Ozonation of Cooling Tower Water: A Case Study

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Ozonation of Cooling Tower Water: A Case Study

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  1. Ozonation of Cooling Tower Water: A Case Study by Stephen Osgood Water Conservation Unit East Bay Municipal Utility District June 1991 Completed under Contract to the California Department of Water Resources Water Conservation Office
  2. 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 the method it uses to treat the water in two cooling towers, replacing a multiple chemical treatment program with ozone gas treatment. As a result, the hospital reduced water use feeding the cooling towers by 13 percent. In addition, after changing the cooling tower water treatment, the hospital: more than doubled the cycles of concentration (based on conductivity), 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 exchange tubes, discovered corrosion of condenser tube end bells, and replaced two fan motors due to corrosion. On the whole, the hospital is pleased with the performance of the ozone system. It values ozone's excellent microbiological control and environmental compatibility. It does not believe there has been any serious destruction of equipment. Consequently, the hospital has not only continued to use ozone in the cooling towers of the main building, it has also recently selected 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 heat from medium sized heating, ventilation, and air conditioning (HVAC) systems; the ozone system is well designed, monitored, and maintained: the makeup water quality is low in dissolved solids. 1
  3. Report Contents The purpose of this report is to describe the technology employed and the results it achieved. The next few sections provide background information on the use and treatment of recirculating cooling water systems. Details then follow of the technology employed at the study site, the water savings, other results, and the costs and savings. The report identifies factors that should be taken into account when ozone is considered for cooling tower water treatment, and ends with a brief discussion of the potential for ozone technology to be adopted throughout California. Open Recirculating Cooling Systems Water 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 the temperature of the water that remains behind. When needed, a fan boosts air flow across the water, thereby increasing evaporation and heat loss. The air expelled by the fan can also carry off water droplets ("drift"). "Makeup" water is added to replace what 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 concentration ratio, which is the ratio of the quantity of dissolved solids in the 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 ppm TDS 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 and forming scale deposits on cooling system surfaces. To dilute the water and minimize scaling, the concentrated water of the cooling tower is discharged and is then replaced by an equivalent volume of fresh makeup water. (The discharge is referred to as "bleed off", 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 cycles has 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 most dramatic water savings are achieved when one moves from very low 2
  4. cycles of concentration to more moderate ones. As the number of cycles 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 waters provide an ideal environment for microbiological growth, which can 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 1 Legionella. At a hospital, where weakened patients are particularly susceptible to infectious organisms and health professionals are frequently exposed to pathogens, the control of microbial 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 and also contribute to fouling. Scale, slimes, and other types of fouling, when present on heat exchanging surfaces, act as insulators, decreasing the efficiency of the heat transfer. This can lead to inadequate cooling or, at the least, to an increase in the amount of energy expended to produce the same amount of cooling.2 Multiple Chemical Treatment Recirculating 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 over metal areas, acids or other scale inhibitors which prevent mineral precipitation, conditioners which decrease the density of any scale particles which form, allowing the particles to be more easily carried off by the flowing water, dispersants which increase foulants' electrical charges, causing them to repel each other, and wetting agents which reduce the water's surface tension so that particles are less likely to adhere to surfaces.
  5. 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 is undesirable to discharge toxic chemicals to aquatic ecosystems or to 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 which is mixed into the recirculating water. Typically, ozone is produced by the corona discharge method, in which 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 in the air is converted to ozone gas. Potential benefits. As a highly powerful oxidant, ozone destroys microorganisms which may threaten health (including Leoionella pneumophila3), foul cooling system surfaces, encourage the buildup of other deposits, or contribute to corrosion. Ozonation has also been reported to achieve 4higher cycles of concentration than multi-chemical treatment. Since there is less blowdown at higher cycles, ozonation offers the potential to save water. In addition, when slightly alkaline water (pH greater than 7) is concentrated, the alkalinity becomes even more pronounced. Operating cooling towers at higher cycles of concentration thus creates a more alkaline condition, reducing corrosivity.5 Ozone also has been promoted as 6 effective method of directly an controlling corrosion and scale. Environmental and Safety Aspects. Highly reactive, ozone resides only 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 5
  6. 8 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 (maximum allowable average concentration over any 15 minute period) is 9 0.3 ppm. By contrast, ozone gas1Ocan be detected by smell at concentrations as low as 0.02 ppm , well below the exposure limit. It is conceivable, however, that a gradual increase in ozone concentration might not be noticed by someone working close to an ozonated tower. Study Site Facility. Providence Hospital ("Providence") in Oakland, California (a coastal city) receives fresh water and wastewater treatment 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. The main 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 the operating 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) by means of a condensed refrigerant. The chillers pump the chilled water to points in the hospital where it cools indoor air, or performs other functions. After the chilled water absorbs heat at the point of application, it returns in a closed loop to the chillers, where the heat is transferred to an internally recirculated refrigerant. The refrigerant warms and expands. In the condenser section of the chiller, the refrigerant is passed over copper tubes through-which passes the water from the cooling towers. 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 of water vapor. Table 1 lists characteristics of the hospital's cooling system and cooling towers.
