CONSIDERATIONS IN THE PROCESS DESIGN OF A STATIC MIXER OZONE CONTACTING SYSTEM FOR MICROBIAL REDUCTION Stephen A. Craik1 , Gordon Finch1 , Jerome Leparc2 and Mysore Chandrakanth2 1

Dept. of Civil and Environmental Engineering, Alberta, Rm. 304 Environmental Engineering Bldg., University of Alberta, Edmonton, AB, T6G 2M8 2 Vivendi Water Technology Center, 790 Atlantic Drive, SEB Suite 220, Atlanta, GA30332, USA Water

ABSTRACT The impact of ozone dissolution conditions in a static mixer on ozone transfer efficiency and reduction of Bacillus subtilis spores was studied in an experimental contactor. Ozone transfer efficiency was found to be a function of static mixer superficial liquid velocity, gasliquid flowrate ratio and height of the down-stream bubble column. Spore reduction was found to be a function of the ozone concentrationtime (Cavgtm ) product in the reactive flow segment and was independent of the initial mixing and ozone dissolution conditions in the static mixer. In an integrated ozone contacting system, the static mixer can be designed to maximize ozone transfer while the reactive flow segment can be designed for efficient microbe reduction.

1

INTRODUCTION Design of efficient ozonation systems for microbe reduction in water treatment requires consideration of both the ozone dissolution process and the microbe reduction process. The dissolution process is a mass transfer problem in which the objective is to minimize ozone loss in the process-off gas, while achieving the desired dissolved ozone residual. Factors such as type of contactor, gas bubble size, gas and liquid flowrates, degree of turbulence and energy dissipated during gas-liquid mixing influence the efficiency of ozone dissolution [9,18]. Reduction of chemically resistant organisms of health concern, such as the encysted protozoans Giardia spp. and Cryptosporidium spp., has been found to be function of both the dissolved ozone residual and contact time in batch experiments [10,13,22]. The mixing pattern within a flow contactor can also impact the microbe reduction efficiency, especially at high reduction levels [24]. The influence of initial ozone dissolution conditions on microbe reduction efficiency are not well understood. This may partly be due to conflicting research findings. Masschelein reported that ozone doses necessary for a given level of bacterial reduction can be lowered by increasing the energy dissipated during ozone dissolution [19]. Farooq et al. found that presence of ozone gas bubbles together with dissolved ozone improved reduction relative to dissolved ozone alone [8]. They later suggested, that, because of their surface active properties, microrganisms would tend to concentrate in the gas-liquid film where the ozone concentration is higher than in the bulk liquid [9]. Finch and Smith, on the other hand, found that reduction of Echerichia coli in a semi-batch gas-liquid ozone contact tank, stirred with a Rushton turbine, was less efficient at higher energy inputs [12]. They concluded that the design of ozone contactors for microbe reduction should promote contact of the microorganisms with a dissolved ozone residual. Static mixers have been proposed as a means of improving the dissolution of ozone in water and wastewater treatment processes [5,17,26]. The combination of high turbulence and the large surface area within static mixers promotes high mass transfer rates and rapid gas dissolution [21]. Some researchers have suggested that these same parameters are important for bacterial reduction in ozone static mixers [27]. Recently, enhanced reduction was observed when Cryptosporidum parvum oocysts were pre-contacted with dissolved chlorine in a static mixer relative to precontact in an empty pipe [14]. The purpose of this work was to study the impact of ozone dissolution conditions on both mass transfer efficiency and microbial reduction in an experimental ozone contacting system. The system was comprised of a static mixer and small bubble column for ozone dissolution and off-gas separation, followed by a reactive flow segment to provide the necessary contact time between the dissolved ozone residual and the microbes of interest. Bacillus subtilis spores were selected as the indicator of disinfection performance. Bacillus spores have been demonstrated to be relatively resistant to the effects of ozone [4,11,20]. This work was part of a tailored collaboration project sponsored by the American Water Works Association Research Foundation (AWWARF) and Vivendi Water. The purpose of the tailored collaboration was to investigate the use of static 2

