Prepared for the General Services Administration

By The National Renewable Energy Laboratory

December 2018

Alternative Water Treatment 

Technologies for Cooling Tower 

Applications

Dylan Cutler

Jesse Dean

Jennifer Daw, P.E.

Dan Howett, P.E.
The Green Proving Ground program leverages GSA’s real estate portfolio 

to evaluate innovative sustainable building technologies and practices. 

Findings are used to support the development of GSA performance 

specifications and inform decision-making within GSA, other federal 

agencies, and the real estate industry. The program aims to drive 

innovation in environmental performance in federal buildings and help 

lead market transformation through deployment of new technologies.
Cooling Tower Water Treatment Technologies Page i

Disclaimer

This document was prepared as an account of work sponsored by the United States Government. While this 

document is believed to contain correct information, neither the United States Government nor any agency 

thereof, nor the National Renewable Energy Laboratory, nor any of their employees, makes any warranty, express 

or implied, or assumes any legal responsibility for the accuracy, completeness, or usefulness of any information, 

apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. 

Reference herein to any specific commercial product, process, or service by its trade name, trademark, 

manufacturer, or otherwise, does not constitute or imply its endorsement, recommendation, or favoring by the 

United States Government or any agency thereof, or the U.S. Department of Energy’s National Renewable Energy 

Laboratory. The views and opinions of authors expressed herein do not necessarily state or reflect those of the 

United States Government or any agency thereof or the National Renewable Energy Laboratory.

 

The work described in this report was funded by the U.S. General Services Administration [and the Federal Energy 

Management Program of the U.S. Department of Energy] under Contract No. IAG-11-01815 and Task No. 

WFK91002.

Acknowledgements

GSA's Proving Ground: Kevin Powell – Director, Jay Fine

United States General Services Administration (GSA) Region 8: Silas Campbell, Douglas Rothgeb, Jessica Higgins, 

Tyler Cooper, Charlie Reinhardt, Douglas Baughman and Laura Magee

Tenfold Information Design Services: Andrea Silvestri

National Renewable Energy Laboratory: Chris Gaul, Otto VanGeet, Gregg Tomberlin

For more information contact:

Dylan Cutler

Senior Engineer

National Renewable Energy Laboratory

Phone: (303) 275 – 3873

Email: dylan.cutler@nrel.gov
Cooling Tower Water Treatment Technologies Page ii

Table of Contents

Page

Executive Summary................................................................................................................................... 3

Background................................................................................................................................................ 8

A. INTRODUCTION ...................................................................................................................................8

B. OPPORTUNITY ....................................................................................................................................10

C. SUSTAINABILITY ...............................................................................................................................11

D. DEMONSTRATION PROJECT LOCATION(S) .............................................................................12

Measurement & Verification Project Plan............................................................................................. 12

A. TECHNICAL OBJECTIVES ................................................................................................................12

B. TECHNOLOGY DESCRIPTION ........................................................................................................12

C. FACILITY DESCRIPTION .................................................................................................................14

D. TECHNOLOGY SPECIFICATION ....................................................................................................15

E. TECHNOLOGY DEPLOYMENT ........................................................................................................18

F. TEST PLAN ...........................................................................................................................................21

Instrumentation Plan......................................................................................................................................21

Results ...................................................................................................................................................... 23

A. Water Savings .....................................................................................................................................23

B. Water Quality .....................................................................................................................................29

C. Preventative Maintenance Savings ............................................................................................30

D. Economic Analysis ............................................................................................................................31

Conclusions .............................................................................................................................................. 34

A. OVERALL TECHNOLOGY ASSESSMENT AT DEMONSTRATION FACILITIES ................34

B. BEST PRACTICE .................................................................................................................................36

C. BARRIERS AND ENABLERS TO ADOPTION..............................................................................36

D. MARKET POTENTIAL WITHIN THE GSA PORTFOLIO..........................................................36

E. RECOMMENDATIONS FOR INSTALLATION, COMMISSIONING, TRAINING, AND 

CHANGE MANAGEMENT ........................................................................................................................44

Appendices............................................................................................................................................... 45

A. RESEARCH DETAILS .........................................................................................................................45

References................................................................................................................................................ 48
Cooling Tower Water Treatment Technologies Page 3

Executive Summary

Background 

This GSA Proving Ground (GPG) project assessed the performance of three alternative water treatment 

systems (AWT) for cooling tower water treatment applications at the Denver Federal Center (DFC) in Denver, 

Colorado. Cooling towers are commonly applied to water cooled chilled water plants in medium to large 

commercial buildings and are the point in the system where heat is dissipated to the atmosphere through the 

evaporative cooling process. Cooling towers also consume a large amount of water. Cooling tower related 

water consumption is one of largest potable water loads within buildings in the United States, with over 26% 

of water use associated with heating and cooling. Reducing water consumption is a priority for the General

Services Administration (GSA) due to Executive Order 13693, Energy Policy Act of 1992, and regional water 

shortages. These factors have brought about the investigation of cost effective opportunities to reduce water 

use, such as AWT technologies for cooling towers.

The current state of water treatment in GSA buildings is to use conventional chemical based cooling tower 

water treatment to maintain cooling tower water quality and contract out this specialized service to a thirdparty company specializing in such service. Traditional water treatment approaches rely on chemicals to 

extend the ability of the water to hold scaling minerals in solution and kill off biological growth. This treatment 

protects the chillers and cooling tower equipment, however even when chemicals are used regularly, a certain 

percentage of condenser water must be blown down and made up with fresh water to maintain system water 

quality parameters. In addition, the use of chemicals creates a waste issue and can cause building owners to 

incur additional disposal fees or sewer charges. The application of AWT’s in place of traditional chemical 

water treatment has the potential to: 1) Lower make-up water costs, 2) Lower sewer costs, 3) Lower material 

costs, 4) Reduce chemical use, and 5) Increase chiller efficiency.

Study design and objectives

Three different AWT’s were evaluated for this report. The AWT installed at Building 25 is manufactured by 

Water Conservation Technology International (https://www.water-cti.com/technology.html) uses a 

proprietary salt-based high efficiency softening system to remove hardness from make-up water, requiring 

little to no standard cooling tower water treatment chemicals. The Building 67 AWT system is manufactured 

by Terlyn (http://www.terlyn.com/cooling-tower-water-conservation-program-overview-video/) and provides 

a water treatment approach using a corrosion inhibitor, biocide, and a proprietary scale inhibitor. This system 

still utilizes chemical additions to regulate water quality; however, these chemicals have different 

compositions and are used in different quantities than a traditional system. The Building 95 AWT uses a 

controlled hydrodynamic cavitation process to provide chemical-free water treatment using two side-stream 

water loops. Building 25 - a multi-use facility of laboratories, data centers, and office space; Building 67 - a 14-

story high rise; and Building 95 – an office and laboratory building. Traditional chemical water treatment was 

existing in all buildings prior to the AWT installations. The system at Building 95 had been uninstalled prior to 

the writing of this report and it was not possible to collect adequate water consumption data for comparison

with the other two technologies (at buildings 25 and 67).

The three AWT technologies evaluated in this report were assessed according to two main criteria set out by 

GSA in the original RFP.
Cooling Tower Water Treatment Technologies Page 4

1) Effectiveness of AWT in meeting a predetermined set of criteria that constitutes proper water quality. 

These criteria were developed by a consultant to GSA under a previous contract and were used in this 

study to determine whether the AWT systems assessed met the water quality criteria. 

2) Effectiveness of AWT systems in reducing GSA water costs and operation and maintenance (O&M) 

costs.

This report addresses both water and cost savings associated with AWT technologies, and the water quality 

provided by the AWT technologies. Common make-up water quality and local weather conditions allowed for 

comparison of different AWTs installed in three different buildings. The principal variable that changed 

between the three different test buildings was the quantity of cooling required, and therefore the amount of 

heat rejected (and water evaporated) by the cooling tower. In order to normalize the results for cooling 

demand met by the tower, the principal metric that was evaluated for water use reduction was water savings 

per ton-hour of cooling delivered (gal/ton-hr.). The normalization of water savings to cooling demand allowed 

for effective comparison of the technologies. It also enables the estimation of savings for other buildings. With 

appropriate metering of the water consumption and cooling demand at other GSA sites it is possible to predict 

potential water savings that may be achievable with the AWT technologies assessed here. 

The two components that need to be measured to account for water savings are (1) water consumption and 

(2) cooling demand of the building. Water consumption was measured via an onsite water meter and tracked 

on a monthly basis for the whole building. Make-up water and blow down were also directly metered and 

water consumption was recorded on a daily basis for each AWT in this study. The exact date ranges and 

quantity of these data are discussed in section IV. E. 

The cooling rejected from each building was measured via the onsite building automation system (BAS) by 

monitoring the condenser loop (cooling tower) supply temperature, return temperature, and flow rates. The 

condenser water cooling load data was correlated to hourly outdoor air temperature and humidity values to 

establish the amount of heat rejected by the cooling tower as a function of outdoor air temperature. These 

values are established by trending the BAS output for the chiller plant in each building during the period from 

May of 2013 to November of 2013 (a typical cooling season in Colorado). 

The water quality was analyzed through assessment of the water quality reports delivered monthly by the 

AWT vendors, as well as monthly water quality reports performed by a third-party testing company. These 

reports evaluated: conductivity, pH value, total hardness, calcium hardness, magnesium hardness, “P” 

alkalinity, “M” alkalinity, silica high range, chloride anions, salt anions, sulfate anions, phosphate, copper, iron, 

biological growth, and cycles of concentration (CoC). 

Project Results/Findings

In order to accurately assess the water savings from these AWT systems, it was necessary to calculate preinstallation cooling tower water usage. For the AWT system at Building 67, make-up water was metered for 

one year prior to installation. For Building 25, the make-up water was only metered post installation of the 

AWT. In the case of Building 25, the post installation data was used to generate estimates of pre-installation 

performance, which were then compared to post-installation performance to calculate water savings (while 

normalizing for the effect of different temperatures and cooling demand during the pre/post install time 

periods).

Due to the various levels of pre- and post-installation data (pre and post data for Building 67, post data for 

Building 25, and neither for Building 95), different analysis approaches were required at each site. The key 

steps in the analysis approach for each building are as follows:

1) Building 67: 
Cooling Tower Water Treatment Technologies Page 5

a. BAS metered data was taken from May 2013 to November 2013 to generate a correlation 

between cooling demand and outdoor air temperature.

b. Historical daily outdoor temperature data and day of the week (weekend vs weekday) was 

used to estimate the monthly cooling load (going back to January 2009 – the beginning of the 

advanced metering data period).

c. A regression analysis was conducted on monthly cooling demand versus the metered water 

consumption (one regression for pre-installation, and one for post-installation). The slope of 

the regression lines was calculated to obtain a gallon per ton-hour value for pre- and postinstallation. The difference in the two slopes was used to calculate water saved in the post 

retrofit months.

2) Building 25

a. Due to the lack of pre-installation data on this system, cycles of concentration were recorded 

from the building logs for pre-installation months.

b. Pre-installation water consumption was calculated based on the change in cycles of 

concentration. This approach inherently accounts for weather normalization because the 

cycles of concentration calculation assumes the same period of operation for calculating the 

pre-installation water consumption.

c. This was compared to post-installation water consumption to calculate water savings.

