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Ozonation and oxidation competition values
Water Res. Vol. 18, No. 4, pp. 473--478. 1984 Printed in Great Britain. All rights reserved
0043-1354 84 $3.00 + 0.00 Copyright c 198-/. Pergamon Press Ltd
OZONATION AND OXIDATION COMPETITION VALUES RELATIONSHIP TO DISINFECTION AND MICROORGANISMS REGROWTH K . BANCROFT*, P. CHROSTOWSKIt, R. L.
WRIGHT~
and I. H. SUFFET Environmental Studies Institute, Drexel University, Philadelphia, PA 19104, U.S.A. ( ReceiL'ed August 1983)
Al~tract--This study was conducted to determine the role of the oxidation competition value (f2) in an ozone disinfection process and to evaluate the potential bacterial growth capacity of water as a result of ozonation. For water from five different sources, good correlations were obtained between TOC, f~ and three measures of microbial inactivation (90~0 kill, 50~,~kill and breakpoint). Initial death rates were related to the quantity of ozone applied. Application of ozone to Delaware River water appeared to enhance growth of P. aeruginosa. The results are interpreted via a mechanistic approach related to ozone chemistry. INTRODUCTION The use of ozone in water treatment can serve many purposes because of its strong oxidation potential (Bean, 1959; Bollyky, 1977). Among the applications are: precipitation of heavy metals, reduction of taste and odor, reduction of natural color and the oxidation of organic compounds (Bollyky, 1977: Kinman, 1975; Schalekamp, 1979; Rice et al., 1981; Safe Drinking Water Committee, 1980). Also, many of the studies of the use of ozone in water treatment has been for disinfection purposes. The disinfection properties of ozone are well documented (Kinman, 1975; Lawrence and Cappelli, 1977; Venosa, 1972), however, it is difficult to relate these studies to practical water treatment since much of the research has been done with pure systems and single cultures. A concise summary of the efficacy of ozonation of bacteria, viruses and other parasites and how disinfection is affected by temperature, pH and ozone concentration can be found in Vol. 2 of the National Academy of Science Report "Drinking Water and Health" (Safe Drinking Water Committee, 1980). An understanding of the mechanism of action of ozone is limited to the site of action on the cell structure and components but does not include a discussion of the possible active forms of the disinfectant (Safe Drinking Water Committee, 1980). Disinfection results may be related to either ozone dose or ozone residual measurements. Irrespective, it is well documented that ozone requires a shorter contact time than
*Present address: Department of Biology, Southern Louisiana University, Hammond, LA 70402, U.S.A. tPresent address: Department of Chemistry, Vassar College, Poughkeepsie, NY 14160, U.S.A. **Present address: Geraghty & Miller, Inc., 884 West Street, Annapolis, MD 21401, U.S.A. 473
chlorine at equivalent doses for disinfection (Safe Drinking Water Committee, 1980; Venosa, 1972). Also, a threshold dose of ozone is required before disinfection can occur and this is related to the organic concentration of the water treated (Scaccia and Rosen, 1977; Farooq et al., 1977). Studies show an "all or none" type action for disinfection where a certain dose is required before organism inactivation occurs. These phenomena have been explained by the fact that "disinfection occurs simultaneously with other oxidation reactions that consume ozone" (Bollyky, 1977). Thus, the ozone demand of the water and the rate of chemical oxidation will determine how much ozone is available for disinfection. Another explanation for the "all or none" type reaction is that instead of competition for the ozone, free hydroxyl radical activity has not been established (Dahi, 1976). Disinfection studies have shown that an important factor in the ozonation process is the ozone demand of the water. This demand is defined as the quantity of ozone consumed by all reactions other than by disinfection. Several methods have been developed for measuring ozone demand (Chrostowski et al., 1982), including the oxidation competition value (f2) method of Hoigne and Bader (1979a). The l'2 value characterizes the effectiveness of hydroxyl radical type reactions in a given water and has been found to be related to the concentration of organic contaminants in the water. The f~ value only measures the ratio of the reaction rates with which OH radicals are consumed by all scavengers present in the water to the rate by which they react with a reference solute. While ozonation of water has been shown to reduce the concentration of organic carbon in the water, it has also been demonstrated that nonbiodegi'adable organics can be made biodegradable by oxidation with ozone (Rice et al., 1981). This may be the reason for some bacteria in ozonated water to
474
K. BANCROFT et al.
have the tendancy to regerminate or reproduce. In fact. on incubation, ozonated ground water has been found to show a higher bacterial cell c o u n t than n o n o z o n a t e d water (Schalekamp, 1979: Van der Kooij, 1978). The purpose o f this study was to determine the role o f the oxidation competition value in an ozone disinfection process and to evaluate the potential bacterial growth capacity of water as a result o f ozonation. MATERIALS AND METHODS Water samples for this study were taken from various sites in the Philadelphia area including the Delaware River, Schuylkill River. Pennypack Creek and Wissahickon Creek. All samples were collected in acid washed one gallon glass bottles, 6--9 in. below the surface of the water. Pertinent water quality parameters are shown in Table 1, Ozonation was carried out using a Welsbach T408-ozonator (Welsbach Corp., Philadelphia, PAl. The feed gas (ultra dry oxygen) to the ozonator was passed through two filters, granular activated carbon and molecular sieve (Alltech, Co., Deerfield, ILl for further purification. The feed gas was then passed into the ozonator where ozone was generated by corona discharge in the oxygen stream. The ozonated air was then passed through glass and teflon tubing to a 1 1. glass contactor fitted with a porous gas diffuser at a flow rate of 0.5 lmin -~. The concentration of ozone in the gas stream (ozone dose) to each sample was measured by the lodometric technique (APHA, 1975). Details of the apparatus are described elsewhere (Chrostowski, 1981). Oxidation competition values for the water samples were determined by the Hoigne and Bader technique (Hoigne and Bader, 1979a,b). The ~ valve is a measure of the competition for ozone by a reference solute and the total organic matter present in the water sample. It is expressed as the amount of ozone required to effect a 67% removal of a reference compound. In this case the reference solute used was benzene. In the laboratory, benzene is used as a reference substrate due to its relatively slow reaction rate with respect to hydroxyl radicals. In the technique, a given amount of benzene is added to the water being studied. Different amounts of ozone are added and the mixture reacted isothermally until completion. Toluene is added as an internal standard, the solutes isolated by liquid-liquid extraction and quantified by flame ionization gas chromatography. The natural logarithm of the percent benzene remaining is then plotted as a function of the amount of ozone added. At the point where 37% of the benzene remains, the value of ozone added is numerically equivalent to the f~ value. The background error of the procedures used was determined by running five distilled water blanks through the entire experimental procedure. The coefficient of variation of the f~ value of these samples was 3% indicating a low background error. Dilution of known concentration of a Table 1. Relevant water quality data for the waters ozonated Conductivity Turbidity u.v. at Sample pH (M~-II (NTU) A254 Pennypack I 8.00 550 11.3 0,066 Pennypack II 7.88 300 4.2 0.203 Delaware River 1 7.53 300 25.3 0.096 Delaware River II 7.40 -7.0 0,045 Wissahickon Creek I 8.30 775 2.0 0,049 Wissahickon Creek II 8.20 -3.4 0,045 Schuylkill River 7.70 -9.3 0,029 Distilled 1 6.50 7.7 1.3 0,037 Distilled II 8.00 -0.4 0.003