mixers for improving the ozone dissolution of drinking water at bench-scale and pilotscale [7], and at full-scale. MATERIALS AND METHODS Experimental Static Mixer Apparatus The experimental static mixer ozone contactor system used in this work (Figure 1) was comprised of a static mixer for rapid ozone dissolution, a vertical bubble column for additional ozone dissolution and gas-liquid separation, and a reactive flow segment to provide the dissolved ozone contact time required for microbe reduction. In all experiments, ozone was contacted with the water in a 15.7 mm ID Sulzer SMV static mixer fitted with three 15 mm corrugated-type structured elements. Including the space between elements, the total internal mixing length and volume were 75 mm and 15 mL, respectively. The water source was building tap water supplied by the Edmonton Rossdale Water Treatment Plant. This plant provides full conventional treatment including coagulation with alum and polymers, flocculation, settling, lime softening, chlorination, dual media filtration and ammonia addition. The water was regulated to 16 ± 1°C by blending hot and cold tap sources, and was then passed over a set of activated carbon columns to reduce the combined chlorine concentration to less than 0.2 mg/L. Ozone gas, produced by passing extra-dry oxygen through an air-cooled corona discharge generator, was introduced into the center of the 1.3 mm ID pipe directly upstream of the static mixer. For a given combination of water and gas flowrates, the concentration of ozone in the gas and the ozone dose applied to the water were controlled by adjusting the power level on the generator. The gas-liquid mixture leaving the static mixer flowed into the bottom of a 38 mm ID vertical bubble column via a 16 mm ID x 1 m flexible connecting tube. Off-gas was separated from the liquid at the top of the column by means of a circular overflow weir. The height of the column could be adjusted to between 45 mm and 226 mm by adding or removing column sections. The flow of the gas-free, ozonated liquid from the weir was split. About 2 L/min was diverted to the reactive flow segment, which consisted of a 51 mm ID × 15 m long serpentine pipe. The remainder of the flow was diverted to waste. Flow splitting allowed for independent variation of the hydraulic conditions within the static mixer/bubble column and the reactive flow segment. All liquid flows were measured with calibrated in-line flowmeters and the calibration was checked for each experimental trial by gravimetric measurement. Gas flow was measured and controlled using a rotameter fitted with a needle valve. Gas flowrate was corrected for system pressure and temperature and reported in normal litres per minute (NL/min) at 0°C and 1 atm. The rotameter was calibrated against a wet test meter. The ozone concentration of the feed gas from the ozone generator and the off-gas leaving the top of the gas-liquid separator column were measured by the potassium iodide absorption method [15]. Ozone residual in liquid samples collected from the various sample points (Figure 1) was measured using the indigo trisulphonate 3

colorimetric method [2] using a molar absorption coefficient of 20,000 M-1cm-1. The color change of the indigo solution at 600 nm was measured on a Pharmacia Biotech Ultrospec 2000 spectrophotometer using a 10 mm quartz cuvette.

G/ L Sep arat or

Cont act Rea ct or

Of f -Gas s

s

s Ozone Feed Gas P

s s

s

s

s

Microbe M ix er

s s P

s Thiosulfit e A ddit ion

Eff lue nt

P

Ozone Stat ic M ix e r

Microbe Feed

Feed Water

Figure 1: Schematic of experimental static mixer ozone contactor. The symbol “s” indicates a sampling location. Bacillus subtilis Spore Reduction Experiments A sample of Bacillus subtilis spore strain number 6633 was obtained from American Type Culture Collection (ATCC). The spores were propagated on R2A agar at 35°C. Spores suspension collected from the agar were washed 3 times in ozone demand-free pH 8, 0.05 M phosphate buffer. The spore suspension was heated to 75°C in a water bath and maintained at this temperature for 20 min to kill any remaining vegetative cells. The suspension was then washed one more time in buffer and stored at 4°C until use. For each experiment, about 50 mL of a stock spore suspension was added to 6 L of test water. The diluted suspension was metered into the static mixer feed water flow using a peristaltic pump to provide initial spore concentrations in the feed of 104 to 105 CFU/mL. To ensure thorough blending of the microbe suspension with the test water, an inline mixer was located in the feed line downstream of the microbe addition point but upstream of ozone addition point. 4

