Prepared for the U.S. General Services Administration and the 

U.S. Department of Energy

By the National Renewable Energy Laboratory

DECEMBER 2018

Demonstration and Evaluation of an 

Advanced Oxidation Technology for 

Cooling Tower Water Treatment 

Jesse Dean (NREL)

Dylan Cutler (NREL)

Gregg Tomberlin (NREL)

James Elsworth (NREL)

ared for the U.S. General Services Administration and the 

U.S. Department of Energy

 
DEMONSTRATION AND EVALUATION OF AN ADVANCED OXIDATION TECHNOLOGY FOR COOLING

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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 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. Funding 

provided by the U.S. General Services Administration.

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

Department of Energy] under Contract No. 47PA0117C0009.

Acknowledgements

GSA's Proving Ground: Kevin Powell - Director

GSA Region #8 - Doug Baughman - Energy Management Specialists / OFM, Tyler Cooper – Mechanical 

Engineer

Tenfold Information Design Services: Andrea Silvestri

National Renewable Energy Laboratory: Michael Deru, Kosol Kiatreungwattana

For more information contact:

Jesse Dean

Senior Engineer

National Renewable Energy Laboratory

Email: jesse.dean@nrel.gov

GSA’s GPG program and DOE’s High Impact Technology (HIT) Catalyst program enable federal and 

commercial building owners and operators to make sound investment decisions in next generation 

building technologies based on their real-world performance.
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Executive Summary

This GSA Proving Ground project assessed the performance of an alternative water treatment (AWT) 

system manufactured by Silver Bullet for a 500-ton cooling tower at Building 95, at the Denver Federal 

Center in Denver, Colorado. Cooling tower water consumption is one of the largest potable water loads 

in commercial buildings in the United States, as more than 28% of water use is from heating and cooling 

systems (EPA n.d.). A cooling tower uses an evaporative cooling process to reject heat to the 

atmosphere from a water-cooled chilled water plant. The continuous evaporation of water from the 

condenser leaves behind the natural mineral content it carried (silica, calcium, magnesium, chloride). 

Thus, the remaining condenser water will have an ever-increasing concentration of impurities as more 

water evaporates. These impurities will eventually precipitate out (because water can hold only so 

much), resulting in solid precipitate. This solid precipitate is commonly called scale and collects on 

surfaces, inhibiting performance. Typically, cooling tower water is treated using three methods: adding

scale inhibitors that allow water to hold a higher concentration of minerals, using corrosion inhibitors

that decrease corrosion in piping systems, and introducing biocides and algicides that mitigate biological 

growth in an open-air cooling tower.

In addition to chemical treatments, a portion of the cooling tower water is typically drained as tower 

blowdown, and the tower is refilled with fresh makeup water. This reduces the chemical and mineral 

concentration of the remaining condenser water. It also reduces the cycles of concentration of the 

cooling tower and increases annual cooling tower water usage. GSA operation and maintenance 

procedures require that cooling towers have a cycle of concentration of 2 or greater. A higher cycle of 

concentration correlates with less blowdown and reduced makeup water consumption for the cooling 

tower.

The AWT technology evaluated at Building 95 at the Denver Federal Center is a nontoxic photochemicalbased cooling tower water treatment technology promoted as a simpler water treatment technology 

that uses an advanced oxidation process to treat cooling tower makeup water. It pulls air from the 

surrounding environment, which then passes through patented sleeves that contain ultraviolet lamps 

and other proprietary components that modify the ambient air, creating negatively charged oxygen 

atoms. These atoms diffuse into the water, forming highly reactive hydroxyl and other radicals. The 

hydroxyl radicals and other oxidants help to oxidize minerals and contaminants in the water, kill 

bacteria, reduce biofilm, and break down calcium buildup (inhibiting scaling). The dissolved oxidants 

combine with water molecules to create hydrogen peroxide, a lasting biocide, though a small amount of 

biocide was still added to the system that was demonstrated. With this system, no additional standard 

cooling tower water treatment chemicals are typically needed, except that biocides such as bromide or 

other algicides may be used to control algae growth in the summer.

The installation of the product is simple . The unit is mounted on the wall, a pipe with an air diffuser to 

treat the cooling tower water is routed from the device to the cooling tower basin, and no significant 

modification of the current cooling tower water treatment system is required.

Building 95 at the Denver Federal Center is a 163,206 ft2

, two-story office/laboratory building that was 

constructed in 1999. The major tenant is the U.S. Department of Interior. Building 95 has two 250-ton
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water-cooled centrifugal chillers that supply chilled water to the facility. The cooling tower is a 500-ton

induced draft cooling tower, with two cooling tower cells and two-speed fan motors.

The AWT technology was evaluated with a combination of on-site chiller plant energy measurements, 

cooling tower makeup water measurements, and outputs from the building automation system.

Electrical energy for the chilled water plant and cooling tower makeup water was metered directly 

through GSA’s automated metering program, and 15-minute interval data were available from 2014 to 

2017. Outside air conditions, including dry bulb temperature, dew point temperature, wind speed, and 

precipitation, were recorded on an hourly basis from the National Renewable Energy Laboratory’s Solar 

Radiation Research Laboratory and daily average values of average dry bulb temperature, minimum dry 

bulb temperature, maximum dry bulb temperature, average dew point, average wind speed, and total 

daily precipitation were taken from Weather Underground for Denver, Colorado. The condenser water 

supply and return temperatures for the main condenser loop were trended via the building automation 

system at a recording interval of 15 minutes for 2016–2017. Condenser pump #1 and #2 speeds were

trended via the building automation system at a recording interval of 15 minutes for 2016–2017.

The AWT technology was installed in December 2014, and calendar year 2014 was used as the baseline 

year for the study. The AWT contractor analyzed the condition of the condenser tubes in early 2015 and 

noted that the condenser tubes were partially fouled with scale buildup. In addition, the 2017 electrical 

meter data and condenser water meter data had the fewest data drop outs, so 2017 was used as the 

post retrofit savings case. The National Renewable Energy Laboratory did not install any secondary data 

acquisition systems and did not perform the same level of measurement and verification as other GSA 

Proving Ground projects because this project was financed by GSA Region 8 and not the national GSA 

Proving Ground. In addition, the existing metering system data and building automation system data 

were used to characterize the cooling tower makeup water savings for both the baseline period and 

post-retrofit period. A listing of quantitative and qualitative performance objectives and measurement 

and verification results are provided in Table 1.