material hypothesized to have a large f~ value was used as a method test. A water sample with twice the hydroxyl radical scavenger potential should have twice the fl value. Solutions of 0,5 and 1.0 mg 1-~ tannic acid in distilled water were examined by the procedure. The f~ value of the lower concentration was 0.46 mg 1- ~ compared to 0.97 mg 1- ~ for the more concentrated solution, closely approximating the 1:2 ratio expected based on the underlying theory. The total organic carbon (TOC) of the water samples were determined prior to ozonation. Samples of each water were placed into acid washed glass hypovials, to which two drops of 6 M HCI were added. The vials were sealed with teflon septa and refrigerated prior to analysis. Triplicate 250~1 aliquots of the water samples were injected into a Beckman 915 B TOC analyzer (Beckman Instruments. Fullerton. CA) using a Hamilton 250 t~l syringe. The average peak height from these analyses was compared to a standard curve similarly generated using concentrations of potassium hydrogen phthalate over a 1.5--20.5mgC1 -* range (Chrostowski, 1981). Bacterial inactivation caused by the ozonation process was determined by decanting water samples from the ozone contactor into sterile 100 ml glass bottles at various time intervals during the ozonation process. Serial dilutions prepared in sterile buffered water were then plated by the standard pour plate procedure (APHA, 1975) using Tryptone Glucose Yeast extract Agar (Difco Laboratories, Detroit, MI). After incubation at 28°C for 72 h the number of colonies developing were counted. Death curves were made by plotting the log of colony forming units (cfu) versus time of sampling during the ozonation process. The values used as an index of bacterial inactivation were the quantity (rag) of ozone required to achieve a 90 and 50~ kill and the breakpoint at which a decrease in organism concentration began to occur. The amount of ozone required to achieve these values was computed by multiplication of the ozone concentration in the gas stream by the flow rate of the gas to the contactor, and the time to reach the desired effect. While the amounts of ozone required to achieve 90 and 50% kills were determined mathematically from plate count data, it was necessary to determine the breakpoint graphically. The reaction conditions resulted in a mass transfer limited fast reaction regime thus there was no ozone residual in the solution. A determination of the "'potential for increased growth" of bacteria after ozonation of water was carried out with Delaware River water. Water samples were taken in sterile glass bottles at various time intervals during the ozonation process. 100 ml aliquots were filtered through 0.45 tt membrane filters (Millipore Corp,, Bedford, MA) to remove potentially viable microorganisms present in the water samples. These 100 ml samples were then placed in sterile 250 ml BOD bottles and inoculated with the bacterium P. aeruginosa isolated from a granular activated carbon contactor at the Torresdale Water Treatment facility, Philadelphia. The initial bacterial concentration was approx. 100cfu m 1-~. The bottles were then incubated at 28°C for up to 10 days. Samples were taken periodically to determine the growth of bacteria by the pour plate technique (APHA, 1975). Absorbance at 254 nm (A254) was measured on all samples after the ozonation process, using a Perkin-Elmer Model 554 UV-Vis spectrophotometer (Perkin-Elmer Corp., Norwalk. CT) to compare the relative organic carbon concentrations before and after the ozonation process (Chrostowski, 1981; Chrostowski et aL, 1982). Correlations between TOC and A254 enabled routine use of absorbance as an analytical technique for organic carbon. RESULTS The results o f this study are presented in Table 2. values for all water samples used in this study
Oxidation competition values and bacteria
475
Table 2. Comparison or" oxidation competition values, TOC and ozone applied dose for water samples
Sample
Oxidation competition value (mgl- I
TOC Imgl -~)
Pennypack Creek I Pennypack Creek 11 Delaware River I Delaware River II Wissahickon Creek I Wissahickon Creek II Schuylkill River Distilled Water 1 Distilled Water II
0.85 1.02 1.00 0.59 0.46 0.76 0.67 0.34 0.32
7.10 8.98 7.54 4.28 5.03 3.45 3.57 1.83 1.50
ranged from 0.32mgl -~ for distilled water to 1.02mgl -~ for a Pennypack Creek Water sample. The initial TOC values of the water samples ranged from 1.50 to 8.98 mg 1-~. Linear regression analysis of TOCvsf~ value was carried out (Fig. 1). The regression line exhibited a positive correlation coefficient of 0.88, which (by analysis of variance) was found to be significant at P < 0.0025. These results indicate that when the initial TOC concentration was higher, the f~ value was higher. The amounts of applied ozone required to obtain 90% kill, 50°'0 kill and the breakpoint in bacterial density were compared to TOC (Fig. 2) and f~ values (Fig 3). The significance of the relationships determined by analysis of variance is presented in Table 3. It was found that the amount of ozone applied (mg) that was required to obtain each of the inactivation parameters was greater when the TOC and f~ values of the water were higher. However, the amounts of ozone required (mg) to obtain these parameters was found to be independent of initial bacterial density. Examples of the reduction in bacterial density with time of ozonation for various water samples of different TOC loading and f2 values are presented in Fig. 4. All death curves were found to be sigmoidal in shape with three separate phases. Initiation of cell inactivation occurred at the end of the first phase and the time to reach this point was found to increase with organic loading of the water sample. The 90~0 kill and 50'Yo kill parameters occurred during the exponential decrease in cell numbers. The third phase
Ozone applied dose I mg l-:) Breakpoint 50% Kilt 90"., Kill 0.89 1.46 0.88 0.78 0.65 0.90 0.25 0.03 0.06
1.06 1.75 I.II 0.88 0.68 0.93 0.28 0.04 0. I0
2.0l 2.54 1.73 1.85 0~81 1.50 1.00 0.14 0.32
was defined when the bacterial numbers remained consistent. Statistical analysis was carried out to compare the initial rate of death vs the initial TOC and f~ values. The correlation between TOC and initial death rate was found to be significant at P <0.01, while for ~ values, significance was at P < 0.025. The initial death rate was found to be related to the amount of ozone required to initiate cell death (P < 0.05) i.e. the greater the amount of ozone required to bring about cell death, the slower the death of the cells occured. However, the initial death rate was not related to the mg ozone required for the parameters of 50 and 90'~ bacterial kill, The contribution of bacteria to TOC was determined by dividing the log number of bacterial cells by the TOC concentration of the sample. For this calculation it was assumed that all the bacteria contained equivalent concentrations of carbon. With this assumption it was not necessary to convert bacterial numbers to organic carbon. When this ratio was compared to the rate of death due to disinfection (Fig. 5) a significant correlation was found (P <0.0005). When the bacterial organic carbon contribution to TOC was reduced, the death rate was found to decrease. The significance of this ratio to 90~ kill was P < 0,0005, and to 50°0 kill and the initiation point for bacterial inactivation was found to be P < 0.01. The results of growth of P. aeruginosa and changes in absorbance at A254 for ozonated Delaware River Water samples are presented in Fig. 6. While an 3
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Regression analysis of oxidation competition value vs total organic carbon concentration.
