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Ozonation of Cooling Tower Water: A Case Studyby Stephen OsgoodWater Conservation UnitEast Bay Municipal Utility District June 1991 Completed under Contract to the California Department

Ozonation of Cooling Tower Water: A Case Study

by Stephen OsgoodWater 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 UnitEast Bay Municipal Utility District

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 microbiological

control 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 program

at the cooling tower in a second building

The experience at this site suggests that ozone treatment of

cooling tower water should be considered at least where the

following conditions are met:

the cooling water's chief function is to remove heatfrom medium sized heating, ventilation, and air

conditioning (HVAC) systems;

the ozone system is well designed, monitored, andmaintained:

the makeup water quality is low in dissolved solids

Report ContentsThe purpose of this report is to describe the technology employedand the results it achieved The next few sections provide

background 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 should

be taken into account when ozone is considered for cooling towerwater treatment, and ends with a brief discussion of the

potential for ozone technology to be adopted throughout

California

Open Recirculating Cooling SystemsWater gains heat when used for cooling To be reused, the

water's temperature must be reduced, typically by passing it

through a cooling tower In a cooling tower the warm water

enters 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 collects

in a basin at the bottom of the tower, from where it is

recirculated to again perform its cooling function

As water evaporates, dissolved solids remain behind and increase

in concentration The extent to which this occurs is referred to

as 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 (For

example, given makeup water with Total Dissolved Solids (TDS) of

58 parts per million (ppm), a cooling tower with water at 145 ppmTDS would be operating at- 2.5 cycles of concentration.) A

continuing increase in dissolved solids can lead to salts of

calcium, 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 volume

of fresh makeup water (The discharge is referred to as "bleedoff", or "blowdown")

A cooling tower operating at relatively high cycles of

concentration will save water compared to a similar one operating

at lower cycles This is because the tower with higher cycleshas less blowdown and less makeup water use However, as shown

in Figures 1 and 2, the relationship between cycles of

concentration and blowdown is not a simple linear one The mostdramatic water savings are achieved when one moves from very low

2

cycles of concentration to more moderate ones As the number ofcycles increases further, more water is saved, but the

incremental reduction in blowdown and makeup becomes less

significant

Operating a recirculating cooling system also presents other

problems 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 (Legionella

pneumophila), may threaten the health of people exposed to

airborne water droplets Workers who clean the inside of

condenser heat exchange tubes may also be exposed to

Legionella 1 At a hospital, where weakened patients are

particularly susceptible to infectious organisms and health

professionals are frequently exposed to pathogens, the control ofmicrobial growth in cooling tower water is critical

Corrosion is another problem to be minimized It not only

destroys metal surfaces, it also produces deposits which can

contribute to the fouling of surfaces Airborne particles (such

as 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 as

insulators, decreasing the efficiency of the heat transfer Thiscan lead to inadequate cooling or, at the least, to an increase

in the amount of energy expended to produce the same amount ofcooling.2

Multiple Chemical TreatmentRecirculating cooling waters are often treated by adding

chemicals which are selected to control one or more of the

problems 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 tension

so that particles are less likely to adhere tosurfaces

Maintaining correct water quality involves controlling the rates

of blowdown and makeup water flow and involves adding chemicals

in correct amounts at proper times. This, in turn, requires

insuring the compatibility of the chemicals, and requires

monitoring and controlling pH and conductivity

Chemical treatment carries with it the risks and responsibilities

of storing and handling hazardous materials In addition, it isundesirable to discharge toxic chemicals to aquatic ecosystems or

to wastewater treatment plants that rely on bacterial activity

Ozone TreatmentOzonation, in contrast to traditional chemical treatment,

involves the on site generation of a single oxidizing agent which

is mixed into the recirculating water

Typically, ozone is produced by the corona discharge method, inwhich dry air is passed through a gap between a highly

electrically charged surface and a grounded surface When

electrical 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 the

buildup of other deposits, or contribute to corrosion.