3) For Building 95: As previously noted, this system was uninstalled prior to the writing of this report 

making calculation of water savings impossible.

The pre-retrofit water consumption was calculated for the cooling demand seen in the post-retrofit years 

2012 and 2013. The calculated efficiency of the traditional water treatment system was used to calculate the 

water consumption that would have occurred with the old system in place, enabling the calculation of 

weather normalized water savings for each of the months post-retrofit. The monthly savings are shown in 

Table 1. Each system experienced a couple month period of calibration and elimination of built up scale; 

therefore, the savings were calculated starting in 2012 despite installation of the AWT systems in January of 

2011 (for Building 25) and October of 2011 (for Building 67).

Table 1. Monthly water savings for 2011-2013 (gallons)

Month 2012 2013

Bldg. 25 Bldg. 67 Bldg. 25 Bldg. 67

Jan 5,497 44,707 8,830 42,604

Feb 13,421 37,690 6,491 38,867

Mar 3,830 66,601 7,836 50,092

Apr 21,667 71,881 18,275 55,270

May 25,117 86,761 26,930 81,031

Jun 50,585 110,658 40,409 105,032

Jul 73,187 119,705 76,667 113,927

Aug 62,193 114,247 82,485 113,619

Sep 48,304 94,524 73,088 95,857

Oct 35,643 66,333 59,251 61,389

Nov 15,175 57,349 26,608 35,170

Dec 12,018 42,526 8,830 -

Totals 366,637 912,983 435,702 792,858
Cooling Tower Water Treatment Technologies Page 6

The results of the water saving analysis for Building 25 AWT #1 demonstrated a 23% reduction in water use 

saving 401,170 gallons/yr .The results for Building 67 AWT #2 showed a 24% savings over the old water 

treatment system, saving 824,448 gallons/yr. 

Table 2 shows the initial cost, water savings per year, and the operations and maintenance (O&M) savings 

associated with Buildings 25 and 67. Using these values the simple payback period and savings to investment 

ratio (SIR) of each technology is calculated. The O&M savings are calculated by comparing the annual costs 

associated with the new system to the previous water treatment O&M costs, as well as the decrease in GSA 

maintenance savings via a reduction in personnel time; this value can be positive or negative depending on the 

relative change in expenditures. 

Table 2. Economics of AWT Systems in Buildings 25 and 67

Economic Parameter

Building 25 (AWT 

#1) at Local Water 

Rate $7.14/kGal

Building 25 (AWT 

#1) at GSA Avg 

Water Rate 

$16.76/kGal

Building 67 

(AWT #2) at 

Local Water Rate 

$7.14/kGal

Building 67 (AWT #2) 

at GSA Avg Water 

Rate $16.76/kGal

Initial Cost ($) $29,600 $29,600 $32,511 $32,511 

Cooling Tower Size (tons) 1,500 1,500 1,200 1,200

Water Savings (Gal/yr.) 401,170 401,170 824,448 824,448

Water and Sewer Cost 

Savings ($/yr.) $2,864 $6,724 $5,887 $13,818 

Annual Increase in O&M 

($/yr.) ($6,445) ($6,445) $1,883 $1,883 

Simple Payback with O&M 

(yrs.) 3.2 2.2 8.1 2.7

Savings to Investment Ratio 4.7 6.7 1.8 5.5

At a combined water and sewer rate of $7.14/kGal AWT #1 had an installed cost of $19,73/Ton, annual water 

savings of 401,170 Gal/yr, a simple payback period of 3.2 years and an SIR of 4.7 using a total project lifetime 

of 15 years. At the GSA national average water and sewer rate of $16.76/kGal, AWT #1 had an annual water 

cost savings of $6,724, a simple payback period of 2.2 years and an SIR of 6.7. AWT #2 at Building 67 had an 

installed cost of $27.1/ton, an annual water savings of 824,448 gallons per year, a simple payback period of 

8.1 years and an SIR of 1.8. At the GSA national average water and sewer rate of $16.76/kGal, AWT #2 had an 

annual water cost savings of $13,818, a simple payback period of 2.7 years and an SIR of 5.5. 

The savings due to reduced water consumption are highly favorable in both cases (higher in Building 67), yet 

the O&M costs decreased significantly in the case of Building 25 for AWT #1, whereas the costs increased in 

Building 67 for AWT #2. This has a large impact on economics associated with each technology. The increased 

O&M costs in AWT #2 were due to an increase in chemical costs for the system (which utilized a proprietary 

chemical that helps in the suspension of solids) and periodic replacement of a glass bead media filter. On the 

other hand, AWT #1 eliminated almost all chemical use and required a less expensive salt regeneration 

process for the O&M requirement in that system. Although there was an increase in chemical costs for AWT 

#2, there was a significant reduction in GSA maintenance hours for each technology. GSA maintenance hours 

reduced from 152 to 80 hours per year for AWT #1, saving $3,677 per year, and GSA maintenance hours were 

reduced from 132 to 69 hours for AWT #2, saving an additional $3,217 per year. For AWT #2, since the O&M 

contract increased by $5,100 per year, the net increase in O&M costs was $1,883.

This economic assessment does not take into account any of the potential energy savings from increased 

chiller efficiency (due to improved heat exchanger effectiveness). The building engineers for both Building 25 

and Building 67 indicated that they were able to run the flat plate chillers more often due to improved chiller 

performance after AWT install, and in Building 67 it was stated that they were able to run with one less chiller 
Cooling Tower Water Treatment Technologies Page 7

the majority of the summer. Tracking the reduction in electricity consumption from the chiller plants was not 

in the scope of this analysis, and therefore is not included in the results. The decrease in energy costs would 

only improve the system economics and should be evaluated in future studies.

Each AWT was monitored during the demonstration period for adherence to acceptable water quality ranges 

defined by GSA. There was no pre-installation water quality data to enable comparison to the previous 

treatment system, yet the operations and maintenance contractors at Buildings 25 and 67 noted significant 

improvement in cleanliness of the towers and chillers. 

Water quality data sampled by a third-party testing company, SJCI, and the O&M contractor demonstrated 

that Building 25 fell outside the desired ranges for conductivity (from 300% – 800%), pH (up to 13%) and 

alkalinity (from 68% - 300%). Building 67 fell outside the desired range for conductivity (from 5% – 290%). It 

should be noted that the specified project ranges are not absolute proof of success or failure of the AWTs in 

achieving adequate performance. By design many of the AWT systems maintain certain water quality 

parameters outside the range of the project specifications. The true measure of success of an AWT’s 

performance is a function of many considerations including water quality, CoC, corrosion, scale, biological 

growth, ability to meet discharge permit requirements, O&M requirements, and cost. Due to the overall 

improvements in water quality and reductions in blow down water usage for two of the three AWT’s 

evaluated in this report, both AWT #1 (Building 25 technology) and AWT #2 (Building 67 technology) was 

installed at six additional installations at the Denver Federal Center that had cooling tower make up water 

meters that collect 15 minute interval data on GSA’s advanced metering system. The measured annual water 

savings and associated cost savings were calculated and combined with the O&M costs from this study to 

calculate an annual cost savings, simple payback period and savings to investment ratio (A combined water 

and sewer rate of $7.14/kGal was used for the annual water cost savings (Table 3). For the AWT #1 

technology, GSA elected to use the vendor provided side stream filter system instead of the typical GSA 

scoped side stream filtration system, which reduced the cost and for the AWT #2 systems, GSA continued to 

install the glass media filter system. For Building 20, chemical costs were decreased to $400 per year, versus 

$1,883 per year for other facilities since this is a smaller facility with less cooling tower water usage than the 

other facilities. 

Table 3 - Annual Water Savings and Economics for Additional Deployments at DFC

Denver 

Federal 

Center 

Facility

AWT 

System

Date 

Installed

Cooling 

Tower 

Size 

(Tons)

Installed 

Cost ($)

Annual 

Water 

Savings 

(Gal/yr.)

Annual 

Water 

Savings 

($)

Annual 

Increase 

in 

O&M 

($/yr.)

Total 

Annual 

Cost 

Savings 

($/yr.)

Simple 

Payback 

(yrs.)

Savings to 

Investment 

Ratio 

(SIR)

Bldg.20 AWT 

Tech. #1 16-Nov 600 $31,057 718,597 $5,131 ($6,445) $11,576 2.7 5.6

Bldg.41 AWT 

Tech. #1 17-Jan 1,000 $36,976 1,809,921 $12,923 ($6,445) $19,368 1.9 7.9

Bldg. 85 AWT 

Tech. #2 14-Jan 500 $8,756 62,450 $446 $400 $46 >40 0.1

Bldg.56 AWT 

Tech. #2 15-Jan 1000 $28,557 661,160 $4,721 $1,100 $3,621 7.9 1.9

Bldg. 

810

AWT 

Tech. #2 14-Jun 2 x 500 $31,047 1,131,450 $8,079 $1,883 $6,196 5.0 3.0

Bldg.810 

USDA

AWT 

Tech. #2 16-Mar 3 x 500 $31,047 1,048,000 $7,483 $1,883 $5,600 5.5 2.7
Cooling Tower Water Treatment Technologies Page 8

For the additional deployments at the DFC, the simple payback ranged from 1.9 to 7.9 years for all installations 

other than Building 85. The AWT #2 vendor was performing water treatment services at Building 85 prior to 

the other installations and the sand filtration system was able to be installed at a much lower cost than the 

other facilities for this facility.

Background

A. INTRODUCTION

Air conditioning accounts for approximately 15% of all source energy used for electricity production in the 

United States alone (nearly 4 quadrillion British thermal units (Btu)), which results in the release of about 343 

million tons of carbon dioxide into the atmosphere every yeari

. Cooling towers are an integral component of 

many refrigeration systems, providing comfort or process cooling across a broad range of applications. They are 

the point in the system where heat is dissipated to the atmosphere through the evaporative process, and are 

commonly applied to water cooled chilled water plants in large commercial buildings. Reducing water 

consumption is a priority for the General Services Administration (GSA) and is especially important in the arid 

regions of the western US and any locations facing drought conditions.

Executive Order 13693 mandates that federal facilities reduce their potable water consumption 2% annually 

through FY 2025 or 36% by the end of FY 2025, relative to a FY 2007 baseline. This Executive Order revokes 

Executive Order 13514, which requires Federal agencies to reduce potable water consumption intensity through 

life cycle cost-effective measures relative to the agency's baseline. Given the large amount of water used by 

cooling towers, the investigation of new cooling tower water treatment technologies that can reduce water 

consumption is of particular interest.

In general building applications cooling towers are connected to the central water-cooled chillers that provide 

building cooling. In water cooled chilled water plants the condenser water absorbs the heat from the chillers and 

then passes it to the cooling towers where this water is exposed to the outside air. Some of the condenser water 

evaporates; the rest is returned to the chiller to repeat the cooling process. Since evaporation is a heatabsorbing process, the returning condenser water is cooler than the condenser water supply water to the 

cooling tower.

The evaporated condenser water must be replaced by the cooling tower make up water, typically coming from 

the cooling tower basin using a float valve, similar in concept to the float valves in older model toilets. Cooling 

tower make up water is typically provided by the city potable water supply and increases the building’s water 

use.