0
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Fig. 2. Effect of total organic carbon concentration on the ozone applied dose required to effect: breakpoint (O); 50~ kill (~); 905~okill (l'-q).
476
K. BANCROFT et al. 3
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Fig. 3. Effect of oxidation competition value on the ozone applied dose required to effect: breakpoint (O); 50°,0 kill ( ~ ) ; 90% kill
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Fig. 6. Effect of ozonation time on the regrowth of P. aeruginosa (O) and 254-nm absorbance (A). 6
~
increase in ozonation time resulted in a decrease in absorbance, as measured by A254, the maximum number of bacterial cells that grew from the original inoculum was found to increase to a constant level. The leveling out in bacterial concentration occurred when a change in slope occurred for A254.
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Fig. 4. Effect o f ozonation time on the bacterial survival in water; distilled water ( ); Wissahickon Creek ( A ) ; Delaware River ([]); Pennypack Creek (O).
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Fig. 5. Effect o f bacterial contribution o f TOC on initial death rate.
The f2 value is a measure of the amount of hydroxyl radical scavenger material present in water and does not give any information on direct reactions of ozone. Thus, the more competition that occurs between the reference solute and other organics for hydroxyl free radicals, the higher the ~ will become (Hoigne and Bader, 1979a,b). The relationship demonstrated in this study between fl values and organic loading (TOC concentration) was found to be in agreement with research reported by Hoigne and Bader (1979a). As the TOC concentration was increased more competition occurs and the [2 value was increased. Comparisons of amounts of ozone required for a 90% kill, 5 0 ~ kill and "breakpoint of cell death" to TOC and ~ were all found to show statistically significant correlations (Table 3, Figs 2 and 3). The response of bacterial density to time of ozonation exhibited negative sigmoidal curve characteristics similar to those reported by Dahi (1976). Since the amount of ozone required for any of the bacterial inactivation parameters was correlated with TOC, 1) and the proportion of bacteria to TOC, it is proposed
Table 3. Comparison or correlation coefficients and significance of TOC and oxidation competition value vs breakpoint, 50% kill and 90°,~ kill
Breakpoint Total organic carbon Oxidation comp, value
r P< r P<
0.87 0.0025 0.88 0.005
50*
90%
0.91 0,001 0.86 0.0005
0.87 0.005 0.88 0.0025
Oxidation competition values and bacteria that the residual time of contact based on these parameters can be used as a measure of disinfection. It is interesting to note that complete elimination of bacteria never occurred, It can be suggested that there are many bacterial species present with differing innate susceptibilities to disinfection. The observed trend may be the result of selective resistance. Species identification was not carried out to determine this in the present study. The relationship of TOC to bacterial inactivation demonstrates that organic loading inhibits the disinfection ability of ozone. Many studies have shown that disinfection is related to the organic loading (Venosa, 1972; Hoigne and Bader. 1979a) and that the higher the organic loading of water, the higher the concentration of ozone required to cause disinfection (Venosa, 1972: Hoigne and Bader, 1979a). Dahi (1976) postulated a mechanism where hydroxyl free radicals were important in the inactivation of organisms with ozone. However, since TOC and f2 values are significantly related, disinfection could be related to ~ values. These relationships do not clearly demonstrate that the mechanism of inactivation is by hydroxyl free radical activity. The first phase of the death curve, however, where no disinfection occurs, is presumably the result of the spontaneous ozone demand of the water (Dahi, 1976). The role of hydroxyl free radical activity in disinfection is implicated more or less by definition, i.e. bacteria act as scavengers competing with organic material for ozone. While there is no apparent direct evidence for or against free radical activity or direct ozonation, what appears to occur is competition between the mass transfer rate and the rate of reaction of ozone with substrate for the rate limiting step. The process may be viewed as occurring (at the pH of natural waters above pH 6.5) in the following three steps: Ozone mass transfer; O~(gF--,O3(aq)
(1)
Ozone decomposition; O3(aq)---, Oxidants (including OH" and 02)
(2)
Competitive reactions: (a) Oxidants + TOC--,products (3a) (b) Oxidants + bacteria--* disinfection.
