ASME 1979 Citrus Engineering Conference CEC1979 March 22, 1979, Lakeland, Florida, USA CEC1979-2506 Downloaded from http://asmedigitalcollection.asme.org/CES/proceedings-pdf/CEC1979/99663/66/2370405/cec1979-2506.pdf by guest on 30 September 2021 CHLORINE DIOXIDE AN EFFECTIVE BIOCIDE FOR RECYCLED OR REUSED WATER SYSTEMS John F. Synan 9 Oriole Drive Norwalk, Conn. 06851 Consultant To Olin Water Services Overland Park, Kansas The desire and indeed the need of industry to recycle or reuse process waters wherever possible is becoming increasingly more urgent. Limited supplies of quality water in many parts of the country and more stringent EPA regulations for waste disposal and water dis- charge are but two of the many reasons for this. The feasibility of recycling or reusing process waters, particular- ly in the food processing industry, depends largely on the ability of main- taining effective biological control in these systems. The contaminants picked up by these waters in the processing of fruits and vegetables provide nutrients for biological growths and create excessive dosage rates for the biocides. Further, reaction of the commonly used biocides with these contaminants may produce objectionable tastes and odors in the product. Time does not permit a detailed discussion of the many biocides available to the food processing industry. The halogens, particularly Published with permission. chlorine and iodine, are the materials most favored. Bromine and bromine compounds have found limited applicability. An excellent paper comparing hypochlorite and the iodophors was presented to this group at your 1978 conference by Mr. R. B. Barrett of Economics Laboratories, Inc. In summary, this paper states that the iodophors and hypochlorites are essentially comparable in germicidal activity when each product is used under favorable condi- tions. These conditions, such as pH, concentration and organic load Downloaded from http://asmedigitalcollection.asme.org/CES/proceedings-pdf/CEC1979/99663/66/2370405/cec1979-2506.pdf by guest on 30 September 2021 of the systems, are defined. The paper further states that the hypo- chlorites are recommended for water treatment and direct food contact applications because of the low toxicity of the degraded by-products. This presentation will be confined to a comparison of the chemical and biological properties of chlorine and chlorine dioxide and how these properties affect each product's suitability for recycle and reused water systems. Generation systems for chlorine dioxide will be shown and finally several case histories of chlorine dioxide applications will be presented. CHLORINATION Chlorine is the most widely used, cost effective biocide available. It is the standard of reference. Its long history of use in potable water systems and in process streams in the food industry testify to its effec- tiveness, safety and low toxicity. It is the material first considered when selecting a biocide for the treatment of a water system. Chlorine is available in many forms; as elemental chlorine in the compressed gas state in cylinders or tank cars; in the liquid state as sodium hypochlorite (liquid bleach); in the dry form as calcium hypo- chlorite or the organic chloro products such as chloroisocyanurate. Regardless of form, gaseous, liquid or dry, when added to a water system the resulting active ingredient is the same, hypochlorous acid, HOC1. This is illustrated by the reaction of elemental chlorine with water. (Figure 1) The chlorine atom in HOCl has a valence of +l. This indicates that the chlorine in this species is in an oxidized state and that HOCl is in fact a derivative of the first oxide of chlorine, chlorine mononoxide. This equation also shows that a rnol of HC1 is produced along with each mol of HOC1. It is the basic chemical activity of HOCl that causes problems in its use. First it is pH sensitive. HOG1 will dissociate in water to the hypochlorite ion (OC 1)- and this dissociation increases with increasing pH. (2) (Figure 2) Above pH 8. 5 little HOCl remains. It has been con- verted to the hypochlorite ion (0~1)-. Downloaded from http://asmedigitalcollection.asme.org/CES/proceedings-pdf/CEC1979/99663/66/2370405/cec1979-2506.pdf by guest on 30 September 2021 The importance of this fact is that the hy chlorite ion has only about 1/10 the biocidal effectiveness of HOC1. P3g (Figure 3) Next chlorine reacts by substitution as well as by oxidation, It reacts with ammonia to form chloroamines, with phenol, hydrocarbons, organic acids, sugars, starches and in fact with most organics contain- ing a labile hydrogen to form the chloro derivatives. (Figure 4) (Figure 5) These reactions have the effect of rendering the chlorine applied much less effective or completely biocidally inactive. Finally, HOG1 has a high oxidizing potential. The couple MOCl -Cl- has a redox potential of 1. 