The continuous evaporation of condenser water from the condenser water side of the cooling cycle also has 

another effect: since only pure water evaporates, it leaves behind any mineral content it carried upon entry into 

the condenser water system. The make-up water has a natural amount of mineral impurities (silica, calcium, 

magnesium, chloride), so the remaining condenser water will have an ever-increasing amount of impurities as 

progressively more water evaporates. These impurities eventually will precipitate out (since water can hold only 

so much), resulting in solid precipitate. This solid precipitate is commonly called scale and will collect on various 

surfaces it touches. Scale has a detrimental effect on heat transfer surfaces; it lowers the efficiency of the heat 

transfer process, causing the chiller to use increasingly more energy over time to produce the same amount of 

cooling. 
Cooling Tower Water Treatment Technologies Page 9

There are two intertwined means of dealing with scale in a conventional chemical-based cooling tower water 

treatment system; injecting chemicals into the condenser water and regularly dumping a certain portion of the 

condenser water down the drain. These methods are intertwined because in traditional cooling tower water 

treatment systems they both must be used; one method alone will not produce proper cooling tower water 

quality.

1) Inject chemicals into the condenser water. Chemicals serve three purposes in cooling tower water 

management. 

a. Chemicals called “scale inhibitors” increase the waters ability to hold a higher concentration of 

minerals in the solution without causing scaling.

b. Chemicals called “corrosion inhibitors” decrease corrosion in piping systems.

c. Chemicals called “biocides” and “algaecides” mitigate biological growth in the cooling tower, 

where warm water is exposed to air.

2) Regularly dump a certain proportion of the condenser water down the drain. As explained above, over 

time the mineral content of cooling tower water will increase as the cooling tower operates more and 

more hours. Since the make-up water will have a lower chemical/mineral content, this dumping process 

, often called tower blowdown or bleed off, has the effect of lowering the chemical/mineral content of 

the remaining condenser water. 

Chemical use has two consequences.

1) Chemicals cost money.

2) The remaining condenser water has a higher level of chemicals than the normal city water or 

groundwater in the area. Once a building is into the maintenance practice of adding chemicals, every 

blowdown of condenser water deposits a higher level of chemicals into the local sanitary or storm 

system than the normal local water contains. This may require special permission from local municipal 

sanitary/storm water departments and incur additional sewer water costs.

The common metric used to measure blowdown is cycles of concentration (CoC). CoC compares the 

concentration of solids of the recirculating cooling tower to the concentration of solids of the original raw makeup water. If the recirculating water has 3 times the concentration of solids that the raw make-up water has, then 

the CoC is 3.

This project assesses the readiness of several off-the-shelf, alternative cooling tower water treatment 

technologies for lowering GSA operating costs while maintaining proper water treatment. Alternative cooling 

tower water treatment (AWT) technologies potentially can lower GSA’s resource demands and operating costs 

through:

• lowering make-up water costs

• lowering sewer costs

• lowering or eliminating chemical costs

• increasing chiller efficiency (via less scale on condenser tube heat transfer surfaces), which will decrease 

electricity use and thereby lower electricity costs
Cooling Tower Water Treatment Technologies Page 10

B. OPPORTUNITY

Cooling tower related water consumption is one of largest potable water loads within buildings in the United 

States. A breakdown of water consumption in office buildings is provided in Figure 1. Figure 1 shows that over 

26% of water use is associated with heating and cooling is by far the dominant water use case due to the 

evaporative cooling demands associated with all water-cooled air conditioning systems and evaporative based 

air conditioners.

Figure 1. Office water end uses1

Cooling towers are an integral component of many refrigeration systems, providing comfort or process cooling 

across a broad range of applications. They are the point in the system where heat is dissipated to the 

atmosphere through the evaporative process, and are commonly applied to water cooled chilled water plants in 

large commercial buildings. 

Cooling towers can be found in all states throughout the country, and this technology can save water in every 

climate zone. Although the technology can save water in every climate zone, facilities located in ASHRAE climate 

zones 1A, 2A, 2B, 3A, and 3B that utilize cooling towers will typically use significantly more cooling tower water 

than similarly sized systems in other climate zones due to the amount of cooling required in these regions. 

These regions require significantly more annual cooling than other regions which can lead to a more costeffective implementation of AWT technologies. An ASHRAE climate zone map is provided in Figure 2.

 

1 http://www.gsa.gov/graphics/pbs/waterguide_new_R2E-c-t-r_0Z5RDZ-i34K-pR.pdf. Accessed 12/10/13
Cooling Tower Water Treatment Technologies Page 11

Figure 2. ASHRAE climate zone map

The Denver Federal Center (DFC) is located in climate zone 5B and GSA Region 8 has a total of fifty cooling 

towers. The cooling towers in GSA Region 8 are installed at thirty of the larger buildings in the region. Although 

the number of cooling towers in each GSA region is unknown, it is expected that each region has numerous 

cooling towers that could assist in reducing water consumption for each GSA region.

C. SUSTAINABILITY

The concept of sustainability focuses on working within environmental, social, and financial constraints to both 

strive for balance and reduce impacts. In the context of this project, cooling towers rely on many resources,

including chemicals, energy, and water, as well as manpower and funds to support their operation. AWT’s are of 

particular relevance in their potential to optimize resource utilization and provide resiliency to potential 

resource shortages.

AWT technologies can be categorized as chemical and non-chemical systems. In traditional chemical treatment 

the application of biocides, corrosion and scale inhibitors, and brine for water softening, while effective, 

presents potential environmental concerns including the introduction of toxins to humans and the environment, 

nutrient loading to natural water bodies, and operational issues for the wastewater district.

ii An approach to 

reduce the environmental impact of chemicals can be to use biodegradable alternatives; however the efficacy 

and availability of these products must be considered with respect to their application.iii Non-chemical systems 

may be viewed as a preferred solution from an environmental and operating cost perspective; however their 

ability to meet treatment goals must be weighed against these potential benefits.iv Both chemical and nonchemical systems have environmental and financial tradeoffs. 

In arid climates, water is a particularly precious resource both to people and the natural environment. Cooling 

towers use a substantial amount of water; which is a tradeoff compared to the substantial electricity demands 

of air cooled systems. However, according to Morrisonv “air-cooled systems consume 30% to 40% more power 

than water-cooled systems, and are one of the largest users of water in the country is power generation.” 

Energy and water are intrinsically linked in the operation of cooling systems. Efforts focused on making optimal 

use of both resources will contribute to the longevity and long-term effectiveness of a technology. 
Cooling Tower Water Treatment Technologies Page 12

D. DEMONSTRATION PROJECT LOCATION(S)

Three buildings at the Denver Federal Center were selected as the installation site for this study (Building 25, 

Building 67, and Building 95). Building 25 is a multi-use facility of laboratories, data centers, and office space. 

The cooling tower loop consists of three single cell cooling towers rated at 500 tons. Building 67 is a 14-story 

high rise with the cooling towers located on the roof. There are two separate towers, each tower is a single cell 

tower and each one is rated at 600 tons. These two towers serve as the condenser loop for the 450 and 900-ton

chillers located in the nearby penthouse. Building 95 is an office and laboratory building with two 250-ton 

chillers and two 250-ton cooling towers.

Measurement & Verification Project Plan

A. TECHNICAL OBJECTIVES 

AWT technologies were assessed according to two main criteria set out by GSA in the original RFP.

1) Effectiveness of AWT in meeting a predetermined set of criteria that constitutes proper water quality. 

These criteria were developed by a consultant to GSA under a previous contract and were used in this 

study to determine whether the AWT systems assessed met the water quality criteria. 

2) Effectiveness of AWT systems in reducing GSA operating costs (water/ sewer costs and operation and 

maintenance (O&M) costs).

This report addresses water savings, O&M costs, and water quality associated with AWT technologies. As 

described in the Section III.B., each AWT technology is unique in its operational premise and each has its own 

distinct advantages or disadvantages. The purpose of this study is not to compare the inner workings of a given 

technology, but rather to compare the system inputs, outputs, and delivered result.

B. TECHNOLOGY DESCRIPTION

Cooling tower structures vary greatly in size and design, but they all provide the same function: liberation of 

waste heat extracted from a process or building system through evaporation of water. In technical terms, 

cooling towers are engineered and designed based on a specified cooling load, expressed in refrigeration tons. 

The cooling load is determined by the amount of heat that needs to be extracted from a given process or peak 

cooling demand. The cooling tower must be adequately sized to reject this same amount of heat to the 

atmosphere.

Cooling towers and chillers require a constant water supply of adequate quality to ensure proper operation and 

to limit scale and corrosion impacts on the system. AWT technologies vary significantly with respect to how they 

provide the required water quality. To quantify the performance of the AWT technologies evaluated in this 

report, each technology was metered to provide data on the inputs, outputs, and delivered results (Figure 3). 
Cooling Tower Water Treatment Technologies Page 13

Figure 3. Conceptual model of the AWTs (Credit: Joelynn Schroeder, NREL)

System Inputs

The inputs to each water treatment system vary. In general, they can be described as follows. Specific details 

with respect to each AWT are included in the Technology Specification section.

• Make-up Water: The quantity of make-up water required to operate the system within required 

operating ranges is a function of the AWT’s effectiveness and the potable water quality in a given area.

• Materials: AWTs may require the use of additional material inputs for their operation, including biocide, 

acid, or brine. The quantity of chemical required varies by AWT and water quality at the site.

• Energy: Operation of each AWT requires additional energy for Programmable Logic Controllers (PLCs) or 

pumps that is incremental to the existing cooling tower operation.

• Operations & Maintenance: Periodic cleaning to reduce scale and corrosion or the replacement of filter 

materials to keep the system in good working order varies by AWT and site conditions.

System Outputs

The outputs from each water treatment system vary. In general, they can be described as follows. 

• Blowdown Water: The quantity and quality of blowdown water is a function of the AWT’s ability to 

maintain acceptable water quality and the CoC of the system. The quantity and quality of blowdown 

water that can be discharged may be limited by local sanitary sewer permitting requirements.

• Waste – if this exists (in addition to blowdown water stream): The amount and quality of waste 

generated by the system, such as brine solution or filter backwash, have specific disposal concerns that 

vary by AWT. 

• Drift: The amount of water lost to the environment as a part of the evaporative cooling process. Drift is a 

function of tower geometry and would not vary based on the AWT.

Delivered Result
Cooling Tower Water Treatment Technologies Page 14

• Adequate Water Quality: The condenser water loop needs to be maintained at appropriate 

concentrations of Total Dissolved Solids (TDS), and pH, and other water quality metrics as defined by the 

RFP to prevent damage to piping and equipment.

• Cooling Delivered: The chiller is able to operate effectively, supplying adequate chilled water to the 

building for maintained occupant comfort.

The main goal of this study is to evaluate the potential for water savings in the technologies under 

consideration. This is evaluated in terms of gallons per ton-hour (gal/ton-hr.) of cooling delivered to the building. 

This metric quantifies the gallons used for each ton-hour of cooling, and therefore enables a comparison 

between buildings with different total cooling demand. 

The goal of these technologies is not only to conserve water in cooling tower operations, but to deliver cooling 

to each building as efficiently as possible. The impact of the improved chiller operation (due to reduced scale) or 

reduction in cooling tower operation was not able to be quantified for this report, due to the fact that the 

metering of the systems was put in place after the systems were installed. Therefore, there is no measured data 

on energy consumption before installation of the new systems that would enable an accurate evaluation of 

improved chiller operation. The probable benefits are discussed qualitatively in the report

The AWTs addressed in this report are commercialized technologies. Given their commercialized state, the 

AWTs evaluated in this report are at a Technology Readiness Level 8 (meaning the system is incorporated in 

commercial design, with actual system/process completed and qualified through test and demonstration.