(3b)
Decomposition of ozone is always fast and never rate limiting [equation (2)]. Usually ozone mass transfer is rate limiting and occurs at a slower rate than the reactions with bacteria and/or organic carbon. If it is assumed that the rates of reaction in equations (3a) and (3b) are equal, a reduction in bacterial density should o¢,cur from the onset of ozonation proportionally to the bacterial contribution to TOC. While the initial rate of cell death was decreased by an
477
increase in the initial TOC concentration, the higher the TOC concentration the longer the time required for the initiation of cell death. If taken as elementary processes, the competitive second order reactions (3a) and (3b) show that both rates are a function of inherent rate constants, concentration of substrate and concentration of oxidant. There is obviously a greater concentration of TOC, and whether or not the rate constant for TOC is greater than for bacteria, the rate of bacterial disinfection will be slower if the TOC reaction is using up the oxidant preferentially. Correlation of inactivation with TOC and ~ values therefore may be explained as a result of increased organic material preferentially competing for ozone, thus reducing the oxidant available for inactivation processes. Apparently, the rate of reaction in step (3a) is faster than in step (3b). Thus as ozone passes into solution it reacts immediately with the TOC. When sufficient TOC has reacted, and the accumulation of sufficient ozone in the water occurs, bacterial inactivation is initiated. The reaction of ozone with high molecular weight humic materials has been investigated by Chrostowski (1981) and Chrostowski et al. (1982) and was found to be mass transfer limited and possibly hydroxyl radical mediated. Hoigne and Bader (1979c) view aquatic humus as radical scavengers. Since bacteria and colloidal high molecular weight humic materials are similar with respect to size, charge, functional groups and gross morphology there is no reason why OH radical should be able to discriminate between their surfaces. The correlations between f2 and disinfection indicate the mechanism of disinfection is related to the f2 value. Since t2 is related to hydroxyl radical activity and not ozone, the results support the hydroxyl radical mediated mechanism postulated by Dahi (1976). However, the nature of the chemical reaction and its specificity for functional groups is presently unknown. It has been postulated by Giese and Christenser, 1954; Bringman, 1955, that unlike chlorine, ozone acts as a general protoplasmic oxidant with attack at the cell surface. This would tend to substantiate the hypothesis that reactions between ozone and TOC and ozone and bacteria are quantitatively similar. Hoigne (1982) postulates that the reaction between ozone and microorganisms is direct as opposed to free radical mediated. If this is the case, the ~ can be considered to be related to disinfection indirectly through competition for oxidizing species by TOC equation (3a)]. If the TOC utilizes oxidizing species, the preferential utilization mechanism advanced above may still be applied regardless of the chemical mechanism of the reaction between an oxidizing species and the bacteria. While ozonation may result in a reduction of TOC concentration, as suggested by a reduction in absorbance at 254 nm, the results using Delaware River water indicated the capacity for potential regrowth of microorganisms may be enhanced. Schalekamp
478
K. B,',NCROVl"et al.
(1979) has demonstrated enhanced growth of natural populations of bacteria after ozonation. This has been attributed to the production of more readily biodegradable material, although it is impossible to discriminate between the former and the possible removal of antagonistic effects of growth inhibiting substances when using mixed natural populations. However, it is well documented that ozonation causes enhanced growth of pure cultures of bacteria including certain species of Pseudomonas (Van der Kooij. 1978), Aeromonas (Van der Kooij et al., 1980) and Flavobacterium (Van der Kooij and Higner, 1981). The implications of the findings are that ozonation, while reducing the T O C concentration, produces smaller more biodegradable compounds from the larger refractory compounds (Benedek et al., 1979), thereby promoting enhanced bacterial growth (Van der Kooij, 1978). Ozonation and enhanced growth may be beneficial in the treatment of potable water in terms of taste and odor and TOC removal; however, the production of higher bacterial densities may have serious consequences in terms of the operation of water treatment (e.g. slime growth on surfaces and shortened filter runs).
the reaction of ozone with aquatic humic substances. Ph.D. Thesis. Drexel University. Philadelphia. PA. Chrostowski P. C.. Wright R. L. and Suffet I. H. (1982) Water quality factors affecting chemical ozone demand, Proceedings. SUN Y Conversation in the Disciplines, "'Program in Chemical Disinfection New Concepts and Materials SUNY, Binghamton, NY. pp. 85-93.
Dahi E. (1976) Physiochemical aspects of disinfection in water by means of ultrasound and ozone. Water Resour. 10, 677-684. Farooq S., Cain E. S. and Engelbrecht K. (1977) Basic concepts in disinfection with ozone. J. Wat. Pollut. Control Fed. 49, 1818-1831. Giese A. C. and Christenser E. (1954) Effects of ozone on organisms. Phys. Zool. 27, 101. Hoigne J. (1982) Personal Communication to P. Chrostowski. Hoigne J. and Bader H. (1979a) Ozone requirements and oxidation of trace impurities. Oxidation Techniques in Drinking Water Treatment (Edited by Kuhn W. and Sontheimer H.). EPA-570/9-79-020. Hoigne J. and Bader H. (1979b) Ozonation at water: oxidation competition values of different types of water used in Switzerland. Ozone Sci. Engng 1,357-372. Hoigne J. and Bader H. (1979c) Ozonation of water: selectivity and rate of oxidation of solutes. Ozone Sci. Engng I, 73-85, Kinman R. N. (1975) Water and wastewater disinfection with ozone: a critical review. CRC Criti. Rev, Envir. Control 5, 141-152. Lawrence J. and Cappelli F. P. (1977) Ozone in drinking water: a review. Sci. Total Era'it. 72, 99-108. Acknowledgements--This research was completed under a Rice R. G., Robson C. M., Miller G. W. and Hill A. G. cooperative program between the City of Philadelphia Wa(1981) Uses of ozone in drinking water treatment. J. Am. ter Department and Drexel University, Philadelphia, PA Wat. Wks Ass. 73, 44-57. supported by the U.S. EPA (Grant CR806256-02), Cincin- Safe Drinking Water Committee (1980) Drinking Water and nati, Ohio, Project Officer Keith Carswell. Review of the Health, Vo[. 2, pp. 42-51. National Academy Press, manuscript by Professor W. O. Pipes of Drexel Univeristy Washington, DC. is sincerely appreciated. Scaccia C. and Rosen H. M. (1977) Ozone contacting: what is the answer? I01 Symposium for Advanced Ozone Technology. Toronto, Canada. Schalekamp M. (1979) Experiences in Switzerland with ozone, paricularly in connection with change of hyREFERENCES gienically undesirable elements present in water. Ozonews APHA (1975) Standard Methods for the Examination oJ" 7, No. 5, 1-8. Water and Wastewater, 14th Edition. American Public Van der Kooij D. (1978) Drinking water. Technical Report Health Association, Washington, DC. 11/78. The Netherlands Waterworks Testing and ReBean E. L. (1959) Ozone effectiveness, production and cost search Institute, KIWA, Ltd, Rijswijk, Netherlands. in water treatment. Am. Chem. Sac., Adv. Chem. 21, Van der Kooij D. and Higher W. A. M. (1981) Utilization 430--442. of low concentrations of starch by a Flat'obacterium Benedek A. et aL (1979) Mechanistic analysis of water species isolated from tap water. Appl. envir. Microbial. 41, treatment data. Ozonews 6, I. 216-221. Bollyky L. J. (1977) Reactions of ozone with trace organics Van der Kooij D., Visser A. and Higher W. A. M. (1980) in water and wastewaters. In Virus and Trace ConGrowth of Aeromonas hydrophila at low concentrations of taminants in Water and Wastewaters (Edited by Bouchard substrate added to tap water. Appl. encir. Microbial. 39, J. C. and Cleland J. K.). Ann Arbor Press, Ann Arbor, 1198-1204. MI. Venosa A. D. (1972) Ozone as a water and wastewater Bringman G. (1955) Determination of lethal activity of disinfectant. A literature review. In Ozone in Water and chlorine and ozone on E. coli. Water Pollut. Abs. 28, 12. Wastewater Treatment (Edited by Evans F.). pp. 123-144. Chrostowski P. C, (1981) Physical-chemical mechanisms of Ann Arbor Science, Ann Arbor, MI.