All experimental trials were conducted at steady-state conditions, with continuous addition of water, ozonized gas and spore suspension for a minimum of 30 min. prior to sampling. Residence time distribution (RTD) analysis of the results of pulse tracer test experiments showed that this was sufficient to allow the tracer concentration at the exit of the contactor to reach 98 % of steady-state concentration (i.e. the t 98 ). After achieving steady-state, triplicate samples were collected from various locations in the system for both spore enumeration and dissolved ozone analysis. Samples for spore enumeration were collected directly into bottles containing a small volume of 0.1 N sodium thiosulfate, sufficient to neutralize the remaining ozone residual. Spores were enumerated using a membrane filtration method [3]. Decimal dilution series were prepared using 10 mL of aliquot and 90 mL of sterile dilution water [1]. For each analysis, 3 to 4 dilutions were filtered through presterilized 47 mm × 0.45 micron membrane filters (part no. 66586, Gelman Sciences, Ann Arbor, MI) using a vacuum filtration apparatus. The filters were then placed on sterile trypticase soy broth saturated pads in petri dishes. The dishes were placed in a dry air oven at 70 to 75°C for 20 minutes, to inactivate any vegetative cells, and then into a 35°C incubator. After 22 h to 24 h the plates were removed from the incubator and the number of colonies on each counted to determine the concentration of live spores in the original sample. Batch Ozonation Experiments Samples of feed water were collected during each day of trials on the lab-scale apparatus and were stored at 4°C. Batch disinfection trials were conducted using 200 mL of the test water sample in 250 mL Erlenmeyer reactor flasks. All glassware and materials used in these experiments were made ozone demand-free (ODF) by soaking in a solution of approximately 20 mg/L ozone for 30 min. followed by drying in an air oven. A volume of spore stock solution was added to the water in each test flask to 6 6 give a concentration of 0.5 × 10 to 2.0 × 10 spores/mL. The flasks were then placed in a temperature controlled water bath (16.0 ± 0.5°C) and left to equilibrate for 30 min. with constant stirring by a magnetic stir bar. Ozone stock solution was prepared by bubbling ozone gas through 500 mL of de-ionized water which was maintained at 1 to 4°C in an ice bath. An aliquot of ozone stock solution was added to each test flask using calibrated ODF pipettes. The concentration of the ozone stock solution was determined by direct UV absorbance at 260 nm and a molar absorption coefficient of 3,300 M-1cm-1. Measurements were made on a Pharmacia Biotech Ultrospec 2000 UV/visible spectrophotometer using a 10 mm quartz cuvette. The stock solution concentration was measured three times both immediately before and after addition of the stock to the reactor. The average of these measurements was used to calculate the applied ozone dose. Samples were removed from the reactor flasks at intervals and analyzed using the indigo trisulphonate colorimetric method. At the end of the prescribed contact time, 0.2 mL of 1 M sodium formate solution was added to the flask to neutralize the remaining dissolved ozone. Three 10 mL samples were removed from the flask for spore enumeration by the membrane filtration method. For 5

each set of batch trials, a control flask was established using the same procedures and conditions as the test flasks, but without addition of ozone.

EXPERIMENTAL DESIGN AND CALCULATIONS Transfer Efficiency Experiments Ozone transfer efficiency (OTE) in the experimental static mixer system was calculated based using the following equation: OTE =

Coz , f − Coz,o Coz, f

×100%

Equation [1]

where Coz,f is the concentration of ozone in the feed gas (g/NL) and Coz,o concentration of ozone in the off gas (g/NL). Note that these transfer efficiency measurements incorporate the ozone transfer that occurs in the static mixer, the bubble column and the interconnecting piping and tubing. The effect of the static mixer operating parameters on ozone transfer efficiency was determined using a factorial design approach in which the experimental factors chosen for study were the static mixer superficial liquid velocity, v s , and the gas-to-liquid flowrate ratio, G/L. The objective was to maintain a constant ozone residual of 1 mg/L at the outlet of the bubble column for each experimental trial by making appropriate adjustments to the feed gas ozone concentration and applied ozone dose. In preliminary trials, the ozone transfer efficiency was found to be independent of applied ozone dose and ozone concentration in the gas, given the same v s and G/L. To study the relative contribution of the gas-liquid separator column to ozone transfer, the height of the separator column, H, was introduced as a third experimental factor. The experimental design plan is described in Table I. A model of the following general linear form was fit to the experimental results using least squares regression:

