Ozonation of Cooling Tower Water: A Case Study

by Stephen Osgood

Water Conservation UnitEast Bay Municipal Utility District

June 1991

Completed under Contract to the

California Department of Water Resources

Water Conservation Office

Ozonation of Cooling Tower Water: A Case Study

by Stephen Osgood, Water Conservation Unit

East Bay Municipal Utility District

Summary

In 1988, Providence Hospital in Oakland, California changed themethod it uses to treat the water in two cooling towers,replacing a multiple chemical treatment program with ozone gastreatment. As a result, the hospital reduced water use feedingthe cooling towers by 13 percent. In addition, after changingthe cooling tower water treatment, the hospital:

more than doubled the cycles of concentration (based onconductivity),eliminated fouling and scaling of exposed surfaces,experienced no new scaling of exposed surfaces,dramatically improved water clarity,greatly reduced bacteria levels,achieved low corrosion rates,experienced minor pitting and scaling of heat exchangetubes,discovered corrosion of condenser tube end bells, andreplaced two fan motors due to corrosion.

On the whole, the hospital is pleased with the performance of theozone system. It values ozone's excellent microbiologicalcontrol and environmental compatibility. It does not believethere has been any serious destruction of equipment.

Consequently, the hospital has not only continued to use ozone inthe cooling towers of the main building, it has also recentlyselected ozone to replace a multiple chemical treatment programat the cooling tower in a second building.

The experience at this site suggests that ozone treatment ofcooling tower water should be considered at least where thefollowing conditions are met:

the cooling water's chief function is to remove heatfrom medium sized heating, ventilation, and airconditioning (HVAC) systems;the ozone system is well designed, monitored, andmaintained:the makeup water quality is low in dissolved solids.

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Report Contents

The purpose of this report is to describe the technology employedand the results it achieved. The next few sections providebackground information on the use and treatment of recirculatingcooling water systems. Details then follow of the technologyemployed at the study site, the water savings, other results, andthe costs and savings. The report identifies factors that shouldbe taken into account when ozone is considered for cooling towerwater treatment, and ends with a brief discussion of thepotential for ozone technology to be adopted throughoutCalifornia.

Open Recirculating Cooling Systems

Water gains heat when used for cooling. To be reused, thewater's temperature must be reduced, typically by passing itthrough a cooling tower. In a cooling tower the warm waterenters at the top and spreads down over numerous vertical panels.The large surface area facilitates evaporation, which lowers thetemperature of the water that remains behind. When needed, a fanboosts air flow across the water, thereby increasing evaporationand heat loss. The air expelled by the fan can also carry offwater droplets ("drift"). "Makeup" water is added to replacewhat is lost by evaporation and drift. The cooled water collectsin a basin at the bottom of the tower, from where it isrecirculated to again perform its cooling function.

As water evaporates, dissolved solids remain behind and increasein concentration. The extent to which this occurs is referred toas the cycles of concentration, also known as the concentrationratio, which is the ratio of the quantity of dissolved solids inthe cooling tower water to that in the makeup water. (Forexample, given makeup water with Total Dissolved Solids (TDS) of58 parts per million (ppm), a cooling tower with water at 145 ppmTDS would be operating at- 2.5 cycles of concentration.) Acontinuing increase in dissolved solids can lead to salts ofcalcium, magnesium, or silica precipitating out of solution andforming scale deposits on cooling system surfaces. To dilute thewater and minimize scaling, the concentrated water of the coolingtower is discharged and is then replaced by an equivalent volumeof fresh makeup water. (The discharge is referred to as "bleedoff", or "blowdown")

A cooling tower operating at relatively high cycles ofconcentration will save water compared to a similar one operatingat lower cycles. This is because the tower with higher cycleshas less blowdown and less makeup water use. However, as shownin Figures 1 and 2, the relationship between cycles ofconcentration and blowdown is not a simple linear one. The mostdramatic water savings are achieved when one moves from very low

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cycles of concentration to more moderate ones. As the number ofcycles increases further, more water is saved, but theincremental reduction in blowdown and makeup becomes lesssignificant.

Operating a recirculating cooling system also presents otherproblems that need to be controlled. Warm recirculating watersprovide an ideal environment for microbiological growth, whichcan result in the formation of slimes on equipment surfaces.Microbes, such as Legionnaires Disease bacteria (Legionellapneumophila), may threaten the health of people exposed toairborne water droplets. Workers who clean the inside ofcondenser heat exchange tubes may also be exposed toLegionella.1 At a hospital, where weakened patients areparticularly susceptible to infectious organisms and healthprofessionals are frequently exposed to pathogens, the control ofmicrobial growth in cooling tower water is critical.

Corrosion is another problem to be minimized. It not onlydestroys metal surfaces, it also produces deposits which cancontribute to the fouling of surfaces. Airborne particles (suchas dust from construction) can enter the recirculating water andalso contribute to fouling. Scale, slimes, and other types offouling, when present on heat exchanging surfaces, act asinsulators, decreasing the efficiency of the heat transfer. Thiscan lead to inadequate cooling or, at the least, to an increasein the amount of energy expended to produce the same amount ofcooling.2

Multiple Chemical Treatment

Recirculating cooling waters are often treated by addingchemicals which are selected to control one or more of theproblems of biological growth, scale, corrosion, and fouling.

The following types of chemicals are available:biocidal poisons (must be EPA registered),oxidizing biocides (must be EPA registered),corrosion inhibitors which form a protective film overmetal areas,acids or other scale inhibitors which prevent mineralprecipitation,conditioners which decrease the density of any scaleparticles which form, allowing the particles to be moreeasily carried off by the flowing water,dispersants which increase foulants' electricalcharges, causing them to repel each other, andwetting agents which reduce the water's surface tensionso that particles are less likely to adhere tosurfaces.

Maintaining correct water quality involves controlling the ratesof blowdown and makeup water flow and involves adding chemicalsin correct amounts at proper times. This, in turn, requiresinsuring the compatibility of the chemicals, and requiresmonitoring and controlling pH and conductivity.

Chemical treatment carries with it the risks and responsibilitiesof storing and handling hazardous materials. In addition, it isundesirable to discharge toxic chemicals to aquatic ecosystems orto wastewater treatment plants that rely on bacterial activity.