Table 1: Performance Objectives

Quantitative 

Objectives Success Criteria Metrics & Data Measurement and Verification Results

Water Savings

>10% reduction in cooling

tower water makeup

(gal/kWh)

- Cooling tower makeup water

- Chiller plant energy usage

- Outside air temperature, dew

point, wind speed, and 

precipitation

Met: Annual water 

reduction 22.7% to 29.7%

Reduction in 

Chemical Costs

>90% reduction in annual 

chemical costs

- Pre and post installation 

annual chemical costs

Met: Elimination of all 

chemicals other than 

biocides

Water Chemistry

Meets or exceeds GSA cooling 

tower water chemistry 

requirements

- Pre and post installation water 

chemistry reports

Met: Passed GSA water 

chemistry requirements

for all metrics other than 

oxidation reduction 

potential

Cost-Effectiveness Simple payback (SPP); Savings 

to Investment ratio (SIR) - Payback <10 years; SIR >1. Met: SPP of 6.2 yrs.; SIR of2.4

Ease of Installation Less than 2-day installation 

time Labor hours to install technology Met: Less than 1 day to install
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The estimated annual cooling tower makeup water savings is 527,791 gallons/year, with a range of 

estimated savings from 433,288 gallons/year to 622,307 gallons/year. The estimated total annual 

cooling tower makeup water savings is 26.3%, with a lower bound estimate of 22.7% and a higher bound 

estimate of 29.7%. The equipment costs for the two AWT processing units was $29,780, the installation 

cost was $2,970, for a total cost of $32,750 ($65.5/ton). The 12-month service agreement was $3,300

($275 per month). The annual water savings, cost savings, and economics for the system are provided in

Table 2.

Table 2: Annual Cost Savings and Economics

Baseline Tested 

Technology 

(After)1

Difference w/ 

Local Sewer + 

Water Rate 

($7.14/kGal.)

Difference 

w/ GSA Avg. 

Water Rate 

$16.76/kGal. (Before)

Installation ($) N/A $32,750 $32,750 $22,487 

Installation Cost ($/ton) $65.50 $65.50 $44.97 

Annual Maintenance $5,855/yr. $3,333/yr. $2,522/yr. $2,522/yr.

Annual Water Consumption 

(Gallons/yr.) 2,003,273 1,475,482 527,791 527,791

Annual Water Costs($/yr.) $14,303 $10,535 $3,768 $8,846

Annual Energy Costs ($/yr.) $0 $1,041 $1,041 $578 

Simple Payback Yrs. 6.2 2.1

Savings-to-Investment Ratio Integer value between 0 and 100 2.4 7.2

The new cooling tower operations and maintenance contract for the AWT saved the site $2,522 per 

year, with a 50% reduction in man hours. The two AWT units had a combined power draw of 1.2 kW and 

operated for 8,760 hrs./yr., for a total power consumption of 10,512 kWh/yr. and an estimated energy 

cost of $1,137/yr. The annual water cost savings was $3,768 per year (using a combined water sewer 

rate of $7.14/kgal.) and the simple payback period is 6.2 years, with a savings-to-investment ratio of 2.4, 

using a 15 year project lifetime. Given that for future installations of this size, both GSA and the vendor 

have confirmed that just one AWT processing unit would be needed, the installed cost for that case was 

estimated to be $22,487, with an energy usage of 5,250 kWh/yr., at a cost of $578/yr. Using GSA’s 

national average combined water and sewer rate of $16.76/kgal., the annual water savings would have 

been $8,846/yr, with a simple payback of 2.1 years and a savings-to-investment ratio of 7.2.

The installation requires only simple wall mounting and the injector hose is connected to the cooling 

tower basin. Thus, the total installation time of only a few hours was much less than the performance 

metric requirement of less than 2 days.

For 2017, the annual average cycle of concentration was 9.54 and all of the tower water chemistry 

values were within the GSA designated ranges. The operations and maintenance contractor servicing the 

facility continued its service contract and, after an initial descaling period, no additional water treatment 

chemicals were added to the system other than the bromine/chlorine biocides to prevent biological 
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growth, significantly reducing the chemical usage of the tower and lowering the environmental impact 

of the chemicals being drained via the blow down cycle. As noted above, this also resulted in a 50% 

reduction in labor hours to service the cooling towers and a $2,522 operations and maintenance savings 

per year.

This AWT product was analyzed as a potential retrofit option for smaller building applications that may 

not have full-time or on-site cooling tower operations and maintenance contractors. This product is 

unique in that it can be used without additional monitoring aside from a low-cost service agreement. 

For future installations, Denver Federal Center staff have indicated that they would suggest leasing the 

technology as part of the service contract with the vendor instead of purchasing it, given that the 

installation is very quick and potential removal of the technology is not disruptive to the balance of 

system. 
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Table of Contents

EXECUTIVE SUMMARY III

I. INTRODUCTION 1

A. WHAT WE STUDIED................................................................................................................... 1

B. WHY WE STUDIED IT ................................................................................................................. 2

II. EVALUATION PLAN 4

A. EVALUATION DESIGN ................................................................................................................ 4

B. TEST BED SITE............................................................................................................................ 4

C. METHODOLOGY ........................................................................................................................ 7

III. DEMONSTRATION RESULTS 11

A. QUANTITATIVE RESULTS ......................................................................................................... 11

B. COST-EFFECTIVENESS .............................................................................................................. 18

C. QUALITATIVE RESULTS ............................................................................................................ 20

IV. SUMMARY FINDINGS AND CONCLUSIONS 21

A. OVERALL TECHNOLOGY ASSESSMENT AT DEMONSTRATION FACILITY .................................. 21

B. LESSONS LEARNED AND BEST PRACTICES ............................................................................... 21

C. DEPLOYMENT RECOMMENDATIONS ...................................................................................... 21

V. DEPLOYMENT GUIDANCE (GSA ONLY) 27

A. INSTALLATION AND COMMISSIONING.................................................................................... 27

B. IMPACT ON FACILITY OPERATIONS ......................................................................................... 27

C. IT SECURITY AND CONTINUITY OF CONNECTIVITY.................................................................. 27

D. TECHNOLOGY MARKET READINESS......................................................................................... 27

E. TECHNOLOGY SPECIFICATIONS ............................................................................................... 27

VI. APPENDICES 28

A. REFERENCES ............................................................................................................................ 28

B. GLOSSARY................................................................................................................................ 29

C. MANUFACTURER CUT SHEET .................................................................................................. 30
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I. Introduction

A. WHAT WE STUDIED

A cooling tower uses the evaporative cooling process to reject heat to the atmosphere from a watercooled chilled water plant. Cooling towers are commonly applied to water-cooled chilled water plants in 

large commercial buildings. The continuous evaporation of pure water from the condenser leaves 

behind any natural mineral content the water carried (silica, calcium, magnesium, chloride). Thus, the 

remaining condenser water will have an ever-increasing concentration of impurities as more water 

evaporates. These impurities eventually will precipitate out (because 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 substantial 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 

achieve the same level of cooling. In addition to scale, biofilms also have a significant impact on heat 

transfer efficiency. The high-water content of biofilms creates an insulating layer that inhibits energy 

transfer to a much greater degree than mineral scale alone (because of the high specific heat of water).

Typical water treatment consists of injecting chemicals into the condenser water for the following three 

purposes:

• Chemicals called “scale inhibitors” alter the natural ability of water so that it can hold a higher 

concentration of minerals.