OTE = β0 + β1x1 + β2x2 +β 3x3 + β11x12 + β22x22 + β 33x32 +β12x1x2 + β13x1x3 + β23x2x3 + β123x1x2 x3

Equation [2]

Here x 1, x2 x 3 represent the levels of the experimental variables v s, G/L and H, expressed in coded units [6]. The coded unit of a given variable is defined as

 Value (+1) - Low Level (-1)  Coded Value = 2  −1 High Level(+1) - Low Level(-1) 

Equation [3]

6

where the high and low levels are defined as in Table I. Significance of the model terms and fit was tested using analysis of variance (ANOVA) and the regression analysis tool in Microsoft Excel. Table I: Factorial Experimental Design Plan for Transfer Efficiency Experiments Variable Low Level Medium Level High Level Coded Unit -1 0 +1 0.65 0.98 1.31 Liquid Superficial Velocity, v s (m/s) Gas-Liquid Flow Ratio,G/L (%) 1.2 2.7 4.1 Bubble Column Height, H (m) 0.46 1.36 2.26

Bacillus Spore Reduction Experiments A factorial designed experimental approach was also adopted to test the effect of static mixer operating variables, v s and G/L, on the reduction of Bacillus subtilis spores by ozone. These experiments were conducted independently of the ozone transfer experiments because it was difficult to ensure steady flow in the reactive flow segment during gas sampling operations. The experimental design is described in Table II. For each combination of ozone dissolution conditions in the ozone static mixer and gas-liquid separator, the objective was to maintain constant conditions of ozone contact, i.e flowrate and dissolved ozone residual, in the single-phase reactive flow segment. The target conditions for the reactive flow segment were a flow of 2 L/min and an inlet dissolved ozone residual of 1 mg/L. For each experimental trial, samples were collected at various locations in the static mixer system. This provides information of spore reduction versus a range of dissolved ozone concentration-time products (Cavgt m ) for each experimental trial. For the spore reduction experiments, the height of the gas-liquid separator column was maintained at 0.45 m so that the total contact time in the static-mixer/bubble column was between 26 s and 43 s, depending on the feed water flowrate. Table II: Design Plan for Bacillus Spore Reduction Experiments Test Condition Static Mixer Parameters Cavgtm Range vs G/L m/s % mg/L × min 1 0.7 1.2 1 to 20 2 0.7 2.6 1 to 20 3 1.33 1.2 1 to 20 4 1.33 2.6 1 to 20 The ozone concentration profile along the reactive flow segment was determined for each experimental trial conducting dissolved ozone analysis on samples collected at various locations along the reactive flow segment. This information was then used to estimate a first order decay rate ozone profile according to: 7

C = C0 exp(−kdt m)

Equation [4]

Curve fitting was done using non-linear least squares regression and the Solver function in Microsoft Excel. An example of ozone profile in the reactive flow segment is shown in Figure 2. The following equation was then used to estimate the integrated average ozone concentration, Cav g , for each sample point [15]: Cavg =

[

]

C0 1− exp(−kdt m) kdt

Equation [5]

The contribution of the G/L separator to the total Cavgtm was estimated by multiplying the average of the measured ozone residual at the inlet and outlet of the separator by the mean residence time in the separator. In equations [4] and [5] the mean residence time, t m , was determined for each sample location from the RTD according to [16]: ∞

t m = ∫ tE( t )dt 0

Equation [6]

The residence time distribution, E(t), was determined at each sampling location from the results of pulse-input tracer tests conducted using methylene blue dye. Tracer tests were conducted in triplicate for each sample point and the average of the three mean residence times was used in the Cavgt m determination. 0 .9 0 Measured Residual 0 .8 0

First Order Decay Model

0 .7 0

O z o n e R e s id u a l (m g / L )

0 .6 0

0 .5 0 0

5

10

Mean Residence Time, t

15 m

20

( min)

Figure 2: Example of the ozone profile in the reactive flow segment during a Bacillus spore reduction experiment. Fit of the first order ozone decay model is shown. 8