Ozone Treatment

Ozonation, in contrast to traditional chemical treatment,involves the on site generation of a single oxidizing agent whichis mixed into the recirculating water.

Typically, ozone is produced by the corona discharge method, inwhich dry air is passed through a gap between a highlyelectrically charged surface and a grounded surface. Whenelectrical discharges occur across the gap, some of the oxygen inthe air is converted to ozone gas.

Potential benefits. As a highly powerful oxidant, ozone destroysmicroorganisms which may threaten health (including Leoionellapneumophila3), foul cooling system surfaces, encourage thebuildup of other deposits, or contribute to corrosion.

Ozonation has also been reported to achieve higher cycles ofconcentration than multi-chemical treatment.4 Since there isless blowdown at higher cycles, ozonation offers the potential tosave water. In addition, when slightly alkaline water (pHgreater than 7) is concentrated, the alkalinity becomes even morepronounced. Operating cooling towers at higher cycles ofconcentration thus creates a more alkaline condition, reducingcorrosivity.5

Ozone also has been promoted as an effective method of directlycontrolling corrosion and scale.6

Environmental and Safety Aspects. Highly reactive, ozone residesonly briefly in water. (Its half-life in distilled water is 20to 30 minutes, and in cooling tower water, where there areoxidizable impurities, 1 to 3 minutes.)7 As a result, thetreated cooling water can be discharged safely to the sewersystem. Even if there were some residual ozone in the discharge,it would be quickly consumed by other wastes in the sewer line.Thus ozone poses virtually no threat to sewage treatment plantsor aquatic ecosystems.

Since an ozone generator will produce the gas at concentrationsof just 1 to 3 percent by weight in air, the resulting ozone/air

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mixture is not explosive.8

Ozone is a toxic gas. The maximum average allowable ozoneconcentration to which workers in California may be exposed overan 8 hour day is 0.1 ppm. The short term exposure limit (maximumallowable average concentration over any 15 minute period) is0.3 ppm.9 By contrast, ozone gas can be detected by smell atconcentrations as low as 0.02 ppm1O, well below the exposurelimit. It is conceivable, however, that a gradual increase inozone concentration might not be noticed by someone working closeto an ozonated tower.

Study Site

Facility. Providence Hospital ("Providence") in Oakland,California (a coastal city) receives fresh water and wastewatertreatment services from East Bay Municipal Utility District(EBMUD). Equipped to provide both acute and chronic medicalcare, the hospital houses 228 beds and employs 720 people. Themain hospital building, which utilizes the cooling systemdiscussed in this report, has a floor area of approximately275,000 square feet.

Cooling system. Air conditioning is commonly referred to as"comfort cooling," which suggests it is a luxury. In a hospital,however, the air temperature is of vital concern, both in theoperating room and in patient rooms. Providence's coolingsystem depends on two chillers which use the water from thecooling towers (at 85°F) to produce chilled water (at 48°F) bymeans of a condensed refrigerant. The chillers pump the chilledwater to points in the hospital where it cools indoor air, orperforms other functions. After the chilled water absorbs heatat the point of application, it returns in a closed loop to thechillers, where the heat is transferred to an internallyrecirculated refrigerant. The refrigerant warms and expands. Inthe condenser section of the chiller, the refrigerant is passedover copper tubes through-which passes the water from the coolingtowers. The heat from the refrigerant is transferred to thewater returning to the cooling towers. Finally, the coolingtowers release the waste heat to the environment, in the form ofwater vapor. Table 1 lists characteristics of the hospital'scooling system and cooling towers.

Table 1. Cooling System Characteristics, Providence Hospital

Chiller capacity 354 Tons (425,000 BTU/hr.)

Chiller operation (ave.) 85% of capacity (300 Tons)

Cooling temp. change (at) 6°F11

Water recirculation capacity 1800 gpm

Recirculation pump 20 HP

Cooling tower capacity (each) 300 Tons (360,000 BTU/hr.)

Cooling tower operation 300 Tons (combined)(average over year) primary - 75% of capacity

secondary - 25% of capacity"

Cooling tower type 2 induced draft, crossflowtowers, with connected basins

Ozone generation principle corona discharge

Ozone generator manufacturer PCI Ozone Corp.,modified by NWMC

Ozone generator capacity 3 lb./day

Ozone generator operation 65% of capacity

Water flow, 03, injection loop 60 gpm

Cooling towers. The cooling towers are about 15 years old, eachwith a capacity to remove 360,000 BTU's of heat per hour (300tons). Their basins are interconnected, and fans at the top ofthe towers induce an upward flow of air when they are engaged.Although water flows continuously through both towers, duringmost of the year only one fan is needed to boost air flow, and itengages intermittently. Only on the hottest days of the yeardoes the extra heat load cause the fan on the secondary tower toengage. With the primary tower operating at approximately 75% ofcapacity and the secondary tower operating in the vicinity of 25%of capacitytons.

together they bear an average heat load of 300

Effective biological control of cooling tower water is importantat the hospital. Windows in one of the hospital buildings whichoverlook the towers are often kept open for ventilation. Theserooms, which are used for office space, may at times be exposedto cooling tower drift. Additionally, the engineering sectionmust report quarterly on the biological condition of the coolingtower to the hospital's quality assurance team, which is chargedwith ensuring compliance with hospital accreditationrequirements. Windows in both the main hospital building and the

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new Medical Office Building (MOB) overlook the towers and may beexposed to cooling tower drift.

Makeup water. The hospital uses drinking water supplied by EBMUBas its source of makeup water for the cooling towers. Since 95%of EBMUD water is treated runoff from California's Sierra-Nevada, it is low in dissolved solids. Table 2 shows selected EBMUDwater quality characteristics during the 1980's, when the hospitalswitched its cooling tower water treatment.

The hospital has its own internal water meter which registersquantities of makeup water flowing to the cooling towers.Providence staff read the meter twice daily.

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Table 2. Selected EBMUD Water Quality Characteristics

Parameter

Microbiological

Units Average*

Total Coliform Bacteria 0.07per 100 milliliters

Chlorine parts per million (ppm) 0.35

Corrosivity Mils per year 3(0.001 in./yr.)