C. FACILITY DESCRIPTION

Building 25, Building 67, and Building 95 at the Denver Federal Center were selected as the installation sites for 

this study. The buildings housing the AWT technologies are a combination of office and laboratory buildings ( 

building 25 and 95 has office / labs, building 67 is an office building) of varying sizes. For the laboratory 

buildings, the internal loads require the cooling towers to operate more hours per day and for months of the 

year than a typical 9-5 office building in CO. They are all medium to large office buildings (small buildings do not 

tend to use water-cooled chillers). Traditional chemical water treatment was existing in all buildings prior to the 

AWT installations. 

The three different buildings selected for this study each had a different AWT technology installed in order to 

decrease the water consumption of their respective water-cooled chiller plants. The ability to have all three 

AWT technologies in close proximity (all sit on the main Denver Federal Center campus) enables a consistent 

comparison with respect to weather (dry bulb, wet bulb, and wind speed) and incoming water quality (all from 

the same water mains).

A summary of each of the three buildings is provided in Table 4.

Table 4. Overview of Selected Facilities

AWT 

System/Technology

AWT Installation Sites, Building Sizes, and Cooling 

System Details

Building 25

Denver Federal Center, Building 25, W. 6th Avenue, 

Denver, Colorado

Building Facts: 360,797 square feet (SF), office / lab

building, housing 425 occupants

Cooling System: (2) 500-ton chillers; (3) 500-ton 

cooling towers; (1) flat plate heat-exchange
Cooling Tower Water Treatment Technologies Page 15

Building 67

Denver Federal Center, Building 67, W. 6th Avenue, 

Denver, Colorado

Building Facts: 342,722 SF, office building, housing 

1200 occupants

Cooling System: (1) 900-ton chiller, (1) 450-ton 

chiller; (2) 600-ton cooling towers, (1) flat plat heatexchanger

Building 95

Denver Federal Center, Building 95, W. 6th Avenue, 

Denver, Colorado

Building Facts: 163,206 SF, office/lab building, 

housing 130 occupants

Cooling System: (2) 250-ton chillers; (2) 250-ton 

cooling towers 

Both of the buildings that received detailed monitoring under this technology demonstration (Building 25 and 

Building 67) had the ability to record time series data for specific points associated with the chiller plant through 

their building automation system (BAS). This enabled the detailed monitoring described in the Test Plan section 

(section IV. D).

D. TECHNOLOGY SPECIFICATION

Three different types of AWTs are evaluated in this report, including: a salt-based system in Building 25vi, a 

chemical-based system in Building 67vii, and a hydrodynamic cavitation system in Building 95viii. A description of 

the primary components of each system is described for each facility.

Building 25

The Building 25 AWT #1 is manufactured by Water Conservation Technology International (https://www.watercti.com/technology.html) and the system controls scale, corrosion, and biological growth without the use of 

chemicals. Using a proprietary salt-based high efficiency softening system the hardness is removed from makeup water (Figure 4). The removal of low solubility ions reduces scale potential in the cooling tower, and 

increases solubility of TDS allowing soluble silica (from 200 -1,000 mg/l) in make-up water to polymerize to 

saturation equilibrium. Polymerized silica protects metals from high TDS, corrosion and scale. And high TDS/ pH 

levels in the water reduce biological growth. This AWT technology is comprised of twin fiberglass ion exchange 

media tanks, alternating polyethylene regeneration tanks, a brine tank, and metered usage controls providing 

web-based remote access for reporting and control. The water softener regenerates based on volume, typically 

2-3 times per week in the summer and 1 time per week in the spring and fall. The regeneration process uses 70 

gallons of water and the brine solution is discharged to the sewer.ix x

 

Technology Summary:

• Scale Prevention: Silica that is normally associated with scaling is altered through the water 

softening process to non-scaling forms.

• Corrosion Inhibition: The altered silica composition inhibits corrosion.

• Biological Growth Control: Biological growth is generally controlled by the heightened pH. Biocide 

may be added if necessary.
Cooling Tower Water Treatment Technologies Page 16

Figure 4. Representative diagram of system layout for Building 25 (Credit: Joelynn Schroeder, NREL)

Building 67

The Building 67 AWT #2 system is manufactured by Terlyn (http://www.terlyn.com/cooling-tower-waterconservation-program-overview-video/) and provides a more conventional water treatment approach that relies 

on scale and corrosion control while reducing blowdown (Figure 5). An advanced formulation of several 

hydrolytically stable, high strength polymers and bonding materials designed to systematically control hardness 

ions and other soluble elements through sequestration, threshold stabilization, and crystal modification. A PLC 

continuously monitors CoC in the cooling tower system water and the amount of fresh water being added to the 

system. Remote monitoring of the controller provides notification if parameters fall outside the desired range. 

The controller is set for 50 CoC based on TDS concentrations, and automates blowdown when this level has 

been reached. Peristaltic pumps feed chemicals into the system. The water meter communicates with the 

controller to help determine the need for chemical dosing. The controller opens and closes the bleed valve to 

blow down the system water when 50 CoC have been reached. The controller monitors potable water use and 

injects scale inhibitor, corrosion control, and biological control reagents accordingly.xi A glass media filter was 

added to this system by the installer to filter out particulate matter from the cooling tower such as dirt, sand,

and other forms of debris. The filter backwashes for 30 seconds once a day, with roughly 300 gallons of 

backwash water discharged to the sewer.

Technology Summary:

• Scale Prevention: A semipermeable membrane filters out suspended solids, thereby preventing 

these from nucleating and precipitating out. 

• Corrosion Inhibition: Industry standard corrosion inhibitors are used.

• Biological Growth Control: Conventional biocides must be added to the cooling tower water.
Cooling Tower Water Treatment Technologies Page 17

Figure 5. Representative diagram of system design for Building 67 (Credit: Joelynn Schroeder, NREL)

Building 95

The Building 95 technology uses a controlled hydrodynamic cavitation process to provide chemical-free water 

treatment. The system consists of two side-stream water loops that connect to the cooling tower sump. In one 

loop, water is passed through the treatment chamber and returned to the sump (Figure 6). In the second sidestream the filtration loop pumped water passes through a pair of horizontally opposed nozzles, increasing the 

water velocity. Collision of water from the opposing nozzles creates kinetic force and subsequent cavitation in 

the water. Within the system, strong vacuum forces form, which strip carbon dioxide from the water, and 

maintain an alkaline pH. High pH creates an environment where calcium carbonate forms colloids, which are 

filtered from the water stream to control the build-up of scale. Cavitation can create high localized temperatures 

and energetic forces, which also can control bacterial growth. The basin sweeping system directs dirt and debris 

toward the filter intakexii. 

Technology Summary:

• Scale Prevention: This creation of intense vortices results in collisions between any forming 

mineral crystals.

• Corrosion Inhibition: The increased concentration of (small) mineral crystals in the water 

raises its pH.

• Biological Growth Control: Conventional biocides must be added to the cooling tower water.
Cooling Tower Water Treatment Technologies Page 18

Figure 6. Representative diagram of system design in Building 95 (Credit: Joelynn Schroeder, NREL)

E. TECHNOLOGY DEPLOYMENT

The AWT system for Building 67 was deployed in October of 2011. It was installed in the penthouse of Building 

67 and new make-up and blowdown pipes were routed from the AWT system to the two cooling towers. The 

system includes water treatment chemicals, a double-walled holding bin where the chemicals can mix with the 

condenser loop water, a crushed glass filter, and a controller that monitors various water characteristics 

(including a continuous reading on TDS) and water flows through the system. This controller is connected to 

both the make-up and blowdown water meters and the system logs these values on a 60-minute basis, allowing 

for calculation of monthly make-up and blowdown values. The TDS meter and make-up water meter associated 

with the system are shown in Figure 7.
Cooling Tower Water Treatment Technologies Page 19

Figure 7. Meters associated with Building 67 AWT system (Credit: Tyler Cooper, GSA)

The AWT system for Building 25 was installed in January of 2011. It was installed against the east wall of the 

chiller room of Building 25. This system consists of two holding tanks where the brine solution can treat the 

incoming make-up water (Figure 8 and Figure 9), a controller that monitors water characteristics and meters 

make-up and blowdown water, and a supplemental chemical feed monitor for water treatment on an as-needed 

basis as it circulates through the condenser loop. 

Figure 8. Piping, controller and chemicals associated with the Building 25 AWT system (Credit: Dylan Cutler, NREL)
Cooling Tower Water Treatment Technologies Page 20

Figure 9. Tanks for treating the make-up water entering the Building 25 AWT system (Credit: Dylan Cutler, NREL)

Both systems take up relatively small areas within the mechanical rooms. The main elements of the treatment 

systems’ controls can be mounted in a 4 ft x 4 ft area in the mechanical room. For the system in Building 25, 

there is also some space requirement due to the two brine tanks (approximately 8 ft2 of floor space). The system 

in Building 67 requires floor space for both the 5-gallon contains of chemicals (approximately 3, 5-gallon 

containers) and for the double-walled mixing basin and sand filter(6-8 ft2 of floor space).

The Building 95 system was installed in October of 2011. This was a rack mount system that did not occupy

much of the mechanical room space. The additional floor space required for the Building 95 install was for a 

floor mounted inline filter (Figure 10). 
Cooling Tower Water Treatment Technologies Page 21

Figure 10. Building 95 AWT System (Credit: Doug Baughman, GSA)

F. TEST PLAN

The test plan focuses on calculating water savings and water quality associated with the AWT systems. It was 

especially important to be able to evaluate the AWT systems in a manner that allowed for comparison between 

the systems and between other buildings in the GSA building stock. 

The system at Building 95 had been uninstalled prior to the writing of this report and it was not possible to 

collect adequate data for comparison with the other two technologies. 

The two components that need to be measured to account for water savings are (1) water consumption and (2) 

cooling demand of the building. AWT water consumption is tracked in three different ways in this study. The 

exact date ranges and quantity of these data are discussed in the following section (IV. E):

1) Monthly make-up and blowdown water use

2) Daily make-up and blowdown meter readings

The cooling demand for the building can be tracked by monitoring the condenser loop (cooling tower) supply

temperature, return temperature, and flow rates. Given this data, it is possible to establish the amount of heat 

rejected by the cooling tower over time. These values are established by trending the BAS output for the chiller 

plant in each building during the period from May of 2013 to November of 2013 (a typical cooling season in 

Colorado). 

Water quality was assessed through water quality reports delivered monthly by the O&M contractor and a thirdparty testing company. These reports evaluated: conductivity, pH value, total hardness, calcium hardness, 

magnesium hardness, “P” alkalinity, “M” alkalinity, silica high range, chloride anions, salt anions, sulfate anions, 

phosphate, copper, iron, biological growth, and CoC.