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

• Chemicals called “biocides” and “algicides” mitigate biological growth in the cooling tower, 

where warm water is exposed to air.

In addition to the chemical treatments, a portion of the cooling tower water is typically drained off as 

tower “blowdown” or “bleed-off.” This water volume is then replaced by fresh “makeup” water. This

process lowers the chemical and mineral concentration of the remaining condenser water. It also lowers 

the cycle of concentration (CoC) of the cooling tower and increases annual cooling tower water usage.

GSA requires that cooling towers have a CoC of 2 or greater. A higher CoC will lower both blowdown and 

total water consumption.

The Silver Bullet alternative water treatment (AWT) technology is a non-chemical-based cooling tower 

water treatment technology. It is promoted as a simpler water treatment technology that can reduce 

cooling tower blowdown by increasing the CoCs, which reduces monthly chemical costs, monthly 

operation and maintenance costs, and the amount of scale that collects on condenser water tubes and 

cooling towers. 

The product uses an advanced oxidation process. It pulls air from the surrounding environment, which 

then passes through patented sleeves that contain ultraviolet lamps and other proprietary components 

that modify the air, creating negatively charged oxygen atoms. These diffuse into the water, forming 

highly reactive hydroxyl and other radicals. The hydroxyl radicals and other oxidants help to oxidize 

minerals and contaminants in the water, kill bacteria, reduce biofilm, and break down calcium buildup 
DEMONSTRATION AND EVALUATION OF AN ADVANCED OXIDATION TECHNOLOGY FOR COOLING

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(inhibiting scaling). The dissolved oxidants combine with water molecules to create hydrogen peroxide,

which acts as a lasting biocide, though a small amount of additional biocide is typically still added with 

this system. With this system, no additional standard cooling tower water treatment chemicals need be 

used, except that biocides such as bromide or other algicides can be used to control algae growth in the 

summer.

The product is the size of a large electrical panel and can be retrofitted in less than a day. It is typically 

rented on a monthly basis at a cost that is comparable to traditional chemical treatments. A picture of 

the product is provided in Figure 1 (Silver Bullet n.d.-a).

Figure 1: Non-Chemical-Based Cooling Tower Water Treatment Technology

The process uses an EPA recognized effective biocide via the advanced oxidation process and is easily 

automated and controlled. The vendor offers a smaller unit for cooling towers up to 400 tons or 1,200 

gallons per minute (gpm) and a larger unit for systems up to 2,000 tons or 6,000 gpm. The smaller units 

draw 396 watts of power and the larger unit draws 720 watts of power while operating.

B. WHY WE STUDIED IT

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

the United States. Figure 2, a breakdown of water consumption in office buildings, shows that more 

than 28% of building water use is associated with heating and cooling. This is by far the dominant water 

use case, owing largely to the evaporative cooling demands from either water-cooled cooling towers or 

evaporation-based air-conditioning systems.
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Figure 2: Office Building Water End Uses (EPA n.d.)

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

every climate zone. Facilities located in hotter climates with a cooling season that lasts for more than 5–

6 months per year, though, will use their cooling towers more and have greater cooling tower water 

savings as a result. 

The Denver Federal Center is located in Colorado and GSA Region 8. In GSA Region 8, cooling towers 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 benefit 

from reducing water consumption.
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II. Evaluation Plan

A. EVALUATION DESIGN 

The primary focus of this evaluation was measuring cooling tower makeup water usage pre- and postinstallation of the AWT. In addition, annual reductions in cooling tower chemical costs, monthly water 

chemistry reports, ease of installation, and overall cost effectiveness were evaluated. 

Table 3 lists the performance objectives, success criteria, and metrics used to evaluate each of the 

objectives.

Table 3: Performance Objectives

Quantitative Objectives Success Criteria Metrics & Data

Water Savings >10% reduction in cooling tower 

water makeup

- Cooling tower makeup water

- Chiller plant energy usage

- Outside air temperature, dew point, 

wind speed, and precipitation

Reduction in Chemical 

Costs

>90% reduction in annual chemical 

costs

- Pre and post installation annual 

chemical costs

Water Chemistry Meets or exceeds GSA cooling tower 

water chemistry requirements

- Pre and post installation water 

chemistry reports

Cost-Effectiveness Simple payback; Savings to 

investment ratio (SIR) - Payback <10 years; SIR >1

Qualitative Objectives Success Criteria Metrics & Data

Ease of Installation Less than 2-day installation time Labor hours to install technology

B. TEST BED SITE

Building 95 at the Denver Federal Center is a 163,206 ft2, two-story office/laboratory building that was 

constructed in 1999. The major tenant is the U.S. Department of Interior. Building 95 currently houses 

the National Water Quality Laboratory — the flagship analytical facility for the U.S. Geological Survey of 

the U.S. Department of the Interior (Figure 3).
DEMONSTRATION AND EVALUATION OF AN ADVANCED OXIDATION TECHNOLOGY FOR COOLING

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Figure 3: Building 95 at the Denver Federal Center

Building 95 has two 250-ton water-cooled centrifugal chillers that supply chilled water to the facility. 

The chiller is rated at 0.5 kW/ton and has a rated evaporator flow rate of 350 gpm and a rated 

condenser flow rate of 750 gpm. There are two 20 hp centrifugal condenser water pumps, rated at 750 

gpm at 75 ft of head, serving the two chillers. The cooling tower is an induced draft cooling tower, with 

two cooling tower cells and two-speed fan motors (Figure 4).
DEMONSTRATION AND EVALUATION OF AN ADVANCED OXIDATION TECHNOLOGY FOR COOLING

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Figure 4: Building 95 Induced Draft Cooling Tower

Figure 5 provides a picture of the AWT treatment system installed in building 95.

Figure 5: Building 95 Alternative Water Treatment System
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C. METHODOLOGY

Quantitative Study Design

The AWT technology was evaluated through a combination of on-site chiller plant energy 

measurements, cooling tower makeup water measurements, and measurements taken from the 

Building Automation System (BAS). A high-level description of the monitoring points is provided below:

On-Site Submetering

• Electrical Energy — Building 95 has an automated submeter for the chilled water plant that 

records 15-minute electricity usage for the entire chilled water plant, including chillers, cooling 

tower fans, condenser water pumps, and chiller primary and secondary pumps.

• Cooling Tower Water — The cooling tower makeup water is metered directly by an on-site 

water meter and provides 15-minute interval cooling tower water usage data dating back to

2013.

Outside Air Data

• Outside Air Conditions — Outside air conditions, including dry bulb temperature, dew point 

temperature, wind speed, and precipitation, were analyzed on an hourly basis by the National 

Renewable Energy Laboratory’s (NREL’s) Solar Radiation Research Laboratory and daily 

average values of average dry bulb temperature, minimum dry bulb temperature, maximum 

dry bulb temperature, average dew point, average wind speed, and total daily precipitation 

were recorded from Weather Underground for Denver, Colorado (NREL n.d., Weather 

Underground 2013).