Chloride ppm 3.6

Total Dissolved Solids ppm 58

Specific Conductance micromho per centimeter 73

Hardness ppm of CaCO3 33

* Averages were determined over a 9 year period (1980 - 1988)Source: "EBMUD: Quality on Tap"', EBMUD Public Affairs Dept.,

Sept/Oct 1989.

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Multiple chemical treatment program. Prior to 1988, the hospitalused several chemicals to treat its cooling water: a corrosionand deposit inhibitor, two microbiocides, a dispersant, and anantifoaming agent.

The corrosion and deposit inhibitor was fed automatically to themakeup water. When the water returning from the chillers to thecooling tower rose above a set level of conductivity, a valvewould open to bleed off some of the water. Simultaneously, thecorrosion and deposit inhibitor would be injected into the waterthat returned to the tower.

All other chemicals were added manually. The microbiocides anddispersant were added approximately once a week; the anti-foamingagent was added as needed.

The representative of the chemical vendor checked monthly on thecondition of the cooling towers and the chemical feed system.

Ozone treatment program. In early 1988 Providence began use ofan ozonation system owned and installed by National WaterManagement Corporation (NWMC). The hospital terminated manualchemical additions and started relying on ozone at the beginningof March, 1988.

The hospital supplies three utilities to the ozone equipment:compressed air, high voltage direct current, and telephone lines.Providence pays NWMC a monthly fee of $1,080 for lease of theequipment and for services. Other costs involved in operatingthe ozone system are discussed later in this report.

Figure 3 schematically illustrates the type of ozone system usedat the hospital. The ozone generator was manufactured by PCIOzone Corporation and modified by NWMC for compliance withproposed Uniform Fire Code safety standards. The generator canproduce up to three pounds of ozone gas per day, but has been setto operate at 65% of capacity.13

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The components of the ozonation system at Providence include:

Ozone generator. Produces ozone through corona discharge.

Air preparation packase. Compressed air (supplied by customer)is passed through an air dryer. Dried air allows effectiveproduction of ozone gas.

Ozone injector. Mixes ozone gas with cooling tower water whichhas been pumped out of the tower basins. After injection ofozone, the water recirculates back to the basins.

Monitoring system. Continuously monitors cooling tower waterquality and the operating status of the ozonation equipment.Telecommunications equipment allows the data to be remotelyaccessed by personal computer.

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Operation and maintenance.runs the ozonation equipment

Under the ozonation contract, NWMCand monitors and maintains the water

quality of the cooling towers. Once the ozone system wasinstalled, Providence hired a company to inspect the heatexchange tubes in the condensers of the chillers. This was thefirst time since the chillers were installed in 1979 that thecondenser tubes had been inspected.14

ozone treatment,In the first year of

both chillers were inspected; since then eachchiller has been inspected on alternate years. Although thehospital continues to briefly check the cooling towers once eachshift, its own routine maintenance efforts consist only ofquarterly check-up of the fan motors.

Twice daily NWMC uses its remote monitoring system to check thecondition of the ozone equipment and the water. The monitoringsystem sends yes/no signals to indicate if there is a problemwith:

0 the ozone generator operating,0 the flow of coolants and electricity to the generator,0 the temperature of the produced ozone,0 the air dryer operating,0 the flow and dryness of the air flowing to the

generator,0 the pumping of the water through the ozone injection

loop, or0 the security of the ozonator cabinet door.

If a problem exists with any of these items, the ozone generatorautomatically shuts down. NWMC's computer would then flag thecondition and the company would send a technician to the site.

The monitoring system also transmits measured values of thefollowing parameters:

0 pressures of the recirculation pumps,0 conductivity of the recirculated water,0 the water's temperature,0 the water's oxidation-reduction potential (ORP). (ORP

provides an indirect indication of ozoneconcentration.)

After installing the ozonation system, NWMC tested to make surethat ozone concentration levels in the air near the coolingtowers were within allowed levels. Since then there has been nodirect measurement of ozone concentration levels in the air atthe towers. However, the ORP values which are obtained on adaily basis should indicate if ozone output becomes excessive.

Regular site services include monthly inspection of the ozonationsystem, plus vacuuming, as needed, of any solids whichprecipitate or settle out in the cooling tower basin, where thewater flows slowly. NWMC also performs an annual maintenanceprocedure on the ozone system, which includes testing of theozone generator.

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Water Savings

After switching to the ozone treatment system, the hospitalreduced the water use of the cooling towers by 13 percent, from6258 gallons per day (gpd) under multiple chemical treatment to5457 gpd under ozone treatment. Table 3 presents the watersavings. The reduced water use is equivalent to nearly 300,000gallons annually.

Table 3. Water Savings from Ozonation at Providence Hospital

Treatment Period Gallons Days Use Note

Consumed (gpd)

Multi- 3/6 - 11/3/87 1,514,400 242 6,258

chemical

Ozone gas 3/6 - 11/3/88 1,255,100 230 5,457 excludes

6/17 - 6/29

Difference 801 12.8% drop

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Data. Water use figures, shown in Table 4, are based on readingsof the makeup meter taken over the first 8 months after thehospital began to rely on ozonation in 1988,.compared to datafrom the same 8 month period in 1987, before ozone treatment.Data were adjusted to account for a 12 day period during whichthe makeup meter did not register water use:

Table 4.Makeup Meter Readings at Providence Cooling Towers

(in

date reading11/03/88 27977410/12/88 27860509/13/88 27716708/11/88 27531407/13/88 27333406/29/88 27241806/17/88 27241806/13/88 27220305/12/88 27002404/13/88 26917003/15/88 26777703/06/8ii 267223

11/03/87 26103310/13/87 25952309/12/87 25743608/11/87 25566807/13/87 25413706/12/87 252239O5/13/87 25008104/14/87 24814203/16/87 24642003/06/87 245889

units of 100 gals.

change days gpd1169 22 53141438 29 49591853 33 56151980 29 6828916 14 6543

0 12 0215 4 5375

2179 32 6809854 29 2945

1393 29 1203554 9 6156

1510 21 71902087 31 67321768 32 55251531 29 52791898 31 61232158 30 71931939 29 66861722 29 5938531 10 5310

It should be noted that the 1987 baseline rate of water use undermulti-chemical treatment was much less than the water use hadbeen a few years earlier. In 1985, for instance, makeup wateruse averaged over 12,000 gpd between mid-March and mid-November.apparently due to problems with the bleed off and basin floatcontrols.

