Disinfection of effluents by combinations of chlorine dioxide and chlorine

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Pergamon 0273-1223(95)00249-9

War. Sci. Tech. Vol. 31, No. 5-6, pp. 105-114, 1995. Copyngbt C 1995 IAWQ Printed in Great Britain. All rigbll reserved. 0273-1223195 $9'50 + 0-00

DISINFECTION OF EFFLUENTS BY COMBINATIONS OF CHLORINE DIOXIDE AND CHLORINE N. Narkis. A. Katz. F. Orshansky. Y. Kott and Y. Friedland Environmental and Water Resources Engineering. Technion· Israellnstiture of Technology, Technion City, Haifa 32000, Israel

ABSTRACT The behaviour of both chlorine dioxide and chlorine, either as an individual or as combined disinfectants were studied on effluents from activated sludge. The results of the disinfection efficiency, residual disinfectants and their by-products, obtained by treating the same effluents at the same day with identical dosing sequence of chlorine dioxide and chlorine, alone and in a combined dosing, were compared after periods of the same contact. In most cases, it seems that each disinfectant maintains its individual capabilities when used in a combined manner. The combination produced a relatively stable high residual of both disinfectants. The important rmding is the fact that the combinations of chlorine dioxide and chlorine decreased the concentration of the undesirable chlorite ion and increased the concentration of the newly formed chlorine dioxide. A greater advantage is obtained by using chlorine dioxide prior to chlorine. Therefore, combined disinfection in this sequence is recommended.

KEY WORDS Chlorine dioxide; chlorine; chlorite ion; combination of disinfectants; disinfection; effluent; indicator microorganisms.

INTRODUCTION In view of the shortfall in natural water resources in Israel reclamation of wastewaters is more and more widely practised. Disinfection is the most important step' in the preparation of wastewater for reuse in ~gation, IndUStry and groundwater recharge. The hazards of reused wastewaters are primarily h.eal~ risks of mfection. The future expansion of this technology will depend to a large extent on the apphcatlon of an effective disinfectant, which can impart a lasting residual to the wastewater. Chlorine, the popular disinfectant, has dramatically improved the health of people throughout the world. However, it was found that the interaction of chlorine with organic materials produced disinfection by• products, among which are the trihalomethanes, THMs (Rook, 1974), which are presumed to be carcinognic and other chloroorganic compounds, which are suspected to be hazardous to public health (Jolley, 1985). This problem triggered the search for an alternative disinfectant. Chlorine dioxide was investigated, as one of the promising disinfectants to substitute chlorine, because of the following reasons (White. 1986; Narkis, 1988, 1990; Narkis and Kott, 1992; Weinberg and Narkis, 1992; Narkis and Weinberg, 1993). It is a strong disinfectant over a wide pH range. It produces a stable residue that can be measured for control. It does not react with organic substances to produce THMs and forms only a small amount of higher chloroorganic compounds. Neither does it react with ammonia to produce chloramines; nor its efficiency as disinfectant is

lOS

N. NARKIS tl aI.

106

reduced as a result of not being involved in side reactions. It is very successful in killing bacteria and eSpecially most efficient in deactivating viruses. In spite of the advantages of chlorine dioxide over chlorine. there is, however. a problem with its use. During the treatment of water and effluents. a part of the chlorine dioxide is reduced to chlorite ions (Werdehoff 1987). which are suspected of being toxic (Gates. 1992). In 1983. the US EPA recommended the maximuni concentration level. MCL, of residual chlorine dioxide together with its undesirable inorganic by-products chlorite and chlorate ions. in the supplied drinking water should not exceed 1.0 mg/L (Werdehoff. 1987). Thi~ low MCL is still under discussion. Nevertheless, both disinfectants, chlorine dioxide and chlorine, possess many advantages and this has encouraged the search for technology which will reduce the production of their undesirable disinfection by-products (Gordon, 1990; Griese, 1991, 1992; Iatrou, 1992; Leitner. 1992; Oehler 1983). • This paper summarizes the results of disinfection of effluents using combinations of an initial constant dose of chlorine dioxide or chlorine and various doses of the second disinfectant, added after 30 minutes contact time: of the fIrst disinfectant alone. for a total contact time of 60 minutes. The aim of this research is to investigate the possibility of using chlorine dioxide and chlorine as combined disinfectants for disinfection of effluents. in order to obtain an environmentally safe source of water. which can be reused for a variety of purposes. The main purpose is to study whether the advantages of each disinfectant are maintained and if combination thereof would reduce their disadvantages. EXPERIMENTAL

Materials Effluents. Activated sludge effluents from the Haifa Municipal Sewage Treatment Plant were used and their characterization is given in Table 1.