  7. Table 1. Cooling System Characteristics, Providence Hospital Chiller capacity 354 Tons (425,000 BTU/hr.) Chiller operation (ave.) 85% of capacity (300 Tons) 11 Cooling temp. change (at) 6°F 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, crossflow towers, 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, each with a capacity to remove 360,000 BTU's of heat per hour (300 tons). Their basins are interconnected, and fans at the top of the towers induce an upward flow of air when they are engaged. Although water flows continuously through both towers, during most of the year only one fan is needed to boost air flow, and it engages intermittently. Only on the hottest days of the year does the extra heat load cause the fan on the secondary tower to engage. With the primary tower operating at approximately 75% of capacity and the secondary tower operating in the vicinity of 25% of capacity together they bear an average heat load of 300 tons. Effective biological control of cooling tower water is important at the hospital. Windows in one of the hospital buildings which overlook the towers are often kept open for ventilation. These rooms, which are used for office space, may at times be exposed to cooling tower drift. Additionally, the engineering section must report quarterly on the biological condition of the cooling tower to the hospital's quality assurance team, which is charged with ensuring compliance with hospital accreditation requirements. Windows in both the main hospital building and the 7
  8. new Medical Office Building (MOB) overlook the towers and may be exposed 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 hospital switched 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
  9. Table 2. Selected EBMUD Water Quality Characteristics Parameter Units Average* Microbiological Total Coliform Bacteria 0.07 per 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. 9
  10. Multiple chemical treatment program. Prior to 1988, the hospital used several chemicals to treat its cooling water: a corrosion and deposit inhibitor, two microbiocides, a dispersant, and an antifoaming agent. The corrosion and deposit inhibitor was fed automatically to the makeup water. When the water returning from the chillers to the cooling tower rose above a set level of conductivity, a valve would open to bleed off some of the water. Simultaneously, the corrosion and deposit inhibitor would be injected into the water that returned to the tower. All other chemicals were added manually. The microbiocides and dispersant were added approximately once a week; the anti-foaming agent was added as needed. The representative of the chemical vendor checked monthly on the condition 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 manual chemical 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 the equipment and for services. Other costs involved in operating the 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 PCI Ozone Corporation and modified by NWMC for compliance with proposed Uniform Fire Code safety standards. The generator can produce up to three pounds of 13 ozone gas per day, but has been set to operate at 65% of capacity. 10
  11. 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 effective production of ozone gas. Ozone injector. Mixes ozone gas with cooling tower water which has been pumped out of the tower basins. After injection of ozone, the water recirculates back to the basins. Monitoring system. Continuously monitors cooling tower water quality and the operating status of the ozonation equipment. Telecommunications equipment allows the data to be remotely accessed by personal computer. 11
  12. Operation and maintenance. Under the ozonation contract, NWMC runs the ozonation equipment and monitors and maintains the water quality 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 the first time since the chillers were installed in 1979 that the condenser tubes had been inspected.14 In the first year of ozone treatment, both chillers were inspected; since then each chiller has been inspected on alternate years. Although the hospital continues to briefly check the cooling towers once each shift, 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 the condition of the ozone equipment and the water. The monitoring system sends yes/no signals to indicate if there is a problem with: 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 generator automatically shuts down. NWMC's computer would then flag the condition 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 ozone concentration.) After installing the ozonation system, NWMC tested to make sure that ozone concentration levels in the air near the cooling towers were within allowed levels. Since then there has been no direct measurement of ozone concentration levels in the air at the 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 ozonation system, plus vacuuming, as needed, of any solids which precipitate or settle out in the cooling tower basin, where the water flows slowly. NWMC also performs an annual maintenance procedure on the ozone system, which includes testing of the ozone generator. 12
  13. Water Savings After switching to the ozone treatment system, the hospital reduced 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 water savings. The reduced water use is equivalent to nearly 300,000 gallons 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 13