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Eight months after the switch to ozone treatment, the makeupmeter began to frequently stop or under register. This causesone to question whether the makeup meter understated the amountof water used during ozonation. If it did, one would expect areplacement meter to show a higher rate of use than was measuredduring the first eight months of ozonation.

This, however, is not the case. The makeup meter was indeedreplaced in 1990. As shown in Table 5, the new meter indicatesan average makeup water flow rate in Spring 1991 which is over20% less than that measured during Spring 1988 when ozonetreatment began. This suggests that the makeup meter did notseriously under register during the 8 months of 1988 in question,except for the 12 days mentioned above when the meter registerdid not advance.

Table 5.Comparison of makeup use on new meter to use during ozonation.

New makeup meter readings since March '91(in units of 1000 gallons)

date reading change days gpd36/13/91 922 148 32 462505/12/91 774 118 31 380604/11/91 656 85 29 293103/13/91 571

Total 351 92 3815

Makeup, meter readings during ozonation, (1988)(in units of 100 gallons)

date reading change days gpd06/13/88 272203 2179 32 680905/12/88 270024 654 29 294504/13/88 269170 1393 29 480303/15/88 267777

Total 4426 90 4918

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Cause of reduced water use. The reduction in water use from 1987to 1988 was not caused by a change in weather. To rule out thepossibilities that milder weather might have caused decreasedevaporation, either directly or as a result of reduced coolingloads, data from three California Irrigation ManagementInformation System (CIMIS) weather stations in the Bay Area wereexamined. Tables 6 through 8 show the results.

In the coastal North Bay Area (Novato), the months of Marchthrough October were somewhat hotter and, on average, more humidin 1988 than they were in 1987.

Data from the other two stations in San Jose and Walnut Creekwere available for fewer of the months in question, but also showthat 1988 was not cooler than 1987. In the coastal South BayArea (San Jose), July through October was on average over 10°Fhotter in 1988 than in 1987, and hotter during each month forwhich data are available. In the inland East Bay Area (WalnutCreek), the average temperature for the period August throughOctober (the months for which data are available) was almost thesame during the two years, just slightly higher in 1988 than1987.

Relative humidity data at the San Jose and Walnut Creek stationsshow less consistent results. In San Jose, the period Julythrough October was slightly more humid in 1988, on average, thanin 1987. In Walnut Creek, the August through October period wasslightly less humid overall in 1988 compared to 1987.

The water savings were achieved by disconnecting the automaticbleed system, thereby eliminating intentional discharge of thecooling tower water. It would be erroneous, however, to say thatthe towers operated at zero blowdown. Some loss of cooling towerwater did occur through mechanisms other than evaporation ordrift.

Causes of inadvertent blowdown. First, there may have beenoverflow of water from the cooling tower basin. The float in thecooling tower basin, which controls the makeup water valve, hasbeen known to fall out of calibration and to cause excessivemakeup water flow, resulting in overflow." In addition, 1.5gallons per minute (gpm) of fresh water constantly flows throughthe ozone generator into the tower basin. This water flow,equivalent to 2160 gpd, is required to cool the ozone generator'sgrounded electrode. Although this rate of water use is much lessthan average evaporation,16 it is conceivable that at night orduring very cold days, this input of makeup water would exceedevaporation and cause overflow.17 Moreover, the ozone generatorcooling flow at times has been as high as 3 gpm.18

Second, some blowdown is believed to have occurred during thestudy period as a result of mistaken opening of the blowdown

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Table 6. Weather Data for CIMIS Station #63, Novato

Summary for Novato:Somewhat hotter and more humid in 1988 than in 1987.

1987 SOLAR VAPOR AIR TEMP. REL. HUM. DEW WIND WIND AVEDATE ETo PRECIP RAD AVE MAX MIN AVE MAX MIN AVE PT AVE RUN SOIL

in. in. Ly/dy mBars --Fahrenheit-- -----%----- F mph mi F-------------------------------------------------------------------------------------------TOTALS:----I--AVERAGES:-----------------------------------------------MARCH 2.82 3.29 331APRIL 4.47 0.36 531 10.7 74 41 56 94 44 71 46 2.6M A Y 5.53 0.15

9.8 64 38 50 96 57 78 44 2.5 59 5363 60

611 12.7 77 47 61 91 48 71 51 2.8 66 69JUNE 5.70 0.18 666 13.1 78 49 62 88 49 70 51 2.9 69 ??JULY 6.10 0.14 657 13.0 78 50 62 84 48 68 52 3.1 75 71AUGUST 6.02 6.49 575 13.4 82 50 63 82 44 68 52 2.8 67 70SEPT. 4.34 0.08 451 12.1 81 48 61 82 41 66 49 2.5 61 68OCT. 3.17 1.24 1304 11.2 77 46 59 82 47 67 47 2.1 51 64

1987 Mar. - Oct. Average 59 69 49

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valve," or to purge chemicals that were improperly added.*'

Lastly, approximately 500 gallons of water are removed each timethat NWMC's service representative vacuums out loose particlesfrom the cooling tower basin.21

It is not known how much cooling tower water was actuallydischarged by these means.known,

Neither is the amount of evaporationfor measured makeup water use is much less than the amount

of evaporation that would be expected from the operationalcooling load that the engineering staff believe the coolingtowers carry.22 Consequently, no water balance for the towershas been included in this report.

Other Results

Cycles of concentration. With ozone, Providence Hospitalgenerally achieved higher cycles of concentration than it didunder the multi-chemical regime. During multi-chemicaltreatment, monthly measurements of the conductivity of both thecooling tower water and the makeup water were taken by thechemical company representative. From March 1987 to March 1988,these monthly measurements indicate that the cooling towerwater's cycles of concentration varied greatly, from 2 to 36. Onthe majority of these monthly reports, the measured concentrationratio exceeded the intended upper limit. Between March 1987 andMarch 1988, the median concentration ratio was 9.8, based onconductivity.