Spore reduction was calculated as log reduction according to the following equation: N Equation [7] log reduction = − log N0 N and N0 were the measured concentration of live spores at the sample location and in the feed water (after spore addition but prior to ozone addition), respectively. Measured concentrations were based on spore enumerations in triplicate samples. Approximate 95 % confidence intervals were calculated for the each log reduction result based on triplicate analysis of N and N0 and using standard statistical techniques. Batch Ozonation Experiments In the batch experiments, the Cavgt m product was calculated according to equations 4 and 5. For the batch flasks, the mean residence time was, t m , was taken as the contact time in the flask prior to addition of reducing agent. Spore reduction was calculated according to equation [7] where that N and N0 were the measured concentrations of spores in the trial batch reactor and the corresponding control reactor, respectively.

RESULTS Effect of Static Mixer Operating Variables on Transfer Efficiency The results of the ozone transfer efficiency experimental design trials are summarized in Table III. Levels of the factors are shown in coded units (Equation [3]) and reflect the actual conditions achieved during the experimental trial. Deviation of the measured ozone residual from the 1 mg/L target was attributed to variations in the performance of the ozone generator and daily variations in the composition and ozone demand of the tap water. The trials in Table III were conducted in random order to account for these uncontrolled variables. During the course of the experiment, it was discovered that it was not possible to achieve stable operation at the target settings of high v s and high G/L ratio. A high v s and intermediate G/L setting combination were substituted, resulting in a less then perfectly balanced design. The outcomes of the three center point replicate (trials 12, 13 and 19) demonstrate good reproducibility of the transfer efficiency measurements. Of the various linear model forms explored, the following model was found to provide the best fit to the data:

9

( )

2

( )

OTE = 83.2 + 3.9vs − 6.1 G / L + 4.1H − 1.8v s + 1.8vs G / L H v s = 3.0(vs − 0.98) G / L = 0.69(G / L− 2.65) H = 0.011(H −136)

Equation [8] In equation [8], the double overstrikes signify the value of the variable in coded units. ANOVA analysis indicated the overall model and each of the model terms was significant at the 95 % confidence level (P ≤ 0.05). Fit of the model to the data is shown in Figure 3. Table III: Results of Designed Experiment Trials on Ozone Transfer Efficiency Controlled Factors (Coded) Response Variables Trial vs G/L H Measured *Ozone + Transferred OTE Residual Ozone Dose % mg/L mg/L 16 -1.04 -1.02 -1 76.1 1.2 1.0 20 -1.15 -0.98 1 88.7 1.2 1.2 11 -1.21 1.22 -1 66.3 0.8 0.86 29 -0.94 0.85 1 77.4 0.8 0.93 17 0.04 -1.01 -1 86.7 1.0 1.2 31 -0.36 -0.88 -0.16 87.1 1.2 1.4 22 0.04 -1.01 1 93.0 1.1 1.2 15 0.04 0.95 -1 73.7 0.5 0.51 21 0.04 0.99 1 82.0 0.8 0.82 25 1.28 -1.02 -1 90.3 1.0 1.1 26 1.13 -0.99 -0.16 91.8 1.2 1.3 24 1.18 -1.01 1 93.8 1.3 1.3 10 1.10 -0.03 -1 81.3 1.0 0.98 23 1.18 -0.02 1 88.1 1.0 0.97 12 -0.16 0.05 -0.16 80.6 1.1 0.89 19 -0.09 0.00 -0.16 82.0 0.8 0.90 18 -0.08 -0.01 -0.16 80.7 0.9 0.97 *ozone residual measured at top of bubble column (at overflow weir) + transferred ozone dose is based on inlet and outlet ozone gas concentration. 10