Building Automation System Trend Logs

• Condenser Supply and Return — The condenser water supply and return temperature for the 

main condenser loop was trended via the BAS at a recording interval of 15 minutes for 2016 

through 2017.

• Condenser Pump Speed — The condenser pump #1 and #2 speeds were trended via the BAS at a 

recording interval of 15 minutes for 2016 through 2017.

The condenser cooling load was calculated via the condenser pump start/stop, pump speed, and 

condenser supply and return temperature difference from the BAS for 2016. NREL did not install any 

secondary data acquisition systems and did not perform the same level of measurement and verification 

(M&V) as on other GSA GPG projects because this project was financed by GSA Region 8 and not 

the national GSA Proving Ground. For this project, only the existing metering system data and BAS 

data were used to characterize the cooling tower makeup water savings (for both the baseline period 

and post retrofit period). A list of monitoring points, instruments, and instrument accuracy is provided in

Table 4.
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Table 4: Monitoring Points and Instrumentation

Monitoring Point Instrument 

Description Location Instrument Accuracy

Chiller Plant 

Energy

Existing automated 

electrical submeter

Building 95 

chiller plant +/- 0.5%

Cooling Tower 

Makeup Water

Existing automated 

water submeter

Building 95 

cooling tower 

makeup

+/- 0.4%

Condenser Water 

Supply and Return 

Temperature

BAS temperature 

sensor

Building 95 

condenser loop +/- 10%

Condenser Pump 

Speed BAS trend point

Building 95 

condenser pump 

#1 and #2

+/- 0%

Outside Air 

Temperature Temperature sensor

Measurement & 

Instrumentation 

Data Center

(NREL MIDC)

(NREL n.d.-b);

Weather 

Underground

+/- 2%; +/- 5%

Outside Air Dew

Point

Combination of 

relative humidity and 

temperature sensor

NREL MIDC;

Weather 

Underground

+/- 2%; +/- 5%

Wind Speed Wind vane

NREL MIDC;

Weather 

Underground

+/- 2%; +/- 5%

Precipitation Rain gauge 

NREL MIDC; 

Weather 

Underground

+/- 2%; +/- 5%

Study Design

On-site building operators and cooling tower maintenance technicians logged water chemistry and 

condenser tube fouling pre and post installation. GSA has developed the water chemistry standards

given in Table 5 as a guideline to determine an acceptable blowdown water quality for a given AWT, and 

they were adopted for this project location. Operations staff and AWT vendors performed monthly 

monitoring of these parameters to characterize the performance of the 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, due to the materials used in its design, may result in water quality that falls outside the 

ranges defined in the project specifications. In the AWT selection process, a site should be sure to 

consider site-specific water quality constraints, whether due to influent potable water or discharge 

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

Test Acceptable Ranges

T alkalinity (parts per million [ppm]) 100–1000

pH 7.3–9.0

Chloride (ppm) 10–500

Cycles >2

Total Hardness (ppm) 500–1500

Phosphate (ppm) 43,327

Conductivity (millimhos [mmHos]) <2400

Bacteria Count (cfu) <80,000

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

Oxidation Reduction Potential

(millivolts [mV])

>300

90-day Copper Coupon (mils per 

year [mpy])

<0.2

90-day Mild Steel Coupon (mpy) <3

90-day Galvanized Steel (mpy) <4

90-day Stainless Steel (mpy) <0.1

Data Analysis

The chiller plant energy and cooling tower makeup water 15-minute totalizing meter readings were 

available for 2014 through 2017. The BAS trend logs were only available for 2016 and 2017 and there 

were some major data drop outs and inconsistencies in the BAS measurement points. Because the 

technology was installed in December 2014, the baseline year for the study was 2014. The AWT 

contractor analyzed the condition of the condenser tubes in early 2015, noting that the condenser tubes 

were significantly fouled with scale buildup from the previous cooling tower water treatment system. 

GSA and the cooling tower contractor undertook successful scale-reduction measures later that year, so 
DEMONSTRATION AND EVALUATION OF AN ADVANCED OXIDATION TECHNOLOGY FOR COOLING

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the 2015 cooling season was not used in this analysis. The 2017 electrical meter data and condenser 

water meter data had the fewest data drop outs and were thus used as the post retrofit savings case.

The cooling tower makeup water savings was calculated using the following procedure:

• The 15-minute chiller energy usage and cooling tower makeup water was time aligned and 

missing data were interpolated from existing data and rolled up into daily totals for 2014 

through 2017. Daily average outside air conditions were downloaded from Weather 

Underground for 2014 through 2017. A multivariable linear regression model was created that 

correlates daily cooling tower makeup water to chiller energy usage and outside air conditions 

for the baseline period of 2014. This multivariable linear regression model was applied to 2017 

chiller energy usage and outside air conditions to estimate a baseline daily cooling tower usage 

and was compared to the actual 2017 daily cooling tower makeup water usage to calculate 

annual water savings. A 95% confidence interval was applied to the multivariable linear 

regression model to characterize the range of daily calculated water savings predictions.

• BAS trend logs for 2016 were used to create a second multivariable linear regression model to 

determine how well the daily condenser water ton-hrs. correlated to daily chiller energy usage 

to support the use of daily chiller energy usage as one of the primary explanatory variables for 

predicting baseline daily condenser makeup water usage.
DEMONSTRATION AND EVALUATION OF AN ADVANCED OXIDATION TECHNOLOGY FOR COOLING

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III. Demonstration Results

A. QUANTITATIVE RESULTS

Water Savings

The 2014 baseline monthly cooling degree days (CDD), chiller energy usage, and cooling tower makeup

water usage were calculated from 15-minute interval data and are provided in Table 6.

Table 6: 2014 Baseline Year Cooling Degree Days, Chiller Energy, Building Water, and Cooling 

Tower Makeup

Month CDD

Chiller Energy 

(kWh/Month)

Building Water 

(Gal/Month)

Cooling Tower Makeup

(Gal/Month)

Jan-14 0 1,312 40,865 1,420

Feb-14 0 958 33,363 647

Mar-14 0 1,175 59,050 240

Apr-14 0 3,129 104,810 29,951

May-14 18 16,549 94,085 151,503

Jun-14 21 38,777 172,041 306,721

Jul-14 22 65,183 293,218 469,240

Aug-14 139 43,167 200,223 381,024

Sep-14 68 29,045 158,000 293,902

Oct-14 1 9,435 122,798 99,273

Nov-14 0 1,177 71,413 2,880

Dec-14 0 945 53,671 1

Totals 269 210,851 1,403,536 1,736,802

The 2014 cooling season had much milder outside air temperatures than other representative years,

with only 269 CDDs, versus over 700 CDDs in 2017 in Denver, Colorado. Consequently, the total annual 

cooling tower makeup water cannot be compared directly to post retrofit years as the impact of building 

operations and outside air conditions would not be appropriately accounted for. In order to characterize 

the cooling tower makeup water as a function of outside air conditions and chiller plant energy usage, a 

linear regression model was created based on the 2014 data set.
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Linear Regression Model

The initial multivariable linear regression model was created using the following input variables to 

calculate daily cooling tower makeup water usage as a function of chiller plant energy usage and outside 

air conditions.