In comparison; NWMC's water quality analysis in 1988 indicatedthat the ozonated water operated at 25.3 cycles of concentration,based on conductivity. 1990 results showed an even higher levelof conductivity in the tower water than in 1988.23

Fouling of exposed surfaces and microbial growth. Prior to theuse of ozone, water in the cooling tower basin was murky andbiological fouling was plainly visible on tower surfaces. Theozone vendor's analysis of the cooling tower water indicated anaerobic bacterial population in excess of 100,000 colonies permilliliter (col/ml), and an anaerobic bacterial populationgreater than 10,000 col/ml. A laboratory hired by NWMC to verifythe results could not quantify the number of bacteria because ofinterference from the particles in the water. Appendix Acontains the ozone vendor's detailed description of the conditionof the cooling tower prior to ozonation,data.

including the bacteria

After installing the ozone system, the cooling tower water andexposed surfaces improved dramatically in appearance. PacificGas and Electric Company (PG&E), which analyzed corrosion rates

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at the hospital's ozone treated cooling tower in 1988, describesthe improvement: "The circulating water turned clear as theexisting deposits on the wetted surfaces within view graduallydisappeared. "24 "There were no crevices, scaling, or signs ofbiological growth observed on surfaces of testing materials orcooling systems structures. The water was crystal clear..."25

The virtual elimination of microorganisms from the cooling towerwas confirmed by bacterial analysis of the tower water. Justhalf of a month after the switch to ozone treatment, an analysisof a sample of tower water found a bacterial count of 29,000 per100 milliliters (or, 290 per ml), an improvement of roughly 3orders of magnitude over bacterial levels observed during multi-chemical treatment.26 Subsequent analyses showed an additionalimprovement of an order of magnitude: a water quality analysisdone by NWMC in 1988 produced a result of 20 cfu/ml; in 1989NWMC reported less than 100 counts/ml;28 and in 1990 alaboratory hired by NWMC counted just 17 cfu/ml.29

Corrosion. Although there are no data available on corrosionrates at the site during multi-chemical treatment, the corrosionrates of metals exposed to ozonated water have been low. Of fivemetal types tested by PG&E in the summer of 1988, all but greycast iron had rates of less than 0.1 mils (milli-inches) peryear (mpy) , as shown in Table 9.

Table 9. Corrosion Rates Measured by PG&E

Metal Type Corrosion Rates (MPY)

CDA 706 (90-10 Copper-Nickel) 0.01 - 0.02

CDA 443 (Admiralty Brass) 0.01 - 0.02

Grey Cast Iron 1.30 - 1.80

Carbon Steel 1018 0.07 - 0.08

Aluminum 6061 0.01 - 0.02

Note: MPY = milliinches per year (0.001 in./yr.)

According to PG&E, "The higher corrosion rates of [grey castiron] (below two mills per year) were caused by lowerconcentration of total dissolved solids, not allowing completepassivation of this material." (Selected characteristics of theozonated water during the monitoring are presented in Table10.)30

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Table 10. Selected Water Quality CharacteristicsDuring PG&E Monitoring

Parameter Units Value

Ave. Temperature Degrees Centigrade 28-30 (82.4-86.00F)

Ozone Concentration parts per billion 47-48

pH Standard Units 8.6 - 8.9

Source (Tables 9 and 10):Paul A. Burda and Salem A. Attiga, Evaluation of Ozone Technologyfor Chemical Treatment Replacement in Cooling Tower Systems (SanRamon: Pacific Gas & Electric Co., January 1989), Status Report,Report No. 006.2-89.2Measurements taken by NWMC in 1989 and 1990 using corrosioncoupons have also shown low corrosion rates: 1.0 - 1.5 mpy formild steel and 0.05 - 0.1 mpy for copper.31

A slight increase in pitting of chiller heat exchanger tubes,however, did occur after ozone was introduced. Very shortlyafter the multi-chemical treatment was replaced with ozone, testsof all of the condenser tubes found no defects in Chiller #l, andfound one tube (out of 660) in Chiller #2 with 20% internaldiameter (I.D.) pitting. A year later, when Chiller #l wasreopened, 3 tubes (also out of 660) showed 20% I.D. pitting. By1990, the number of tubes in Chiller #2 with 20% I.D. pitting hadalso increased to three, although this condition was described as"not a problem at this time." In both chillers, the status ofeach pitted tube was deemed acceptable by the company performingthe testing.32 Table 11 shows results from the 1988 - 1990tests of the chiller tubes."

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Table 11. Results from Tests of Chiller Condenser Tubes

Year Chiller No. 1

1990 Not tested.

Chiller No. 2

3 tubes w/ 20% I.D.pitting; ea. tube deemed'*acceptable;" overalldeemed "not a problem atthis time." "Lightscale.. Recommend checkwater treatment."

1989 3 tubes with 20% I.D. Not tested.pitting; ea. tube deemed"acceptable." "Rust andlight scale...Check watertreatment."

1988 No defects. "Recommend 1 tube w/ 20% I.D.brushing the tubes." pitting; tube deemed

"acceptable." "Veryminor defect; allremaining tubes nodefects."

Note: I.D. = internal diameterSource: Non Destructive Eddy Current Test Reports from Pacific

Coast Trane Service to Providence Hospital.

The 1989 inspection of Chiller #l also found rust in thecondenser section. The chemical supplier representative, presentwhen the chiller was open, took photographs of what he describedas "corrosion of the tube sheet (due to electrolysis)."34 Thephotographs show what would appear to be serious galvaniccorrosion of the condenser tubes end bell, where metals ofdifferent types are in close proximity.

The end bell corrosion, which extended to all of the condensertube end bells, was also present when the chillers were firstopened up in 1988. Providence staff believe the end bellcorrosion would have occurred regardless of the water treatmentmethod. After the 1989 chiller inspection sacrificial pieces ofmetal were installed, arresting the problem.35

The 1989 and 1990 examinations also found "light scale" in thechillers' condenser tubes.36 The chemical supplier took asample of the scale in 1989 at the same time as he photographedthe electrolysis. A consultant hired by the chemical suppliercharacterized the substance as principally composed of silicatesand, secondarily, of what was probably calcium carbonate.Appendix B contains the characterization report.

