pH 5～7 as well[13]. It was evident that the decomposition in acidic conditions was much better than that in alkaline conditions because a disproportional amount of chlorine dioxide was consumed by the reactions under alkaline conditions. For drinking water treatment, it has been suggested that the mixture of chlorine 0.8 mg/L and chlorine
dioxide 0.5 mg/L will achieve disinfection and control THMs formation in preference to use of pure chlorine dioxide[14]. According to USEPA drinking water standard, the residue of ClO2 is limited as 0.8 mg/L that tends to the goal of 0.4 mg/L. 4Control of pathogens with disinfectants
Human pathogens that are transmitted by water including bacteria, viruses and protozoa. Organisms transmitted by water usually grow in the intestinal tract and leave the body in the feces. Thus, they are infections. Fecal pollution of water supplies may then occur, and if the water is not properly treated, the pathogens enter a new host when the water is consumed, therefore, it may be infectious even if it contains only a small number of pathogenic organisms. Most outbreaks of waterborne diseases are due to breakdowns in treatment systems or are a result of post contamination in pipelines.
The microorganisms of concern are those which can cause human discomfort, illness or diseases. These microbes are comprised of numerous pathogenic bacteria, viruses, certain algae and protozoa etc. The disinfection efficiency is typically measured as a specific level of cyst inactivation. Protozoan cysts are the most difficult to destroy. Bacteria and viral inactivation are considered adequate if the requirement for cyst inactivation is met. Therefore, water quality standard for the disinfection of water have been set at microorganisms, usually take the protozoan cysts as indicator, so viruses will be adequately controlled under the same operation conditions required for inactivation of protozoan cysts. The widely found drinking water contamination is caused by protozoan that is a significant intestinal pathogens in diary cattle, likely a source of this outbreak.
There are two of the most important protozoa - Cryptosporidium and Giardia cysts those are known to outbreak diseases, frequently are found in nature and drinking water storage ponds. Protozoa form protective stages like oocysts that allow them to survive for long periods in water while waiting to be ingested by a host. Protozoa cysts are not effectively removed by storing water because of their small size and density. Cryptosporidium oocysts have a setting velocity of 0.5 um/s. Therefore, if the water tank is 2 m deep, it will take the oocyst 46 days to settle to the
bottom. Giardia cysts are much large and have a great settling velocity of 5.5um/s. It was evident that chlorine and chloramines were ineffective against Cryptosporidium oocysts, which was discovered to be amazingly resistant to chlorine, and only ozone and chlorine dioxide may be suitable disinfectants [15]. The investigations have verified that Cryptosporidium is highly resistant to chorine, even up 14 times as resistant as the chlorine resistant Giardia, therefore methods for removing it in past rely on sedimentation and filtration. Watson′s Law to study protozoan disinfection, reads as follows:
K = Cηt (9)
In the formula:
K ——constant for a given microorganism exposed to a disinfectant under a fixed set of pH and temperature conditions; C ——disinfectant concentration (mg/ L); η——empirical coefficient of dilution ;
t ——time required to achieve the fixed percentage inactivation.
For the preoxidation and reduction of organic pollutants , the recommended dosages are between 0. 5～2. 0 mg/ L with contact time as 15～30 min depending on the pollutants characteristics in the water. In the case of post - disinfection , the safe dosages of ClO2 are 0. 2～0.4 mg/L. At these dosages, the potential by - products chlorite and chlorate do not constitute any health hazard [16]. The relation between disinfectant concentration and contact time can be established by using Ct products based on the experimental data. From this the effectiveness of disinfectants can be evaluated based on temperature, pH value and contact time. Since Cryptosporidium has become a focus of regulatory agencies in the United States and United Kingdom, the prospects of controlling this pathogen show more considerable. The comparison of the Ct values by using ozone , chlorine dioxide , chlorine and chloramines for Giardia and Cryptosporidium cyst s are listed in Tab. 5[17 ,18 ] , and for some microorganisms disinfection are displayed in Tab. 6[19 ] .