0043-1354 84 $3.00 + 0.00 Copyright c 198-/. Pergamon Press Ltd
OZONATION AND OXIDATION COMPETITION VALUES RELATIONSHIP TO DISINFECTION AND MICROORGANISMS REGROWTH K . BANCROFT*, P. CHROSTOWSKIt, R. L.
WRIGHT~
and I. H. SUFFET Environmental Studies Institute, Drexel University, Philadelphia, PA 19104, U.S.A. ( ReceiL'ed August 1983)
Al~tract--This study was conducted to determine the role of the oxidation competition value (f2) in an ozone disinfection process and to evaluate the potential bacterial growth capacity of water as a result of ozonation. For water from five different sources, good correlations were obtained between TOC, f~ and three measures of microbial inactivation (90~0 kill, 50~,~kill and breakpoint). Initial death rates were related to the quantity of ozone applied. Application of ozone to Delaware River water appeared to enhance growth of P. aeruginosa. The results are interpreted via a mechanistic approach related to ozone chemistry. INTRODUCTION The use of ozone in water treatment can serve many purposes because of its strong oxidation potential (Bean, 1959; Bollyky, 1977). Among the applications are: precipitation of heavy metals, reduction of taste and odor, reduction of natural color and the oxidation of organic compounds (Bollyky, 1977: Kinman, 1975; Schalekamp, 1979; Rice et al., 1981; Safe Drinking Water Committee, 1980). Also, many of the studies of the use of ozone in water treatment has been for disinfection purposes. The disinfection properties of ozone are well documented (Kinman, 1975; Lawrence and Cappelli, 1977; Venosa, 1972), however, it is difficult to relate these studies to practical water treatment since much of the research has been done with pure systems and single cultures. A concise summary of the efficacy of ozonation of bacteria, viruses and other parasites and how disinfection is affected by temperature, pH and ozone concentration can be found in Vol. 2 of the National Academy of Science Report "Drinking Water and Health" (Safe Drinking Water Committee, 1980). An understanding of the mechanism of action of ozone is limited to the site of action on the cell structure and components but does not include a discussion of the possible active forms of the disinfectant (Safe Drinking Water Committee, 1980). Disinfection results may be related to either ozone dose or ozone residual measurements. Irrespective, it is well documented that ozone requires a shorter contact time than
*Present address: Department of Biology, Southern Louisiana University, Hammond, LA 70402, U.S.A. tPresent address: Department of Chemistry, Vassar College, Poughkeepsie, NY 14160, U.S.A. **Present address: Geraghty & Miller, Inc., 884 West Street, Annapolis, MD 21401, U.S.A. 473
chlorine at equivalent doses for disinfection (Safe Drinking Water Committee, 1980; Venosa, 1972). Also, a threshold dose of ozone is required before disinfection can occur and this is related to the organic concentration of the water treated (Scaccia and Rosen, 1977; Farooq et al., 1977). Studies show an "all or none" type action for disinfection where a certain dose is required before organism inactivation occurs. These phenomena have been explained by the fact that "disinfection occurs simultaneously with other oxidation reactions that consume ozone" (Bollyky, 1977). Thus, the ozone demand of the water and the rate of chemical oxidation will determine how much ozone is available for disinfection. Another explanation for the "all or none" type reaction is that instead of competition for the ozone, free hydroxyl radical activity has not been established (Dahi, 1976). Disinfection studies have shown that an important factor in the ozonation process is the ozone demand of the water. This demand is defined as the quantity of ozone consumed by all reactions other than by disinfection. Several methods have been developed for measuring ozone demand (Chrostowski et al., 1982), including the oxidation competition value (f2) method of Hoigne and Bader (1979a). The l'2 value characterizes the effectiveness of hydroxyl radical type reactions in a given water and has been found to be related to the concentration of organic contaminants in the water. The f~ value only measures the ratio of the reaction rates with which OH radicals are consumed by all scavengers present in the water to the rate by which they react with a reference solute. While ozonation of water has been shown to reduce the concentration of organic carbon in the water, it has also been demonstrated that nonbiodegi'adable organics can be made biodegradable by oxidation with ozone (Rice et al., 1981). This may be the reason for some bacteria in ozonated water to
474
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have the tendancy to regerminate or reproduce. In fact. on incubation, ozonated ground water has been found to show a higher bacterial cell c o u n t than n o n o z o n a t e d water (Schalekamp, 1979: Van der Kooij, 1978). The purpose o f this study was to determine the role o f the oxidation competition value in an ozone disinfection process and to evaluate the potential bacterial growth capacity of water as a result o f ozonation. MATERIALS AND METHODS Water samples for this study were taken from various sites in the Philadelphia area including the Delaware River, Schuylkill River. Pennypack Creek and Wissahickon Creek. All samples were collected in acid washed one gallon glass bottles, 6--9 in. below the surface of the water. Pertinent water quality parameters are shown in Table 1, Ozonation was carried out using a Welsbach T408-ozonator (Welsbach Corp., Philadelphia, PAl. The feed gas (ultra dry oxygen) to the ozonator was passed through two filters, granular activated carbon and molecular sieve (Alltech, Co., Deerfield, ILl for further purification. The feed gas was then passed into the ozonator where ozone was generated by corona discharge in the oxygen stream. The ozonated air was then passed through glass and teflon tubing to a 1 1. glass contactor fitted with a porous gas diffuser at a flow rate of 0.5 lmin -~. The concentration of ozone in the gas stream (ozone dose) to each sample was measured by the lodometric technique (APHA, 1975). Details of the apparatus are described elsewhere (Chrostowski, 1981). Oxidation competition values for the water samples were determined by the Hoigne and Bader technique (Hoigne and Bader, 1979a,b). The ~ valve is a measure of the competition for ozone by a reference solute and the total organic matter present in the water sample. It is expressed as the amount of ozone required to effect a 67% removal of a reference compound. In this case the reference solute used was benzene. In the laboratory, benzene is used as a reference substrate due to its relatively slow reaction rate with respect to hydroxyl radicals. In the technique, a given amount of benzene is added to the water being studied. Different amounts of ozone are added and the mixture reacted isothermally until completion. Toluene is added as an internal standard, the solutes isolated by liquid-liquid extraction and quantified by flame ionization gas chromatography. The natural logarithm of the percent benzene remaining is then plotted as a function of the amount of ozone added. At the point where 37% of the benzene remains, the value of ozone added is numerically equivalent to the f~ value. The background error of the procedures used was determined by running five distilled water blanks through the entire experimental procedure. The coefficient of variation of the f~ value of these samples was 3% indicating a low background error. Dilution of known concentration of a Table 1. Relevant water quality data for the waters ozonated Conductivity Turbidity u.v. at Sample pH (M~-II (NTU) A254 Pennypack I 8.00 550 11.3 0,066 Pennypack II 7.88 300 4.2 0.203 Delaware River 1 7.53 300 25.3 0.096 Delaware River II 7.40 -7.0 0,045 Wissahickon Creek I 8.30 775 2.0 0,049 Wissahickon Creek II 8.20 -3.4 0,045 Schuylkill River 7.70 -9.3 0,029 Distilled 1 6.50 7.7 1.3 0,037 Distilled II 8.00 -0.4 0.003