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Due to the observed scale and rust, the inspector(s) of thechillers in both 1989 and 1990 recommended that the hospitalcheck its cooling water treatment.

Although one would probably assume that the slightly increasedpitting and the presence of scale and rust in the chillers aredue to the ozone treatment, this is not known with certainty.For example, it is conceivable that some of the rust may havebeen related to the end bell corrosion, which Providence does notattribute to ozone treatment. It is also conceivable that anexternally caused change in the pH of the cooling tower waterchemistry could have played a role. A sudden drop in pH and asimultaneous increase in instantaneous corrosion rates did occurduring PG&E's monitoring of corrosion rates during the summer of1988. This suggested to NWMC that a foreign chemical had beenadded to the cooling tower water.37 In March 1989, too, NWMCfound a foreign substance in one of the tower basins. Appendix Cis a copy of NWMC's letter to Providence on the subject.

An additional equipment problem at the hospital also developedafter the switch to ozone treatment. A fan motor was damaged bymoist air penetrating into its housing. Although the suitabilityof the motor for a moist environment and the adequacy of itsseals were the fundamental issues, some Providence staff alsoworried that ozone might have contributed to the damage. Afterthe pump was replaced with one with adequate seals, epoxy, andpaint, the problem appeared to have been resolved to thehospital's satisfaction. In May 1991 the fan motor on the secondcooling tower also had to be replaced due to corrosion. This wasnot unexpected by Providence.38 The motor from the second toweralso is being replaced by a new motor with a special epoxycoating.

Customer satisfaction. On the whole, the hospital is pleasedwith the performance of the ozone treatment. As explained byProvidence's Chief Engineer, the hospital values the excellentmicrobiological control that ozone has delivered. It alsoappreciates the environmental benefits, foreseeing more reasonsin the future for facilities to switch to practices that areenvironmentally benign.

Providence believes that ozone has not resulted in any seriousdestruction of equipment. As mentioned above, it does notattribute the end bell corrosion to the ozone treatment, and itfeels that the replacement of the fan motors with ones that havespecial coatings was a necessity and a satisfactory solution. Italso sees the pitting and observed deposits in the heat exchangetubes as a minor problem which is not getting appreciablyworse.39

The hospital's satisfaction is further evidenced by the fact that

24

it has not only continued to use the ozone system in the mainbuilding, it has recently selected ozone to replace a multiplechemical treatment program at the cooling tower in its recentlyconstructed Medical Office Building.

Cost Effectiveness

It is not known whether Providence's employment of the ozonesystem has yielded net benefits or net costs. This is due to theuncertainty about the ozone treatment's effect on heat exchangeefficiency and on equipment longevity. However, costs andsavings resulting from ozone treatment that are known, or thatcan be reasonably estimated, are listed in Table 12 in 1991dollars.40

Table 12. Known Annual Costs and Monetary Savings

ITEM COST / AVOIDED COST

Avoided Chemical $5,582

Avoided Labor $1,260

Avoided Fresh Water $387

Avoided Sewer Conveyance $211

Avoided Wastewater Treatment $227

TOTAL KNOWN AVOIDED COSTS $7,667

Ozone Lease and Service $12,960

Electricity to Generate Ozone $418

Electricity to RecirculateWater in Ozone Injection- Loop

$593

Electricity to Compress Air

Phone Usage

TOTAL OZONE OPERATING COSTS

$1,011

$180

$15,162

KNOWN NET ANNUAL COST $7,495

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If ozone treatment caused a significant change in the heatexchange efficiency of the chiller tubes, it would make a greatdifference to the cost of running the cooling system. Forinstance, a 5.5% change in chiller energy use at Providencewould be equivalent to a change in energy costs of more than$9,000 41 A change in equipment lifespan would also imposesignificant, real costs or savings on the hospital.

Chiller energy use. Unfortunately, no baseline data exists onthe condition of the chiller heat exchange tubes prior to ozonetreatment. As indicated earlier, the hospital began to employ acompany to inspect and test the chillers only after ozonationbegan. Since there is no separate electricity meter serving thechillers, it cannot be determined with certainty whether chillerenergy consumption increased or decreased.

There are conflicting indicators of the probable condition of thechiller heat exchange tubes during multi-chemical treatment. Onone hand, the fouling of exposed cooling tower surfaces and thepresence of numerous bacteria in the cooling tower water suggestthere may have been slimes or other types of biological foulingpresent in the heat exchange tubes. On the other hand, nosignificant problems with the heat exchange tubes were reportedin 1988 less than a month after the multiple chemical treatmentprogram was terminated.

Equipment longevity. The lack of certainty about the effect ofozone on equipment longevity also is due to conflicting evidence.On one hand are the observed pitting of the heat exchange tubesand the corrosion of the fan motors. On the other hand are thelow corrosion rates measured by PG&E and NWMC which would seem tosuggest that ozone should cause little observable corrosion.

Chemical savings. Chemical savings of roughly $5600 are based onlogged manual additions of chemicals, using actual unit costs in1987, adjusted upward by 3%

4 2per year in order to express in 1991

dollars.

Labor savings. Providence Hospital saved approximately 5 hoursof labor per month that had been required to manually addchemicals, equivalent to about $1300 annually.43

Ozone system energy use. Energy costs to run the ozone system,approximately $2000, have been estimated, since there is noseparate electricity meter that measures electricity consumptionof the ozone system components. Estimates of electricityconsumption by the ozone generator and by the water pump used in'the ozone injection loop have been derived using formulasprovided by NWMC. These formulas suggest a higher consumption of

26

electricity per pound of generated ozone than has been estimatedby PG&E.44 The electricity used by the air compressor wasassumed to be equal to both the generator consumption and theconsumption of the recirculating pump.45

Conclusions

Providence Hospital, after replacing multiple chemical treatmentof cooling tower water with treatment by ozone gas:

0 reduced makeup water use by 13%,0 more than doubled the cycles of concentration (based on

conductivity),0 eliminated fouling and scaling of exposed surfaces,0 experienced no new scaling of exposed surfaces,0 dramatically improved water clarity,0 greatly reduced bacteria levels,0 achieved low corrosion rates,0 experienced minor pitting and scaling of heat exchange

tubes,0 discovered corrosion of condenser tube end bells, and0 replaced two fan motors due to corrosion.

