Reducing Water Use by Increasing Cooling Tower Cycles of Concentration
PowerPoint Presented at International Congress on Stainability Science & Engineering 2023
Sponsored by American Institute of Chemical Engineers
Timothy Keister, CWT FAIC
Water is the preferred heat transfer and cooling medium for building air conditioning and many industrial processes. Generally, this water is recirculated and cooled using evaporative cooling towers. Cooling towers in commercial & institutional buildings can consume up to 35% of total input water while industrial units can consume up to 95%. Part of the water is used as blowdown, water removed from the cooling tower and discharged to prevent scale formation. Blowdown amount is controlled by the cycles of concentration (COC) required to prevent scale formation. Increasing COC reduces fresh water use and discharge of blowdown discharge. Shortages of fresh water and increased costs for makeup water and blowdown disposal are driving a need to increase COC.
One older practice to increase COC while avoiding scale formation is pH control by acid addition to render the cycled cooling water non-scaling. Controlling pH by acid addition has always suffered from control, cost, and environmental health & safety problems. Today most cooling towers supplied with hard makeup water are treated with a combination of phosphonate and organic polymers for scale prevention to obtain COC of 2 to 4.
Removal of calcium, which is the primary scale forming ion in hard waters, by cation softening allows COC to be increased without the potential for scale formation. The manifold benefits of softened makeup water for cooling towers are discussed by Harfasti in detail. Problems cited in opposition to use of softened makeup water are increased corrosivity of cycled soft water, softening cost, and, in some areas, brine disposal from water softener regeneration is a problem.
Operation of cooling towers at high COC with softened makeup water was first disclosed in a patent issued in 1974. Our current technology is based on a project started in 1984 at Brockway Glass Company to control scale formation in glass melting furnace electrode cooling jackets. This work has since been developed and refined into a complete patentedii water management technology, marketed as Aqua Ionic, which addresses the need for higher COC while preventing scale and economically controlling corrosion. Glass Plant
Controlling Corrosion
The cited patents disclose a process for use of softened makeup water and specific water treatment chemistry for control of corrosion based upon products incorporating a basic chemistry incorporating polysilicate, polyphosphate, and ortho phosphate, for corrosion control.
Various specific inhibitors such as tolytriazole, for copper corrosion control, and surfactants, phosphonate, and organic polymers, for deposition control, are typically included in blended cooling water treatment products. Cooling towers using softened makeup water, operated at 10 COC or more and this chemistry, routinely obtain mild steel corrosion rates below 2 mil/yr, often obtaining rates between 0.25 and 0.5 mil/yr. Yellow metal corrosion rates are below 0.2 mil/yr, often getting as low as 0.01 mil/yr.
“White rust” is the term used to describe an accelerated corrosion of zinc (galvanize) due to attack by high pH/alkalinity water. White rust is a problem with high COC operation using soft water makeup as the cooling water pH increases into the range of 9.2 to 10.0. White rust occurs anytime the cooling water pH exceeds 8.2. This problem was addressed by the development of a proprietary inhibitor for control of white rust which is maintained in cycled cooling water at 4 to 12 mg/l as active.
Economics
Another argument against the use of softened makeup water is the cost of the softening process. To examine this often heard argument, the following economic analysis compares hard water makeup at a COC of 2.2 with phosphonate-polymer treatment, hard water makeup with pH control by acid addition at a COC of 4.0, and softened makeup water at a COC of 10.0. The analysis is based on a 1,000 ton annual thermal load cooling tower with Phoenix city water used as makeup for all three scenarios. The following table shows the parameters of this makeup water.
Table I
| Parameter | Result | Parameter | Result |
| pH | 7.5 | total alkalinity mg/l as CaCO3 | 120 |
| conductivity mmhos | 1,347 | calcium mg/l | 48.0 |
| magnesium mg/l | 16.0 | silicon mg/l | 1.4 |
| chloride mg/l | 325 | sulfate mg/l mg/l as SO4 | 59 |
| total phosphate mg/l as PO4 | 0.083 | total hardness mg/l as CaCO3 | 185.9 |
The following operational values are calculated for the example cooling tower on an annual basis.
Evaporation = 9,690,750 gallons
Blowdown at COC of 2.2 = 8,075,625 gallons Makeup = 17,766,375 gallons
Blowdown at COC of 4.0 = 3,230,250 gallons Makeup = 12,921,000 gallons Blowdown at COC of 10.0 = 1,076,750 gallons Makeup = 10,767,500 gallons For scale control with hard water makeup, a phosphonate-polymer inhibitor dosed to maintain 100 mg/l at a cost of $3.70/lb will be required. This calculates to a use of 6,739 lbs/yr at a cost of $24,934.