TABLE 1 CHARACfERISllCS OF ACTIVATED SLUDGE EFFLUENT Parameter Temperature pH Total suspended solids Volatile suspended solids Total COD Soluble COD NOzo-N NH4+-N

Conductivity Total colifonns MPN per 100 ml Fecal coliforms MPN per 100 ml Total streptococci MPN per 100 ml E-coliphages MPN per 100 ml

Results 220C 7.90 18.60mg/L 16.80mg/L 104.26 mg/L Oz 89.36mg/L0z O.OOmg/L 46.90 mg/L 1900 j.1mhoslcm 7.0 x 1()6 3.5 x 1()6 3.5 x lOS 1.4 x lOS

Chlorine dioxide. Chlorine dioxide was produced from sodium chlorite activated by HCl 10% solution (Standard Methods. 1992). The chlorine dioxide gas formed was driven off by air bubbling and then absorbed into distilled water cooled in an ice bath. The CIOz stock solution concentration was determined at the beginning of the experiment and is given in Table 2. Chlorine. Stock solution of free chlorine was prepared from a commercial sodium hypochlorite 10% solution the Kleen Malbin product from Viteo. Israel. The concentration of the working solution was 1000 mg/L as Ct. •

Disinfection of effluents by cblorine dioxide and cblorine

107

Analytical Procedures Initial and final concentrations of chlorine dioxide, chlorine. chlorite ion, and the sum total of oxygenated chlorine compounds and chlorine were measured by amperometric titration methods, utilizing dead stop end titration, with PAO for determinations at pH 7.0 and above, and sodium thiosulfate for determinations at pH 2.5 and below (Palin, 1974; Aieta and Roberts, 1981,1984; Standard Methods, 1992). TABLE 2 COMPOSmON OF CHLORINE DIOXIDE STOCK SOLUTION Cl~ 949.26 mg/L as CI~ Cl02- 120.50 mg/L as O~HOCI 30.55 mg/L as CI 1:Cl .. 1:0~ + a~- + HOa

=

2494.59 as CI

253.32 mgIL as Cl .,

2778.39 mg/L as Cl

Methods The behavior of both chlorine dioxide and chlorine, either as an individual or as combined disinfectants were studied on the same effluents, on the same day. Similar initial constant doses of 5 or 6 mg/L chlorine dioxide or chlorine were added to parallel effluent samples, at t = O. After 30 minutes contact time of the first disinfectant alone, various doses of the second disinfectant were added. The total disinfection period was 60 minutes, since the first disinfectant was added, which is in fact 30 minutes contact time of the combined disinfectants. At the end of the specified contact time, residual concentrations of the disinfectants, chlorine dioxide and chlorine, were determined, as well as the residual concentrations of chlorite ion and the sum total of chlorine and oxychlorine compounds, ICl, expressed as mg/L CI. At the same, time, samples were taken for bacteriological examinations to determine the surviving microorganisms, indicating the presence of indicator microorganisms, such as total coliforms, fecal coliforms, fecal streptococci (Standard Methods 1992) and E-coliphages (Kott, 1966, 1974), expressed as the logarithm of the MPN per 100 mi. The maximal inactivation of these microorganisms, zero MPNIlOOmI, appears in the figures as log (10- 1).

RESULTS AND DISCUSSION The effect of combinations of two disinfectants, chlorine dioxide and chlorine on residual disinfectants, concentration of chlorite ion and survival of pathogenic indicator microorganis~s was studied on activated sludge effluents.