  14. 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 data from the same 8 month period in 1987, before ozone treatment. Data were adjusted to account for a 12 day period during which the makeup meter did not register water use: Table 4. Makeup Meter Readings at Providence Cooling Towers (in units of 100 gals. date reading change days gpd 11/03/88 279774 1169 22 5314 10/12/88 278605 1438 29 4959 09/13/88 277167 1853 33 5615 08/11/88 275314 1980 29 6828 07/13/88 273334 916 14 6543 06/29/88 272418 0 12 0 06/17/88 272418 215 4 5375 06/13/88 272203 2179 32 6809 05/12/88 270024 854 29 2945 04/13/88 269170 1393 29 1203 03/15/88 267777 554 9 6156 03/06/8ii 267223 11/03/87 261033 1510 21 7190 10/13/87 259523 2087 31 6732 09/12/87 257436 1768 32 5525 08/11/87 255668 1531 29 5279 07/13/87 254137 1898 31 6123 06/12/87 252239 2158 30 7193 O5/13/87 250081 1939 29 6686 04/14/87 248142 1722 29 5938 03/16/87 246420 531 10 5310 03/06/87 245889 It should be noted that the 1987 baseline rate of water use under multi-chemical treatment was much less than the water use had been a few years earlier. In 1985, for instance, makeup water use averaged over 12,000 gpd between mid-March and mid-November. apparently due to problems with the bleed off and basin float controls. 14
  15. Eight months after the switch to ozone treatment, the makeup meter began to frequently stop or under register. This causes one to question whether the makeup meter understated the amount of water used during ozonation. If it did, one would expect a replacement meter to show a higher rate of use than was measured during the first eight months of ozonation. This, however, is not the case. The makeup meter was indeed replaced 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 not seriously under register during the 8 months of 1988 in question, except for the 12 days mentioned above when the meter register did 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 gpd 36/13/91 922 148 32 4625 05/12/91 774 118 31 3806 04/11/91 656 85 29 2931 03/13/91 571 Total 351 92 3815 Makeup, meter readings during ozonation, (1988) (in units of 100 gallons) date reading change days gpd 06/13/88 272203 2179 32 6809 05/12/88 270024 654 29 2945 04/13/88 269170 1393 29 4803 03/15/88 267777 Total 4426 90 4918 15
  16. Cause of reduced water use. The reduction in water use from 1987 to 1988 was not caused by a change in weather. To rule out the possibilities that milder weather might have caused decreased evaporation, either directly or as a result of reduced cooling loads, data from three California Irrigation Management Information System (CIMIS) weather stations in the Bay Area were examined. Tables 6 through 8 show the results. In the coastal North Bay Area (Novato), the months of March through October were somewhat hotter and, on average, more humid in 1988 than they were in 1987. Data from the other two stations in San Jose and Walnut Creek were available for fewer of the months in question, but also show that 1988 was not cooler than 1987. In the coastal South Bay Area (San Jose), July through October was on average over 10°F hotter in 1988 than in 1987, and hotter during each month for which data are available. In the inland East Bay Area (Walnut Creek), the average temperature for the period August through October (the months for which data are available) was almost the same during the two years, just slightly higher in 1988 than 1987. Relative humidity data at the San Jose and Walnut Creek stations show less consistent results. In San Jose, the period July through October was slightly more humid in 1988, on average, than in 1987. In Walnut Creek, the August through October period was slightly less humid overall in 1988 compared to 1987. The water savings were achieved by disconnecting the automatic bleed system, thereby eliminating intentional discharge of the cooling tower water. It would be erroneous, however, to say that the towers operated at zero blowdown. Some loss of cooling tower water did occur through mechanisms other than evaporation or drift. Causes of inadvertent blowdown. First, there may have been overflow of water from the cooling tower basin. The float in the cooling tower basin, which controls the makeup water valve, has been known to fall out of calibration and to cause excessive makeup water flow, resulting in overflow." In addition, 1.5 gallons per minute (gpm) of fresh water constantly flows through the ozone generator into the tower basin. This water flow, equivalent to 2160 gpd, is required to cool the ozone generator's grounded electrode. Although this rate of water use is much less than average evaporation,16 it is conceivable that at night or during very cold days, this input of makeup water would exceed evaporation and cause overflow.17 Moreover, the 18 ozone generator cooling flow at times has been as high as 3 gpm. Second, some blowdown is believed to have occurred during the study period as a result of mistaken opening of the blowdown 16
  17. 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 AVE DATE 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 3.29 331 9.8 64 38 50 96 57 78 44 2.5 59 53 APRIL 4.47 0.36 531 10.7 74 41 56 94 44 71 46 2.6 63 60 M A Y 5.53 0.15 611 12.7 77 47 61 91 48 71 51 2.8 66 69 JUNE 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 71 AUGUST 6.02 6.49 575 13.4 82 50 63 82 44 68 52 2.8 67 70 SEPT. 4.34 0.08 451 12.1 81 48 61 82 41 66 49 2.5 61 68 OCT. 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 17

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