Last checked: October 22, 2026

How data centers treat cooling water

Data centers with evaporative cooling, such as cooling towers, work like a giant swamp cooler. Water evaporates to carry heat away, and the minerals it leaves behind build up in the water that's left. Operators manage that with blowdown (draining some water off), chemical treatment, and a program to control Legionella bacteria. Here's how it works, in plain English, from federal sources.

Where the water goes

In a cooling tower, water is lost three ways: evaporation (the cooling itself), blowdown (water drained on purpose to keep minerals from building up), and small losses such as drift (mist carried out by air). Fresh make-up water replaces what's lost. DOE's Federal Energy Management Program (FEMP) puts it as: make-up = evaporation + blowdown.¹

Cycles of concentration

As water evaporates, dissolved minerals stay behind and concentrate. "Cycles of concentration" measure how many times the water is concentrated before it's drained. EPA defines it as the conductivity of the blowdown divided by the conductivity of the make-up water, which is about the same as make-up volume divided by blowdown volume.⁴

• Many systems run at 2–4 cycles, and 6 or more may be possible. DOE says going from 3 to 6 cycles cuts make-up water by about 20% and blowdown by 50%.²
• How high you can go depends on the make-up water quality and the treatment program.² Harder, more mineral-rich water reaches its scaling limits sooner.¹
• Operators often install a conductivity controller that drains water only when a set point is passed, plus meters on the make-up and blowdown lines.²

Scale, corrosion and biological growth

• Scale: every dissolved mineral has a saturation limit, and scale forms past it. DOE notes that acid feed (sulfuric, hydrochloric or ascorbic) can convert some alkalinity into more soluble forms and allow more cycles, but overdosing can damage the system.¹²
• Corrosion: more concentrated water also tends to be more corrosive.¹ CRS notes that data center wastewater can contain corrosion products such as copper, nickel and chromium.⁵
• Biofouling: typical treatment programs combine corrosion and scale inhibitors with biological fouling inhibitors.² CRS lists acids, disinfectants, dispersants, corrosion inhibitors and biocides as chemicals data centers may use, and says the exact products are sometimes proprietary.⁵
• Filtration: side-stream filtration of part of the circulating water can remove particles and help raise cycles.³

Source water matters. CRS notes that water from a regulated public system generally suits cooling, partly because those systems control corrosivity and keep a disinfectant in the water. Data centers that use groundwater or reclaimed water may need to pretreat it.⁵

Legionella control

Cooling towers can spread Legionella, the bacteria that cause Legionnaires' disease, through mist. CDC recommends that each building with a cooling tower have a water management program. It points operators to ASHRAE Guideline 12 and Cooling Technology Institute Guideline 159 for practical steps.⁶ ASHRAE Standard 188 sets the framework for those programs: a risk assessment, control measures, monitoring, corrective actions and periodic validation that the program works.⁷ Some states and cities have their own cooling tower registration or testing rules, so check locally. CDC's testing thresholds are summarized in Data centers and water testing.

What happens to blowdown

According to CRS, data centers most commonly send wastewater to a municipal treatment plant. Under EPA's National Pretreatment Program, which local utilities mostly enforce, they count as "indirect dischargers" and can face local limits on what they send. A facility discharging straight to a river or stream needs its own NPDES permit.⁵ Blowdown typically carries concentrated dissolved solids, and it may also contain chlorine from treated source water plus treatment chemicals. Discharges during commissioning and startup, when cooling loops are cleaned, can differ from normal operation.⁵

Utah's 2026 reporting law asks new large data centers to describe their discharge water treatment plans and the expected change in discharge water temperature before construction.⁸

Frequently asked questions

Why don't data centers just reuse the same water forever? Evaporation concentrates minerals until they scale or corrode the equipment, so some water has to be drained and replaced. Treatment lets operators reuse it longer, not forever.¹

Does harder water mean more water use? Make-up water quality limits how many cycles a system can safely run, and fewer cycles means more blowdown and more make-up water.²⁴

Do closed-loop systems need this treatment? Closed loops that don't evaporate water use little water on site, so the blowdown issues above mostly don't apply. They still need their own water treatment, and they use more electricity.⁹

Related guides

• Data centers and water: how much they use, where it comes from, and the rules
• Will a nearby data center affect my well?
• Data centers and water testing
• Iron in well water
• Reading your well water test results