What's Actually in Data Center Water Discharge — and Who Regulates It

The coverage of data center water use tends to focus on a single number: how many millions of gallons per day a facility consumes. That framing — data centers as water consumers — is accurate but incomplete. It misses the other half of the story: what happens to the water that comes back out. Cooling systems don't just consume water. They concentrate it, add chemicals to it, heat it, and discharge it — as blowdown, as thermal effluent, and in some configurations as wastewater with a contaminant profile that municipal pretreatment programs were not designed to handle. The contamination incident at Cheyenne, Wyoming made that gap visible in the most public possible way. This piece covers the engineering behind how data centers actually use water, what's in the water when it leaves, and what regulatory framework governs its discharge.

The Scale of the Problem

A medium-sized data center can consume up to 300,000 gallons of water per day for cooling — roughly equivalent to the daily water usage of 1,000 households, according to a June 2026 University of Georgia Extension publication (Saha, UGA CAES TP-121). Larger hyperscale facilities can each use up to 5 million gallons per day, or approximately 1.8 billion gallons annually — equivalent to a town of 10,000 to 50,000 people.

The United States has approximately 4,286 data centers as of May 2026, representing 38% of all data centers worldwide — by far the largest share of any country. Georgia alone has 213, concentrated in the metro Atlanta area. The world's largest data center hub, Loudoun County, Virginia, used approximately 1 billion gallons of water — mostly potable — in 2023, around 2.75 million gallons per day.

Five Cooling System Types — and Their Very Different Water Profiles

Not all data centers have the same water footprint. The cooling technology deployed determines how much water is consumed, whether it is discharged or evaporated, and what it contains when it leaves the facility. Understanding this distinction matters for anyone evaluating the water quality implications of a specific facility.

What's in Cooling Tower Blowdown

The most common data center discharge stream is cooling tower blowdown — the water intentionally removed from an open-loop evaporative cooling system to prevent dangerous concentrations of dissolved solids. As water evaporates in the cooling tower, the minerals and chemicals it carried remain behind, progressively concentrating in the recirculating water. Blowdown is the controlled purge of this concentrated water, replaced by fresh makeup water to keep the system within acceptable operating parameters.

Blowdown is not clean water. According to UGA Extension (Saha, 2026) and EPA guidance on water-related permits for data centers, cooling tower blowdown may contain:

• Concentrated salts and minerals — calcium, magnesium, silica, and other dissolved solids at 3–5× the concentration of incoming makeup water, depending on the cycles of concentration the system operates at
• Corrosion and scale inhibitors — chemical additives applied to protect the cooling system's metal components from scale buildup and corrosion; typically phosphate-based or polymer compounds
• Biocides — chemical treatments to control Legionella and other biological growth in the cooling tower; commonly chlorine, bromine, or isothiazolone compounds
• Heavy metals — leached from piping, heat exchangers, and cooling system components; copper, zinc, and chromium are common depending on metallurgy
• Glycols — in some configurations where freeze protection is required
• PFAS compounds — some liquid coolants used in immersion and direct liquid cooling systems contain fluorinated compounds; some refrigerants used in air cooling are classified as F-gases (a PFAS subcategory)

Thermal Discharge: The Less-Discussed Impact

In facilities using free cooling — drawing water from rivers, lakes, or reservoirs and returning it after absorbing heat — the discharge returns to the receiving water at elevated temperature. Warmer discharge water reduces dissolved oxygen, stresses fish and aquatic organisms, promotes algal blooms, and can cause long-term changes to aquatic ecosystems. This is thermal pollution, regulated under the Clean Water Act but subject to site-specific permit conditions that vary considerably between facilities and jurisdictions.

Evaporative cooling avoids thermal discharge to receiving waters — but at the cost of losing most of that water entirely to the atmosphere. "Even if they're using reclaimed or recycled water, that water is no longer going back into the base flow of the rivers and streams," Julie Bolthouse, director of land policy at the Piedmont Environmental Council, told the Lincoln Institute. "Everybody is upstream from someone else." Washington, DC, for example, still loses water supply if Northern Virginia data centers use recycled water, because that water won't return to the Potomac River.