Tab. 5 Ct values (mg·min/ L.) for disinfection of Giardia and Cryptosporidium cysts by using 4 disinfectants
ProtozoaOzoneChlorine dioxide ChlorineChloramines Giardia (pH6. 9 , 5 ℃) 0. 34. 316～47365 Cryptosporidium (pH7 , 25 ℃) 5～1078 72022200Tab. 6 Comparison of value intervals for the product Ct (mg·min/ L) for the inactivation of various microorganisms by using 4 disinfectants
MicroorganismsOzoneChlorine dioxideChlorine Chloramines (pH6～7)(pH6～7)(pH6～7) (pH8～9)
E. Coli 0. 020. 4～0. 75 0. 034～0. 05 95～180 Polio 1 0. 1～0. 2 0. 2～6. 7 1. 1～2. 5768～3740 Rotavirus0. 006～0. 060. 2～2. 1 0. 01～0. 053806～6476 Cysts of G. Lamblia0. 5～0. 6 2647～150 2200 Cysts of G. muris1. 8～2. 07. 2～18. 530～630 1400 The mean Ct value for ClO2 at pH 7 and 5 ℃was 11. 9 mg·min/ L, and dropped to 5.2 at pH 7 and 25 ℃. High temperatures normally enhance the efficiency of disinfectants while lower temperatures have opposite effects requiring additional contact time or extra quantity of disinfectants. The best performance for ClO2 is at pH 9 and 25 ℃, which yields a Ct product of 2.8 mg·min/ L [20]. Chlorine dioxide appears to be more efficient for Cryptosporidium oocysts than either chlorine or monochloramine. Exposure of oocysts to 1.3 mg·min/ L at pH 7 reduces excystation from 87 % to 5 % in a hour at 25 ℃. Based on this result, Ct product of 78 mg·min/ L was calculated. However, the Ct product for ozone to do this work was examined as 5 - 10 mg·min/ L from observation that excystation decreased from 84 % to 0 % after 5 minutes with the ozone concentration of 1 mg/ L [15]. As with other disinfectants, increasing temperature decreased the Ct values and improved the cysticidal action. Increasing temperature unexpectedly reduced the Ct values from a high of 6.35 mg·min/ L at pH5 to a low of 2.91 mg·min/ L at pH 9[20]. It is generally the rule, that for protozoa ozone is the best cysticide, chlorine dioxide is superior to chlorine and
iodine, but chlorine, in overall, is much superior to chloramines [21].
Although disinfection efficiency of ozone is higher than chlorine dioxide, this difference can be compensated by the contact time. The experiment indicated that chlorine dioxide could reach the same results for disinfection of coliform bacteria as ozone did if time lasted long enough, which can be seen in Fig. 1. The added concentrations of both of ozone and chlorine dioxide were 2 mg/ L.
Control of Cryptosporidium oocysts in potable water requires an integrated multiple barrier approach. Coagulation is critical in the effective control of Cryptosporidium by clarification and filtration. Dissolved air floatation can achieve oocysts removal of 3 logs compared to about 1 log by sedimentation. Dissolved air floatation and filtration provide two effective barriers to Cryptosporidium oocysts with cumulative log removal of 4 to 5 compared to log removals of 3 to 4 by sedimentation and filtration [22].
Fig. 1 Comparison of disinfection efficiency between ozone and chlorine dioxide on coliform bacteria
5Tendency of disinfection for drinking water
In the future, the burden of producing water with low pathogen level and low tastes and odor will be allocated to a combination of steps, including source water protection, coagulation - flocculation - sedimentation, filtration, floatation, membrane processes and adsorption. Some form of terminal treatment with chlorine, chlorine dioxide, ozone, UV, or other agents will also be required. No single step can or should be expected to shoulder the entire burden to controlling a given contaminant. With the development of techniques, new chemical and physical agents will meet tests of practicability for use in water treatment and will reduce pathogens. These may include electromagnetic fields and other forms of treatment with light or sonic energy [23].
