Data centres use water mainly to carry heat away from servers and reject that heat to the atmosphere. In a common cooling-tower system, water circulates through heat exchangers, some of it evaporates at the tower, and some is drained as “blowdown” so minerals do not build up. Other facilities use outside air, air-cooled equipment or closed-loop liquid cooling, so water use varies substantially by design and site.
The important distinction is between water withdrawn, water consumed by evaporation, water discharged, and water recirculated. Those quantities describe different parts of the same cooling system and should never be treated as one universal “water used” number.
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Where water fits in the heat-removal chain
Servers, storage systems, networking equipment and power electronics produce heat continuously while operating. Cooling does not make that heat disappear; it transfers the heat from the IT equipment to another medium and eventually releases it outdoors or to another heat sink.
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- IT equipment warms the room. Fans or liquid-cooled components move heat away from processors, memory, power supplies and other hardware.
- Room cooling transfers heat to a chilled-water loop. Air handlers or equipment-level heat exchangers pass the heat into water circulating through the building.
- A chiller moves heat to a condenser-water loop. The chiller’s refrigeration cycle transfers heat from the chilled-water side to a warmer condenser-water circuit.
- The condenser loop reaches a cooling tower. The tower exposes part of that water to moving air. Evaporation removes heat because the water that changes into vapour carries latent heat with it.
- The remaining water returns to the plant. Pumps send it back through the condenser loop, while makeup water replaces evaporation and other losses.
This simplified sequence is described by the U.S. Department of Energy’s Federal Energy Management Program (DOE FEMP), whose cooling-tower guidance is dated January 9, 2019. Actual facilities may combine several loops, use different heat exchangers or reject heat without a tower.
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Four different meanings of “water used”
A water-accounting figure is meaningful only when its boundary is stated. A facility can withdraw water from a municipal supply, reuse the same water many times, consume a portion through evaporation and discharge another portion for treatment.
| Term | What it means in a data centre | Typical destination or purpose |
|---|---|---|
| Withdrawal or intake | Water taken from a potable, reclaimed, industrial or other source. | Used as makeup water, humidification water or another facility supply. |
| Consumption | Water that leaves the site boundary in a way that is not immediately available for reuse, principally evaporation in an evaporative tower. | Enters the atmosphere as water vapour. |
| Discharge | Water intentionally drained from a loop, including cooling-tower blowdown. | Usually sent to a wastewater utility or another permitted treatment route. |
| Recirculation | Water retained in a closed or semi-closed loop and used repeatedly. | Continues moving between servers, heat exchangers, chillers and pumps. |
| Other losses | Small or irregular losses from tower drift, leaks, maintenance or equipment operation. | Varies with design, condition and operating practice. |
DOE FEMP identifies evaporation as the main cooling-tower use, while also identifying blowdown, drift and leaks as losses. A withdrawal number therefore cannot be read as an evaporation number, and a discharge number cannot be read as water that was consumed.
What happens inside an evaporative cooling tower
Evaporation removes the heat
Warm condenser water is distributed over fill material while fans move air through the tower. A small fraction of the water evaporates. That phase change carries heat into the atmosphere, cooling the liquid that remains. The cooled liquid returns to the condenser loop.
Minerals concentrate as water evaporates
Dissolved salts and other minerals do not evaporate with the water. If operators only replaced evaporated water, those substances would become increasingly concentrated, encouraging scale, corrosion or biological-control problems. Operators therefore drain a controlled portion of the circulating water. This stream is called blowdown.
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Makeup water replaces several losses
Fresh or reclaimed makeup water replaces evaporation, blowdown and smaller losses such as drift and leaks. The amount depends on tower performance, weather, load, water chemistry and the cycles of concentration maintained by the operator.
Higher cycles can reduce makeup demand
Cycles of concentration describe how much more concentrated the circulating water is than the incoming makeup water. DOE FEMP cites a cooling-tower best-management practice in which moving from three cycles to six reduces makeup-water demand by 20% and blowdown by 50%. That is a cited practice example, not a guarantee for every data centre. Water chemistry, treatment capability and equipment limits determine whether six cycles are safe; DOE notes that systems commonly operate at two to four cycles and that six or more may be possible in suitable conditions.