• Chiller Plant Energy Usage (Energy) — daily total energy usage, calculated from 15-minute

chiller plant energy usage.

• Daily Average Temperature (Avg_Temp) — daily average temperature, downloaded from 

weather underground.

• Daily Maximum Temperature (Max_Temp) — daily maximum temperature, downloaded from 

weather underground.

• Daily Minimum Temperature (Min_Temp) — daily minimum temperature, downloaded from 

weather underground.

• Daily Average Wind Speed (Wind) — daily average wind speed, downloaded from weather 

underground.

• Daily Average Dew Point Temperature (Dewpoint) — daily average dew point temperature,

downloaded from weather underground.

• Daily Total Precipitation (Precip) — daily total precipitation, downloaded from Weather 

Underground.

• Weekday/Weekend (Wkd_Wknd) — Each weekday was given a designation of 1 and each 

weekend day and federal holiday was given a designation of 0 to account for days when the 

facility is unoccupied has different chiller energy usage and cooling tower makeup water usage

patterns.

All of the variables other than the Weekday/Weekend variable were converted into coded variables that 

had values between -1 and 1 in order to give them all the same range of possible values and to help 

improve the fit of the model. A linear regression model that included all of the terms as explanatory 

variables, including squared and interaction terms between explanatory variables, was created to 

identify the variables with the highest statistical significance (p-value less than 0.05). From this statistical 

significance analysis, a final linear regression model that had the highest R2 and normalized root mean 

square error was created and is provided in Equation 1.
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Equation 1. Baseline Daily Cooling Tower Makeup Water Regression

CCCCCCCCCCCCCC TT TTeerr MMMMMM MMMM WW WW

= cc0 + (cc1 ∙ WWWWWW_WWWWWWWW) + (cc2 ∙ CCCCCC CCCC) + (cc3 ∙ CC CCCCCC )

+ (cc4 ∙ CCCCCC CC 2) + (cc5 ∙ CCCCCC CCCC ∙ CC CCCCCC )

where: 

WKD_WKND = weekday or weekend/holiday [1=Weekday or 0=Weekend]

CEnergy = coded daily energy usage [-1=min to +1=max]

CDewpoint = coded daily average dew point temperature [-1=min to +1=max]

CEnergy2 = coded daily energy usage squared [-1=min to +1=max]

CEnergy · CDewpoint = coded daily energy usage times coded daily average dew point temperature 

[-1=min to +1=max]

The linear regression model was selected to ensure that the explanatory variables used in the model

have p-values less than 0.05. The explanatory variables that had the highest statistical impact were 

Weekday/Weekend, chiller plant energy usage, and daily average outside air dew point, which all would 

be expected to impact the daily cooling tower makeup water usage. The adjusted R2 value for the model 

is 94.65% and the normalized root mean square error for the model is 6.89%. The coefficients for each 

explanatory variable are provided in Table 7.

Table 7: 2014 Cooling Tower Makeup Water Linear Regression Model Coefficients 

2014 Linear Regression 

Model Parameters

Values

r squared 0.9465

Intercept 9456.3 coefficient

WKD_WKND -344.5 coefficient

CEnergy 444.8 coefficient

CDewpoint 8697.2 coefficient

CEnergy2 -8909.2 coefficient

CEnergy x CDewpoint 9186.9 coefficient

A check of model prediction was performed that compared the actual cooling tower makeup water to 

the modeled cooling tower makeup water. Figure 6 shows that that there is a good correlation between 

the two, especially given that the analysis was based on existing metering system readings and not on 

more accurate third-party data acquisition systems. 
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Figure 6: 2014 Actual Cooling Tower Makeup Water versus Predicted Cooling Tower Makeup 

Water Usage

As a next step, because BAS trend logs were available for 2016, the hourly condenser water load was 

checked against the hourly chiller plant energy usage to see if there was good agreement between the 

two. The chiller plant has a plate and frame heat exchanger that allows for tower free cooling, and the 

analysis was conducted to ensure that the chiller plant energy usage was a good predictor of chiller 

cooling load, as measured in condenser water ton-hrs. From this statistical significance analysis, a final 

linear regression model that had the highest R2 and normalized root mean square error was created and 

is provided in Equation 2.

Equation 2. Daily Condenser Load as a Function of Chiller Plant Energy Usage 

CCCCCCCC CCCC LL �

hrr � = cc0 + (cc1 ∙ CCCCCC CCCC) + (cc2 ∙ CCCCCC CCCC ∙ CCCC )

where: 

CEnergy = coded daily energy usage [ -1=min to +1=max]

CCCD = coded cooling degree days [ -1=min to +1=max ]

The adjusted R2 for the model was 88.49% and the normalized root mean square error for the model is 

7.22%. Table 8 lists the coefficients for each explanatory variable.
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Table 8: 2014 Cooling Tower Makeup Water Linear Regression Model Coefficients 

2014 Linear Regression 

Model Parameters

Values

r squared 0.8849

Intercept 1,941.4 coefficient

CEnergy 4,294.1 coefficient

CCDD -1,239.0 coefficient

CEnergy x CCDD 1,181.6 coefficient

A final check of model prediction was performed comparing the actual cooling tower makeup water to 

the modeled cooling tower makeup water. Figure 7 shows that that there is again a good correlation 

between the two, with the same caveat that this was based on existing metering system readings and 

not more accurate 3rd party M&V equipment. 

Figure 7: Actual Daily Cooling Ton-Hrs. versus Predicted Cooling Ton-Hrs.

Given that there is good agreement for both condenser ton-hrs and cooling plant energy consumption, it 

was determined that cooling plant energy consumption could be used in place of condenser water 

cooling load to model cooling tower makeup water as a function of chiller plant energy usage, as in

Equation 1. The 2017 monthly CDD, chiller plant energy usage, and cooling tower makeup water usage 

are provided in Table 9. The 2017 cooling season was used as the savings case because, as noted above,

the 2015 year was used to descale the chilled water plant’s condenser coils and there were major data 

drop outs in the 2016 data. 
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Table 9: 2017 Monthly Cooling Degree Days, Chiller Plant Energy Usage, and Cooling Tower 

Makeup Water

Month CDD

Chiller Energy 

(kWh/Month)

Cooling Tower 

Makeup

(Gal/Month)

1-Jan 0 828 928

1-Feb 0 4,151 15,042

1-Mar 0 10,241 46,706

1-Apr 0 10,603 52,254

1-May 6 14,072 54,904

1-Jun 143 62,794 282,322

1-Jul 286 93,724 398,206

1-Aug 173 65,806 298,039

1-Sep 101 43,687 190,375

1-Oct 0 16,402 61,146

1-Nov 0 7,989 35,554

1-Dec 0 2,480 40,005

Totals 709 332,776 1,475,482

The 2017 cooling season had 709 CDD, compared to just 269 in 2014, and the chiller plant energy usage 

was 332,775 kWh/yr. versus 210,851 kWh/yr. in 2014, indicating that the 2017 cooling season was much 

hotter and required significantly more chiller plant energy to meet the facility’s cooling load than in

2014.