In light of availability, efficacy, operability and costs, the priority should be given to ultraviolet method among all of the currently utilized disinfection technologies, particularly in developing countries. The medium and low - pressure UV extends tremendous potential promise for adaptation into various scale water supply plants. The researches have validated that extremely low dosage of UV can be
highly effective for inactivate oocysts [24]. Furthermore, comparison of medium and low - pressure lamps demonstrated no significant differences. By using low - pressure UV at the dosage of 3 , 6 and 9 mJ/ cm2 , oocyst inactivation levels were yielded between 3.4 and 3.7 log. In the trials of UV in water with turbidity of more than 1 NTU, the ability of medium – pressure was not affected, and high level of oocysts inactivation could still be achieved. 6Conclusions
To purify drinking water, chlorine dioxide can be chosen instead of chlorine, ozone and other disinfectants because of it s advantages of high efficiency of disinfection, competent stability, low cost and simple utilizing way etc. Both ozone and ClO2 are absolutely vital to ensure that any microorganisms arising from fecal contamination of water storage are destroyed. The utilization of chlorine dioxide has been found to efficiently restrict protozoa growth, to disinfect from bacteria and viruses. Taking the protozoan cysts as indicator in which Cryptosporidium oocysts were solidly resistant to chlorine, but chlorine dioxide may be suitable disinfectants to mutilate. Thus, viruses will be adequately controlled by chlorine dioxide under the same operation conditions required for inactivation of protozoan cysts. The experiment indicated that chlorine dioxide could reach the same results for disinfection of coliform bacteria as ozone did if time lasted long enough although disinfection efficiency of ozone is higher than chlorine dioxide.
It is an obvious preference for chlorine dioxide to pragmatically remove oil and benzene homologous compounds in water treatment meanwhile the formation of mutagenic and toxic substances is limited. The degradation rate was proportional to input amount of oxidants and increase of operating time. The dosage input , in overall , is suggested to range between 0. 5～2.0 mg/ L. The effective pH at which reactions occur is in the slightly acid range of 5 to 7 at which formation of chlorite and chlorate is minimized. The chlorine dioxide gas should be injected directly into the treated water body, so that high concentrations of ClO2 can be kept in the solution. Under these conditions, the elimination rate for organic pollutants will be much higher. For the storage system, input dosage of chlorine dioxide concentration should be higher
than that in laboratory studies due to complex pollutants in treated water. References:
[1 ]Katz J . Ozone and chlorine dioxide technology for disinfection of drinking water [M]. Noyes New Jersey: Data Corporation, 1980.
[2]Fiessinger F. Organic micropollutants in drinking water and health [M] . Publisher, N. Y., U. S. A: Elsevier Sci., 1985.
[3 ]Hoff J C , Geldreich E E. Comparison of the biocidal efficiency of alternative disinfectants [C] . In Proceedings AWWA seminar, Atlanta, Georgia, 1980.
[4 ]APHA , American Public Health Association. American Water Works Association and Water Pollution Control Federation. Standard Methods for the Examination of Water and Wastewater. (16th Edition) [M]. Washington D. C., 1989. [5]Dietrich A M. Determination of chlorite and chlorate in chlorinated and chloraminated drinking water by flow injection analysis and ion chromatography[J ] . A nal. Chem., 1992, 64:496 - 502.
[6]Monarca S. Mutagenicity of extracts of lake drinking water treated with different disinfectants in bacterial and plant tests[J ] . Water Res, 1998, (32):2 689 - 2 695.
[7]Vikesland P , Ozekin K, Valentine R L. Effect of natural organic matter on monochloramine decomposition : pathway elucidation through the use of mass and redox balance[J ] . Envi ron. Sci. Tech., 1998, 32 (10):1 409 - 1 416.
[8]Schulz S , Hahn H H. Generation of halogenated organic compounds in municipal wastewater [M] . Proc. 2nd Int. Assoc. Water Qual. Int. Conf. Sewer Phys. Chem. Bio. Reactor, Aalborg, Denmark, 1998.
[9 ]Aieta E M. A review of chlorine dioxide in drinking water treatment [J]. J. A WWA, 1986, 78 (6): 62 - 72.
[10 ]Gordon G Minimizing chlorine ion and chlorate ion in water treatment with chlorine dioxide[J ] . J. A WWA, 1990, 82 (4):160 - 165.
[11]Kmorita J D , Snoeyink V L. Monochloramine removal from water by activated carbon[J ] . J. A WWA, 1985, (1):62 - 64.
[12]Gordon G, Adam I , Bubnis B. Minimizing chlorate information[J ] . J. A
WWA, 1995, 87, (6): 97 - 106.