Using pH control by acid addition to prevent scale, target pH of 7.5, using 50% sulfuric acid and a phosphonate-polymer inhibitor dosed to maintain 100 mg/l at a cost of $3.70/lb. Sulfuric acid would be dosed at 688 mg/l at a cost of $0.60/lb. Inhibitor use would be 2,696 lb/yr for a cost of $9,974 while the sulfuric acid use would be 74,184 lb/yr at a cost of $44,510.
Use of softened makeup water will require an inhibitor dosed to maintain 375 mg/l at a cost of $3.40/lb. This calculates to a use of 3,370 lb/yr at a cost of $11,546. Operation of the water softener will require salt for regeneration at a level of 15 lbs/cubic foot of exchange resin. Salt use would be 83,448 lbs/yr at $0.35/lb for a cost of $29,207. In addition to the salt cost, softener regeneration wastewater will be produced at an annual rate of 1,001,376 gallons which in Phoenix will cost $13,909 at the combined water & sewer charge of $13.89/1000 gallons. The operating parameters and costs are summarized in the following table.
Table II
| Parameter | Hard Water | Hard Water with acid | Soft Water |
| Makeup water use | 17,766,375 gpy | 12,921,000 gpy | * 10,767,500 gpy |
| Blowdown to sewer | 8,075,625 gpy | 3,230,250 gpy | * 2,078,126 gpy |
| Makeup water cost | $200,405 | $145,749 | $121,457 |
| Sewer cost | $21,125 | $8,431 | $5,424 |
| Inhibitor cost | $24,934 | $9,974 | $11,546 |
| Acid cost | – | $44,510 | – |
| Salt cost | – | – | $29,207 |
| Total annual cost | $246,464 | $208,664 | $167,634 |
* includes softener regeneration water & wastewater
A significant water use reduction using pH control by acid addition is obtained, it is less than that obtained by softened makeup water. It is also more costly than the soft makeup water case and there is also a significant EH&S risk in handling the amount of sulfuric acid involved while pH controller problems are well documented.
This table clearly shows that for the given makeup water quality, use of softened makeup water to obtain high COC is clearly an economic proposition. An annual cost reduction of $78,830 is obtained while reducing water use by 6,998,875 gallons per year as compared to use of hard makeup water.
An appropriately sized water softener would need an average flow rating of 41 gpm. Such a unit has a capital cost of $14,313 with a typical installation cost of $12,000, total investment $26,313. The simple ROI for the equipment needed to provide softened makeup water is 4 months.
Aqua Ionic technology is well proven. It has been in use since 2008 in glass plants, die casting plants, sintered metal plants, plastic molding plants, casinos, and a college with excellent results.
Case History #1
An electronic components manufacturing plant located in Tempe, AZ, was required by a corporate sustainability program to reduce their water use. A plant survey found a 1,600 ton thermal capacity plant cooling tower system used for both process and comfort cooling operating at an annual load of 58%. The existing phosphonate-polymer based hard water treatment program averaged just 1.4 COC.
The plant requested a six month demonstration and agreed to purchase the chemical feed and control system and water softener needed if the soft makeup water program was successful. A new chemical feed and control system was required as the existing equipment had a maximum conductivity range of 5,000 and operation at 10 COC with Tempe makeup water would exceed that level. An automatic two tank water softener was supplied to provide softened makeup water 100% of the time.
During the first two months of the demonstration, August and September, 2021, the cooling tower average COC was 9.2 and facility water use was reduced an average of 15,464 gpd. At this point, the plant converted the demonstration into service supply contract.
Corrosion being a major concern when operating at high COC with softened makeup water, ninety day corrosion studies were started following the conversion to softened makeup water. Corrosion results for mild steel were 0.1 mil/yr while copper was <0.01 mil/yr. These are excellent results per the Association of Water Technologies corrosion coupon guidelinesiii.
Sampling of the cooling water was also undertaken on a routine basis between April and September, 2022 with the following average results obtained.
Table III
| Parameter | Result | Parameter | Result |
| pH | 9.4 | COC on conductivity | 9.5 |
| conductivity mmhos | 11,650 | calcium mg/l | 4.4 |
| magnesium mg/l | 1.89 | silicon mg/l | 55.9 |
| chloride mg/l | 3250 | sulfate mg/l | 862 |
| total hardness mg/l as CaCO3 | 18.8 | total alkalinity mg/l as CaCO3 | 1,660 |
In the time period May to July, 2023, the cooling tower system average COC was 9.85, average makeup rate of 27,059 gpd, and average cooling water pH of 9.5. The plant reports that on an annual basis their use of fresh water has decreased by 5 million gallons per year.