The 2017 daily chiller plant energy usage, and outside air conditions were imported into the linear 

regression model provided in Equation 1 to estimate baseline water consumption. A 95% confidence 

interval was used to predict the lower and upper bounds of the modeled baseline cooling tower makeup

water usage for the post installation measured 2017 cooling tower makeup water consumption, and the 

monthly results are provided in Table 10.
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Table 10: Measured versus Modeled 2017 Cooling Tower Makeup Water Consumption

Month

2017 Cooling 

Tower Makeup

(Gal/Month)

Modeled 

Baseline Water 

(Gal/Month)

Modeled Baseline 

Water Lower 

Bound 

(Gal/Month)

Modeled Baseline 

Water Upper

Bound 

(Gal/Month)

1-Jan 928 0 0 0

1-Feb 15,042 18,301 15,087 21,516

1-Mar 46,706 54,556 47,606 61,507

1-Apr 52,254 66,350 60,921 71,780

1-May 54,904 107,543 101,170 113,917

1-Jun 282,322 375,419 360,615 390,226

1-Jul 398,206 506,684 487,778 525,593

1-Aug 298,039 445,116 432,816 457,420

1-Sep 190,375 275,749 263,717 287,783

1-Oct 61,146 108,623 99,138 118,108

1-Nov 35,554 44,930 39,922 49,939

1-Dec 40,005 0 0 0

Totals 1,475,482 2,003,273 1,908,771 2,097,789

The estimated annual cooling tower makeup water savings are 527,791 gallons/ year, with a range of 

estimated savings from 433,288 gallons/year to 622,307 gallons/year. This corresponds to an estimated 

total annual cooling tower makeup water savings of 26.3%, with a lower bound estimate of 22.7% and a 

higher bound estimate of 29.7%.

Water Chemistry Analysis

During the cooling load dominated months of May through October, the average CoC increased from 7.9 

CoCs during the 2014 baseline year to 11.1 CoCs in 2017, representing a 29.7% improvement in CoC. 

Although the CoC data were not used to calculate water savings, the improvements in CoC are 

directionally in line with the water savings calculated from the regression model in Table 10.

Prior to installing the AWT, the cooling towers were drained, and the contractor performed a bore-ascope of the two chiller tube condenser bundles (running a camera through the tubes to search for 

scaling) and found a small amount of scale buildup in both chillers. For the first few months of 2015, the 

existing cooling tower water treatment systems were operated in conjunction with the AWT product at 

much lower CoCs to descale the chillers and clean out the system. Bore-a-scope pictures from February 

2017 showed significant improvement in condenser tube fouling. The operations and maintenance

(O&M) contractor servicing the facility continued its service contract and, after the initial descaling

period, no additional water treatment chemicals other than bromine/chlorine biocides were used to 

prevent biological growth, significantly reducing the chemical usage of the tower and decreasing the 
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environmental impact of the chemicals being drained via the blowdown cycle. The average annual 

makeup water and tower water chemistry, as well as GSA acceptable tower water chemistry ranges for 

calendar year 2017 is given in Table 11.

Table 11: 2017 Alternative Water Treatment Annual Average Water Chemistry

AWT Annual Average Acceptable Ranges

Test Makeup Tower Tower

Conductivity 209 1780 <2400

pH 7.83 8.73 7.3–9.0

P-Alk. 0.00 17.25 -

M-Alk. 50.92 287 -

Total -Alk 50.92 304 100–1000

Total hardness 72 600 500–1500

Calcium hardness 55 452 <500

Chlorides 21 181 10–500

Silica 6.30 47.46 <150

Conductivity Cycles - 9.54 >2

Total Hardness Cycles - 9.94 -

Calcium Hardness Cycles - 9.88 -

Chloride Cycles - 9.74 -

Silica Cycles - 7.84 -

Oxidation Reduction 

Potential 295 194 >300

Bacteria - 960 <80,000

Iron 0.13 0.27 <4

Copper 0.11 0.13 -

Hydrogen Peroxide - 1.24 -

For 2017, the annual average conductivity-based CoC was 9.54, and all of the tower water chemistry 

values were within the GSA designated ranges except for oxidation reduction potential, which was lower 

than the required 300 minimum.

B. COST-EFFECTIVENESS

The AWT company gifted the GSA Denver Federal Center one processing cabinet and an additional 

cabinet was installed during the pilot test. The equipment costs for the two AWT processing units was 

$29,780 and the installation cost was $2,970, for a total cost of $32,750 ($65.5/ton). The 12-month
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service agreement was $3,300 ($275 per month). Table 12 shows the water rates for 2011 through 

2014. Over this period, water rates rose an average of 6.7% per year.

Table 12: Denver Water Rates 2011 to 2014

Year Water Cost

Increase Over 

Previous Year

($/1,000 gallons) (%)

2011 3.5 -

2012 3.6 6.0%

2013 3.8 5.0%

2014 4.0 9.0%

Based on the annual water cost inflation rate assumption of 6.7% per year (Based on 2011 to 2014 

increases in water rates, the 2017 water cost estimate was $4.80/kgal. The actual rate was $4.82/kgal, 

so the estimated annual inflation rate has been consistent over the last 6 years. 

Each gallon of water saved reduced the amount discharged to the sewer and subsequent costs incurred 

by the site. This is because the water saved with improved water treatment technologies was associated 

with the blowdown component of cooling tower water use. The blowdown therefore would not be 

discharged to the sewer system, saving that portion of the water costs. The 2017 water rate was 

$4.82/kgal and the 2017 sewer rate was $2.33/kgal, for a total cost of $7.14/kgal. The annual water 

savings, cost savings, and economics for the system are provided in Table 13.
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Table 13: Annual Cost Savings and Economics

Baseline Tested 

Technology 

(After)1

Difference w/ 

Local Sewer +

Water Rate 

($7.14/kGal.)

Difference 

w/ GSA Avg.

Water Rate 

$16.76/kGal. (Before)

Installation ($) N/A $32,750 $32,750 $22,487 

Installation Cost ($/ton) $65.50 $65.50 $44.97 

Annual Maintenance $5,855/yr. $3,333/yr. $2,522/yr. $2,522/yr.