material hypothesized to have a large f~ value was used as a method test. A water sample with twice the hydroxyl radical scavenger potential should have twice the fl value. Solutions of 0,5 and 1.0 mg 1-~ tannic acid in distilled water were examined by the procedure. The f~ value of the lower concentration was 0.46 mg 1- ~ compared to 0.97 mg 1- ~ for the more concentrated solution, closely approximating the 1:2 ratio expected based on the underlying theory. The total organic carbon (TOC) of the water samples were determined prior to ozonation. Samples of each water were placed into acid washed glass hypovials, to which two drops of 6 M HCI were added. The vials were sealed with teflon septa and refrigerated prior to analysis. Triplicate 250~1 aliquots of the water samples were injected into a Beckman 915 B TOC analyzer (Beckman Instruments. Fullerton. CA) using a Hamilton 250 t~l syringe. The average peak height from these analyses was compared to a standard curve similarly generated using concentrations of potassium hydrogen phthalate over a 1.5--20.5mgC1 -* range (Chrostowski, 1981). Bacterial inactivation caused by the ozonation process was determined by decanting water samples from the ozone contactor into sterile 100 ml glass bottles at various time intervals during the ozonation process. Serial dilutions prepared in sterile buffered water were then plated by the standard pour plate procedure (APHA, 1975) using Tryptone Glucose Yeast extract Agar (Difco Laboratories, Detroit, MI). After incubation at 28°C for 72 h the number of colonies developing were counted. Death curves were made by plotting the log of colony forming units (cfu) versus time of sampling during the ozonation process. The values used as an index of bacterial inactivation were the quantity (rag) of ozone required to achieve a 90 and 50~ kill and the breakpoint at which a decrease in organism concentration began to occur. The amount of ozone required to achieve these values was computed by multiplication of the ozone concentration in the gas stream by the flow rate of the gas to the contactor, and the time to reach the desired effect. While the amounts of ozone required to achieve 90 and 50% kills were determined mathematically from plate count data, it was necessary to determine the breakpoint graphically. The reaction conditions resulted in a mass transfer limited fast reaction regime thus there was no ozone residual in the solution. A determination of the "'potential for increased growth" of bacteria after ozonation of water was carried out with Delaware River water. Water samples were taken in sterile glass bottles at various time intervals during the ozonation process. 100 ml aliquots were filtered through 0.45 tt membrane filters (Millipore Corp,, Bedford, MA) to remove potentially viable microorganisms present in the water samples. These 100 ml samples were then placed in sterile 250 ml BOD bottles and inoculated with the bacterium P. aeruginosa isolated from a granular activated carbon contactor at the Torresdale Water Treatment facility, Philadelphia. The initial bacterial concentration was approx. 100cfu m 1-~. The bottles were then incubated at 28°C for up to 10 days. Samples were taken periodically to determine the growth of bacteria by the pour plate technique (APHA, 1975). Absorbance at 254 nm (A254) was measured on all samples after the ozonation process, using a Perkin-Elmer Model 554 UV-Vis spectrophotometer (Perkin-Elmer Corp., Norwalk. CT) to compare the relative organic carbon concentrations before and after the ozonation process (Chrostowski, 1981; Chrostowski et aL, 1982). Correlations between TOC and A254 enabled routine use of absorbance as an analytical technique for organic carbon. RESULTS The results o f this study are presented in Table 2. values for all water samples used in this study
Oxidation competition values and bacteria
475
Table 2. Comparison or" oxidation competition values, TOC and ozone applied dose for water samples
Sample
Oxidation competition value (mgl- I
TOC Imgl -~)
Pennypack Creek I Pennypack Creek 11 Delaware River I Delaware River II Wissahickon Creek I Wissahickon Creek II Schuylkill River Distilled Water 1 Distilled Water II
0.85 1.02 1.00 0.59 0.46 0.76 0.67 0.34 0.32
7.10 8.98 7.54 4.28 5.03 3.45 3.57 1.83 1.50
ranged from 0.32mgl -~ for distilled water to 1.02mgl -~ for a Pennypack Creek Water sample. The initial TOC values of the water samples ranged from 1.50 to 8.98 mg 1-~. Linear regression analysis of TOCvsf~ value was carried out (Fig. 1). The regression line exhibited a positive correlation coefficient of 0.88, which (by analysis of variance) was found to be significant at P < 0.0025. These results indicate that when the initial TOC concentration was higher, the f~ value was higher. The amounts of applied ozone required to obtain 90% kill, 50°'0 kill and the breakpoint in bacterial density were compared to TOC (Fig. 2) and f~ values (Fig 3). The significance of the relationships determined by analysis of variance is presented in Table 3. It was found that the amount of ozone applied (mg) that was required to obtain each of the inactivation parameters was greater when the TOC and f~ values of the water were higher. However, the amounts of ozone required (mg) to obtain these parameters was found to be independent of initial bacterial density. Examples of the reduction in bacterial density with time of ozonation for various water samples of different TOC loading and f2 values are presented in Fig. 4. All death curves were found to be sigmoidal in shape with three separate phases. Initiation of cell inactivation occurred at the end of the first phase and the time to reach this point was found to increase with organic loading of the water sample. The 90~0 kill and 50'Yo kill parameters occurred during the exponential decrease in cell numbers. The third phase
Ozone applied dose I mg l-:) Breakpoint 50% Kilt 90"., Kill 0.89 1.46 0.88 0.78 0.65 0.90 0.25 0.03 0.06
1.06 1.75 I.II 0.88 0.68 0.93 0.28 0.04 0. I0
2.0l 2.54 1.73 1.85 0~81 1.50 1.00 0.14 0.32
was defined when the bacterial numbers remained consistent. Statistical analysis was carried out to compare the initial rate of death vs the initial TOC and f~ values. The correlation between TOC and initial death rate was found to be significant at P <0.01, while for ~ values, significance was at P < 0.025. The initial death rate was found to be related to the amount of ozone required to initiate cell death (P < 0.05) i.e. the greater the amount of ozone required to bring about cell death, the slower the death of the cells occured. However, the initial death rate was not related to the mg ozone required for the parameters of 50 and 90'~ bacterial kill, The contribution of bacteria to TOC was determined by dividing the log number of bacterial cells by the TOC concentration of the sample. For this calculation it was assumed that all the bacteria contained equivalent concentrations of carbon. With this assumption it was not necessary to convert bacterial numbers to organic carbon. When this ratio was compared to the rate of death due to disinfection (Fig. 5) a significant correlation was found (P <0.0005). When the bacterial organic carbon contribution to TOC was reduced, the death rate was found to decrease. The significance of this ratio to 90~ kill was P < 0,0005, and to 50°0 kill and the initiation point for bacterial inactivation was found to be P < 0.01. The results of growth of P. aeruginosa and changes in absorbance at A254 for ozonated Delaware River Water samples are presented in Fig. 6. While an 3
