Part 5 of 13 in Thirteen Records. The long article, Data Centers Don’t Have an Environmental Record. They Have Thirteen of Them., rates thirteen kinds of environmental complaint against data centers. This series takes them one at a time.
I work at a local ISP in Northern Illinois and co-founded FD-IX, an internet exchange. I have no customer, vendor, or interconnection relationship with any facility or developer named here and no financial position in any of them beyond broad-market index funds. My industry grows with infrastructure investment in a general way, which is an indirect interest; the full disclosure is at the top of the complete article.
The data center water debate is about gallons in. Almost nobody asks about gallons out. Every evaporative cooling tower leaves the dissolved minerals behind when the water evaporates, so operators bleed off part of the concentrated water, add treatment chemistry to what stays, and send the bleed to the sanitary district. That stream is blowdown, and the Congressional Research Service put it in a July 2026 report in a sentence worth reading twice: EPA has written no effluent guidelines for data centers, so no categorical pretreatment standard applies. [12a]
This is one of the shorter posts in the series and the one with the clearest ask. The chemistry is not unique to data centers; a hospital’s tower sends the same thing to the same plant. What differs is the volume at one site, and no public source reports it. The question for a council is a classification question, and the sanitary district answers it, not the council.
How the ratings work. Each complaint below is stated the way it usually arrives at a hearing and rated on the scale the full article uses. Systemic: documented at compliant facilities across multiple geographies. Regional: real, but only under specific geographic or operational conditions. Isolated: documented at one or few facilities due to misconduct or atypical design. Exaggerated: the concern is real but the figures are permit ceilings or worst cases, not operating norms. Selective: a legitimate concern applied only to data centers, not to comparable land uses. Misattributed: the harm is real but the source, geography, or cause is wrong. Contested: a plausible mechanism exists but causal documentation at compliant facilities does not. Fabricated: no credible evidentiary basis exists. Where I say I found no record of something, that means a search of public sources through October 2026, not paid databases or records requests.
Cooling Wastewater
Every evaporative cooling system produces a waste stream, and the water-use debate almost never mentions it.
Claim: Evaporative data centers send a chemical wastewater stream to the sewer, and no rule was written for it. Tier: Systemic.
A cooling tower rejects heat by evaporating water. Evaporation leaves the dissolved minerals behind, so operators bleed off part of the concentrated water and replace it with makeup water. That bleed is blowdown. In EPA’s water balance, makeup equals evaporation plus blowdown, and cycles of concentration is makeup divided by blowdown: at three cycles, one of every three gallons of makeup leaves as blowdown and two evaporate; at six cycles, one in six. EPA puts the savings from three cycles to six at about 20% of total water use. [12b] The same arithmetic cuts the other way: fewer gallons of wastewater, each more concentrated.
The Congressional Research Service’s July 2026 report lists what that stream carries: dissolved solids concentrated from the source water, chlorine, corrosion inhibitors, biocides, and copper, nickel, and chromium from pipe corrosion, with pH and temperature regulated in their own right and some treatment chemistry kept proprietary. Closed-loop systems discharge less in normal operation, but both kinds generate wastewater during maintenance, construction, and commissioning, and those flushes can reach a treatment plant as high-volume slug loads that pass through it or upset it. [12a]
Most data centers send that stream to the local sanitary district, which makes them indirect dischargers under the National Pretreatment Program. “EPA has not established ELGs for data centers, so categorical pretreatment standards do not apply.” [12a] Effluent limitations guidelines are the industry-specific numeric standards EPA writes for categories like steam power plants and steel mills. What remains is the general rule for every indirect discharger: nothing that interferes with the plant or passes through it into the river in amounts that violate the plant’s own discharge permit, no discharge below pH 5.0 unless the plant is designed to accept it, nothing that pushes plant influent above 104°F, and the local limits each plant writes to enforce those prohibitions. [12f] Those rules protect the plant and, through its permit, the receiving water. What they are not is a standard written for what a cooling tower sends to the sewer.
Whether a data center gets the closer oversight a large industrial discharger normally gets is a classification question. Illinois’s 25,000-gallon-a-day test for a significant user counts process wastewater and excludes “sanitary, noncontact cooling, and boiler blowdown wastewater”; a process stream of 5% or more of the plant’s dry-weather capacity also qualifies, and the control authority can designate a user regardless. [12c] The rule does not say how a district must classify evaporative cooling tower blowdown, which carries the treatment chemistry listed above; a district can count it as process wastewater. [12c] CRS reports, as a national generalization, that data centers are not typically classified as significant users because blowdown is excluded from the flow count. [12a] I found no Illinois district determination either way, so an applicant should get the district’s classification in writing. Ohio is the one state I found that drafted a statewide data center permit, and it covered a different pathway: direct discharge to surface water, not the sewer. Circle of Blue reported in July 2026 that Ohio EPA had dropped the draft after more than 7,000 comments. [12d][12e]
Claim: Cooling tower blowdown is a uniquely toxic data center discharge. Tier: Selective.