Instrumentation Plan

The goal of the instrumentation plan for this technology demonstration focused on metering the cooling tower, 

specifically to quantify the water use in each building. The cooling demand / heat rejection for each building is 
Cooling Tower Water Treatment Technologies Page 22

required to establish a gallon per ton-hr. metric for evaluating the efficacy of the different AWT systems. To 

develop this measurement, the BAS in each building was configured to export a daily report on the following 

points:

- Condenser water supply and return temperatures (°F)

- Condenser water pump status (ON/OFF), including one point for each pump

- Cooling tower fan status (ON/OFF), including one point for each fan

- Chiller water supply and return temperatures (°F)

- Chiller status (ON/OFF), including one point for each chiller

- Chilled water loop pump status (ON/OFF), including one point for each pump

- Outdoor air temperature (°F) and humidity (%)

All of these data were collected at 15-minute intervals from May of 2013 (August 2013 for Building 67) through 

November of 2013.

The other data required to establish the efficacy of the AWT systems was the water consumption of the 

different systems over time. The water consumption data came from three different data sources:

- Monthly building meter data (including pre- and post-retrofit data) from the advanced metering initiative at 

the DFC. This water use was measured at a whole building level from January of 2009 to October of 2013.

- Monthly water use for the cooling tower make up water, as provided by the two system manufacturers. 

These data spanned from January of 2010 to November of 2013 for Building 67 and from January of 2012 to 

September of 2013 for Building 25.

- Daily make-up and blowdown water for May 2013 through September 2013 (for Building 25) and August 

2013 through November 2013 (for Building 67). These metered data were gathered from the controllers 

installed with the AWT systems. These controllers logged both blowdown and make-up water readings from 

pulse meters installed with the system.

The last piece of data collected for the analysis was the average daily outdoor air temperatures from January 

2009 to October of 2013. This data was collected from the Denver Centennial weather station (historical data 

was downloaded from wunderground.comxiii). This data was used to estimate the cooling demand for the preinstallation time frame, which was then correlated with the historical water consumption data outlined above 

to generate weather-normalized water consumption data for each building.

GSA has developed the following water chemistry standards as a guideline to determine the acceptability of 

cooling tower water quality for a given AWT (Table 5). Operations staff and AWT vendors performed monthly 

monitoring of these parameters to characterize performance of a system. It should be noted that adherence to

these ranges is not the only indicator of an AWT’s success. The operation of each AWT is unique and as a 

function of the materials used in its design may result in water quality that falls outside the ranges defined in the 

project specifications. These chemistry standards were established for guidance in this particular project 

location. In the application of these AWTs a site should consider site-specific water quality constraints, whether 

due to influent potable water or discharge permit limitations, and make an AWT selection accordingly.
Cooling Tower Water Treatment Technologies Page 23

Table 5. Water Quality Criteria (as defined by GSA)

Test Acceptable Ranges

T alkalinity (ppm) 100 - 1000

pH 7.3 – 9.0

Chloride (ppm) 10 - 500

Cycles >2

Total Hardness (ppm) 500 - 1500

Phosphate (ppm) 43327

Conductivity (mmHos) <2400

Bacteria Count (cfu) <80000

Water Appearance Clear

Iron (ppm) <4

Calcium Hardness (ppm) <500

Magnesium Hardness (ppm) <100

Chlorides (ppm) <250

Salt (ppm) <410

Sulfates (ppm) <250

Silica (ppm) <150

ORP (mV) >300

90-day Copper Coupon (mpy) <0.2

90-day Mild Steel Coupon (mpy) <3

90-day Galvanized Steel (mpy) <4

90-day Stainless Steel (mpy) <0.1

Results

A. Water Savings

In order to accurately assess the water savings from these AWT systems, it was necessary to obtain (or calculate) 

both pre- and post-retrofit water consumption. It is also necessary to have these consumption values for the 

same cooling demand so that the savings are normalized for weather variation. The weather normalization 

ensures that a cooler or warmer than average month does not impact the calculated water savings.

Building 67 recorded make up and blowdown water consumption both before (for 21 months prior) and after 

installation of the AWT. Building 25 only had post retrofit water consumption data for the AWT system. In order 

to calculate pre-retrofit water consumption for the Building 25 system, the CoC of the old water treatment 

system was recorded (obtained from the building operational logs). The two different calculation methods are 

presented below. 

Building 67:

The steps in calculating the savings for building 67 are as follows:
Cooling Tower Water Treatment Technologies Page 24

1) BAS metered data was taken from August 2013 to November 2013 to generate a correlation between 

cooling demand and outdoor air temperature

2) Historical daily outdoor temperature data was used to estimate monthly cooling loads

3) A regression analysis was conducted on monthly cooling demand versus the metered water consumption

(one regression for pre-installation, and one for post-installation)

a. The slope of the regression lines was calculated to obtain a gallon per ton-hour value for pre- and 

post-installation

4) The difference in the two slopes was used to calculate water saved in the post retrofit months

Step 1 – Correlation between Cooling Demand and Outdoor Air Temperature

The BAS system data provided the supply and return temperature from the cooling towers, as well as the 

operating status of the condenser water loop pumps. This enables the calculation of heat rejected from the 

cooling tower using the equation: 

QQ = mṁCCpp∆TT, 

• QQ is the heat rejected by the system

• mṁ is the flow from the constant speed pumps

• CCpp is the specific heat of water

• ∆TT is the difference in temperatures between the entering condenser water and the exiting 

condenser water

Due to the fact that the temperature differential (∆TT) was calculated on the condenser water loop (not the 

chilled water loop), it isolates out any impacts of chiller efficiency. In this case, therefore, QQ represents the heat 

rejected through evaporation of the condenser water. 

The heat rejected was calculated for each 15-minute time step and then summed over the course of the day.

Daily values were also calculated for: average outdoor dry bulb, average relative humidity, minimum dry bulb, 

maximum dry bulb, and operating hours. Each of these variables (plus weekday/weekend status) was tested for 

correlation with the calculated cooling demand. The two significant variables identified were average outdoor 

air temperature and weekday/weekend (expressed as a -1 for weekend and 1 for weekday). These correlations

were then able to be used to calculate cooling demand for the buildings for historical periods where the BAS 

trend data was not available.

Figure 11 shows the regression of daily condenser water heat rejection versus daily average outside air 

temperature. The regression equations shown on the graphs are the equations used to correlate cooling 

demand to operating conditions. The regression lines are not shown in the figures due to the fact that they are 

bi-variate regressions which are not able to be plotted on a 2-dimensional graph, but both independent 

variables are shown (outdoor air temp, and the weekend/weekday – shown by series color). 
Cooling Tower Water Treatment Technologies Page 25

Figure 11. Correlation between air temperature and cooling demand for Building 67 

Step 2 – Estimate historical monthly cooling loads 

The linear regression performed in step 1 was used in conjunction with historical temperature data (daily 

average outdoor air temperatures) to estimate daily cooling demand for each building. This was done for the 

period of January 2009 through November of 2013. The monthly cooling demand for each building is shown in 

Figure 12.

Figure 12. Estimated monthly cooling demand for Building 67

Step 3 – Perform regression on monthly cooling demand versus metered water consumption

To quantify water savings, it is necessary to know consumption before and after the installation; the subtraction 

of these two values results in water saved due to the new technology. The issue with this simple subtraction is 
Cooling Tower Water Treatment Technologies Page 26

that the data was not normalized for weather. A July month after the install could have been much warmer 

(causing more water use) than the July month before the install, making the simple difference calculation not 

valid.

In order to perform the weather normalization, a regression was performed that compared monthly cooling 

demand to the metered water consumption. This was done for the pre-retrofit period and the post-retrofit 

period. The regression lines and plotted data are shown in Figure 13. 

Figure 13. Regression of monthly water use to cooling demand (Building 67)

Step 4 – Calculate Water Savings for Post-retrofit Months

The difference in the slope of the pre- and post-retrofit regression lines equates to the gallons saved per tonhour of cooling, due to the implementation of the AWT technologies. This amounted to 0.422 gal/ton-hour for (a 

24% savings over the old water treatment system). The pre-retrofit regression equations were used to calculate 

the water consumption that would have occurred with the old system in place, and the post-retrofit equations 

were used to calculate the water consumption for the new system. These calculations were done for each of the 

post-retrofit months (using the same monthly cooling demand in each equation). The monthly savings are 

shown in Table 6.
Cooling Tower Water Treatment Technologies Page 27

Table 6. Monthly water savings for Building 67 from 2012-2013 (gallons)

Month 2012 2013

Jan 44,707 42,604

Feb 37,690 38,867

Mar 66,601 50,092

Apr 71,881 55,270

May 86,761 81,031

Jun 110,658 105,032

Jul 119,705 113,927

Aug 114,247 113,619

Sep 94,524 95,857

Oct 66,333 61,389

Nov 57,349 35,170

Dec 42,526

Totals 912,983 792,858

It is also important to note that there is a limit to how much water can be saved by these systems. This is due to 

the evaporation component of cooling tower water use; this is what provides the cooling to the condenser loop 

and cannot be eliminated from the water consumption. The evaporation portion can be calculated with the 

equation:

EE = FF × QQ

∆HHvvvv × 8.34 llll/gggg

• EE is the gallons of water evaporated 

• FF is a factor expressing the ratio of latent to sensible cooling (usually a value between 0.75 and 1, 

with a value of one dictating total latent cooling)

• QQ is the cooling required by the tower (can be calculated using mṁCCpp∆TT as discussed above)

• ∆HHvvvv is the latent heat of vaporization of water (approximately 1000 Btu/lb at sea level and 

closer to 976 at Denver’s altitude).

If QQ is set to be 12,000 Btu (or one ton), the lower limit for gallons per ton-hr can be calculated as between 1.10-

1.32 gal/ton-hr. (depending on FF, the portion of cooling that is latent vs. sensible). The equivalent ton on the 

cooling tower side rejects 15,000 Btu/hr due to the heat-equivalent of the energy needed to drive the chiller's 

compressor, in which case the lower limit for condenser side gallons per ton-hr is 1.375 to 1.65 gal/ton-hr. The 

system in Building 67 achieved 98% of available cooling tower water savings.

Building 25:

Due to the fact that Building 25 did not have any pre-retrofit water consumption data, a different analysis 

technique was used to calculate the water savings for Building 25. For Building 25 the monthly water logs for the 

old water treatment system were accessed and the recorded CoC was obtained for each of the months in 2009. 

This data is shown in 

Table 7. It should be noted that these are only spot measurements and do not necessarily show the actual 

average CoC maintained by the prior water treatment system; that being said, the consistency in the data 

demonstrates that this was approximately where the prior system was controlling the cycles.
Cooling Tower Water Treatment Technologies Page 28

Table 7 - Cycles of Concentration for summer 2009

Date Conductivity -

Make Up

Conductivity -

Blow Down

Cycles of 

Concentration

9-May 890 240 3.71

9-Jun 1,240 280 4.43

9-Jul 1,180 280 4.21

9-Aug 1,280 280 4.57

9-Sep 1,340 280 4.79

9-Oct 1,340 280 4.79

Average: 4.42

A mass balance on the water in the cooling tower can be represented byxiv:

MM kkkkkkkk = EEEE + DDDDDDDDDD + BB BBBBBBBBBBBB

In order to determine the water savings for Building 25 it was necessary to calculate the difference in make-up 

water.