Disinfectants' Residuals The effect of disinfection of effluents by using combinations of an initial constant dose of 5 mg/L Cl02 and various doses of chlorine. added after 30 minutes on residual chlorine dioxide, combined chlorine and chlorite ion, determined after total contact time of 60 mi~utes is shown in fig. 1. The analytical tests were carried out after 30 minutes contact of the a02 alone, before th~ addition of the chlorine and after 60 minutes from the beginning of the experiment, which is in fact 30 minutes contact time of the combined disinfectants. Figure 1 shows that the results at a dose of 0 mg/L chlorine. after 30 and 60 minutes contact of CI02 alone, are similar. All the 5 mg/L 00 2 were consumed by the effluent, leaving zero Cl~ residual, partly convened (54%) to undesirable by-product chlorite ion, reaching the highest concentration of 3.37 mgIL as CIOi. When chlorine is added after 30 minutes contact time of the effluent with chlorine dioxide alone. the chlorine oxidizes the chlorite ion and convens it back to chlorine dioxide, the active and desirable residual. Thus, as a result of the addition of the chlorine. a residual of chlorine dioxide is produced. whose concentration rises when larger doses of chlorine are added. Figure 1 shows that a dose of 1 mg/L chlorine produces 0.58 mg/L chlorine dioxide, after a 30 minutes contact time of the two disinfectants together. At the same time, considerable decrease in the undesirable chlorite ion was observed from 3.37 mg/L to 2.72 mg/L as O~-. After an addition of 5 mg/L chlorine, no chlorite ions remained in the effluent, while at the same time the newly formed chlorine dioxide concentration increased to 2.90 mg/L as Cl~ and acted as an effective disinfectant.

N. NARKlS

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Fig. 1. Effect of disinfection of effluents using combinations of an initial constant dose of 5 mg/L chlorine dioxide and various doses of chlorine, added after 30 minutes contact time of the CI02 alone, on residual chlorine dioxide, combined chlorine and chlorite ion. Total contact time: 60 minutes

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Disinfection of effluents by chlorine dioxide and chlorine

109

These results indicate the advantage of combining the two disinfectants, when the chlorine dioxide is added first, at the beginning of the disinfection and already acts as an efficient disinfectant material during the initial 30 minutes contact. Then, when the chlorine is added, after 30 minutes contact time of the chlorine dioxide alone, it completes the disinfection and removes the undesirable chlorite ion from the system, by converting it to the desirable chlorine dioxide, resulting in a higher residual concentration of this disinfectant. Figure 2 summarizes the analytical result of the reversed combination of this process, i.e. when 5 mg/L chlorine were added first and the chlorine dioxide, second. The results shown in fig. 2 at a dose of 0 mg/L CI02, after 30 and 60 minutes of chlorine alone are almost similar. The chlorine reacted with organic amines and ammonia, in the effluent from conventional biological treatment of sewage and was converted to residual combined chlorine. It can be assumed that kinetically, the reaction of ammonia with chlorine is slower than the reaction of chlorine with organic amines, forming mainly the organic chloramines which do not act as disinfectant materials (White, 1992). Therefore, it is possible that a large part of the combined chlorine compounds, analyzed and determined as active chlorine, are in effect, non-active organic chloramines, which do not disinfect and do not oxidize chlorite ions to produce chlorine dioxide. After 30 minutes contact time with 5 mg/L chlorine alone, the residual combined chlorine was 1.43 mg/L. The addition of 5 mg/L chlorine, at the beginning, supplies most of the chlorine needed to oxidize the reduced compounds present in the effluent. After the 30 minutes contact time with chlorine alone, the first dose of 1.0 mg/L CI02 added, was needed for the completion of oxidation of the reduced constituents in the effluent, resulting in zero residual CI02, accompanied by the formation of 0.67 mg/L chlorite ion and an increase in the combined chlorine concentration up to 1.97 mg/L CI02, as shown in fig. 2. Addition of higher doses of chlorine dioxide was almost not needed for oxidation and only increased the amount of stable residual chlorine dioxide. At the same time, the concentration of the combined chlorine and chlorite ions almost remained constant. The residual chlorine dioxide, determined after 60 minutes contact time from the start of the disinfection with chlorine, showed that a dose of 2.5 mg/L chlorine dioxide gave a residual of 0.77 mg/L as CI02, which was increased to a residual of 2.91 mg/L as CI02 with a dose of 5.0 mg/L chlorine dioxide. In this order of combination, with the addition of 5 mg/L chlorine fIrst and chlorine dioxide second, most of the CI02 remained as a stable residue and no more was required for reaction with the effluent. As a result, a small amount of the undesirable chlorite ions was produced. With chlorine dioxide doses up to 2.5 mg/L, the chlorite ion concentration increased up to a maximum of 1.10 mg/L as CI02-. In larger doses, there was reduction in concentration until a residue of 0.4 mg/L CI02- was obtained with a dose of 5 mg/L CI02, which also slightly reduced the concentration of the combined chlorine. The tendency of reduction of combined chlorine and chlorite ion concentrations may indicate the existence of interactions between these two compounds and chlorine dioxide. At this stage of the research the concentration of chlorate ions in the solution was not measured. Indicator Microorganisms The effect of effluent disinfection using combinations of initial constant doses of 5 or 6 mg/L of chlorine dioxide or chlorine and various doses of the second disinfectant added after 30 minutes contact of the first disinfectant alone, on the survival of the indicator microorganis~s, such as total coliforms, fecal coliforms, fecal streptococci and E-coliphages after 60 minutes total contact time, are shown in fIgs. 3 to 6. In the range of doses studied from 1 to 6 mg/L, when the chlorine was added first to the effluents, the deactivation efficiency of the coliforms is higher than when the chlorine dioxide was added first. The effect of each disinfectant alone is in agreement with previous findings of Narkis and Kott (1992) that when giving an application of doses lower than 5 mg/L disinfectant to effluents chlorine was found to be more effective than a similar small dose of chlorine dioxide, in the killing of total coliforms, fecal coliforms and fecal streptococci. At this range of doses, chlorine was not active enough to kill all the indicator microorganisms. The advantage of chlorine over chlorine dioxide disappeared when the dosage was increased. With a dose of 8.90 mg/L Cl~, after 60 minutes contact time, only 7.8 total coliforms, 2 fecal coliforms, zero fecal streptococci and zero E-coliphages MPN/l00 ml survived, compared to 540 total coliforms, 220 fecal coliforms, zero fecal streptococci and 2.0 x 1()4 E-coliphages that survived a dose of 8.88 mg/L chlorine (Narkis and Kott, 1992).