According to the model, maximum efficiency is achieved at the highest v s the lowest G/L and with the largest bubble column. The inter-relationship of the three variables is illustrated by the model predictions presented in Figure 4. High velocities have a diminishing effect on transfer efficiency. In all cases, transfer efficiency is improved by increasing the height of the bubble column and, therefore, the residence time of the rising gas bubbles. However, the magnitude of the effect is a function of static mixer operation. Increasing the size of bubble column has less effect on transfer efficiency when the transfer efficiency is highest (at high v s and low G/L ) or lowest (at low v s and high G/L ). At the high v s and low G/L condition, it appears that conditions for ozone transfer are optimum within the static mixer and, therefore, most of the transfer occurs within the static mixer and outlet tubing. For example, at a v s of 1.3 m/s and G/L of 1.2 %, a five-fold factor increase in the height of the separator, from 0.45 m to 2.26 m, results in only a 4 % increase in efficiency, from about 90 to 94 %. If the column height is extrapolated to zero, the model predicted efficiency is 88 %, indicating that a large proportion of the ozone transfer occurs within the static mixer itself and the outlet tubing. A similar effect exists at low v s and high G/L. At this condition, the bubbles produced by the static mixer may be large small to promote good mass transfer in the bubble column, therefore, increasing the column size does little to improve overall transfer efficiency. At intermediate conditions, such as high v s and low G/L, the contribution of the column to overall transfer efficiency is much greater.

Predicted Transfer Efficiency

100 95 90 85 80 75 70 65 60 60

65

70 75 80 85 90 Measured Transfer Efficiency

95

100

Figure 3: Transfer efficiency model predictions

11

95 90 85

G/L = 4.1 %

80 75 70 65 0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

95 90 85 80 G/L = 1.2 %

75 70 65 0.6

0.7

0.8 0.9 1 1.1 Superficial Velocity, m/s

H = 0.45 m

H = 1.20 m

1.2

1.3

H = 2.26 m

Figure 4: Model predictions of the effect of system variables on ozone transfer efficiency. Effect of Ozone Dissolution Conditions on Spore Reduction Duplicate spore reduction experimental trials were done in the laboratory-scale static mixer apparatus at each of the target conditions (Table II). A summary of the measured operating parameters in each experimental trial is provided in Table IV. Estimates of the ozone transfer efficiency for the static mixer operating conditions, generated with equation [8], are also shown in Table IV. Figure 5 is an example of the spore reduction results for a single set of duplicate trials (Trials 1 and 5). Note that the spore enumeration for the sample at Cavgtm of approximately 18 mg/L × min. was close to the minimum detection limit of the spore assay. This contributed to the relatively larger error bars for this data point. Although the ozone residuals achieved in the two trials were different, reproducibility of the overall spore reduction versus Cavg tm curve between duplicate runs was good. The results of all 8 experimental trials with the static mixer contactor are shown together in Figure 6. There is no discernible difference in the spore reduction curves obtained at the various combinations of v s and G/L ratio. The spore reduction curve was consistently characterized by a pronounced shoulder at Cavgtm less than about 3 mg/L × min where little reduction was observed, followed by a region in which the reduction was proportional to Cavgtm, and finally by a region of tailing at 12

high log reduction levels. The position of the shoulder, the slope of the linear portion and the degree of tailing were independent of the v s or G/L settlings in the static mixer. Furthermore, very little reduction was measured across the static mixer itself (Table IV) at any of the target conditions. Reduction of spores in the static mixer system is comparable to reduction obtained in the batch well-mixed reactors at the same Cavgtm (Figure 7). A total of 21 batch trials were completed at various combinations of contact time (3 to 30 min) and initial ozone residual (0.75 to 3.6 mg/L). Some reduction in the tailing in the reduction curve obtained in the static mixer system relative to the batch well-mixed reactors was noted. Using only the data points at Cavgtm of 15 mg/L × min or higher, the average spore reduction 0.5 log-units greater in the static mixer than in the batch reactor. Using a one-sided t-test on these data points, the difference is statistically significant at the 90 % confidence level but not at the 95 % level. One hypothesis for this observation is that the tailing in the spore reduction curve is caused by the presence of spore aggregates that have a greater resistance to ozone than individual spores. Under microscopic examination, aggregation of spores in the experimental spore stock solutions was evident. For each batch experiment, stock preparations were stirred vigorously with vortex mixer at high speed for two minutes prior to use to break apart the aggregates. The high turbulence and shear forces within the static mixer may have promoted more complete aggregate break-up. It may be argued, therefore, that the observed reduction in tailing is an artifact of the experimental conditions. Contact in the static mixer certainly does not dramatically reduce the resistance of these Bacillus spores to ozone. Table IV: Summary of Experimental Conditions for Bacillus Spore Reduction Trials Trial G/L *Initial O3 O3 Decay pH Reduction vs + OTE Temp. Constant in Static Residual Mixer kd C0 -1 m/s % mg/L min % log-units °C 1 5