Where the water goes after cooling
Evaporated water becomes atmospheric vapour
In an evaporative tower, the consumed fraction changes phase and leaves with the exhaust air. It is not recovered as liquid at the facility. This is why a site can have substantial water consumption even though most of the water in its cooling loop is continuously recirculated.
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Blowdown usually becomes a wastewater stream
Blowdown carries concentrated minerals and treatment chemicals out of the loop. Microsoft says that, at the sites covered by its local explanation, non-evaporated cooling water is typically returned to a local wastewater utility for treatment, subject to local regulations. That statement describes the company’s reported practice, not a universal rule. Discharge limits, pretreatment requirements and receiving systems differ by jurisdiction.
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Some water remains in circulation
A recirculating loop can retain the same coolant for long periods. Pumps, filters, heat exchangers and chillers move the fluid without sending it to a drain after each pass. Even a recirculating system may require occasional maintenance, treatment or replacement, and the facility can still use water for sanitation, humidification or other non-cooling needs.
Reclaimed supplies and rainwater can substitute for potable water
Where infrastructure and permits allow, operators may use reclaimed or recycled water instead of drinking-quality water. Microsoft reports reclaimed-water use at some locations and rainwater capture at selected sites. Its local explainer also says some cooling water at described sites is reused two to five times before the non-evaporated portion goes to a utility. These are site-specific company examples; they do not establish that all data centres reuse water that many times or have access to alternative supplies.
How cooling designs compare
Water consumption is only one design criterion. Engineers also weigh compressor energy, local climate, water quality and availability, heat density, reliability, maintenance and the water associated indirectly with electricity generation.
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| Cooling approach | Water pathway | Where it works well | Important qualification |
|---|---|---|---|
| Cooling tower with evaporative heat rejection | Recirculates condenser water; evaporation rejects heat; blowdown and drift leave the loop; makeup water replaces losses. | Many climates and large facilities where efficient heat rejection and water supply are available. | Can reduce compressor energy but consumes water and requires chemistry control. |
| Outside-air economizer | Uses suitable outdoor air to remove heat, potentially avoiding tower evaporation during favourable conditions. | Climates and operating hours with sufficiently cool, dry or otherwise suitable outdoor conditions. | Performance depends on weather, filtration, humidity control and the facility’s allowable temperatures. |
| Air-cooled chiller or dry cooler | Rejects heat directly to air instead of evaporating tower water. | Sites prioritising low direct water use or lacking a dependable water source. | May require more electricity, especially in hot conditions; the trade-off is site-specific. |
| Direct liquid cooling | Captures heat close to chips or racks and sends it through a coolant distribution unit and heat-rejection plant. | High-density computing where air cooling becomes difficult or inefficient. | The server-side loop may be closed, but another part of the plant can still evaporate water. |
| Closed-loop direct-to-chip design | Liquid circulates between servers and chillers without a routine fresh cooling-water supply for evaporation. | Facilities designed around high-density liquid-cooled hardware. | Specific savings, energy effects and operating boundaries depend on the announced design and site. |
Microsoft’s 2023 cooling infographic presents company-specific method ranges and says local climate informs its choices. Those ranges should not be treated as current, universal benchmarks for every operator.
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What “zero water evaporation” means in a closed loop
In December 2024, Microsoft described a direct-to-chip design that is filled during construction and then continually recirculates water. The company says the design avoids evaporative cooling-water consumption and estimates that it could avoid more than 125 million litres per year per data centre. That figure is a company design estimate, not an independently measured industry benchmark.
Microsoft Vice President Steve Solomon wrote on December 9, 2024: “These new liquid cooling technologies recycle water through a closed loop.” The claim applies to the announced architecture. It should not be generalized to every liquid-cooled facility, because a site can use direct liquid cooling at the rack while still relying on evaporative equipment elsewhere for heat rejection.
Microsoft also states that the design has a nominal annual energy-use increase compared with its evaporative designs. That illustrates the central trade-off: reducing direct water consumption can change electricity demand, and electricity production itself may have an indirect water footprint.
