Wastewater and Cooling Water Discharge for Data Centers

A data center produces two very different wastewater streams. The first is sanitary flow from restrooms and kitchens, which is small because staffing is light. In one recent Texas case, a planned data center complex estimated about 30,200 gallons per day of sanitary sewer use and told the city that was less than the housing previously planned for the tract.¹⁰ The second is process wastewater from cooling, and for a campus using evaporative cooling it can be far larger.

Cooling towers reject heat by evaporating water. Evaporation leaves minerals behind, so the recirculating water grows more concentrated over time. To keep scale and corrosion in check, operators continuously bleed off part of that water and replace it with fresh make-up water. The bleed is called blowdown, which state permits define (following the federal definition at 40 CFR 401.11) as the minimum discharge of recirculating water needed to remove the buildup of dissolved materials.¹ Other process streams can include reject water from reverse osmosis or softening systems and periodic drain-downs for maintenance.

Blowdown volume depends on the cooling design, the climate and the quality of the make-up water, so it should come from the mechanical engineer, not a rule of thumb. Our guide to data center water requirements covers the demand side. The point for siting is that every gallon of make-up water that is not evaporated becomes wastewater, and it is more mineralized than what the utility delivered. Black & Veatch, describing a data center blowdown project, notes that sampling the actual blowdown was part of deciding whether it could go to the local plant without added treatment.³

Most data centers send blowdown to a publicly owned treatment works, or POTW, through the city sewer. That discharge falls under the National Pretreatment Program, a component of the NPDES program run cooperatively by EPA, states and local utilities.² As of the EPA program page cited here, 37 states were approved to oversee POTW pretreatment programs, with EPA regions handling the rest.² Where a POTW has an approved local program, it issues the industrial user permit; where it does not, the state typically does.¹¹

Rules that apply to every user

Federal general and specific prohibitions apply to all nondomestic dischargers, whether or not the POTW has an approved program and whether or not the user holds a permit.⁷ The core test is whether a discharge causes pass through (pollutants that exit the plant and cause a violation of its NPDES permit) or interference (disruption of treatment or sludge handling).¹² Specific limits a POTW develops for its own plant, called local limits, are treated as federal pretreatment standards for enforcement purposes.¹²

Will the data center be a significant industrial user?

EPA’s definition of a significant industrial user (SIU) includes a user that discharges an average of 25,000 gallons per day or more of process wastewater, a user whose process flow is 5% or more of the plant’s average dry-weather hydraulic or organic capacity, and any user the POTW designates because it could harm the plant or cause violations.¹³ The 25,000-gallon count excludes sanitary, noncontact cooling and boiler blowdown flows.¹³ Whether cooling tower blowdown counts toward the threshold is therefore a question for the control authority, but a large campus can be designated on the capacity or harm tests regardless. SIU status brings a permit, sampling, reporting and inspections.¹³

The parameters that matter most for blowdown are usually not the ones that concern a food plant. Dissolved solids, temperature, pH and residual treatment chemicals (biocides, scale and corrosion inhibitors) are the typical discussion points. The plant’s biology can matter too: a Nevada permit fact sheet records a facility that proposed sending blowdown to a storm drain outfall instead of the nearby treatment plant, because the plant works best with nutrient-rich influent and the blowdown lacked nutrients.¹⁴

Where no sewer is available, or the POTW cannot take the flow, a site may discharge blowdown to surface water. That requires NPDES coverage from the state (or EPA in non-delegated states). Many states issue general permits that cover noncontact cooling water and cooling tower blowdown, which is faster than an individual permit when the facility fits the eligibility terms.

• North Carolina’s NCG500000 covers noncontact cooling water and cooling tower and boiler blowdown, excludes major dischargers and Outstanding Resource Waters, and notes that open recirculating cooling discharges to water supply waters may need an individual permit.¹
• Oregon’s 100-J general permit covers recycled noncontact cooling water and cooling tower blowdown, which it describes as the bleed-off that prevents solids buildup.⁸
• Ohio’s general permit OHN000006 is limited to discharges under 5% of the receiving stream’s seven-day, ten-year low flow.⁹

That last condition illustrates the real siting issue. A small creek that runs low in late summer may not be able to take a continuous discharge without an individual permit and tighter limits, and the summer is when cooling loads peak. A larger river or an existing permitted outfall on a former industrial site can be a meaningful asset. Our guides to stormwater management and greenfield vs. brownfield sites cover related permits and legacy outfalls.

Black & Veatch describes one data center project where the work included designing a 2.5-mile pipeline to carry blowdown to a local treatment plant and modifying an NPDES permit, which shows that the two paths are often evaluated together.³

Pretreatment rules address what is in the wastewater. Capacity addresses how much of it the system can carry and treat. A plant may have headroom on paper while the gravity sewer between the site and the plant is full, or a lift station is undersized. Our guide to municipal water and sewer capacity covers how utilities measure headroom; recent cases show how it plays out for data centers.

In Goodyear, Arizona, the city lacked sewer line and plant capacity for more data center wastewater. Under an updated development agreement approved in March 2023, Microsoft agreed to pay $36 million toward a roughly $90 million expansion of the 157th Avenue treatment plant, in place of wastewater impact fees, securing 1.2 million gallons per day of capacity; to post more than $5 million in assurance for a dedicated discharge line; and to air-cool data centers built later on the site.⁴

In Shreveport, Louisiana, a council packet estimated that a proposed facility would send about 2.5 million gallons per day of wastewater and blowdown back to the city system. Under the draft agreement the developer would build the needed improvements, and the sewer charge was set at 35% of metered water use.⁵

Peaks matter as much as averages. Santa Clara, California, hired a consultant to study “extreme discharge events” from data center cooling during heat waves, when many facilities could raise discharges at once, and the study could lead to restrictions on how data centers dispose of wastewater.¹⁵

Sewer service has become a lever in local data center debates. In April 2026 the Ypsilanti Community Utilities Authority in Michigan approved a 12-month moratorium on new water and sewer service for large data centers while it studies capacity.⁶ Bridge Michigan reported that local officials tied the dispute to the state’s data center tax incentives, which require hookup to municipal water, arguing that system capacity had not been assessed when the incentives passed.¹⁶

For a site, that means a sewer connection should be treated like an entitlement, with council or board approval, public hearings and conditions. Our guide to community engagement covers how water and sewer concerns surface, and the tax incentives guide covers where incentives carry utility conditions.

1. 01Get blowdown estimates from the cooling engineer for each design under consideration, including summer peaks.
2. 02Identify the POTW, the route from the site, and any lift stations or interceptors along it.
3. 03Ask the utility whether it has an approved pretreatment program, what its local limits are and whether it would designate the facility an SIU.
4. 04Ask whether the plant and the conveyance system have capacity, whether any moratorium or study is pending, and who pays for upgrades (see impact fees and connection fees).
5. 05If the sewer is uncertain, check the state’s cooling water general permit and the low flow of nearby streams.
6. 06Make wastewater acceptance a condition in the due diligence period, alongside the will-serve letter for water.

Confirm conclusions with the utility’s pretreatment coordinator, the state permitting agency and a wastewater engineer. If you want water, wastewater, power and land looked at together, you can get a site reviewed.