Annual Water Consumption 

(Gallons/yr.) 2,003,273 1,475,482 527,791 527,791

Annual Water Costs($/yr.) $14,303 $10,535 $3,768 $8,846

Annual Energy Costs ($/yr.) $0 $1,041 $1,041 $578 

Simple Payback Yrs. 6.2 2.1

Savings-to-Investment Ratio Integer value between 0 and 100 2.4 7.2

The new cooling tower operations and maintenance contract for the AWT saved the site $2,522 per 

year, with a 50% reduction in man hours. The two AWT units had a combined power draw of 1.2 kW and 

operated for 8,760 hrs./yr., for a total power consumption of 10,512 kWh/yr. and an estimated energy 

cost of $1,137/yr. The annual water cost savings was $3,768 per year (using a combined water sewer 

rate of $7.14/kgal.) and the simple payback period is 6.2 years, with a savings-to-investment ratio of 2.4, 

using a 15 year project lifetime. Given that for future installations of this size, both GSA and the vendor 

have confirmed that just one AWT processing unit would be needed, the installed cost for that case was 

estimated to be $22,487, with an energy usage of 5,250 kWh/yr., at a cost of $578/yr. Using GSA’s 

national average combined water and sewer rate of $16.76/kgal., the annual water savings would have 

been $8,846/yr, with a simple payback of 2.1 years and a savings-to-investment ratio of 7.2. 

C. QUALITATIVE RESULTS

The installation time for the product was only a few hours as the unit only requires a simple wall

mounting and connection of the injector hose to the cooling tower. This few hours to install the unit was 

less than the performance metric requirement of 2 days or less.
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IV. Summary Findings and Conclusions

A. OVERALL TECHNOLOGY ASSESSMENT AT DEMONSTRATION FACILITY

GSA installed the AWT at Building 95 at the Denver Federal Center in an effort to see whether the 

technology could maintain adequate water quality while conserving water and reducing operating costs. 

The determination of adequate water quality was made based on a previously-developed set of cooling 

tower water chemistry requirements from GSA. The calculated annual makeup water savings is provided 

in Table 10 and estimated to range from 22.7% to 29.7%. The calculated simple payback period for this

demonstration was 6.2 years and the AWT showed favorable economics at the demonstration facility. 

Given that the bore-a-scope of the two chiller tube condenser bundles found scaling prior to installing 

the AWT and a reduction in scaling after implementation in February of 2017, the system also has the 

potential to reduce energy usage from diminished condenser tube scaling and reduce the number of 

tube cleanings. Although the chiller plant had 15-minute interval electrical meters, the energy savings 

could not be quantified due to the lack of BAS trend logs for the chiller plant for the baseline year of 

2014.

The simplicity of this product, ease of installation, and reduction in on-site chemical usage all make this 

technology a good candidate for future AWT deployments.

B. LESSONS LEARNED AND BEST PRACTICES

The installation of the AWT system only required a small footprint and a simple tie-in process. Since the 

installation and the removal of the technology are not invasive to the balance of the system, the risk of 

adopting this technology is reduced. For this installation, it only took a few hours to install the product 

and connect it to the existing cooling tower. 

One of the primary drivers from the Denver Federal Center staff for wanting to test this AWT product 

was that they were searching for a system that would work in smaller building applications, where there

may not be a full-time mechanic or on-site cooling tower O&M contractor. This product is unique in that 

it can be used without additional monitoring other than a low-cost service agreement. 

For future installations, Denver Federal Center staff have indicated that they would suggest leasing the 

technology as part of the service contract with the vendor instead of purchasing it, given that the 

installation is very quick and potential removal of the technology would not be invasive to the balance of 

system. 

C. DEPLOYMENT RECOMMENDATIONS

This AWT technology met all the predetermined success criteria for water savings, reduction in chemical 

costs, water chemistry, cost-effectiveness, and ease of installation at the test location. For similar 

buildings in climates with cooling seasons of at least as long as that of the test location (climate zone 

5B), this technology could offer cost and water savings, as well as environmental benefits and reduced 
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system O&M. Due to these factors, and the system’s ease of installation, this AWT should be considered 

for further deployment. 

Cooling tower performance depends on a variety of factors, however, many of which are locationspecific. Variables such as seasonal changes and ambient air quality are specific to the site location and 

tower location on the site (e.g., airborne particulate matter) and have the potential to affect the 

observed performance of the evaluated technology. These factors can also contribute to biological 

growth or mineral deposits that would require chemical treatments and additional maintenance. If the 

site has higher total dissolved solids or particulate matter, this technology should be used in conjunction 

with a side stream filtration system.

Given that cooling tower performance is a function of wet bulb temperature, the amount of cooling 

delivered for each technology and the water savings will also vary by site. Sites in hot/dry climates with 

low wet bulb design temperatures will typically have the largest savings. 

Potable water quality is highly variable across the United States. The performance of AWT technologies 

is a function of the quality of influent water that is treated. Locations with high hardness, pH, etc.,

typically require higher water and chemical usages. These sites will have the greatest opportunity for 

savings. 

Rebate opportunities may also be available through local water utilities to implement water conserving 

technologies. The availability of these financial incentives can make the technology’s implementation 

even more cost-effective.

Market Potential within the GSA Portfolio

The first step in evaluation of further deployment of an AWT technology is the identification of buildings 

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

loads that 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 

previously used the whole-building modeling software EnergyPlusTM to model water savings potential in 

an NREL/GSA GPG report entitled Alternative Water Treatment Technologies for Cooling Tower 

Applications. The “Large Office” building model was selected from the Commercial Reference Buildings 

that are developed and maintained by the U.S. Department of Energy and NREL (DOE n.d.). 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. For the modeling analysis included in this report, the “post-1980” 

construction model was used. 

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 3.0. 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 
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set to maintain a 3.0 CoC, (2) with the cooling tower set to maintain a 10.0 CoC, and (3) with the cooling 

tower set to maintain a 15.0 CoC. The EnergyPlus default of 3 CoC was established as the baseline, 

representative of a standard water treatment approach for water-cooled chillers. The 10.0 and 15.0 CoC 

simulations represent a range of concentrations that have been shown to be achievable by AWT 

technologies in this report. Figure 8 shows the annual evaporation (in thousands of gallons water) and 

the annual water savings (over the baseline blowdown at 3.0 CoC) for 10.0 and 15.0 CoC. The cities with 

larger numbers of cooling degree days and more arid climates show the greatest water savings. 

Additionally, as shown in Figure 8, when the CoC is above 10 CoCs, the blowdown water usage is

reduced dramatically, and the evaporative portion of the cooling tower water makeup usage makes up 

the majority of the water usage. Note also that the vast majority of the water savings are achieved by 10

CoCs.

Figure 8: Modeled Water Evaporation and Blowdown Savings across ASHRAE Climate Zones

The water savings numbers were then translated into annual cost savings using site specific water rates. 