[13]Iatrou A. Removing chlorite by the addition of ferrous iron[J ] . J. A WWA, 1992, 84 (11): 63 - 68.
[14 ]Schalekamp Maarten. Pre - and intermediate oxidation of drinking water with ozone, chlorine and chlorine dioxide [J]. J. Ozone Science and Engineering, 1986, 8: 151 - 186
[15 ]Korich D G, Mead J R , Madore M S , et al . Effects of ozone, chlorine dioxide, chlorine and monochramine on Cryptosporidium parvum oosyst viability [J]. Applied and Environmental Microbiology, 1990, 56: 1 423 - 1 428.
[16 ]AWWA Research Foundation. Chlorine dioxide; drinking water issues, 2nd International Symposium [R]. Houston, TX, 1992.
[17]Lykins B W, Griese H G. Using chlorine dioxide for trihalomethane control[J ] . J, A WWA, 1986, 71 (6): 88 - 93.
[18]Regli S. Chlorine dioxide , drinking water issues , 2nd International Symposium [ R ] . Houston, TX, AWWA Research Foundation, 1992.
[19]Hoff J C. Inactivation of microbial agents by chemical disinfectants[J] . US EPA, 1986.
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饮用水的消毒及杀菌技术研究
朱琨伏小勇
(兰州铁道学院环境工程系, 甘肃兰州 730070)
摘 要:饮用水处理消毒过程中可产生一系列致癌物质,主要是氯与水中的有机物和其它化学成分反应的结果,其中典型产物有三氯甲烷. 通过对常用消毒剂液氯,臭氧,二氧化氯,高锰酸钾,氯胺及过氧化氢的实验对比,证明二氧化氯是高效,方便,廉价的消毒剂. 它不仅对一般病原菌类有明显的抑制和杀菌作用,对清除难以灭杀的潜原性病毒也有理想的效果. 在净化水中石油类有机物时,二氧化氯的效果受到反应时间,注入方式和pH 值的影响. 关键词:消毒;氧化剂;水处理;病原菌;二氧化氯 中图分类号:X523 文献标识码:A
中文译文：
饮用水消毒和杀菌技术的研究
朱琨 伏小勇
（兰州铁道学院环境工程系，甘肃兰州，730070中国）
在饮用水处理过程中，通过氯与溶解性有机物和其他化合物的反应所产生的消毒副产物被看作一个令人担忧的问题，因为一系列诱变致癌的物质组成包括总卤甲烷。在被检测的消毒剂中（氯、臭氧、二氧化氯、高锰酸钾、氯胺、过氧化氢等），由于二氧化氯的高氧化效率，低成本以及简单的使用方法，曾经证明对于饮用水的处理和致病菌的去处，它是最可行有效的氧化剂。一系列实验表明二氧化氯能有效地抑制总卤甲烷的产生，控制细菌生长特别是隐孢子虫属的卵囊虫。实验证明臭氧和二氧化氯完全有能力保证由蓄水中的粪便污染物引起的微生物被去除。这篇文章讨论的是二氧化氯的氧化能力，特别是对于石油类有机物的去除，受反应时间，气体注入方式以及处理水的PH值的影响。
关键词：消毒；氧化剂；水处理；病原菌；二氧化氯 中图分类号：X523文献标识码：A 导言