The chemistry is the same at any building that cools with a tower, and EPA’s water-efficiency guidance describes the same water balance and blowdown for hospitals, office towers, and plants that send it to the same sanitary districts. [12b] What differs at a hyperscale campus is the volume at one site, and no source I located publishes facility-level blowdown volumes, so that is a number the applicant has to supply. [12a] Air-cooled closed-loop designs produce little routine blowdown, which is the wastewater side of the trade described in the water supply post in this series. In Yorkville, the sanitary district’s director told his board in February 2026 that the projects’ cooling “now will not use water” and that they “will only discharge sanitary waste,” while noting the district “has still not received any plans.” [11p] On that account the open wastewater question there is construction and commissioning discharge, not routine blowdown. [12a]
What to ask for. Ask the sanitary district, in writing, whether it counts tower blowdown as process wastewater, because Illinois’s 25,000-gallon-a-day significant-user test excludes noncontact cooling and boiler blowdown and does not say how a district must treat evaporative blowdown with treatment chemistry in it. [12c] Require the cooling design, the planned cycles of concentration, the projected blowdown and commissioning discharge volumes, the receiving treatment plant or receiving water, and the full list of treatment additives including biocides. Get the treatment plant’s sign-off before approval, not after, since the district is often a separate unit of government from the council voting. Where the volume warrants, ask the plant to designate the facility a significant industrial user, which Illinois rules allow and which brings sampling and inspection with it. Where a facility does use evaporative cooling, ask whether reclaimed water from the district can supply the makeup; that keeps potable water out of the tower, though it does not remove the blowdown, since reclaimed water starts with more dissolved solids and concentrates the same way.
What nobody knows. No public dataset reports blowdown volumes or discharge chemistry for data centers, the number a sanitary district needs to size a pretreatment agreement. [12a] In Yorkville, where the approved projects are air-cooled, the open question is construction and commissioning discharge, not routine blowdown, and the district had received no plans as of February 2026. [11p]
This is part 5 of 13 in Thirteen Records, a series that takes the long article Data Centers Don’t Have an Environmental Record. They Have Thirteen of Them. one topic at a time. Previous: water quality. Next: noise, from Chandler’s chillers to Yorkville’s continuous monitors to the infrasound claim with no evidence behind it.
Cover photo: Saud / Wikimedia Commons, CC BY-SA 4.0
Citations
[11p] WSPY News, “Air-cooled design reduces wastewater concerns for Yorkville projects,” March 10, 2026, reporting the Yorkville-Bristol Sanitary District’s February 2026 board meeting. Executive Director Cyrus McMains: “The cooling process systems approaches have been revised and now will not use water”; “This means that the data centers will only discharge sanitary waste to YBSD”; “The sanitary wastewater will be domestic strength or similar from other office buildings and homes”; “YBSD has still not received any plans from the prospective data centers.” https://www.wspynews.com/news/local/air-cooled-design-reduces-wastewater-concerns-for-yorkville-projects/article_ba7331c8-6c74-4ae4-9658-f851ba5c6951.html (archived: https://web.archive.org/web/20261025162525/https://www.wspynews.com/news/local/air-cooled-design-reduces-wastewater-concerns-for-yorkville-projects/article_ba7331c8-6c74-4ae4-9658-f851ba5c6951.html)
[12a] Congressional Research Service, “Data Centers and Water: Frequently Asked Questions,” R49057, July 31, 2026. Blowdown and discharge composition (total dissolved solids, chlorine, corrosion inhibitors, biocides, copper, nickel, chromium, pH, temperature; some additives proprietary); data centers most commonly discharge to publicly owned treatment works under the National Pretreatment Program; “EPA has not established ELGs for data centers, so categorical pretreatment standards do not apply”; data centers not typically classified as significant industrial users because cooling system blowdown is excluded from flow-based thresholds; direct-discharge NPDES limits typically set on best professional judgment; construction and maintenance slug loads; public water systems supply an estimated 97% of U.S. data center water; no facility-level discharge figures given. Canonical: https://www.congress.gov/crs-product/R49057 (archived: https://web.archive.org/web/20260824154654/https://www.congress.gov/crs-product/R49057). Readable copy: https://www.everycrsreport.com/reports/R49057.html (archived: https://web.archive.org/web/20260926130247/https://www.everycrsreport.com/reports/R49057.html)