Combined water and sewer rates were obtained from local water utilities for each city, assuming each 

site is on a 6-in. water line and uses more than 200,000 gallons per month. 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 9. 
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Figure 9: Estimated Yearly Cost Savings by Climate Zone

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, Georgia) 

generate the largest annual cost savings despite not having the largest total water savings. Table 14

gives the water rates used in this evaluation (current as of May 2018). 

Table 14: 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

LA (3B-Coast) 8.88 Helena (6B) 8.30

San Francisco (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 used to 

calculate a savings-to-investment ratio (SIR) for each city. Note 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. The ratios denoted here are rough estimates, considering the assumptions that the 
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original system was operating at 3 CoC, the new system would achieve 10 CoC, and that the annual 

operating costs remain the same pre- to postinstallation, 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 ($35,000) and a low-cost assumption ($20,000) are shown in Figure 10 and Figure 11, 

respectively. The figures show the modeled SIRs for a given water and wastewater combined rate across 

various climate zones. The SIR calculation assumes a 15-year project life, 10 CoC, $2,522/yr in O&M 

savings and onsite electrical costs that are based on the local electric rates.

Figure 10. Savings-to-Investment Ratio for Same System in Evaluated Climate Zones for Various 

Water Rates: High-Cost Scenario

For the high-cost scenario, the AWT is life cycle cost-effective (SIR>1) across all 16 climate zones when 

the combined water and sewer rate is more than $8/kgal.
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Figure 11: Savings-to-Investment Ratio for Same System in Evaluated Climate Zones for Various 

Water Rates: Low-Cost Scenario

For the low-cost scenario, the AWT is life cycle cost-effective (SIR>1) across all 16 climate zones when 

the combined water and sewer rate is more than $4/kgal. For this particular AWT, the technology has 

the potential to be cost-effective in every GSA region throughout the United States, given that all of 

them have combined water and sewer rates of more than $4/kgal.
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V. Deployment Guidance (GSA Only)

A. INSTALLATION AND COMMISSIONING

The technology was easy to install as the unit can be mounted on a wall and air tubing is run from the 

location of the AWT system to the cooling tower basin and an air diffuser is installed to diffuse the air in 

the cooling tower water. The system does not require decommissioning of the existing cooling tower 

treatment system or any other mechanical system within the building and takes less than 1 day to 

install.

B. IMPACT ON FACILITY OPERATIONS

This product can work well in smaller building applications, where there may not be a full-time mechanic

or on-site cooling tower O&M contract. This product is unique in that is has the ability to be used 

without additional monitoring aside from a low-cost service agreement and can also be used in larger 

buildings to reduce the annual chemical costs and O&M contract costs. For any water treatment system, 

care should be taken in negotiating an O&M agreement with the original equipment manufacturer or its 

partner. Providing good cleaning and servicing of the systems, as well as good customer service are key 

to the acceptance of the systems by the site staff. The agreements should not require that site 

personnel take on additional duties like water sampling or cleaning. 

C. IT SECURITY AND CONTINUITY OF CONNECTIVITY

The controls for the AWT system are self-contained within the unit and there is no connection to the 

BAS or other IT security related issues.

D. TECHNOLOGY MARKET READINESS

The AWT technology is commercially available and distribution and installation channels are in place. 

There are numerous installations of the AWT technology in the United States, varying in size and 

application. Fabrication of these systems is relatively simple and the lead times for new installations are

not expected to be lengthy.

E. TECHNOLOGY SPECIFICATIONS

If the technology evaluation has been successful and the technology has potential for deployment 

within GSA, support development of a technical specification that GSA can use for procurement. 
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VI. Appendices

A. REFERENCES

DOE (U.S. Department of Energy). n.d. Office of Energy Efficiency and Renewable Energy. Accessed April 

14, 2014. http://energy.gov/eere/buildings/commercial-reference-buildings.

EPA (Environmental Protection Agency). N.d. Saving Water In Office Buildings, Accessed December 10, 

2018

https://www.epa.gov/sites/production/files/2017-01/documents/ws-commercial-factsheet-offices.pdf

NREL (National Renewable Energy Laboratory). n.d.-a. Solar Radiation Research Laboratory, Accessed 

January 2018. https://midcdmz.nrel.gov/srrl_bms/. 

NREL. n.d.-b. Measurement and Instrumentation Data Center. Accessed November 25, 2018. 

https://midcdmz.nrel.gov. 

Silver Bullet. n.d.-a. Advanced Oxidation Process. Accessed November 24, 2018. 

https://www.silverbulletcorp.com/silver-bullet-advanced-oxidation-process/

Silver Bullet. n.d.-b. Product Specifications. Accessed November 25, 2018. 

https://www.silverbulletcorp.com/wp-content/uploads/Data-Center-AOP-Tech.pdf. 

Weather Underground. 2013. Denver, Colorado. Accessed June 18, 2013. 

https://www.wunderground.com/history/airport/KDEN/2013/06/18/DailyHistory.html. 
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B. GLOSSARY

Advanced Oxidation 

Process (AOP)

A process by which ultraviolet light and other procedures modify an airstream to 

produce negatively charged hydroxyl groups (OH-). These can then bind to 

contaminants in water and remove them. 

Building 

Automation System 

(BAS)

Provides automated centralized control of the facilities heating, ventilating, and airconditioning (HVAC) system.

Biocides & Algicides Added substances or chemicals that prevent biological and algal growth in water.

Blowdown The higher mineral content water that is periodically drained from a cooling tower 

and replaced with makeup water to prevent the solid concentration in the tower 

water from rising to a level that would lead to precipitation. 

Bore-a-Scope A device with an eyepiece on one end and a camera or lens on the other, connected 

by a tube. It facilitates visual inspection of a pipe or other hard-to-see area. 

Chiller A device that cools by removing heat from a working fluid as it undergoes 

compression and expansion in a closed-loop system, with a means of ejecting heat. 

The cooled working fluid can then be used to cool a building space (usually after 

passing through a heat exchanger). 

Cooling Tower A heat rejection device that uses an evaporative cooling process to reject heat to the 

atmosphere from a water-cooled chilled water plant.

Corrosion Inhibitors Additives that prevent corrosion in water-carrying pipes. 

Cycles of 

Concentration 

(CoCs)

How many times greater the concentration of solids in cooling tower water is than 

the fresh makeup water. GSA requires cooling towers to have a CoC of at least 2.

Induced Draft 

Cooling Tower

A cooling tower with a fan at the outlet that aids in pulling moisture-filled air out of 

the tower and away from the air inlet. 

Makeup Water Fresh water that is added to a cooling tower to dilute mineral impurities and 

maintain water volume.

Scale A film or buildup of solid minerals that have precipitated out of water, frequently 

calcium, silica, magnesium, or chlorides. It can build up inside pipes, impeding flow. 
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C. MANUFACTURER CUT SHEET

The product specifications sheet is provided in Figure 12 (Silver Bullet n.d.-b).

Figure 12. Alternative Water Treatment Product Specifications