污水處理外文翻譯帶原文(共25頁)

1、Study on Disinfection and Anti microbial Technologies for Drinking WaterZHU 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 othe

2、r 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 hydrog

3、en 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 rest

4、rain 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 dioxi

5、de, 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 dioxideCLC number: X523 Document code: A1 IntroductionChemical and filtration processes are two main

6、 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

7、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 evalu

8、ate and discuss available disinfectants for drinking water treatment. The different disinfectants are compared regarding purification efficiencies and application approaches.2 Comparison ofO3 > ClO2 > HOCl > OCl - > NHCl2 > NH2ClReferring to Fiessingers 2 suggestion, the properties of

9、 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 - pr

10、oducts, particularly trihalomethanes (THMs) and other mutagens. Here THMs mainly refer to chloroform, bromoform, dibromochloromathane and bromodichloromathane etc. Since the 1970s, the usage of Cl2 in drinking water disinfection has been questioned with ozone being substituted as the preferred disin

11、fectant 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 (1520 min. ) . As with other disinfectants, ozonation also leads to formation of organic by - product s such as aldehyde, ketones, and carboxylic acids, a

12、nd also mutagenicity may be induced if bromic anion exists.Tab. 1 Comparison of various oxidantsComparison Cl2 ClO2 O3 KMnO4 NH2Cl H2O2THM formation + + + - - - - -Disinfection effects + + + + + + + + - + - +Enhanced biodegradability + + + + + + - +Taste removal - + + + + - +Iron and manganese + + +

13、 + + + + + - +Ammonia + + + - - - - -Formation of mutagens or toxic substances + + + - + - + - + - + - no effect ; + little effect ; + + effect ; + + + largest effectMany studies have pointed out that disinfection is absolutely vital to ensure that any microorganisms arising from fecal contamination

14、 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 common

15、ly 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 > Ozone3 Purification of organic pollutants by chlori

16、ne dioxideAccording 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 oxid

17、ation 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 co

18、ncentration 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-2000 was used for measurement of Cl2, ClO2, ClO-2 and ClO-35. Oil concentrations w

19、ere 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

20、 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

21、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,

22、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 us

23、e of NaClO for cont rolling the formation of non-volatile mutagens6.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

24、results are shown in Tab. 2.Tab. 2 Comparison of oxidation capacity for the various oxidantsOrganic Compounds Initial conc. O3 ClO2 H2O2 Mixing Gas KMnO4/ mg·L 1 Oil 11. 34 67. 2 45. 8 0 61. 8 0Benzene 3. 61 78. 3 71. 4 0 82. 3 0Toluene 5. 23 91. 8 83. 0 0 95. 2 0Ethyl benzene 8. 37 95. 1 91. 1

25、 0 94. 5 0pphenylmethane 7. 86 95. 8 90. 5 0 100 o-phenylmethane 8. 36 95. 9 90. 3 0 100 0mphenylmethane 9. 29 95. 4 87. 3 0 100 0Styrene 9. 36 96. 2 84. 7 5. 7 100 46. 1A study was conducted to elucidate the decay pathway of monochloramine in the presence and absence of natural organic matter (NOM)

26、 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

27、 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

28、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)2Cl

29、O -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

30、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

31、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.

32、 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

33、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)Compounds Initial concern Removal rate/ % / mg·L 1 2 min 10 min 15 minOil 18. 12 45. 8 46. 9 47. 3Benzene 41. 25 14. 1 41. 2 60. 7Toluene 31. 75 17.

34、5 54. 9 86. 8Ethylbenzene 16. 15 24. 7 63. 5 89. 8p-phenylmethane 10. 75 25. 9 84. 9 100o-phenylmethane 30. 25 20. 9 79. 1 100m-phenylmethane 33. 20 28. 6 100 100 Styrene 62. 40 100 100 100The injecting method for chlorine dioxide gas into the solution also has an apparent influence on the removal r

35、ate. 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 decompositi

36、on 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)

37、, 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 signifi

38、cantly 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)Compounds Initial concern Rem

39、oval rate/ % / mg·L 1 pH5 pH7 pH10Benzene 5 54. 0 48. 2 24. 1Toluene 5 71. 7 63. 5 52. 6Ethylbenzene 5 84. 2 78. 7 73. 5pphenylmethane 5 84. 4 77. 6 73. 0o-phenylmethane 5 84. 1 80. 3 73. 7mphenylmethane 5 84. 0 9. 8 70. 4Styrene 5 100. 0 98. 7 90. 5There are, however, some disadvantages with C

40、lO2, 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 h

41、as 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 so

42、lution 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 - sulfi

43、te ion at pH values of 5710 ,12. Fe (II) can be used to eliminate chlorite from the water , and the redox reaction is kinetically more rapid at pH 57 as well13. It was evident that the decomposition in acidic conditions was much better than that in alkaline conditions because a disproportional amoun

44、t 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 dioxide14.

45、 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.4 Control of pathogens with disinfectantsHuman pathogens that are transmitted by water including bacteria, viruses and protozoa. Organisms transmitted by water usually grow in t

46、he 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 numb

47、er 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 pat

48、hogenic 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 i

49、s 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

50、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

51、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 v

52、elocity 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

53、 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. Watsons Law to study protozoan disinfection, reads as follows:K = Ct (9)In the formula:K constant for a given microorganism exposed to a disinfectant under a fixed set of pH and temperature conditions;C disinfe