∆MM MM = (EEEE PPPPPPPP + DDDDDDDDDDPPPPPPPP + BB BBBBBBBBBBBBPPPPPPPP) − (EE PPPPPP + DDDDDDDDDDPPPPPP + BB BBBBBBBBBBBBPPPPPP)

In order to calculate the savings for the same level of cooling demand (weather normalized), we know that 

evaporation must be equivalent (same cooling load met). It also follows that the drift component would be 

equivalent (same climatic effects). Therefore, the only difference between pre- and post- retrofit make-up water 

consumption is the amount of blowdown required. Since the post retrofit blowdown was reduced to zero in 

2012 and 2013, the above equation can be simplified to:

∆MM MM = BB BBBBBBBBBBBBPPPPPP

Blowdown can be calculated as follows:

BB BBBBBBBBBBBB = EEEE

CC CC − 1

Therefore, the water savings can be calculated using the above equation with the evaporation for the postretrofit data, and the CoC from the pre-retrofit system (average value of 4.42 used). The calculated monthly 

savings are shown in Table 8.
Cooling Tower Water Treatment Technologies Page 29

Table 8. Monthly water savings for Building 25 from 2012-2013 (gallons)

Month 2012 2013

Jan 5,497 8,830

Feb 13,421 6,491

Mar 3,830 7,836

Apr 21,667 18,275

May 25,117 26,930

Jun 50,585 40,409

Jul 73,187 76,667

Aug 62,193 82,485

Sep 48,304 73,088

Oct 35,643 59,251

Nov 15,175 26,608

Dec 12,018 8,830

Totals 366,637 435,702

The total annual water savings for building 25 ranged from 336,637 gallons in 2012 to 435,000 gallons in 2013.

B. Water Quality

On a monthly basis cooling towers are tested for effectiveness of water treatment, including pH, TDS, 

conductivity, biological dosage level, scale and corrosion inhibitors. Tests are performed for biological growth

including iron-related bacteria, sulfate reducing bacteria, slime forming bacteria, fluorescing pseudomonas, and 

blue-green algae. Chemicals and biological treatment dosage and water blowdown rate are adjusted as 

required. Make-up water is tested frequently to determine the appropriate levels for chemical dosing.

Acceptable water quality ranges for this project are presented in the Instrumentation Plan section of this report. 

GSA typically runs a cooling tower that uses standard chemical-based water treatment between 3 to 6 CoC. 

Blowdown may need to be increased if other indicators determine an adverse impact on the tower. However,

this should be balanced with water conservation goals.

The following paragraphs present water quality data for the AWT technologies from two reporting sources: San 

Joaquin Chemicals, Inc. (SJCI), a third-party company, and the O&M contractor for each building. These data 

have been summarized below and are presented in detail in the Appendix.

It should be noted that no pre-installation water quality data was available to enable comparison to the previous 

treatment system. However, the O&M contractors at Buildings 25 and 67 noted significant improvement in 

cleanliness of the towers and chillers with the new AWT systems. The AWT system at Building 95 was not 

performing adequately according to O&M staff and was subsequently decommissioned. 

Building 25

Analysis of the water quality data shows that some of the control ranges in the project specifications are not 

being maintained for the following criteria:

• Conductivity (microsiemens): Control range <2,400; maintained = 8,700 - 20,000 (300% - 800% outside 

range)

• pH value: Control range = 7.3–9.0; maintained = 9.6–10.2 (up to 13% outside range)

• “M” Alkalinity (as CaCO3): Control range = 100–1,000 ppm; maintained = 1,680 – 3,004 (68% - 300% 

outside range)
Cooling Tower Water Treatment Technologies Page 30

CoC during the demonstration period for this AWT ranged from 12 – 80 (values of 3-5 were observed during the 

first 3 months of operation, but this was impacted by cleanout from previous operation). The lower values of 12-

15 were impacted by tower clean out lowering the operating cycles. Improved efficiency from this AWT 

decreased blowdown by 99% during the demonstration period.

Building 67

Analysis of the water quality data shows that some of the control ranges in the project specifications are not 

being maintained for the following criterion: 

• Conductivity (microsiemens): Control Range <2,400; maintained = 2,540 - 7,000 (5% - 290% outside 

range)

The CoC during the demonstration period ranged from 13 – 18. Improved efficiency from this AWT decreased 

blowdown by 94% during the demonstration period.

It should be noted that the specified project ranges are not absolute proof of success or failure of the AWTs in 

achieving adequate performance. By design many of the AWT systems maintain certain water quality 

parameters outside the range of the project specifications. While high TDS and conductivity levels can lead to 

corrosion and high alkalinity levels can cause scale build up in cooling tower system pipes O&M staff observed 

no adverse effects on the system with these parameters falling outside specified ranges in Buildings 25 and 67.

Building 95

Analysis of the water quality data shows that except for one instance the control ranges in the project 

specifications are being maintained. The CoC during the demonstration period ranged from 3 – 10.

The range of CoCs attained by each of the AWT systems was plotted in Figure 14 for the monitored period 

during the demonstration. 

Figure 14. Observed CoC during SJCI Test Period

C. Preventative Maintenance Savings

Typical preventative maintenance (PM) practices for GSA cooling tower water treatment systems include the 

use of chemicals, chemical feeding, maintaining proper water conditions, and controlling bleed off. Water 
Cooling Tower Water Treatment Technologies Page 31

treatment methods are intended to protect the life of equipment, maximize heat transfer, and minimize scale,

corrosion, solid matter buildup, biological growth, and water usage. 

Building 25

GSA indicated that the amount of time required for PM on Building 25 became significantly less with the new 

treatment system, roughly 16 hours less time per quarter and requiring 1 staff member instead of 2 to perform 

cleaning. Corrosion coupons showed no signs of corrosion, nor were any signs visible on the chiller end bells or 

chiller tubes during scheduled maintenance. Because of cleaner cooling tower operations O&M staff was able to 

increase the programmed run time of the flat plate heat exchanger. According to GSA’s O&M staff, Building 25 

has eliminated the use of scale inhibitor, but still uses some biocides to protect against algae. However, as a 

function of the treatment technology more salt is being consumed to meet the demand.

Building 67

According to GSA the loosening of residual scale was observed in Building 67 at first deployment of the 

technology. As technology use continued, the released scale was cleaned out of the system and operation of the 

chillers/towers improved. The O&M Project Manager indicated that the equipment overall was much cleaner 

and cleaning the tower as part of PM became much easier: O&M staff was able to hook a hose to the suction 

side of the sand filter and vacuum the tower without draining it. The amount of cleaning time associated with 

this AWT has been dramatically reduced; however, glass filter beads will still need to be replaced every 3 years 

to uphold system operations. This system does require higher cost chemicals to attain these improved operating 

conditions.

O&M staff also indicated that the technology helped with maintaining the chiller tubes and flat plate exchanger 

surfaces. There was a noticeable increase of efficiency of the flat plate exchanger, allowing for a larger operating 

range and a higher transition temperature where the building shifts from free-cooling to initial chiller

activationxv. It was stated that the 400-ton chiller can now handle up to 85°F outdoor air temperatures before 

bringing an additional chiller on-line to meet cooling demands. O&M staff has also observed that the flat plate 

collector can go 6°F higher before needing to switch to mechanical cooling, which is believed to be due to a 

cleaner system. While it was not possible to quantify the exact energy and cost savings associated with 

improved performance of the chiller and flat plate heat-exchanger operation (due to lack of sub-meter data and 

coincident energy efficiency improvements throughout the building that impact whole building energy use), it is 

clear that the improved operation will reduce energy use at the site. It is recommended that future work 

evaluate the energy impact of improved chiller operation.

Building 95

Due to the location of the Building 95, the tower receives continuous sun and wind exposure. GSA staff believes

this may have led to algae growth that plagued the system for most of the demonstration period. The amount of 

debris (including algae and dirt) that accumulated in the basin led to an increase in maintenance for this system. 

The O&M Project Manager indicated the tower required 4 times the normal cleaning along with the flow

switches requiring regular cleaning. The vendor addressed the algae issue immediately, treating it with biocide,

and installed a copper silver ionizer to deter the algae growth; however, the problem still persisted. No signs of 

corrosion were present based on the regular samples taken. To operate this technology 2 filtration pumps were

required, posing an energy demand of 7.5 HP and 10 HP when cooling towers are in use.

D. Economic Analysis

The economic evaluation of these technologies was based on the annual savings for each technology, the initial 

cost of the technology, and the yearly operation and maintenance costs. Using these values, the net present 

value (NPV) of each project was calculated, as well as the simple payback period. NPV was calculated in this 
Cooling Tower Water Treatment Technologies Page 32

report according to the National Institute of Standards and Technology (NIST) Handbook 1352 with an assumed 

system life of 15 years, a discount rate of 3%, and a water cost escalation rate of 6.2% (as determined by historic 

Denver Water master meter rate increases over the past 4 years, see Table 9). The O&M costs were assumed to 

escalate at the NIST inflation rate of 0.9%.

Table 9. Denver water rates of the past four years (master meter outside the city)

Year Water Cost 

($/1000 gallons)

Increase over 

previous year 

(%)

2011 $3.5 -

2012 $3.6 6%

2013 $3.8 5%

2014 $4.0 9%

The savings due to reduced water consumption were calculated using utility rates provided by GSA. These rates 

were: $3.45 - $3.81/ 1,000 gallons (from 2011 – 2013) for a Denver Water master meter outside the City and 

$1.84/thousand-gallon for sewer costs. A combined water and sewer rate of $7.14/kGal from 2017 utility bills 

was used for the calculations. It was assumed that each gallon of water saved reduced the amount discharged to 

the sewer and subsequent costs incurred by the site. This is due to the fact that the water saved due to

improved water treatment technologies was associated with the blowdown component of cooling tower water 

use. The blowdown therefore was not required to be discharged to the sewer system, saving that portion of the 

water costs.

Cooling tower water use is made of three components: evaporation, blowdown, and drift. The evaporation 

component provides the cooling to the chiller system and is not a function of the water treatment technology, 

but rather by the cooling required by the building. The drift component is determined by the physical 

characteristics of the cooling tower, and the flow rate through the tower. Due to the fact that the cooling towers 

(and their associated pumps) were not altered during the water treatment technology installations, the drift 

component should not have changed from pre-to post-installation. Therefore, the water savings calculated in 

this report should be direct reduction in blowdown due to the ability to increase the CoC achieved in the 

condenser loop.

Table 10 shows the initial cost, water savings per year, and the O&M savings associated with Buildings 25 and 

67. For AWT #1 the equipment costs were $18,100, the labor costs were $11,500, total installed cost of $29,600. 

For AWT #2 the equipment costs were $17,100, and the labor costs were $15,400, for a total installed cost of 

$32,500. Using these values the simple payback period and SIR of each technology is calculated. The O&M 

savings are calculated by comparing the annual costs associated with the new system to the previous water 

treatment O&M costs; this value can be positive or negative depending on the relative change in expenditures.

It should be noted that the high escalation rate for water costs has a significant impact on the NPV calculation, 

making both systems cost effective over a 15-year analysis (despite the relatively long simple payback value for 

Building 67). 