Total coliforms. Figure 3 indicates that the combination, when chlorine was added first and chlorine dioxide was added second, is slightly better in less than one order of magnitude in total coliforms reduction, than the reversed combination.

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Disinfection of effluents by cblorine dioxide and cblorine

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Fecal Coli/orms. The effect of disinfection of effluent using combinations of an initial constant dose of 6 mgIL chlorine dioxide or chlorine, added first, and after 30 minutes various doses of the second disinfectant were added, for total contact time of 60 minutes, on the survival of fecal coliforms, is shown in Fig. 4. In the case of fecal colifonns addition of 6 mgIL chlorine alone resulted in more efficient destruction of the fecal coliforms than when 6 mg/L chlorine dioxide was added alone. The MPN/IOO ml of fecal coliforms in effluent was 1.3 x 1()6. With a dose of 6.0 mg/L chlorine alone, after 30 and 60 minutes contact time, only 2.0 total coliforms survived, compared to 7.8 x 10 1 in the case of a dose of 6.0 mg/L chlorine dioxide alone. The advantage of chlorine being added first influenced the effect of low doses of chlorine dioxide added secondly. A dose of 4 mg/L CI02 and above. added as the second disinfectant, completely destroyed the fecal coliforms. In the case that 5 mg/L chlorine were added f1J'S!, a dose of I mg/L CI~ was sufficient for a totalldlling of the fecal coliforms. In the reversed combination of this process. Le. when 6 mg/L chlorine dioxide were added flI'St and the chlorine was added second, in addition of 2.5 mg/L chlorine, only 2 MPN/IOO ml fecal coliforms survived; while. in addition of 4 mg/L chlorine and above total killing of the fecal coliforms was achieved.

Fecal Streptococci. Figure 5 shows that at a dose of 5 mg/L chlorine alone was more effective in two orders of magnitudes in killing the fecal streptococci than chlorine dioxide. This advantage disappeared immediately with the combination of the two disinfectants. In both cases. addition of 1.0 mg/L of the second disinfectant was enough to achieve complete destruction of fecal streptococci.

E-coliphages. Narkis and Kolt (1992) showed the greatest advantage of chlorine dioxide over chlorine. expressed by the reactions to the virus indicator. such as E-coliphages. With each dose and at each of the contact times. it was shown that chlorine dioxide inactivated the phages efficiently. whereas chlorine was completely ineffective. Figure 6 shows the effect of effluents' disinfection on the survival of E-coliphages by using combinations of an initial constant dose of 5 mgIL chlorine dioxide or chlorine and after 30 minutes. various doses of the second disinfectant were added. for a total contact time of 60 minutes. A dose of 5 mgIL chlorine dioxide added as the first disinfectant, or as an independent disinfectant. reduced the MPNIIOO ml of E-coliphages from 1.4 x lOS to 4.0 x IQ2 at the end of 30 minutes conlact and up to 1.6 x 101 at the end of 60 minutes. When 5 mg/L chlorine were added alone, or as an independent disinfectant. the E-coliphages were reduced to 4.6 x 1()4. after 60 minutes contact. When 5 mgIL chlorine dioxide were added to the effluent, as the first ~~infectant, after 30 minutes, a~diti?n ~f small doses ~f chlorine, in the range of I to 4 mgIL as the second dISInfectant, had no effect on the lDacUvatlon of the E-cohphages. This is in accordance with the known inefficiency of chlorine to inactivate viruses. Only when 5 mgIL chlorine were added to the effluent, which already contained 5 mg/L chlorine dioxide. complete destruction of the E-eoliphages was achieved. In the reversed sequence, when 5 mg/L chlorine were added first the addition of various doses of chlorine