0.63 0.59

1.25 1.34

1.22 1.22

0.039 0.018

77 75

16.6 15.6

7.6 7.8

0.06 ± 0.27 0.07 ± 0.04

2 6

1.34 1.38

2.51 2.42

0.88 0.98

0.016 0.037

82 82

16.0 15.4

7.6 7.9

0.01 ± 0.04 -0.03 ± 0.04

3 7

1.37 1.40

1.24 1.21

1.34 0.83

0.030 0.032

89 89

16.3 16.4

7.5 7.8

0.00 ± 0.18 0.13 ± 0.03

4 0.55 3.15 1.16 0.023 70 16.4 7.7 0.04 ± 0.08 8 0.69 2.49 1.47 0.039 75 16.4 7.8 0.03 ± 0.01 *measured at inlet of the reactive flow segment + average of measured feed and reactive flow segment effluent temperature 13

7 6 5 T rial 1

4

T rial 5

3

S p o re R e d u c ti o n (l o g -u n it s )

2 1 0 0

5

10

15

20

25

Cavgt m ( m g/ L x min)

Figure 5: Example of duplicate spore reduction trial results in the laboratory-scale static mixer contactor (Trials 1 and 5 in Table IV). 7 6 5

Target v s , G/ L 0 .7 m/ s, 1.2 %

3

0 .7 m/ s, 2.6 %

2

1 .33 m/ s, 1 .2 %

S p o re R e d u c ti o n (l o g -u n it s )

4

1

1 .33 m/ s, 2 .6 %

0 0

5

10

15

20

25

Ca vg t m (m g/ L x min)

Figure 6: Effect of static mixer operating parameters on reduction of Bacillus subtilis spores in the laboratory-scale ozone static mixer. 14

7 6

Cont act or: 5

St at ic Mixer Flow

4

Well-Mixed Bat ch

3

S p o re R e d u c to n (l o g -u n it s )

2 1 0 0

5

10

15

20

25

Cavgt m (mg/ L x m in)

Figure 7: Comparison of Bacillus subtilis spore reduction in the laboratory-scale ozone static mixer contactor and in a well-mixed batch contactor.

DISCUSSION The finding that superficial velocity and gas-liquid flowrate ratio are key factors affecting transfer efficiency in static mixers is consistent with reported literature on ozone dissolution in static mixers [17,23,28]. In static mixers, the degree of mixing and mass transfer is proportional to the energy dissipated across the mixer. Because there are no moving parts, the energy dissipated and mass transfer are proportional to the liquid pressure drop and superficial velocity. Improved transfer efficiency at low gas flows means that static mixers can be effectively integrated with modern, high-concentration ozone generators. In comparison, the conventional fine bubble diffusion systems suffer from the limitation that sufficient gas flow is required to ensure adequate gas-liquid mixing and ozone dissolution [25]. The inclusion of the bubble column height as an experimental variable in the ozone transfer experiments provides new information on the relative contribution to ozone transfer of the various system components. In contrast to conventional diffuser systems, a significant portion of the transfer between the gas and liquid phases occurs during initial contact within the static mixer itself and in the outlet piping. Relatively less transfer occurs in the bubble column. In the experimental system, the residence 15

