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 1 Introduction
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.
2 Comparison 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 O3 KMnO4 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 3 Purification 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-2000 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. O3 ClO2 H2O2 Mixing Gas KMnO4 / mg·L – 1 Oil 11. 34 67. 2 45. 8 0 61. 8 0 Benzene 3. 61 78. 3 71. 4 0 82. 3 0 Toluene 5. 23 91. 8 83. 0 0 95. 2 0 Ethyl benzene 8. 37 95. 1 91. 1 0 94. 5 0 p–phenylmethane 7. 86 95. 8 90. 5 0 100 o-phenylmethane 8. 36 95. 9 90. 3 0 100 0 m–phenylmethane 9. 29 95. 4 87. 3 0 100 0 Styrene 9. 36 96. 2 84. 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