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Water Usage Effectiveness is a facility intensity metric, not a complete water footprint for a digital task. DOE FEMP defines site-based WUE as annual site water use in litres divided by annual IT-equipment energy use in kilowatt-hours. Microsoft describes its reported WUE as annual water used for humidification and cooling divided by annual IT kWh. Because boundaries and numerators can differ, two WUE values should be compared only when their definitions match.
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| Reported figure | Scope and qualification |
|---|---|
| 0.30 L/kWh | Microsoft FY24 global WUE for data centres it fully owns and controls that had been operational for 12 months at calculation time. |
| 0.27 L/kWh | Microsoft FY25 global WUE for the same stated type of owned-and-controlled, full-year operational dataset; FY25 ran from July 1, 2024 through June 30, 2025. |
| 0.49 L/kWh | Microsoft’s previously reported 2021 global average, cited in a December 2024 comparison with the later figure and carrying the company’s stated scope. |
| More than 125 million litres per data centre per year | Microsoft’s announced-design estimate for cooling water avoided by its zero-water-evaporation approach; not a measured universal saving. |
| More than 10,000-fold variation | Lei, Lu, Shehabi and Masanet’s 2025 review reports this range across analysed workload cases, attributing it to major differences in water per kWh and workload efficiency; it is not a prediction that every pair of facilities differs by that amount. |
WUE does not show how much water a particular workload caused, nor does it include every upstream effect. A workload’s broader water footprint can depend on server efficiency, utilisation, inactive-server share, infrastructure efficiency, cooling type, climate zone, electricity-grid water factors and server refresh cycles.
Why the same workload can have very different water impacts
- Server efficiency: More efficient hardware can perform the same computation with less electricity and therefore less associated cooling demand.
- Utilisation: A highly utilised server spreads fixed facility overhead across more useful work; idle or lightly loaded equipment still consumes power and generates heat.
- Cooling architecture: Evaporative towers, dry cooling, economizers and liquid systems have different direct-water and energy profiles.
- Climate: Outdoor temperature and humidity affect how often economizers work and how much mechanical cooling is required.
- Electricity supply: Water used at power plants is separate from water consumed onsite, but it can materially affect a full lifecycle assessment.
- Infrastructure efficiency and refresh timing: Power distribution, cooling overhead and the replacement of older equipment alter water per unit of useful computation.
The 2025 review concludes that there is no single recipe for minimising water use; the best combination depends on these interacting factors. Direct facility water and indirect power-generation water should be reported as separate boundaries rather than silently combined.
How operators reduce cooling-water demand
- Measure the right streams. Track intake, recirculation, evaporation estimates, blowdown, drift, leaks and wastewater discharge separately.
- Set a chemistry target. Test conductivity, scaling potential, corrosion indicators and biological-control conditions before raising cycles of concentration.
- Optimise cycles gradually. Use the highest concentration that equipment, treatment and permits can safely support; the three-to-six example from DOE FEMP is a benchmark practice, not an automatic setting.
- Find and repair losses. Inspect valves, pumps, piping, tower drift eliminators, level controls and meters for leaks or abnormal discharge.
- Use non-potable sources where appropriate. Evaluate reclaimed water, recycled streams or captured rainwater against local quality, reliability and regulatory requirements.
- Match cooling to climate and heat density. Economizers, dry coolers or direct liquid cooling may reduce water demand in the right conditions, while a tower may offer an energy advantage elsewhere.
- Account for the whole system. Compare direct water, electricity consumption, grid-water factors, reliability and workload efficiency rather than optimising one metric in isolation.
What cannot be inferred from a generic water number
A facility’s reported litres per kilowatt-hour does not reveal whether the water was potable or reclaimed, how much evaporated, how much became blowdown, what wastewater treatment occurred or which assets were included. Microsoft’s FY25 figure, for example, excludes facilities outside its stated owned-and-controlled sites that had operated for a full year at calculation time.
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There is also no single current industry-wide total supported here for global data-centre water use. Facility designs, accounting boundaries and regional conditions differ too much for one headline number to describe them reliably.
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