 

2 Energy Price Indices and Discount Factors for Life-Cycle Cost Analysis – 2013. Annual Supplement to Handbook 135 and 

NBS Publication 709. U.S Department of Commerce. http://www1.eere.energy.gov/femp/pdfs/ashb13.pdf. Accessed 

12/15/13
Cooling Tower Water Treatment Technologies Page 33

The savings due to reduced water consumption are highly favorable in both cases (higher in Building 67), yet the 

O&M costs decreased significantly in the case of Building 25, whereas the costs increased in Building 67. This has 

a large impact on economics associated with each technology. The increased O&M costs in Building 67 were due 

to an increase in chemical costs for the system (which utilized a higher quality chemical product) and periodic 

replacement of a glass bead media filter. On the other hand, Building 25 eliminated almost all chemical use and 

required a less expensive salt regeneration process for the O&M requirement in that system. Although there 

was an increase in chemical costs for AWT #2, there was a significant reduction in GSA maintenance hours for 

each technology. GSA maintenance hours reduced from 152 to 80 hours per year for AWT #1, saving $3,677 per 

year, and GSA maintenance hours were reduced from 132 to 69 hours for AWT #2, saving an additional $3,217 

per year. For AWT #2, since the O&M contract increased by $5,100 per year, the net increase in O&M costs was 

$1,883.

This economic assessment does not take into account any of the potential energy savings from increased chiller 

efficiency (due to improved heat exchanger effectiveness). The building engineers for both Building 25 and 

Building 67 indicated that they were able to run the flat plate chillers more often due to improved chiller 

performance after AWT install, and in Building 67 it was stated that they were able to run with one less chiller 

the majority of the summer. Tracking the reduction in electricity consumption from the chiller plants was not in 

the scope of this analysis, and therefore is not included in this analysis. The decrease in energy costs would only 

improve the system economics and should be evaluated in future studies.

Table 10. Economics of AWT systems in Buildings 25 and 67

Economic Parameter

Building 25 (AWT 

#1) at Local Water 

Rate $7.14/kGal

Building 25 (AWT 

#1) at GSA Avg 

Water Rate 

$16.76/kGal

Building 67 

(AWT #2) at 

Local Water Rate 

$7.14/kGal

Building 67 (AWT #2) 

at GSA Avg Water 

Rate $16.76/kGal

Initial Cost ($) $29,600 $29,600 $32,511 $32,511 

Cooling Tower Size (tons) 1,500 1,500 1,200 1,200

Water Savings (Gal/yr.) 401,170 401,170 824,448 824,448

Water and Sewer Cost 

Savings ($/yr.) $2,864 $6,724 $5,887 $13,818 

Annual Increase in O&M 

($/yr.) ($6,445) ($6,445) $1,883 $1,883 

Simple Payback with 

O&M (yrs.) 3.2 2.2 8.1 2.7

Savings to Investment 

Ratio 4.7 6.7 1.8 5.5

At a combined water and sewer rate of $7.14/kGal AWT #1 had an installed cost of $19,73/Ton, annual water 

savings of 401,170 Gal/yr, a simple payback period of 3.2 years and an SIR of 4.7 using a total project lifetime 

of 15 years. At the GSA national average water and sewer rate of $16.76/kGal, AWT #1 had an annual water 

cost savings of $6,724, a simple payback period of 2.2 years and an SIR of 6.7. AWT #2 at Building 67 had an 

installed cost of $27.1/ton, an annual water savings of 824,448 gallons per year, a simple payback period of 

8.1 years and an SIR of 1.8. At the GSA national average water and sewer rate of $16.76/kGal, AWT #2 had an 

annual water cost savings of $13,818, a simple payback period of 2.7 years and an SIR of 5.5. 
Cooling Tower Water Treatment Technologies Page 34

Conclusions

A. OVERALL TECHNOLOGY ASSESSMENT AT DEMONSTRATION 

FACILITIES

GSA installed AWT systems in three buildings located in Colorado in an effort to see whether AWT technologies 

could maintain adequate water quality while conserving water and reducing operating costs. Systems from three 

different manufacturers were installed in the three buildings. The determination of adequate water quality was 

to be made based on a previously-developed set of benchmarks (see section IV.E. - Instrumentation).

The AWT systems at Buildings 25 and 67 both generated water savings as compared to the previous water 

treatment systems. They generated water use reduction of 23% and 24% respectively over the previous systems. 

The Building 95 system received negative reviews from on-site maintenance personnel and this system was 

removed by on-site maintenance personnel within a year of the date it was brought online. This made it 

impossible to calculate quantitative water savings for this system. Based on this and the previously discussed 

results, the systems in Buildings 25 and 67 show promise for widespread use in GSA buildings.

Table 11 through Table 13 present a summary of study results based on the model used for evaluation.

Table 11. Summary of Inputs to the Three AWT Systems

AWT Make-up Water Materials Energy Required O&M

Building 25 Decreased 23% Brine 120 V controller

Decreased with 

AWT, but 

chemical costs 

increased

Building 67 Decreased 24%

Biocide, Scale 

Inhibitor 

Corrosion 

Inhibitor

120 V controller

Decreased with 

AWT, but glass 

media filter may 

offset this 

reduction

Building 95

Decreased (exact 

percentage not 

quantified)

Biocide 7.5 and

10 HP pumps

Increased due to 

algae growth

Table 12. Summary of Outputs Associated with the Three AWT Systems

Outputs

AWT Blowdown Water Waste

Building 25 Decreased 99% 70 gallons per regeneration

Building 67 Decreased 94% 300 gallons per backwash

Building 95 Decreased (exact percentage not 

quantified) None

Table 13. Summary of Delivered Results from the Three AWT Systems

Delivered Result

AWT Water Quality Cooling Delivered

Building 25 Outside ranges for conductivity, pH, 

alkalinity

As required (potential increase in 

system efficiency)

Building 67 Outside range for conductivity As required (potential increase in 

system efficiency)

Building 95 Ranges met As required
Cooling Tower Water Treatment Technologies Page 35

It should be noted that, in spite of the fact that the final RFP dictated that adherence to the project 

specifications determined whether the technologies provided adequate water quality, these ranges should not 

be taken as absolute proof of success or failure of the AWT technologies in achieving adequate water quality. 

Many of these AWT systems by design maintain certain of these indices outside the range of the project 

specifications (e.g., pH is elevated as a means of corrosion control, so consequently, phosphate levels are lower 

than pH level targets using conventional chemical control).

Due to the overall improvements in water quality and reductions in blow down water usage for two of the three 

AWT’s evaluated in this report, both AWT #1 (Building 25 technology) and AWT #2 (Building 67 technology) was 

installed at six additional installations at the Denver Federal Center that had advanced 15-minute cooling tower 

make up water meters. The measured annual water savings and associated cost savings were calculated and 

combined with the O&M costs from this study to calculate an annual cost savings, simple payback period and 

savings to investment ratio (Table 14).

For the AWT #1 technology GSA elected to use the vendor provided side stream filter system instead of the 

typical GSA scoped side stream filtration system, which reduced the cost and for the AWT #2 systems, GSA 

continued to install the glass media filter system. For Building 20, chemical costs were increased to $400 per 

year, versus $1,883 per year for other facilities since this is a smaller facility with less cooling tower water usage 

than the other facilities.

Table 14 - Annual Water Savings and Economics for Additional Deployments at DFC

Denver 

Federal 

Center 

Facility

AWT System Date 

Installed

Cooling 

Tower Size 

(Tons)

Installed 

Cost ($)

Annual 

Water 

Savings 

(Gal/yr.)

Annual 

Water 

Savings 

($)

Annual 

Increase 

in O&M 

($/yr.)

Total 

Annual 

Cost 

Savings 

($/yr.)

Simple 

Payback 

(yrs.)

Bldg.20 AWT 

Technology #1 Nov-16 600 $31,057 718,597 $5,131 ($6,445) $11,576 2.7

Bldg.41 AWT 

Technology #1 Jan-17 1,000 $36,976 1,809,921 $12,923 ($6,445) $19,368 1.9

Bldg. 85 AWT

Technology #2 Jan-14 500 $8,756 62,450 $446 $400 $46 >40

Bldg.56 AWT 

Technology #2 Jan-15 1000 $28,557 661,160 $4,721 $1,100 $3,621 7.9

Bldg. 810 AWT 

Technology #2 Jun-14 2 x 500 $31,047 1,131,450 $8,079 $1,883 $6,196 5.0

Bldg.810 

USDA

AWT 

Technology #2 Mar-16 3 x 500 $31,047 1,048,000 $7,483 $1,883 $5,600 5.5

For building 41 the water usage went up for the post retrofit year of 2017 due to an issue with the blowdown 

solenoid valve that didn't get fixed for 3 months. For building 41 for the first four months of 2018, the system is 

showing considerable water savings and the savings for the new cooling tower AWT were extrapolated for the 

remaining months in 2018. For all of the additional AWT technology #2 installations, the measured blow down 

for the months preceding the installation of the AWT were reduced to close to 0 gallons a month, representing a 

97.8% to 100% reduction in blow down across all systems.

For the additional deployments at the DFC, the simple payback ranged from 1.9 to 7.9 years for all installations 

other than Building 85. The AWT #2 vendor was performing water treatment services at Building 85 prior to the 
Cooling Tower Water Treatment Technologies Page 36

other installations and the sand filtration system was able to be installed at a much lower cost than the other 

facilities for this facility.

B. BEST PRACTICE

One common theme for these systems was the fact that they all took some troubleshooting effort after the 

initial installation. Cooling tower water treatment is a unique sub-trade in which other tradesmen (HVAC 

technicians, plumbers, controls technicians) rarely engage. This phenomenon is borne out by the fact that 

traditional chemical water treatment (sales, installation, and maintenance of systems) is done by specialized 

professionals who concentrate on water treatment. Therefore, if GSA is to successfully implement this 

technology on a broad scale, rigorous installation methods and criteria must be laid out by GSA (see Section 

VI.E).

Best practice should also include integration of these AWT systems with building management systems or use of 

PLCs to allow operation of AWT systems to be adjusted and monitored remotely. Use of this strategy explains 

some of the success at the Denver site.

C. BARRIERS AND ENABLERS TO ADOPTION

Cooling tower performance depends on a variety of factors, many of which are location specific. Variables such 

as ambient air quality are specific to the site location and tower location on the site (e.g. airborne particulate 

matter), as well as seasonal changes (e.g. pollen) have the potential to affect the observed operation of each 

technology evaluated. These factors can contribute to biological growth or mineral deposits that require 

chemicals and additional maintenance towers.

The main challenge to widespread GSA deployment of these technologies is proper installation of the AWT 

systems. Cooling tower water treatment is a specialized niche in the building maintenance industry. To perform 

it properly one must possess in-depth knowledge in several disparate subject areas: building heating, ventilating, 

and air conditioning; water chemistry; and biological algae growth. If the on-site forces have received no training 

in these new AWT systems, this situation creates additional difficulties in successful implementation.

D. MARKET POTENTIAL WITHIN THE GSA PORTFOLIO

The first step in evaluation of further deployment of these AWT technologies is the identification of buildings in 

that GSA portfolio that have water cooled chillers. These are typically larger buildings where the high cooling 

loads benefit from the improved efficiency of water cooled chiller plants (and where the higher initial cost of a 

chiller plant is warranted due to higher loads). 