The Regulatory Framework — and Its Gaps

Data center water discharge is regulated under the Clean Water Act through two primary mechanisms: NPDES permits for direct discharge to surface waters, and industrial pretreatment program requirements for discharge to municipal sewer systems.

The EPA specifically identifies cooling tower blowdown from data centers as a wastewater stream that may require permits before discharge (EPA, 2026). Facilities that discharge directly to waterways are subject to NPDES permit conditions. Facilities that discharge to the municipal sewer must comply with local pretreatment standards, including categorical and local limits on specific pollutants.

The operational phase raises a parallel issue. Most pretreatment programs have well-established limits for conventional pollutants — TSS, BOD, pH, certain metals — but variable coverage for the specific chemical additives used in modern cooling tower chemistry, and essentially no requirements for PFAS monitoring in cooling system discharges. As direct liquid and immersion cooling systems become more common and PFAS-containing coolants enter more facilities, this gap will widen.

Transparency: A Documented Absence

A significant obstacle to managing data center water impacts is the near-total absence of facility-level water use and discharge data in the public domain. The UGA Extension publication notes that public records regarding data center water usage are currently limited, with many operators protected by local nondisclosure agreements that prevent communities from accessing information about specific facilities.

Georgia Senate Bill 421 — the "Data Center Transparency Act" — would prevent local governments from using nondisclosure agreements to conceal data center water or electricity usage data, making it publicly available. As of May 2026, the bill was pending in committee.

Nationally, the EPA monitors aggregate usage but does not publish facility-level data. Some major operators release voluntary sustainability reports — Google reported using more than 5 billion gallons across all data centers in 2023 — but the coverage is inconsistent and the methodology varies between companies, making facility-to-facility comparisons difficult.

Geographic Concentration in Water-Stressed Areas

Roughly two-thirds of data centers built since 2022 have been located in water-stressed regions, according to a Bloomberg News analysis cited by the Lincoln Institute. Arizona has attracted significant data center investment despite being one of the most water-constrained states in the country. Northern Virginia — home to the densest concentration of data centers on Earth, with approximately 300 facilities in a handful of counties — draws from the same Potomac watershed that supplies Washington, DC.

Peter Colohan, director of partnerships and program innovation at the Lincoln Institute, framed the geographic dynamic precisely: "Wherever they choose to put a data center, it is like a giant soda straw sucking water out of that basin. And when you take water from a place, you have to reduce demand or put water back in that same place — there's no other solution."

In Newton County, Georgia, a Meta data center that opened in 2018 uses 500,000 gallons of water per day — 10% of the entire county's water consumption. Newton County continues to receive permit requests for new data centers that would use up to 6 million gallons per day, more than doubling what the entire county currently consumes.

What This Means for Municipal Water Systems and Treatment Professionals

For municipal water utility managers, pretreatment program administrators, and water treatment professionals, data center growth presents a set of specific operational challenges that differ from conventional industrial users:

• Cooling tower blowdown chemistry varies by facility design, cooling system type, and the specific biocide and corrosion inhibitor program the operator runs — and may not be well-characterized in the permit application. Pre-permit water quality characterization should include the full additive chemistry, not just conventional pollutants.
• Construction-phase discharges (fill-and-flush, pipe pressure testing, commissioning flushes) generate water with a fundamentally different contaminant profile than operational-phase blowdown — and existing pretreatment programs may not address this distinction. The Cheyenne incident is the most visible example.
• PFAS from immersion and direct liquid cooling will become an increasing concern as these technologies are deployed more broadly. Facilities using fluorinated coolants should be required to characterize that chemistry in permit applications and subject to PFAS monitoring in discharge characterization.
• Thermal discharge to receiving waters from free-cooling configurations requires site-specific analysis of receiving water temperature, flow, and ecological sensitivity — standard NPDES thermal analysis but increasingly relevant as data center siting reaches water bodies in drought-affected regions.
• Nondisclosure agreements covering water use and discharge data obstruct the ability of water utility managers to plan infrastructure capacity and monitor cumulative impacts. Where state law permits, utilities should negotiate for water use reporting as a condition of connection, independent of any NDA between the facility and the local government.