在中国化学及过滤过程是用于处理饮用水的两种主要方法，与此同时紫外线照射已经被成功的应用于相对流速较低的水处理。对于个别家庭水平，通常化学处理是一种可供选择的可行方法。下属的准则存在于合适消毒剂的选择：反应必须强烈，足够消灭细菌和控制微生物生长，污染物通过分解，蒸发，沉淀等被去除，为了排除或减少毒性，氧化剂或反应副产物不应该对人体健康有害，而净水过程要使用经济。这篇文章的目的是评价和讨论对于饮用水处理可供利用的消毒剂。不同消毒剂的对比主要是净水效率和使用范围。 消毒剂的比较
例如：氯、氯胺、次氯酸、二氧化氯和臭氧等消毒剂主要考虑饮用水的处理。
这些药剂的氧化能力按效率降低排序如下： O3 > ClO2 > HOCl > OCl - > NHCl2 > NH2Cl
根据菲辛格的建议，这些消毒剂的性质在表1中进行了比较。对于长时间防止诸如伤寒水传播疾病，氯气被证明是一个很好的消毒剂。氯反应不仅在氧化，而且还亲电取代，以产生多种有机氯化物产品，特别是三卤甲烷和其他致变因素。在此三卤甲烷主要涉及三氯甲烷、三溴甲烷、二溴氯甲烷和溴二氯甲烷等。从20世纪70年代起，在供水厂饮用水的消毒中，Cl2的使用随着被O3取代作为更受欢迎的消毒剂而受到质疑。但是由于臭氧的高成本和半衰期短（15—20分钟），它不能被引进到农村的农民群体中。向其他消毒剂一样，臭氧也会导致有机副产物的生成，如醛、酮和羧酸，假如存在致突变性的溴阴离子也可能被诱变。
许多研究证明消毒完全能够保证有蓄水中的粪便污染物产生的任何微生物被去除。合理消毒剂的选择必须涉及能减少来自饮用水微生物污染的风险和潜能在来自于由使用任何消毒剂而造成化学污染风险中的增加。对常用消毒剂—臭氧、二氧化氯、氯、氯胺的杀菌效率排名几乎为同一顺序的氧化能力，但这些稳定后排序如下：
氯胺 >二氧化氯 >氯>臭氧
表.1 不同氧化剂的比较
比较Cl2 ClO2 O3 KMnO4 NH2Cl H2O2 三溴甲烷的构造 + + +- - - - - 消毒剂的效果++ + + + + + + - + -+ 增强生物可降解性+ + ++ ++ - + 除味-+ + ++ - + 铁和锰 ++ + + + + + + - + 氨 + + +- - - -- 诱变剂或毒性物质的构造 + ++ - + - + -+ -+ - - 没有影响 ；+ 影响很小 ；++ 有影响 ；+++影响最大 3.有机污染物通过二氧化氯的净化法
根据世界卫生组织关于饮用水质量的指标，更多要考虑的应该是关注苯的同
种异体混合物，因此关于二氧化氯净化效率的研究被集中在石油化学污染物。一系列实验被执行去模拟受污染水的氧化过程。受污染溶液被制成一个有7种苯的同源性化合物 - 苯、甲苯、乙苯、对 - 苯基甲烷、邻 - 苯基甲烷、米 - 苯基甲烷和苯乙烯的黑色桶（10升容量）内。样本被用于确定该化合物的初始浓度比前测试。标准生产的二氧化氯溶液是由亚氯酸钠与盐酸溶液反应的10％。沈阳LZ–2022的FID检测器GR–16A气相色谱仪被用于Cl2, ClO2, ClOˉ2 和 ClOˉ3的检测。油浓度用美国卫生协会编程设计的紫外光-120-20分光光度计（日本岛津）确定。水样中的有机成分用气相色谱—质谱仪（QP—1000A）测定。对于饮用水化学消毒的目的，由于三卤甲烷和其他诱变物质的形成氯气被瞬间忽视。结果表明，高锰酸钾和过氧化氢没有足够的氧化分解能力对石油污染物分别只有实现46％，5.7％的苯乙烯分解。不可能被选为虽然臭氧是一个优秀的水处理氧化剂，但由于它的高费用、复杂的操作以及半衰期短而不可能被选中。二氧化氯是未来最成功的消毒剂替代品。它的好处包括：有效的氧化能力、溶藻效果和微不足道的卤化副产品的形成。根据经济和运行的要求，混合气体的方法要容易使用。上述结果表明，饮用水使用臭氧或二氧化氯消毒，似乎是对次氯酸钠控制非挥发性致突变物形成的一个合适的替代品。
在实验室实验中，氧化剂臭氧、二氧化氯、高锰酸钾和混合气体（主要是二氧化氯和含有一定量的氯气、臭氧和过氧化氢）被测试用于石油化合物清除，结果如表2所示。
表.2比较不同的氧化剂的氧化能力
有机化合物初始浓度O3ClO2 H2O2 新鲜气体 KMnO4 / mg·L – 1石油 11. 34 67. 245. 8061. 80 苯3. 61 78. 371. 4082. 30 甲苯5. 23 91. 883. 0095. 20 乙苯8. 37 95. 191. 1094. 50 p-苯基 7. 8695. 890. 501000 o-苯基 8. 3695. 990. 301000 m–苯基9. 2995. 487. 301000