dio~ide. ~t the end of 30 rnin~tes contact of the chlorine alone, red~ced the number of E-coliphages gradually.

until their complete destruction was achieved with a dose of 5 mg/L chlorine dioxide, added as the second disinfectant It should be emphasized that chlorine alone was not able to inactivate the E-coliphages. Only when chlorine dioxide was added, as the second disinfectant in this combination. the E-coliphages were affected.

SUMMARY The results of this research show the advantages of effluents' disinfection by combinations of two disinfectants, chlorine dioxide and chlorine. A greater advantage is obtained by using chlorine dioxide prior to chlorine. Such a sequence in the combined disinfection can be recommended for effective disinfection of reused effluents and contaminated natural waters. It can be expected that in this sequence, when chlorine dioxide is added first. ruMs and higher chloroorganic compounds formation will be greatly reduced in disinfection of natural waters and effluents. In the combinations studied each disinfectant maintained its individual disinfection capabilities. when used in a combined manner. The combined disinfection produced a relatively stable high residual of chlorine dioxide. The important fmding is the fact that the combinations of chlorine dioxide and chlorine decreased the concentration of the undesirable chlorite ion and increased the concentration of the newly formed chlorine dioxide.

N. NARKIS

112

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Disinfection of effiuents by chlorine dioxide and chlorine

113

CONCLUSIONS I. The combined disinfection was found

to

be more effective than the use of each disinfectanl alone.

2. II seems thai each disinfectant maintains its individual disinfection capabilities when used in a combined manner. 3. Besl results were obtained when chlorine dioxide was employed prior to chlorine. 4. When 5 mg/L chlorine dioxide was employed flI'Sl, all the Cl~ was consumed by the effluenl and chlorite ion concentration increased to 3.37 mg/L, as a result of the CICh oxidation reduction reactions with the effluent. AI this stage addition of varying doses of chlorine fonned residual chlorine dioxide. which increased with increasing chlorine doses. on account ofdecreasing the toxic chlorite ion concentration. up to 0.0 mg/L CI~·.

oS. The newly formed residual chlorine dioxide is a very effective disinfectant. 6. When chlorine dioxide was employed prior to chlorine. the E-coliphages were much more effectively destroyed than when chlorine is dosed first. 7. Fecal streptococci. total coliforms and fecal coliforms are less affected by the sequence of the addition of the combined disinfectants. 8. The results of this research show that a greater advantage is obtained by using chlorine dioxide prior to chlorine. Therefore combined disinfection in this sequence can be recommended for effective disinfection of reused effluents. ACKNOWLEDGEMENTS This paper is partially based on the M.Sc. thesis of A. Katz, carried oul at the Environmental and Water Resou~c~s Engineering. Technion· Israel .Institute o~ Tech~ology. Th~ authors wish to express their appreclanon to R. Offer, E. Melamed and Z. Vtder for their help 10 the expenmental and graphical work. The research was sponsored by a grant from the Israel Water Commissioner's Office. This grant is gratefully acknowledged. REFERENCES Aieta, E.M. and Roberts, P.V. (1981). Chlorine dioxide: chemistry, generation and residual analysis. In: Chemistry in Water Reuse, W.J. Cooper (Ed.), Vol. I, Ann Arbor Science Publ., Chap. 20, pp.429• 452. Aieta, E.M., Roberts, P.V. and Hernandez. H. (1984). Determination of chlorine dioxide, chlorine, chlorite and chlorate in water. J. Am. Wat. Wks. Assoc. Research and Technology, 76. 1,64-69. Gates, D.J. (1992). Drinking water disinfection practic~s: Chlorine Dioxide for Nineties. Presented at the Fifth ., . . . National Conference on Drinking Water. Manitoba, Canada. Gnese, M.H., Hauser, K., Berkemeier, M. and Gordon, G. (1991). Usmg reducmg agents to ehmmate chlorine dioxide and chlorite ion residuals in drinking water. J. Am. Wat. Wks. Assoc.• 83.5.56-61. Griese. M.H.• Kaczur. J.J. and Gordon. G. (1992). Combining methods for the reduction of oxychlorine residuals in drinking water. J. Am. Wat. Wks. Assoc., 84, 11.69-77. Gordon. G.• Siootmaekers. B.• Tachiyashiki. S. and Wood. W.W. (1990). Minimizing chlorite ion and chlorate ion in water treated with chlorine dioxide. J. Am. War. Wks. Assoc.• 82.4.160-165. Iatrou, A. and Knocke. R.W. (1992). Removing chlorite by the addition of ferrous iron. J. Am. War. Wks. Assoc., 84. II. 63-68. Jolley. R.L. (1985). Basic issues in water chlorination: a chemical perspective. In: Water Chlorination. Environmental Impact & Health Effects, R.E. Jolley (Ed.). Vol. 5. Lewis Publ.. Michigan, pp.19·38. Katz, A. (1992). Effluent's disinfection by combinations of chlorine dioxide and chlorine. M.Sc. Thesis. Technion, Israel Institute of Technology. Environmental and Water Resources Engineering, Haifa. Israel. Katz. A. and Narkis. N. (1993). Disinfection of effluent by combinations of equal doses of chlorine dioxide and chlorine added simultaneously over varying contact times. Submitted to War. Res.