Based on the experience at this site, ozone should be consideredfor treatment of cooling tower water at least where the followingconditions are met:

0 the cooling water's chief function is to remove heatfrom medium sized heating, ventilation, and airconditioning (HVAC) systems;

0 the ozone system is well designed, monitored, andmaintained;

0 the makeup water quality is low in dissolved solids.

Due to some observed pitting and scaling of heat exchange tubesand the corrosion of fan motors after introduction of ozone, anunequivocable recommendation for use of ozone to treat coolingtower water cannot be made.

Where ozone treatment is considered, a decision to use or not touse ozone at any one site should be based upon its expected costeffectiveness and on the perceived importance of factors thatcannot be adequately expressed in dollar terms.

The cost effectiveness of any ozone system installation dependsupon several factors. One is the degree of avoided or increasedfouling or scaling, which would affect energy consumption. Asecond is the effect on equipment corrosion: if low corrosionrates extend the expected life of the equipment, a benefit would

27

be obtained; if equipment experienced increased corrosion, thecost to replace it would be borne sooner. A third factor is thesize of the cooling system, which not only affects the sizing ofthe ozone system, and thus the size of the ozonation contractfee, but also affects the water and energy consumption. A finalcost consideration is the makeup water quality, which affects theupper limits on the achievable cycles of concentration and thusaffects the water related savings.

Other benefits which are not easily quantified in dollar termsneed to be factored into the decision--as well. One is the valueto the customer of the excellent microbiological control thatozone provides. Environmental benefits include reduced waterconsumption and discharge, as well as reduced toxicity offacility discharges.

Tradeoffs, too, need to be weighed. Ozone allows the toweroperator to avoid storing and handling hazardous chemicals, yetpresents the potential to increase exposure to ozone gas. Inaddition, an ozone lease and service contract reduces customerlabor and automates the treatment process, but this may also havea down side. Facility operations staff might see the ozonesystem as a foreign black box over which they have limitedcontrol. This could lead to operations staff developingattitudes of suspicion, perhaps even hostility, towards the ozonesystem or the ozone vendor, and ultimately could interfere withrelations with the vendor, handling of the cooling tower system,or with the work environment itself if negative feelings weremisdirected.

Potential for Adoption Statewide

Direct purchase, installation, and operation of an ozone systemwould be limited to organizations with sufficient technicalexpertise to monitor, diagnose, and maintain an ozonation systemover the long term. For this reason, facilities that haveattempted to piece together their own ozone control systems havenot always achieved successful results.

Consequently, today the standard arrangement for using ozone totreat cooling tower water is to hire the services of a companythat specializes in the installation,. maintenance, and monitoringof ozone systems, such as National Water Management Corporationor TriOx, its chief competitor. Adoption of this approachthroughout California might be geographicly limited by theavailability of such services. Principal service areas inCalifornia in 1991 are the San Francisco Bay and Los Angelesareas, but rapid expansion by NWMc and TriOx is anticipated.

AS of 1991 there is little regulatory review of new installations

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of ozone generating systems. This is largely due to the youth ofthis technology in the United States: use of ozone to treatcooling tower water in the U.S. appears to have begun less than15 years ago: only since the late 1980s have ozone treatmentservices been available.

Sources of ozone are not specifically regulated by air qualityagencies, for instance, because in the past ozone has notgenerally been emitted. Rather it has been formed in urban smogby other emitted pollutants interacting with sunlight.Nevertheless, laws aimed at attaining and maintaining state andfederal ambient air quality standards for ozone could potentiallybe brought to bear on ozone generators. Small generators,however, may not need specific approval from air qualitydistricts, since ozone used for cooling tower treatment isproduced in low concentrations and is almost completely consumedby oxidizable impurities in the cooling water. The Bay Area AirQuality Management District (BAAQMD) suggests as a guideline thatcooling water treatment systems or other small ozone generatorsof less than 20 lbs. per day locating in its area of jurisdictionneed not seek approval from it. Larger generators, however, suchas municipal drinking water or wastewater treatment systems,should contact the BAAQMD for the need to obtain a permit or forexemption, which the agency would evaluate on a case by casebasis. (These larger systems generally have abatement systems toreduce emissions.)46

One new area of regulation is likely to be the Uniform Fire Code(UFC) . In 1990 changes were proposed to the UFC that wouldgovern the installation of ozone generating systems.47 These,however, are not yet in effect.48

The above factors appear for the near future to be the onlyexternal constraints on the adoption of ozone technology forcooling tower water treatment in California.

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6.

ENDNOTES

H. Banks Edwards, "Ozone: An Alternate Method of TreatingCooling Tower Water, " Journal of the Cooling TowerInstitute, 8, No. 2 (1987), 10-21.

For a discussion of the relationship between watersidefouling of condenser heat exchange tubes and condenserperformance, see Charles E. Middleton, Review of R&DOpportunities in Ozone Technology (San Ramon, Ca.: PacificGas & Electric Co., Dept. of Research and Development,1990), Report 008.1-90.2, pp. 2-8.

For a discussion of ozone's effectiveness in destroyingLegionella, as well as background information aboutLegionnaires Disease and the bacteria which causes it, seeEdwards.

See Douglas T. Merrill and Joseph A. Drago, Evaluation ofOzone Treatment in Air Conditionins Coolinq Towers (WalnutCreek, Ca.: Brown and Caldwell, 1979) and Leroy V. Baldwin,Ellen S. Feeney, and Rick Blackwelder, The Investigation andApplication of Ozone for Coolinq Water Treatment (KennedySpace Center, Fl.: EG&G Florida, 1985), from 46th AnnualMeeting International Water Conference, Pittsburgh, Pa.,4-7 November 1985, IWC No. 85-36.