times were between 0.05 and 0.11 s in the static mixer, between 0.7 s and 1.5 s in the outlet tubing, and between 26 s and 58 s in the bubble column. Therefore, overall mass transfer rates were considerably higher in the static mixer and outlet piping. Unfortunately, the contributions of the static mixer and outlet piping to ozone transfer efficiency were not determined separately in this work. However, the ozone residual was generally observed to increase between the exit of the static mixer and the inlet of the bubble column. This indicates that, in a full-scale system, the static mixer outlet piping may contribute significantly to the overall ozone transfer. A more detailed study of the mass transfer in a static mixer system is needed to better understand the contributions of the system components and interactions of the operating variables, and to optimize the process design of ozone transfer. In the experimental designs of previous workers [8,12,19], the conditions of ozone dissolution were physically coupled to the conditions of dissolved ozone contact. This may have confounded their conclusions regarding the effect of initial mixing and ozone bubbles on microbe reduction. The advantage of the experimental system use in this work is that the conditions of initial mixing and gas-liquid were separated contact from the conditions of dissolved ozone contact. While the static mixer superficial velocity and gas-liquid flow ratio both impacted ozone transfer efficiency significantly (equation [8]), there was no discernible effect of either parameter on reduction of Bacillus spores, as long as sufficient dissolved ozone contact was achieved in the reactive flow segment (Figure 6). There was no little evidence of enhanced spore reduction by virtue of vigorous contact with gaseous ozone in the static mixer. This indicates that reduction of ozone-resistant microorganisms is not influenced by the rate of by ozone gas-liquid mass transfer but is determined by the rate of reaction between dissolved ozone and the microorganisms. Systems intended for maximum microorganism reduction, therefore, should be designed to promote contact with dissolved ozone in a reactive flow segment with sufficient detention time and appropriate hydraulic characteristics. This design concept is similar to that proposed by Finch and Smith [12]. This work and that of others [7] have demonstrated that this design concept for ozonation systems can be achieved in practice through the use of static mixers. The high shear environment of the static mixer promotes rapid and efficient dissolution. In the experimental static mixer system, ozone dissolution was completed in less than 1 min, including the residence time in the bubble column, with transfer efficiencies as high as 93%. Rapid establishment of a homogeneous dissolved ozone residual means that the bulk of the system volume can be dedicated to dissolved ozone contact and microbe reduction. The overall result, which was demonstrated in the experimental system (Figure 7), is that microbe reduction equivalent to that of a well-mixed batch reactor can potentially be achieved. A well-mixed batch reactor can be considered to be the ideal hydraulic regime for microbe reduction because all microbes are exposed to the same chemical concentration and for the same contact time with no chance of by-pass or short-circuiting. For systems with close to first-order kinetics, such as microbe reduction, this hydraulic regime provides the greatest conversion[16]. 16

Another advantage of the static mixer that was demonstrated in this work is that, in the integrated system, ozone mass transfer efficiency can be optimized without compromising the conditions for efficient microbe reduction in the downstream reactive flow segment. In a well designed system, ozone transfer efficiency will be determined mainly by the type and number of mixing elements, the superficial velocity and gas-liquid flow ratio within the mixer and by the design of the outlet piping or bubble chamber. Microbe reduction, on the other hand, will be determined dissolved ozone Cavgtm and the hydraulic characteristics of the reactive flow segment.

CONCLUSIONS Experiments with a small-scale ozone static mixer system showed that liquid superficial velocity within the mixer, gas-liquid flowrate ratio and bubble column size all impact ozone transfer efficiency significantly. At high velocities and low gasliquid flowrate ratios, most of the ozone transfer occurs within the static mixer and the outlet tubing between the mixer and the bubble column. Further research is needed to better understand the contribution of all system components and to fully optimize the process design of a static mixer system for ozone transfer efficiency. Reduction of Bacillus spores, a microbial surrogate for encysted protozoans, was found to be a function of dissolved ozone concentration × time (Cavgtm ) product and was independent of initial mixing and ozone dissolution conditions. In an integrated ozone contacting system, the static mixer can be designed to maximize ozone transfer. Efficient microbe reduction will be then achieved by contact with the dissolved ozone for sufficient contact time in an appropriately designed downstream contactor. Microbe reduction equivalent to that of a well-mixed batch system can potentially be achieved in a static mixer ozone dissolution system integrated with a well-designed downstream contactor. It remains to be determined if similar conclusions will extend to encysted protozoans.

ACKNOWLEGMENTS The authors gratefully acknowledge the American Water Works Association Research Foundation and Vivendi Water for providing funding for this project and the University of Alberta for providing research facilities. Additional support was provided by the Natural Sciences and Research Council of Canada in the form of a Postgraduate scholarship to Mr. Craik

KEYWORDS: Ozone, Static Mixers, Bacillus subtilis, spores, microorganism reduction, ozone transfer efficiency 17

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