Decreasing fresh water use, and subsequent blowdown to the sanitary sewer, by 5 million gallons per year reduced plant utility costs by $70,600 per year at the current combined Tempe water & sewer rate of $14.12/1000 gallons.
Case History #2
A national retailer with multiple locations in the Southwest was committed to reducing water use by their evaporative condenser units as part of a corporate sustainability program. A facility in Tempe, AZ, was selected for a 90 day demonstration as it was already equipped with a facility water softener. The evaporative condenser was rated 350 tons and operated on a 7 day/24 hours basis servicing freon cycle refrigeration chillers. The existing water treatment program was found to be operating at 2.0 COC using a phosphonate-polymer program. A new chemical feed and control system was required as the existing equipment had a maximum conductivity range of 5,000 and operation at 10 COC with Tempe makeup water would exceed that level.
The demonstration started on November 20, 2020, and was deemed successful as the water use reduction desired was obtained with very low corrosion rates. The retailer has now operated this location on a continuous basis with softened makeup water to date. In the April to June, 2023, time period this evaporative condenser has operated at an average COC of 9.8, average cooling water pH of 9.4, and average makeup of 4,417 gpd.
Operation over the same time period at a COC of 2.0 would have resulted in a makeup of 9,736 gpd. Water savings on an annual basis are calculated at 1,941,435 gallons.
Corrosion being a major concern when operating at high COC with softened makeup water, corrosion coupon studies are carried out on a routine basis. Corrosion results for mild steel are less than 0.2 mil/yr with copper less than 0.05 mil/yr. These are excellent results per the Association of Water Technologies corrosion coupon guidelines. Shown are steel and copper corrosion coupons as removed from the corrosion coupon rack at 90 days exposure.
Sampling of the cooling water was also undertaken on a routine basis during the demonstration period with the following results obtained.
Table IV
| Parameter | Result | Parameter | Result |
| pH | 9.05 | COC on conductivity | 8.2 |
| conductivity mmhos | 10,418 | calcium mg/l | 4.4 |
| magnesium mg/l | 1.89 | silicon mg/l | 55.9 |
| chloride mg/l | 3250 | sulfate mg/l as SO4 | 862 |
| Total hardness mg/l as CaCO3 | 18.8 | Total alkalinity mg/l as
CaCO3 |
945 |
The national retailer was pleased with the demonstration results and is proceeding to convert all their evaporative condensers in Arizona, California, and Nevada to softened makeup water.
Case History #3
A chemical manufacturing plant in Mesa, AZ, desired to reduce their use of fresh water and discharge of wastewater to the sanitary sewer. Examination of the facility showed three reverse osmosis units operating with softened influent water to provide high quality process water with concentrate discharged to the sanitary sewer. Three on-site cooling tower systems were used for process cooling with hard city makeup water using a phosphonate-polymer program at 5 COC to control scale with a thermal load of 482 tons. The average reverse osmosis concentrate flow was determined to be sufficient to supply 100% of the cooling tower makeup needs.
Reverse osmosis concentrate contains elevated levels of all the parameters in the influent water and corrosion control was a major concern as expected levels of chloride in the cooling water would exceed the generally recognized maximumiv of 800 mg/l. Given this concern, COC for the cooling towers was set at 5.0 to 6.0.
The reuse system as installed consisted of a 1,000 gallon concentrate accumulation tank, pump skid, makeup and blowdown meters, and new chemical feed and control systems. The existing chemical feed and control systems did not have sufficient range when using reverse osmosis concentrate as makeup. Following start-up in June, 2017, the following data was obtained for a period of one month.
Table V
| Cooling tower makeup | 446,500 gallons | Cooling tower blowdown | 62,500 gallons |
| Evaporation | 384,000 gallons | Thermal load | 482 tons |
Using the thermal load and operation at 3 COC with hard makeup water over a month, cooling tower makeup would have been 576,000 gallons with blowdown at 192,000 gallons. Thus in this one month period, fresh water use was reduced by 576,000 gallons while blowdown was reduced by 129,500 gallons.
Given the high levels of chloride and sulfate expected in the cooling waters, the plant engaged a third party to do three 90 day corrosion studies in 2017 and 2018. The average results are shown in the following table.
Table VII
| Coupon Material | CT 1 | CT 2 | CT 3 |
| Mild steel | 0.27 mils/yr | 0.37 mils/yr | 0.63 mils/yr |
| Copper | 0.1 mils/yr | <0.1 mil/yr | <0.1 mils/yr |
These are excellent results per the Association of Water Technologies corrosion coupon guidelines.