The next step in site selection is identifying sites where the AWT technology will perform well economically. To 

assist GSA in identifying sites that have high potential water and/or cost savings, NREL modeled water savings 

potential using the whole-building modeling software EnergyPlus. The “Large Office” building model was 

selected from the Commercial Reference Buildings that are developed and maintained by the DOE/NRELxvi. The 

Commercial Reference Buildings are a set of EnergyPlus building models that represent typical building types 

and constructions, and include climate-specific models (per building type) for each of the sixteen different 

ASHRAE climate zones (see Figure 15). The reference buildings also include three different vintages of 

construction: “pre-1980”, “post-1980”, and “new construction” (compliant with ANSI/ASHRAE/IESNA Standard 

90.1-2004). For the modeling analysis included in this report, the “post-1980” construction model was utilized. 
Cooling Tower Water Treatment Technologies Page 37

Figure 15. ASHRAE climate zone map

The large office building model is a 498,588 ft2 office building that is cooled via a water-cooled chiller. The 

standard cooling tower model in EnergyPlus defaults to blowdown operation that maintains a CoC of 3 and the 

cooling tower size ranged from 3,619 tons to 5,212 tons depending on location. In order to evaluate the 

potential impact of AWT in the national GSA building portfolio, the large office building model was simulated in 

16 different U.S. cities, one representative city for each of the 16 ASHRAE climate zones. For each climate zone, 

the model was run three times, (1) with the cooling tower set to maintain 3 CoC, (2) with the cooling tower set 

to maintain 15 CoC, and (3) with the cooling tower set to maintain 30 CoC. The EnergyPlus default of 3 CoC was 

established as the baseline, representative of a standard water treatment approach for water cooled chillers. 

The 15 and 30 CoC simulations represent a range of concentrations that have been shown to be achievable by 

AWT technologies in this report. Figure 22 shows the annual evaporation (in thousands of gallons water), and 

decrease in annual blowdown water use for 3, 15, and 30 CoC. The cities with larger numbers of cooling degree 

days and more arid climates (the “B” climates) show the greatest water savings. It can be noted that the vast 

majority of the water savings are achieved by 15 CoC; across the 16 different cities an average of 92% of the 

savings achieved at 30 CoC were captured at 15 CoC (Figure 16). 
Cooling Tower Water Treatment Technologies Page 38

Figure 16. Modeled water evaporation and blowdown usage across ASHRAE climate zones, showing decreased 

blowdown from 3 to 15 to 30 CoC. 

The water usage numbers were then translated into annual cost savings using site specific water rates. 

Combined water and sewer rates are based on current local municipal rates as of May, 2018, and are provided 

in Table 15. The annual water savings for each location were multiplied by the combined water rate for each 

city. The results from this analysis are presented in Figure 17.

Figure 17. Estimated yearly cost savings by climate zone
Cooling Tower Water Treatment Technologies Page 39

The wide variation in water costs between the different cities results in a significantly different picture in cost 

savings than is seen in water savings. Cities with high water rates (such as Atlanta, GA) generate the largest 

annual cost savings despite not having the largest total water savings. Table 15, below, gives the water rates 

used in this evaluation (current as of May, 2018). 

Table 15. Combined water and sewer rates for sample cities across each of the 16 ASHRAE climate zones

Location 

(Climate Zone)

Combined Water and 

Sewer Rate ($/kGal)

Location 

(Climate Zone)

Combined Water and 

Sewer Rate ($/kGal)

Miami (1A) 13.62 Albuquerque (4B) 4.98

Houston (2A) 10.38 Seattle (4C) 25.18

Phoenix (2B) 7.76 Chicago (5A) 7.76

Atlanta (3A) 29.12 Boulder (5B) 9.32

Las Vegas (3B) 8.25 Minneapolis (6A) 9.98

L.A. (3B-Coast) 8.88 Helena (6B) 8.30

San Fransisco (3C) 24.01 Duluth (7A) 13.51

Baltimore (4A) 12.30 Fairbanks (8A) 22.07

To gain an appreciation of the market potential for GSA, approximate system costs were utilized to calculate 

a savings-to-investment ratio (SIR) for each city. It should be noted that this calculation assumes that the 

annual operating costs associated with these systems are the same after the install as they were with the 

original system. For the installations evaluated in this report, one of the systems reduced the annual 

operating costs (due to reduced chemical use) and another system increased the annual costs (higher cost 

chemical). The SIRs denoted here are a very rough estimate, considering the assumptions that the original 

system was operating at 3 CoC, the new system would achieve 30 CoC, and that the annual operating costs 

remain the same pre- to post-install, yet they give a feeling for the critical variables driving economic 

viability of the system in various U.S. locations. The SIRs for a high installed cost assumption ($60,000) and a 

low cost assumption ($45,000) for each technology are shown in Figures 18a – 18d. The figures show the 

modeled SIRs for a given water and wastewater combined rate across various climate zones. The SIR 

calculations assume a 15 year project life and 30 CoC. 

AWT #1:
Cooling Tower Water Treatment Technologies Page 40

Figure 18a. AWT #1 - Savings to investment ratio for same system in evaluated climate zones for various water rates: 

high cost scenario

1A

2A

2B

3A

3B

3B-coast

3C

4A

4B

4C

5A

5B

6A

6B

7A

8A

SIR=1

GSA Avg. Water Cost 0

2

4

6

8

10

12

14

4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00

SIR

Water Rate ($/kgal)
Cooling Tower Water Treatment Technologies Page 41

For the high cost scenario, AWT #1 is life cycle cost effective (SIR>1) across all 16 climate zones when the 

combined water and sewer rate is above $5.50/kGal.

Figure 18b. AWT #1 - Savings to investment ratio for same system in evaluated climate zones for various water rates: 

low cost scenario

For the low-cost scenario, AWT #1 is life cycle cost effective (SIR>1) across all 16 climate zones when the 

combined water and sewer rate is above $4/kGal, as it was in each of the sixteen sample cities used in this 

study.

1A

2A

2B

3A

3B

3B-coast

3C

4A

4B

4C

5A

5B

6A

6B

7A

8A

GSA Avg. Water Cost SIR=1

0

2

4

6

8

10

12

14

16

18

20

4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00

SIR

Water Rate ($/kgal)
Cooling Tower Water Treatment Technologies Page 42

AWT #2:

Figure 19a. AWT #2 - Savings to investment ratio for same system in evaluated climate zones for various water rates: 

high cost scenario

For the high cost scenario, AWT #2 is life cycle cost effective (SIR>1) in some climate zones at some combined 

water and sewer rates. At the GSA average water cost of $16.76/kgal, this technology is cost effective in 12 of 

the 16 climate zones. 

1A

2A

2B

3A

3B

3B-coast

3C

4A

4B

4C

5A

5B

6A

6B

7A

8A

SIR=1

GSA Avg. Water Cost

-1

1

3

5

7

9

11

13

4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00

SIR

Water Rate ($/kgal)
Cooling Tower Water Treatment Technologies Page 43

Figure 18b. AWT #2 - Savings to investment ratio for same system in evaluated climate zones for various water rates: 

low cost scenario

For the low-cost scenario, AWT #2 is life cycle cost effective (SIR>1) across many of the climate zones at typical 

water and sewer combined rates. At the GSA average water cost of $16.76/kgal, this particular AWT technology 

is cost effective in 13 of the 16 climate zones. 

The buildings that have a large number of cooling degree days, high evaporation rates and good water quality 

can expect maximum water savings, whereas buildings in areas where typical wet bulb temperatures approach 

dry bulb design temperature will generally see less water use and therefore less water savings. The locations 

that have high water costs in addition to high water savings will result in the most economically favorable 

locations for GSA to pursue in its deployment of AWT technologies. It is also important to consider the water 

quality of the locations under consideration. In this case Miami, Huston, Phoenix, Atlanta, and Las Vegas, all had 

payback periods under 5 years based on the higher installed cost estimate of $60,000 and all locations had pay 

back periods of less than 12 years based on the lower installed cost estimate of $45,000. Given that the 

additional deployments at the DFC had installed costs that were all less than $37,000, the simple payback would 

be even lower across all cities at the installed costs realized for the additional deployments at the DFC.

1A

2A

2B

3A

3B

3B-coast

3C

4A

4B

4C

5A

5B

6A

6B

7A

GSA Avg. Water Cost SIR=18A

-1

1

3

5

7

9

11

13

15

17

4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00

SIR

Water Rate ($/kgal)
Cooling Tower Water Treatment Technologies Page 44

E. RECOMMENDATIONS FOR INSTALLATION, COMMISSIONING, 

TRAINING, AND CHANGE MANAGEMENT

A common challenge with the AWTs evaluated in this study was the lack of sufficient information on system 

installation and operations. This was due in part to the fact that the manufacturer representatives did not have 

much prior experience with government installations and did not provide the installation services that would 

have reduced project coordination challenges. In all cases there seems to have been a scarcity of information 

available to provide assistance and education in the design and installation of these AWT systems, including the 

following.

1) Clear statement of the design principles that should be followed in any installation of the particular AWT 

system. Design guidance documentation is required, similar to the design guidance provided by 

manufacturers of HVAC equipment in their literature. For instance, if an engineer or designer is 

designing a chiller, there are step-by-step guidelines (with example problems) about how to select a 

chiller (e.g., sizing, ancillary piping and devices)

2) Schematic layouts the designs should follow (e.g., specific and robust schematic plans showing all 

components, manufacturer-provided and off-the-shelf, required)

3) Theoretical explanations of the underlying fluid dynamics and thermodynamics involved when 

integrating these systems in cooling tower systems. For example, pumping head design information 

related to the specific AWT system-cooling tower system layout (series or parallel).

4) Trade manuals showing best practices in pump/pipe layout and integration of AWT systems into existing 

cooling tower systems.

5) Inclusion of the AWT metering systems into the BAS to assist with continuous monitoring of the systems.

In short, to ensure the proper design, installation, and functioning of AWT systems, the kind of process/product

education and manufacturer support network that exists in the larger field of building HVAC design must be 

developed. It is also recommended to discuss the accompanying energy reduction possibilities early on in the 

project and plan for operational changes that may result from improved chiller plant operation and how to 

document those energy and cost savings.

Further considerations for the application of these AWTs include the influent water quality and waste generated 

by these systems. Influent water quality is a function of the source water being used. Based on the chemical 

make-up of the influent water the addition of a filter may be required to support effective operation of an AWT 

and prevent damage to the cooling tower and chillers. As noted in previous sections, the Building 25 and 67 

technologies both generate waste streams through softener regeneration and filter backwash, respectively. 

While these systems both demonstrated a substantial reduction in blowdown, a small amount of waste (brine 

solution and filter backwash water) is produced as a function of their operations. The volume and chemical 

composition of these waste streams will vary based on cooling tower capacity and influent water quality. If in 

sufficient volume or concentration, discharge of these waste streams may be subject to restrictions from the 

sewer district.
Cooling Tower Water Treatment Technologies Page 45

Appendices

A. RESEARCH DETAILS

The detailed water quality results are presented in Figure 20 to 21.

Figure 20. Building 25 Water Quality Data
Cooling Tower Water Treatment Technologies Page 46

Figure 21. Building 67 water quality data
Cooling Tower Water Treatment Technologies Page 47

Figure 22. Building 95 water quality data
Cooling Tower Water Treatment Technologies Page 48

References 

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iv Holliday, R., Geiger, G., Geuns, P. 2012. Cooling Tower Institute Annual Conference. “How Green is the Cooling 

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v Morrison, F. ASHRAE Journal, July 2004. “What’s Up with Cooling Towers.” 

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