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Kott, Y. (1966). Estimation of low numbers of Escherichia coli bacteriophage by use of the most probable number method. Appl. Microbiol, 14, 141-144. Kott, Y. (1974). Bacteriophages as viral pollution in~icators. Wat. Res., 8,.165-171.. .. Leitner, N.K., De Laat, J., Dore, M., Suty, H and Povlllot, R. (1992). Chlonte and chlonne dioxide removal by activated carbon. Wat. Res., 26, 8, 1053-1066. Narkis, N., Offer, R. and Betzer, N. (1988). Chlorine dioxide as a disinfectant at each step of the advanced treatment of effluents intended for reuse. Water. Pollut. Control. Fed. 6lst Annual Conf. Dallas, Texas. Narkis, N., Offer, R., Linenberg, E. and Betzer, N. (1990). The use of chlorine dioxide in disinfection of wastewater. In:Water Chlorination Chemistry, Environmental Impact and Health Effects. R.E.Jolley (Ed.), Vol 6, Chap. 73, pp. 955-966. Narkis, N. and Kon, Y. (1992). Comparison between chlorine dioxide and chlorine for use as a disinfectant of wastewater effluents. The 16th IAWPRC Biennial International Conference in Washington, DC. May, Wat. Sci. & Technol., 26, (7/8), 1483-1492. Narkis, N. and Weinberj?;, H. (1993). A comparison of the effects of chlorine dioxide and chlorine on effluent quality. (Unpublished). Oehler, KB., Kohler, A. and Schuttler, A. (1986). Formation of chlorite during raw water treatment with chlorine dioxide and the removal of chlorite by water treatment. Wat. Supply, 4, Mulhouse, pp.127• 139. Palin, A.T. (1974). Analytical control of water disinfection with reference to differential DPD method for chlorine, chlorine dioxide. bromine, iodine and ozone.lnst. Water Eng.• 28, 139-154. Rook, J.J. (1974). Formation of haloforms during chlorination of natural waters. J.Wat. Treat. Exam., 23, No.2, 234-243. Standard Methods for the Examination of Water and Wastewater. (1992). Am. Public Health Assoc., Am. Wat. Wks. Assoc. and Wat. Pollut. Control Fed., 18th ed. Weinberg, H. and Narkis N. (1992). Oxidation by-products resulting from the interactions of chlorine dioxide and non-ionic surfactants. In: Chemical Oxidation Technology of the Nineties. A. Roth (Ed.), Technomic Pub!. Comp. Inc., Lancaster, P.A. USA. Werdehoff. KS. and Singer. P.C. (1987). Chlorine dioxide effect on THMFP. TOXFP and the formation of inorganic by-products, J. Am. Wat. Wks. Assoc., 79,9, 107-113. White. a.c. (1992). Handbook of Chlorination and Alternative Disinfectants, 3rd ed. Van Nostrand_ Reinhold. New York.