The following table compares makeup water (reverse osmosis concentrate) and analysis results for the three cooling towers as sampled in July, 2017.
Table VI
| Parameter | makeup | CT 1 | CT 2 | CT 3 |
| pH | 8.2 | 9.5 | 9.5 | 9.5 |
| total alkalinity mg/l as CaCO3 | 338 | 2,185 | 2,163 | 2,125 |
| conductivity mhos | 2,210 | 11,650 | 11,690 | 11,500 |
| calcium mg/l | 0.07 | 0.56 | 0.53 | 0.40 |
| magnesium mg/l | 0.068 | 0.375 | 0.304 | 0.339 |
| iron mg/l | <0.03 | <0.03 | <0.03 | |
| copper mg/l | 0.04 | 0.02 | 0.03 | |
| zinc mg/l | 0.040 | 0.031 | 0.069 | |
| silicon mg/l | 32.0 | 105 | 101 | 100 |
| chloride mg/l | 255 | 3,350 | 3,380 | 3,352 |
| sulfate mg/l as SO4 | 418 | 3,170 | 3,160 | 3.316 |
| COC | 5.3 | 5.3 | 5.2 |
Softener Wastewater
In several areas of the country disposal of softener regeneration wastewater has been banned due to its high dissolved sodium chloride content. Alternative methods of softening makeup water with no production of sodium chloride brine include nanofiltration and chemical precipitation.
Side Benefits
During routine Legionella sampling of 17 cooling tower systems operating with Aqua Ionic technology, all the test results were reported as none detected. Review of the literature on Legionella found a paperv showing that Legionella bacteria cannot exist in an alkaline environment, pH level over 9.2. To confirm this, a study using a cooling tower with no biocide feed was then sampled for a period of nine months, again all Legionella samples were none detected.
Operation with softened makeup water eliminates formation of scale on heat exchange surfaces. Improved heat transfer decreases chiller energy use.
Conclusions
Use of softened makeup water with Aqua Ionic technology has been shown to have superior economics to use of acid or phosphonate-polymer technologies for increasing COC to reduce water use. Operating costs are lower than competing acid pH control and phosphonate-polymer technologies.
The known corrosivity issue with highly concentrated soft water is addressed by Aqua Ionic technology with superior corrosion control achieved. White rust corrosion is controlled to acceptable levels by use of a proprietary corrosion inhibitor currently marketed as GalvaGard.
Use of softened makeup water allows high COC to be obtained to maximize reduction of fresh water use and subsequent discharge of blowdown to sewer.
It has been found that Legionella bacteria cannot survive in cooling waters with a pH over 9.2. As shown in the three Case Histories, operation at high COC with softened makeup water produces cooling water pH levels exceeding 9.2.
Elimination of scale formation on heat exchange surfaces by use of softened makeup water provides an energy use reduction due to improved heat transfer.
i Harfst, William “Benefits of So Water for Cooling Tower operaon”, Internaonal Water Conference Paper 07-10,
2007 ii US patents 7,595,000 and 8,128,841, Keister to ProChemTech Internaonal, Inc. iii Associaon of Water Technologies “Recommendaons and Guidelines for Corrosion Coupons in Cooling Systems”, Technical Commitee, 2016 iv Colin Frayne, “Cooling Water Treatment Principals and Pracce”, Chemical Publishing Company, New York, NY, 1999. v “An Alkaline Approach to Treat Cooling Towers for Control of Legionella Pneumophila”, States et. Al., Applied and Environmental Microbiology, Aug. 1987, p. 1775-1779.
Timothy Keister has a B.Sc. in Ceramic Science and was employed by a Fortune 500 manufacturing firm for 13 years as the water & wastewater manager for 38 plant locations. In 1987 he founded ProChemTech International and is currently Chief Chemist/President of the firm. He is the inventor on 11 patents and is an active member of the Technical and Certification committees of the Association of Water Technologies. A Certified Water Technologist, he is also a Fellow of the American Institute of Chemists, Emeritus Member of the American Institute of Chemical Engineers, and maintains memberships in the Water Environment Federation,
American Society of Hospital Engineers, American Chemical Society, and Cooling Technology Institute. Presentations have been done at the International Water Conference, Association of Water Technologies, Water Environment Federation, and Pennsylvania Association of Environmental Professionals. He has also had articles published in Water Conditioning & Purification and the Analyst.
Contact: tek@prochemtech.com
Office: 814-265-0959