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The f2 value is a measure of the amount of hydroxyl radical scavenger material present in water and does not give any information on direct reactions of ozone. Thus, the more competition that occurs between the reference solute and other organics for hydroxyl free radicals, the higher the ~ will become (Hoigne and Bader, 1979a,b). The relationship demonstrated in this study between fl values and organic loading (TOC concentration) was found to be in agreement with research reported by Hoigne and Bader (1979a). As the TOC concentration was increased more competition occurs and the [2 value was increased. Comparisons of amounts of ozone required for a 90% kill, 5 0 ~ kill and "breakpoint of cell death" to TOC and ~ were all found to show statistically significant correlations (Table 3, Figs 2 and 3). The response of bacterial density to time of ozonation exhibited negative sigmoidal curve characteristics similar to those reported by Dahi (1976). Since the amount of ozone required for any of the bacterial inactivation parameters was correlated with TOC, 1) and the proportion of bacteria to TOC, it is proposed
Table 3. Comparison or correlation coefficients and significance of TOC and oxidation competition value vs breakpoint, 50% kill and 90°,~ kill
Breakpoint Total organic carbon Oxidation comp, value
r P< r P<
0.87 0.0025 0.88 0.005
50*
90%
0.91 0,001 0.86 0.0005
0.87 0.005 0.88 0.0025
Oxidation competition values and bacteria that the residual time of contact based on these parameters can be used as a measure of disinfection. It is interesting to note that complete elimination of bacteria never occurred, It can be suggested that there are many bacterial species present with differing innate susceptibilities to disinfection. The observed trend may be the result of selective resistance. Species identification was not carried out to determine this in the present study. The relationship of TOC to bacterial inactivation demonstrates that organic loading inhibits the disinfection ability of ozone. Many studies have shown that disinfection is related to the organic loading (Venosa, 1972; Hoigne and Bader. 1979a) and that the higher the organic loading of water, the higher the concentration of ozone required to cause disinfection (Venosa, 1972: Hoigne and Bader, 1979a). Dahi (1976) postulated a mechanism where hydroxyl free radicals were important in the inactivation of organisms with ozone. However, since TOC and f2 values are significantly related, disinfection could be related to ~ values. These relationships do not clearly demonstrate that the mechanism of inactivation is by hydroxyl free radical activity. The first phase of the death curve, however, where no disinfection occurs, is presumably the result of the spontaneous ozone demand of the water (Dahi, 1976). The role of hydroxyl free radical activity in disinfection is implicated more or less by definition, i.e. bacteria act as scavengers competing with organic material for ozone. While there is no apparent direct evidence for or against free radical activity or direct ozonation, what appears to occur is competition between the mass transfer rate and the rate of reaction of ozone with substrate for the rate limiting step. The process may be viewed as occurring (at the pH of natural waters above pH 6.5) in the following three steps: Ozone mass transfer; O~(gF--,O3(aq)
(1)
Ozone decomposition; O3(aq)---, Oxidants (including OH" and 02)
(2)
Competitive reactions: (a) Oxidants + TOC--,products (3a) (b) Oxidants + bacteria--* disinfection.
(3b)
Decomposition of ozone is always fast and never rate limiting [equation (2)]. Usually ozone mass transfer is rate limiting and occurs at a slower rate than the reactions with bacteria and/or organic carbon. If it is assumed that the rates of reaction in equations (3a) and (3b) are equal, a reduction in bacterial density should o¢,cur from the onset of ozonation proportionally to the bacterial contribution to TOC. While the initial rate of cell death was decreased by an
477
increase in the initial TOC concentration, the higher the TOC concentration the longer the time required for the initiation of cell death. If taken as elementary processes, the competitive second order reactions (3a) and (3b) show that both rates are a function of inherent rate constants, concentration of substrate and concentration of oxidant. There is obviously a greater concentration of TOC, and whether or not the rate constant for TOC is greater than for bacteria, the rate of bacterial disinfection will be slower if the TOC reaction is using up the oxidant preferentially. Correlation of inactivation with TOC and ~ values therefore may be explained as a result of increased organic material preferentially competing for ozone, thus reducing the oxidant available for inactivation processes. Apparently, the rate of reaction in step (3a) is faster than in step (3b). Thus as ozone passes into solution it reacts immediately with the TOC. When sufficient TOC has reacted, and the accumulation of sufficient ozone in the water occurs, bacterial inactivation is initiated. The reaction of ozone with high molecular weight humic materials has been investigated by Chrostowski (1981) and Chrostowski et al. (1982) and was found to be mass transfer limited and possibly hydroxyl radical mediated. Hoigne and Bader (1979c) view aquatic humus as radical scavengers. Since bacteria and colloidal high molecular weight humic materials are similar with respect to size, charge, functional groups and gross morphology there is no reason why OH radical should be able to discriminate between their surfaces. The correlations between f2 and disinfection indicate the mechanism of disinfection is related to the f2 value. Since t2 is related to hydroxyl radical activity and not ozone, the results support the hydroxyl radical mediated mechanism postulated by Dahi (1976). However, the nature of the chemical reaction and its specificity for functional groups is presently unknown. It has been postulated by Giese and Christenser, 1954; Bringman, 1955, that unlike chlorine, ozone acts as a general protoplasmic oxidant with attack at the cell surface. This would tend to substantiate the hypothesis that reactions between ozone and TOC and ozone and bacteria are quantitatively similar. Hoigne (1982) postulates that the reaction between ozone and microorganisms is direct as opposed to free radical mediated. If this is the case, the ~ can be considered to be related to disinfection indirectly through competition for oxidizing species by TOC equation (3a)]. If the TOC utilizes oxidizing species, the preferential utilization mechanism advanced above may still be applied regardless of the chemical mechanism of the reaction between an oxidizing species and the bacteria. While ozonation may result in a reduction of TOC concentration, as suggested by a reduction in absorbance at 254 nm, the results using Delaware River water indicated the capacity for potential regrowth of microorganisms may be enhanced. Schalekamp