Ozonation has also been reported to achieve higher cycles of

concentration than multi-chemical treatment.4 Since there is

less blowdown at higher cycles, ozonation offers the potential tosave water In addition, when slightly alkaline water (pH

greater than 7) is concentrated, the alkalinity becomes even morepronounced Operating cooling towers at higher cycles of

concentration 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 20

to 30 minutes, and in cooling tower water, where there are

oxidizable impurities, 1 to 3 minutes.)7 As a result, the

treated cooling water can be discharged safely to the sewer

system 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 plants

or aquatic ecosystems.

Since an ozone generator will produce the gas at concentrations

of just 1 to 3 percent by weight in air, the resulting ozone/air

mixture is not explosive.

Ozone is a toxic gas The maximum average allowable ozone

concentration to which workers in California may be exposed over

an 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 exposure

limit It is conceivable, however, that a gradual increase inozone concentration might not be noticed by someone working close

to an ozonated tower

Study SiteFacility 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 medical

care, the hospital houses 228 beds and employs 720 people Themain hospital building, which utilizes the cooling system

discussed in this report, has a floor area of approximately

275,000 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 cooling

system depends on two chillers which use the water from the

cooling 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 heat

at the point of application, it returns in a closed loop to thechillers, where the heat is transferred to an internally

recirculated 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 the

water returning to the cooling towers Finally, the cooling

towers 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

towers, with connected basinsOzone generation principle corona discharge

Ozone generator manufacturer PCI Ozone Corp.,

modified by NWMCOzone 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 capacity

tons

together they bear an average heat load of 300

Effective biological control of cooling tower water is important

at 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 exposed

to 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 accreditation

requirements Windows in both the main hospital building and the

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

Makeup water. The hospital uses drinking water supplied by EBMUB

as 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 EBMUD

water quality characteristics during the 1980's, when the hospitalswitched its cooling tower water treatment

The hospital has its own internal water meter which registers

quantities of makeup water flowing to the cooling towers

Providence staff read the meter twice daily

8

Table 2 Selected EBMUD Water Quality Characteristics

Chlorine parts per million (ppm) 0.35

(0.001 in./yr.)

Specific Conductance micromho per centimeter 73

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

Sept/Oct 1989

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 of

an ozonation system owned and installed by National Water

Management Corporation (NWMC) The hospital terminated manualchemical additions and started relying on ozone at the beginning

of 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 used

at the hospital The ozone generator was manufactured by PCIOzone Corporation and modified by NWMC for compliance with

proposed Uniform Fire Code safety standards The generator canproduce up to three pounds of ozone gas per day, but has been set

to operate at 65% of capacity.13

10

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

Operation and maintenance.

runs the ozonation equipment

Under the ozonation contract, NWMCand monitors and maintains the waterquality of the cooling towers Once the ozone system was

installed, Providence hired a company to inspect the heat

exchange 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, both chillers were inspected; since then eachIn the first year ofchiller has been inspected on alternate years Although the

hospital continues to briefly check the cooling towers once eachshift, its own routine maintenance efforts consist only of

quarterly 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, or

0 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 the

following 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 cooling

towers 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 a

daily basis should indicate if ozone output becomes excessive.Regular site services include monthly inspection of the ozonationsystem, plus vacuuming, as needed, of any solids which

precipitate or settle out in the cooling tower basin, where thewater flows slowly NWMC also performs an annual maintenance

procedure on the ozone system, which includes testing of the

ozone generator

12

Water SavingsAfter switching to the ozone treatment system, the hospitalreduced the water use of the cooling towers by 13 percent, from

6258 gallons per day (gpd) under multiple chemical treatment to

5457 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

Data Water use figures, shown in Table 4, are based on readings

of the makeup meter taken over the first 8 months after the

hospital 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

controls

14

Eight months after the switch to ozone treatment, the makeup

meter began to frequently stop or under register This causesone to question whether the makeup meter understated the amount

of 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 indicates

an average makeup water flow rate in Spring 1991 which is over

20% less than that measured during Spring 1988 when ozone

treatment 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)

(in units of 100 gallons)

date reading change days gpd