[12b] U.S. EPA, “Water Efficiency Management Guide: Mechanical Systems,” EPA 832-F-17-016c, November 2017 (WaterSense at Work). Cooling tower water balance: makeup equals evaporation plus blowdown (drift and leaks negligible); cycles of concentration equals makeup divided by blowdown; increasing cycles from three to six reduces water use by about 20%. https://www.epa.gov/sites/default/files/2017-12/documents/ws-commercialbuildings-waterscore-mechanical-systems-guide.pdf (archived: https://web.archive.org/web/20260103012643/https://www.epa.gov/sites/default/files/2017-12/documents/ws-commercialbuildings-waterscore-mechanical-systems-guide.pdf)
[12c] 35 Ill. Adm. Code 310.112, “Significant Industrial User.” Three tests beyond categorical standards: average discharge of 25,000 gallons per day or more of process wastewater to the POTW “(excluding sanitary, noncontact cooling, and boiler blowdown wastewater)”; a process wastestream “which makes up five percent or more of the average dry weather hydraulic or organic capacity of the POTW treatment plant”; or designation “by the Control Authority on the basis that the industrial user has a reasonable potential for adversely affecting the POTW’s operation or for violating any pretreatment standard or requirement.” https://ilga.gov/ftp/JCAR/AdminCode/035/035003100A01120R.html (archived: https://web.archive.org/web/20261025155933/https://ilga.gov/ftp/JCAR/AdminCode/035/035003100A01120R.html)
[12d] Ohio EPA, Draft General NPDES Permit No. OHD000001 for discharges from data center facilities to waters of the state, Public Notice issued October 31, 2025, written comments through December 17, 2025, public hearing December 17, 2025. https://dam.assets.ohio.gov/image/upload/epa.ohio.gov/Portals/35/permits/Data_Centers/OHD000001_Draft_PN.pdf (archived: https://web.archive.org/web/20260407094842/http://dam.assets.ohio.gov/image/upload/epa.ohio.gov/Portals/35/permits/Data_Centers/OHD000001_Draft_PN.pdf). Fact sheet: eligible discharges (noncontact cooling water, cooling tower blowdown, boiler blowdown, air compressor condensate); limits for pH and total filterable residue from Ohio water quality criteria; conditional total residual chlorine and oxidant limits; monitoring for temperature and flow; Ohio EPA approval required for bromine-, chlorine-, and phosphorus-releasing treatment additives; exclusions for high-quality receiving waters and for surface water intakes of 2 MGD or more. https://dam.assets.ohio.gov/image/upload/epa.ohio.gov/Portals/35/permits/Data_Centers/OHD000001_Draft.fs.pdf (archived: https://web.archive.org/web/20260926130249/https://dam.assets.ohio.gov/image/upload/epa.ohio.gov/Portals/35/permits/Data_Centers/OHD000001_Draft.fs.pdf) Summary of scope: Bricker Graydon LLP (law firm), “Ohio EPA Issues Draft General NPDES Permit for Data Centers,” December 2, 2025: covers discharges “to waters of the state”; excludes discharges to combined or sanitary sewer systems and to ground water. https://www.bricker.com/insights/publications/ohio-epa-issues-draft-general-npdes-permit-for-data-centers (archived: https://web.archive.org/web/20260513031736/https://www.bricker.com/insights/publications/ohio-epa-issues-draft-general-npdes-permit-for-data-centers)
[12e] Christian Thorsberg, Circle of Blue, “Ohio Scraps Controversial Plan to Ease Data Center Wastewater Discharges,” July 22, 2026. Ohio EPA: “Ohio EPA has decided not to move forward with finalizing the general permit”; more than 7,000 public comments; data center discharge permits to continue on an individual basis. https://www.circleofblue.org/2026/supply/water-quality/ohio-scraps-controversial-plan-to-ease-data-center-wastewater-discharges/ (archived: https://web.archive.org/web/20261025155644/https://www.circleofblue.org/2026/supply/water-quality/ohio-scraps-controversial-plan-to-ease-data-center-wastewater-discharges/)
[12f] 40 CFR 403.5, National pretreatment standards: prohibited discharges. (a)(1): “A User may not introduce into a POTW any pollutant(s) which cause Pass Through or Interference.” (b)(1): wastestreams with “a closed cup flashpoint of less than 140 degrees Fahrenheit”; (b)(2): “in no case Discharges with pH lower than 5.0”; (b)(5): heat that inhibits biological activity, and “in no case” such that “the temperature at the POTW Treatment Plant exceeds 40 °C (104 °F)” without approval; (b)(8): “Any trucked or hauled pollutants, except at discharge points designated by the POTW.” (c): POTWs with approved pretreatment programs “shall develop and enforce specific limits” implementing the prohibitions. https://www.law.cornell.edu/cfr/text/40/403.5 (archived: https://web.archive.org/web/20260215053414/https://www.law.cornell.edu/cfr/text/40/403.5)