苯乙烯9. 36 96. 284. 7 5.7 100 46.1 进行的一项研究阐明了在天然有机物存在和不存在的情况下氯胺的腐蚀路径。结果发现，天然有机物主要充当的是还原剂而不是催化剂。这一结论使用氧化还原平衡得到了验证，大多氯胺的氧化能力是对天然有机物的去除氧化。在城市废水在来自于家庭饲养，医院，厨房的清洁剂和消毒剂是可吸附有机卤化物的来源。可吸附有机卤化物剧烈产生的数量依赖于它的性质和溶解固体有机物浓度、活性物质的浓度、温度、pH值和反应时间。当混合气体、水分子和有机微污染物反应时，次氯酸是由氯，氯酸盐组成，氯酸盐离子是在一系列氧化还原反应中从二氧化氯产生的。主要反应式总结如下:
ClO2 + 有机物→ClO -2 + 氧化有机物(1) 2ClO -2 + Cl2 = 2ClO2 + 2Cl - (2) 2ClO -2+ HOCl = 2ClO2 + 2Cl - + OH- (3) 2ClO2 + HOCl + H2O = 2ClO - 3 + HCl + 2H+ (4)
氯酸盐的收益率可以用方程描述：
d[ClO3]/ dt = 2k[ClO2][HOCl](5)
在其中k = 1.28米/分，在25℃。
在低亚氯酸盐浓度或过量氯存在时形成氯酸盐的不良反应的化学计量如下：
ClO -2 + Cl2 + H2O = ClO - 3 + 2Cl - + 2H+ (6) ClO - 2 + HOCl = ClO - 3 + Cl - + H+ (7) 在碱性条件：
ClO -2 + HOCl + OH- = ClO - 3 + Cl - + H2O (8) 通常情况下，二氧化氯用于饮用水处理的浓度范围从0.1到2.0毫克/升。但是二氧化氯反应的相应副产物亚氯酸盐和氯酸盐被发现在人体内当浓度超过100毫克/升时可能诱导高铁血红蛋白血症。有机污染物的氧化结果受反应时间到影响。初始浓度和不同时间的去除率列于表3。结果表明，二氧化氯具有很强的氧化能力，其中包括打破了苯环。有没有其他常用氧化剂做得像这样，除了臭氧。
表.3在不同的运行时间检测有机化合物的去除率（在PH值为7）
化合物初始浓度去除率 %
/mg·L – 12 min 10 min 15 min 石油18. 12 45. 846. 947. 3 苯41. 25 14. 141. 260. 7 甲苯31. 75 17. 554. 986. 8 乙苯16. 15 24. 763. 589. 8 p–苯基 10. 75 25. 984. 9100 o–苯基 30. 25 20. 979. 1100 m–苯基 33. 20 28. 6100 100苯乙烯62. 40100 100 100 二氧化氯气体注入溶液的方法对去除率也有一个明显影响。依据间接的方法，首先是气体溶解在一定量的蒸馏水，然后加入检测的有机溶液，结果显示，去除率比直接吹法低。此种差异的主要原因是由于在反应之前溶解过程中二氧化氯的转换和分解。从表格3得到证实，去除率与操作时间成正比。由于二氧化氯对有机化合物的强氧化能力，根据方程（4）它被还原为亚氯酸盐阴离子，去除率最初出现相当高的现象。然后亚氯酸盐能把氧化能力保持在一定水平，即使氧化反应随着反应时间逐渐变弱也允许有机化合物分解继续。实验表明，pH值明显影响了有机物的去除率。通过间接投入的方式对不同pH值降解率的差异列表显示在表4中。
表4.在不同pH下通过间接方式苯同源物质的降解率（在15分钟后）
化合物初始浓度 去除率/ %/mg·L–1 pH5pH7pH10 苯 5 54.048.224.1 甲苯 5 71.763.552.6 乙苯 5 84.278.773.5 p–苯5 84.477.673.0 o–苯5 84.180.373.7 m–苯5 84.079.870.4 苯乙烯 5 100.0 98.790.5 但是二氧化氯有一些不利条件，如由于挥发性容易从溶液中丢失，和在pH
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