478
K. B,',NCROVl"et al.
(1979) has demonstrated enhanced growth of natural populations of bacteria after ozonation. This has been attributed to the production of more readily biodegradable material, although it is impossible to discriminate between the former and the possible removal of antagonistic effects of growth inhibiting substances when using mixed natural populations. However, it is well documented that ozonation causes enhanced growth of pure cultures of bacteria including certain species of Pseudomonas (Van der Kooij. 1978), Aeromonas (Van der Kooij et al., 1980) and Flavobacterium (Van der Kooij and Higner, 1981). The implications of the findings are that ozonation, while reducing the T O C concentration, produces smaller more biodegradable compounds from the larger refractory compounds (Benedek et al., 1979), thereby promoting enhanced bacterial growth (Van der Kooij, 1978). Ozonation and enhanced growth may be beneficial in the treatment of potable water in terms of taste and odor and TOC removal; however, the production of higher bacterial densities may have serious consequences in terms of the operation of water treatment (e.g. slime growth on surfaces and shortened filter runs).
the reaction of ozone with aquatic humic substances. Ph.D. Thesis. Drexel University. Philadelphia. PA. Chrostowski P. C.. Wright R. L. and Suffet I. H. (1982) Water quality factors affecting chemical ozone demand, Proceedings. SUN Y Conversation in the Disciplines, "'Program in Chemical Disinfection New Concepts and Materials SUNY, Binghamton, NY. pp. 85-93.
Dahi E. (1976) Physiochemical aspects of disinfection in water by means of ultrasound and ozone. Water Resour. 10, 677-684. Farooq S., Cain E. S. and Engelbrecht K. (1977) Basic concepts in disinfection with ozone. J. Wat. Pollut. Control Fed. 49, 1818-1831. Giese A. C. and Christenser E. (1954) Effects of ozone on organisms. Phys. Zool. 27, 101. Hoigne J. (1982) Personal Communication to P. Chrostowski. Hoigne J. and Bader H. (1979a) Ozone requirements and oxidation of trace impurities. Oxidation Techniques in Drinking Water Treatment (Edited by Kuhn W. and Sontheimer H.). EPA-570/9-79-020. Hoigne J. and Bader H. (1979b) Ozonation at water: oxidation competition values of different types of water used in Switzerland. Ozone Sci. Engng 1,357-372. Hoigne J. and Bader H. (1979c) Ozonation of water: selectivity and rate of oxidation of solutes. Ozone Sci. Engng I, 73-85, Kinman R. N. (1975) Water and wastewater disinfection with ozone: a critical review. CRC Criti. Rev, Envir. Control 5, 141-152. Lawrence J. and Cappelli F. P. (1977) Ozone in drinking water: a review. Sci. Total Era'it. 72, 99-108. Acknowledgements--This research was completed under a Rice R. G., Robson C. M., Miller G. W. and Hill A. G. cooperative program between the City of Philadelphia Wa(1981) Uses of ozone in drinking water treatment. J. Am. ter Department and Drexel University, Philadelphia, PA Wat. Wks Ass. 73, 44-57. supported by the U.S. EPA (Grant CR806256-02), Cincin- Safe Drinking Water Committee (1980) Drinking Water and nati, Ohio, Project Officer Keith Carswell. Review of the Health, Vo[. 2, pp. 42-51. National Academy Press, manuscript by Professor W. O. Pipes of Drexel Univeristy Washington, DC. is sincerely appreciated. Scaccia C. and Rosen H. M. (1977) Ozone contacting: what is the answer? I01 Symposium for Advanced Ozone Technology. Toronto, Canada. Schalekamp M. (1979) Experiences in Switzerland with ozone, paricularly in connection with change of hyREFERENCES gienically undesirable elements present in water. Ozonews APHA (1975) Standard Methods for the Examination oJ" 7, No. 5, 1-8. Water and Wastewater, 14th Edition. American Public Van der Kooij D. (1978) Drinking water. Technical Report Health Association, Washington, DC. 11/78. The Netherlands Waterworks Testing and ReBean E. L. (1959) Ozone effectiveness, production and cost search Institute, KIWA, Ltd, Rijswijk, Netherlands. in water treatment. Am. Chem. Sac., Adv. Chem. 21, Van der Kooij D. and Higher W. A. M. (1981) Utilization 430--442. of low concentrations of starch by a Flat'obacterium Benedek A. et aL (1979) Mechanistic analysis of water species isolated from tap water. Appl. envir. Microbial. 41, treatment data. Ozonews 6, I. 216-221. Bollyky L. J. (1977) Reactions of ozone with trace organics Van der Kooij D., Visser A. and Higher W. A. M. (1980) in water and wastewaters. In Virus and Trace ConGrowth of Aeromonas hydrophila at low concentrations of taminants in Water and Wastewaters (Edited by Bouchard substrate added to tap water. Appl. encir. Microbial. 39, J. C. and Cleland J. K.). Ann Arbor Press, Ann Arbor, 1198-1204. MI. Venosa A. D. (1972) Ozone as a water and wastewater Bringman G. (1955) Determination of lethal activity of disinfectant. A literature review. In Ozone in Water and chlorine and ozone on E. coli. Water Pollut. Abs. 28, 12. Wastewater Treatment (Edited by Evans F.). pp. 123-144. Chrostowski P. C, (1981) Physical-chemical mechanisms of Ann Arbor Science, Ann Arbor, MI.