49V. Thus HOCl is capable of oxidizing most organic contaminant present in recycle systems. These reactions natural- ly increase the chlorine demand of a system and in many cases to a point where chlorination will fail without massive dozages being applied. (Figure 6) Two other factors must be considered when considering the use of chlorine. As shown in Figure 1 one mol of HC 1 is produced for each mol of HOCl and the application of large chlorine dosages could create corrosion problems in neutral or acidic pH systems. Finally, it is well known that many of the organic chloro derivatives have objectionable tastes and/or odors, as for instance the chlorophenols. This is a very important consideration in the food processing industry. Having extolled the virtues of chlorine and pointed out its drawbacks, let's now look at the properties of chlorine dioxide. CHLORINE DIOXIDE Chlorine dioxide is the second oxide of chlorine with the formula CIOZ. Its chlorine atom has a valence of +4 indicating a high level of oxidation. The next figure (Figure 7) shows the basic structure of the two chlorine oxides. Chlorine dioxide is a yellow-green gas in dilute concentration, its color changing- - to yellow- orange- with increasing concentration. Its molecular weight is 67. 5. It has a disagreeable odor similar to chlorine and somewhat resembling ozone. The sensitivity of chlorine dioxide to temperature and pressure preclude its production and shipment in bulk. It must be generated and used on site. It is quite water soluble with its solubility depending on temperature and pressure. At room temperature and 30mm partial pressure it is soluble to the extent of 2, 9 gms/liter (2900 ppm) and in chilled water solutions of over 10. 0 gmslliter (10, 000 ppm) can readily be produced. (Figure 8). Chlorine dioxide does not react with water. It remains a true dissolved gas. It does not dissociate as does HOC1. As is chlorine, chlorine dioxide is subject to photodecomposition. This decomposition is a function of time and the intensity of the ultraviolet light source. Solutions Downloaded from http://asmedigitalcollection.asme.org/CES/proceedings-pdf/CEC1979/99663/66/2370405/cec1979-2506.pdf by guest on 30 September 2021 of C102 will retain their strength for several months when stored in the dark and at low temperature as in a refrigerator. Chlorine dioxide is harmless and safe to use when handled in a water solution. In its chemical properties chlorine dioxide is most different from HOC1. As stated before, C102 does not react with water. It does not form its acid derivative HC102-chlorous acid or dissociate to the chlorite ion (~10~)-except at high pH values, 10. O+. Even here, it is a reaction with the alkali rather than the water. The significant difference between C102 and HOCl is its chemical reactivity. C102 does not react by substitution. It does not react with ammonia or ammonia compounds to form the chloro derivatives. It will react by oxidation but in a more limited scope than HOC1. It will not react with or oxidize ammonia, ammines except the tertiary, poly saccharides, such as sugar, starch, cellulose, organic acids, alcohols, or hydrocarbons, even the unsaturated types. It will react and destroy phenols, even the chloro derivatives, humic acids, mercaptans, etc. It will react with reducing materials such as sulfides, sulfites and cyanides. This more limited reactivity may be explained partially by the lower -. redox potential of CIOZ as compared to HOC1. The couple C102 -C102 has a potential of 1. 15V (3). This lack of chemical reactivity as compared to HOCl means that in contaminated water systems the chlorine dioxide demand will be substantially lower than for HOCl and effective biological control can be more easily maintained, particularly in systems where chlorine fails. BIOLOGICAL PROPER TIES The biological properties of chlorine dioxide have been extensively studied over the past twenty-five years with significant results reported by Ridenour and associates at Michigan, Trakhtman and co-workers in Russia, the Bernarde and associates at Rutgers, among others. Early work by Ridenour and associates of the Department of Environmental Health, School of Public Health, University of Michigan at Ann Arbor, Michigan, during the period 1946-1950 (4* 5, 6* 7, proved chlorine dioxide to have biocidal effectiveness equivalent to chlorine. This work has been reported in the Journal of the American Water Works Association, Water and Sewage Works and the American Journal of Public Health. This work established that chlorine dioxide is not only an effective bactericide but is also a potent viricide and sporicide. This work included not only the test organism E. Coli, Downloaded from http://asmedigitalcollection.asme.org/CES/proceedings-pdf/CEC1979/99663/66/2370405/cec1979-2506.pdf by guest on 30 September 2021 but also several common water -borne pathogens. Ridenour also studied the effect of pH and temperature on the biological efficiency of chlorine dioxide. Chlorine dioxide proved less pH sensitive than chlorine with its effectiveness enhanced with increasing pH values.