George Wofford, Cindy Slezak, and Michael Bukay, CorrosionControl in Ozonated Cooling Water Systems, rpt. fromUltrapure Water Expo '90 West, pp. 24-31. Also, ChrisMorrison, District Sales Manager, NALCO Chemical Co.,Pleasant Hill, Ca., verbal communication, 1 April 1991.

Ozone's effectiveness in microbiological control isacknowledged by vendors of ozone and multiple chemicaltreatment programs alike. Ozone's ability to directlycontrol corrosion and inorganic scale, however, is disputed.For a description of corrosion control mechanisms usingozone, see Wofford et al. For a description of ozone scalecontrol mechanisms, see Marshall F. Humphrey, Cooling TowerWater Conditioning Study (Pasadena, Ca: Jet PropulsionLaboratory, 1981). For other reports discussing mechanismsOf ozone scale or corrosion control, see Merrill and Drago;Kenneth R. French, Ronald D. Howe, and Marshall F. Humphrey,"Ozone Inhibits Corrosion in Cooling Towers,*' NASA Tech.Briefs, Fall 1979; and Edwards. In addition, PG&E isexpected to soon release a study of the use of ozone totreat cooling water at a natural gas compressor station,which is likely to discuss the issues of scale and corrosion

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ENDNOTES

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

Alan Pryor, Executive Vice President - Development, NationalWater Management Corp. (NWMC), verbal communication,16 October 1989.

Alan Pryor, "Ozone Toxicology, Exposure Threshold LimitValues, and Safety Precautions," Ozone News, Nov./Dee. 1990.

Woody Hill, Industrial Hygienist, Cal/OSHA ConsultationService, San Mateo, verbal- communication, 7 May 1991.Also, Air Contaminants - Permissible Exposure Limits (Title29 Code of Federal Regulations Part 1910.1000), OccupationalSafety and Health Administration Reprint, OSHA 3112 (1989).

Edwards.

Based on 85°F average water temperature to chillers and 91°Faverage water temperature from chillers. These averageswere derived from values noted in the annual chillerpreventive maintenance inspection reports from Pacific CoastTrane Service, 1988 - 1990.

Steve Weber, Chief Engineer, Providence Hospital, verbalcommunication, August 1990.

Although the generator produces a constant amount of ozone,it could be modified to automatically vary its outputaccording to changes in the measured oxidation reductionpotential (ORP) of the water in the ozone injection loop.(Mark Fisher, Commercial Service Manager, NWMC, verbalcommunication, 17 October 1989.)

Weber, verbal communication, 26 June 1991.

The overflow control comes out of adjustment periodically,and is a problem of equal magnitude from year to year.(Weber, verbal communications, 26 October 1990 and 26 June1991.)

Average evaporation of the hospital cooling towers, ifoperating at 300 tons of cooling with a 60F temperaturedrop, would be expected to be 5.4 gpm (7776 gpd), onaverage. If the towers are actually carrying a cooling loadof 200 tons, average expected evaporation with a 6°Ftemperature drop would be 3.6gpm (5184 gpd). The formulaused to estimate evaporation is:

Evap. =(delta) t x . 001 x 3 gpm x Tons

It is NWMC's understanding that to prevent the ozonegenerator cooling water from creating overflow, the waterlevel at which the float control would trigger makeup water

31

ENDNOTES

Burda and Attiga, p. 9.30.

31.

32.

33.

34.

35.

36.

37.

38.

39.

40.

41.

Pryor, 20 February 1991.

Pacific Coast Trane Service, Non-Destructive Eddy CurrentTest Analysis Reports for Providence Hospital TRANECentrifugal Chillers, 1988-90.

The ozone vendor questions the reliability of the reports ofpitting, citing the fact that the tube in Chiller #2identified in 1988 as having pitting was not so identifiedin 1990. Specifically, Tube No. 18 of Row 5 was found in1988 to have some pitting, but in 1990 the tubes withpitting were Tube 13 of Row 16, Tube 16 of Row 17, and Tube16 of Row 18.

However, the system of reference used in 1988 was not thesame as that used in 1990. In 1988, the test end was theright, the row direction horizontal, the row count top leftto bottom right, and the facing panel tube count bottom leftto top right. By contrast, in 1990, the test end was theleft, the row direction diagonal, the row count top right tobottom left, and the panel tube count top left to bottomright. It is impossible for an untrained observer withaccess only to the written test results to decide whether ornot the same tube was referred to in 1988 and 1990.

Peter Gunderson, Aqua Treat Chemicals, Inc., Belmont, Ca.,Consulting Service Report to Providence Hospital, 7 March1989.

Weber, 26 June 1991.

Trane.

Pryor and Bukay. Also, Steve Weber, verbal communication,28 September 1989.

Weber, 26 June 1991.

Providence perceptions of ozone performance from SteveWeber, 26 June 1991.

Ozone lease and service costs are the same in 1991 as theywere in 1987, when the ozonation contract was signed.

A change in heat exchanger fouling from a design clean stateto slight fouling, or a decrease in fouling from moderate toslight fouling, could be considered equivalent to a 5.5%change in condenser energy use. (NWMC, Technical Bulletin

32

ENDNOTES

No. 6: Energy Savings Through Cooling Tower Ozonation,Revision No. 1 (1988; San Jose, Ca.: NWMC, 9 November 1988),p. 2, Table 1.) The value of such a change in energy usewas estimated for Providence assuming 300 tons of coolingand 8.48c per kilowatt-hour, and using the formula:

Tons x . 76 KW/Ton x % energy change x hrs. x c/KW-hr.(Formula from Keith Victor, Vice President of ProjectDevelopment, NWMC, proposal to Steve Weber, 21 Aug. 1987,and from Alan Pryor, verbal communication, 23 Oct. 1989.)

42. A 3% annual increase to boost 1987 chemical costs to their1991 equivalents was suggested by the chemical vendor, PeterGunderson, Agua Treat Chemical Co., verbal communication,19 May 1991.

43. Weber, verbal communications, 29 June 1990 and 29 August1990.

44. Middleton.

45. Information on typical air compressor energy consumption inan ozone system relative to that of an ozone generator andan injection loop water pump provided by William K. McGrane,Applications & Development Engineer, TriOx Corp., Dublin,Ca., facsimile transmission (FAX) of 17 May 1991.

33