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Data centres use water mainly to move heat away from servers and reject that heat to the environment. In a 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 designs use outside air, air-cooled equipment or closed liquid loops that can avoid ongoing cooling-water evaporation. The amount withdrawn, consumed, recirculated and discharged depends on the facility’s design, climate, water quality and operating practices.
Why a data centre needs cooling
Servers, storage systems, networking equipment, power supplies and other electrical components turn much of the electricity they consume into heat. That heat is produced continuously, including overnight and during periods when demand is lower. If it is not removed, component temperatures can exceed operating limits, causing throttling, failures or automatic shutdowns.
Cooling is therefore a heat-removal chain rather than a single appliance. Fans, pumps, heat exchangers, chillers, cooling towers and control systems work together to transfer heat from the electronics to a final heat sink, usually outdoor air. Water may be part of that chain, but it is not always consumed in the same way.
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The U.S. Department of Energy’s Federal Energy Management Program (DOE FEMP) describes a common arrangement in four linked stages. The exact equipment varies by site, but the direction of heat flow is broadly the same.
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- IT equipment generates heat. Chips and other electrical components warm the air or liquid around them.
- Room or rack cooling captures that heat. Air handlers or liquid-cooling hardware transfer heat into a chilled-water loop or another intermediate circuit.
- A chiller moves heat into a condenser-water loop. The chiller’s refrigeration cycle separates the indoor cooling loop from the loop that will reject heat outdoors.
- A cooling tower rejects heat to the atmosphere. Warm condenser water is distributed over tower fill while fans or natural airflow move air through it. A small portion of the water evaporates; that phase change carries heat away.
The cooled condenser water returns to the chiller and repeats the cycle. Water that evaporates must be replaced with makeup water. The tower also loses water through drift, when tiny droplets leave with the exhaust air, and through leaks or maintenance activities.
“Water used” can mean four different things
Reports can appear contradictory because they use different boundaries and definitions. A facility may withdraw water from a municipal supply, consume some through evaporation, discharge some as wastewater and recirculate the rest many times. These quantities should not be treated as interchangeable.
- Withdrawal: water taken from a source such as a utility, river, aquifer or reclaimed-water network.
- Consumption: water that does not immediately return to the source being measured. In an evaporative tower, the principal consumed portion becomes atmospheric water vapour.
- Discharge: water sent to a sewer, treatment plant or permitted receiving system. Cooling-tower blowdown is a common discharge stream.
- Recirculation: water kept in a loop and used repeatedly. Recirculated water can support many heat-transfer cycles while only makeup water enters the loop.
A facility can therefore have high internal circulation but relatively modest withdrawals, or low direct consumption but substantial indirect water use associated with the electricity that powers it. Any number needs its boundary and measurement period.
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What evaporation does in a cooling tower
Evaporation is the tower’s primary heat-rejection mechanism. As liquid water changes to vapour, it removes a large amount of heat without requiring all of the circulating water to leave the system. The vapour joins the atmosphere; it is not normally captured and piped back to the data centre.
Evaporation also concentrates dissolved minerals. Water entering a tower contains salts and other dissolved material, and those substances remain when water vapour leaves. If concentration rises too far, scale can form on heat-transfer surfaces, corrosion can accelerate and biological-control requirements can change.
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Why cooling towers drain water as blowdown
Operators periodically or continuously drain a controlled portion of the circulating water. This stream is called blowdown. Its purpose is to keep dissolved-mineral concentration within the range that the tower, piping and treatment programme can tolerate.
Fresh makeup water then replaces both the evaporated water and the blowdown. DOE FEMP describes blowdown as an operational part of maintaining consistent dissolved-mineral content in the recirculating condenser-water loop. Blowdown is not the same as evaporation: it remains liquid and normally goes to a wastewater-management route after any required onsite treatment.
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“Cycles of concentration” compares the mineral concentration in circulating tower water with that in the incoming makeup supply. Running at more cycles means water stays in the tower longer before blowdown, which can reduce both makeup demand and wastewater volume. The limit is set by water chemistry, treatment capability, equipment materials, local rules and operating risk.
DOE FEMP cites a cooling-tower best-management practice in which increasing operation from three cycles to six reduces makeup water by 20% and blowdown by 50%. That is a result attributed to the cited practice, not a guaranteed outcome for every data centre. DOE notes that systems commonly operate at two to four cycles and that six or more may be possible when site conditions support it.
- Test makeup and circulating water before changing the control target.
- Check conductivity, scaling tendency, corrosion indicators and biological-control requirements.
- Confirm that valves, sensors and automated blowdown controls are calibrated.
- Verify that the receiving utility or permit allows the resulting discharge chemistry.
Where the water goes after cooling
Evaporated water enters the atmosphere
In an evaporative tower, the consumed fraction leaves as water vapour with the rejected heat. At the facility boundary, this is generally counted as consumption rather than discharge. Atmospheric moisture may later return as precipitation, but that natural cycle does not mean the same water is available to the facility in the same place or time.
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Blowdown goes to a treatment route
Blowdown contains concentrated dissolved minerals and any chemicals used in the cooling programme. Depending on local requirements, it may receive onsite treatment before being sent to a municipal wastewater utility or another permitted system. Microsoft says that, at the sites covered by its local explanation, non-evaporated water is typically returned to a local wastewater utility for treatment, subject to local regulations. That is a company-described practice, not a universal rule.
Some water remains in a recirculating loop
Water in a closed cooling loop can circulate between servers, pumps, heat exchangers and chillers without being discharged during normal operation. The loop still requires maintenance, leak response and eventual fluid management, and the surrounding facility may use water for humidification, sanitation or other purposes.
Reclaimed water and rainwater can replace potable supplies
Some operators use treated reclaimed water instead of drinking-water supplies where quality, infrastructure and permits make that practical. Rainwater capture can supplement supplies at selected sites. Microsoft reports both reclaimed or recycled-water use at some locations and rainwater capture at selected sites. Its local explanation also says some cooling water is reused two to five times before the non-evaporated portion is sent to a utility. These are site and company examples, not an industry-wide guarantee.
Whether a particular discharge is reused, treated, released or subject to additional limits must be confirmed in the operator’s environmental reports, local utility information and permit records. It is not accurate to assume that all discharged water returns directly to a watershed, is potable, or is reused for cooling.
How alternative cooling designs change the water pathway
| Cooling approach | How heat is rejected | Direct water implication | Main constraints |
|---|---|---|---|
| Cooling tower with evaporative heat rejection | Water evaporation carries heat from the condenser loop into outdoor air. | Requires makeup water for evaporation, blowdown, drift and leaks. | Water availability, chemistry, treatment, tower controls and climate. |
| Outside-air economizer | Suitable outdoor air removes heat directly when temperature and humidity permit. | Can reduce or avoid cooling-water use during suitable conditions. | Climate, air quality, humidity control, filtration and building design. |
| Air-cooled chiller or dry cooler | Fans move air across coils or radiators without a cooling tower’s evaporative process. | Usually avoids routine tower evaporation at the heat-rejection step. | Potentially higher electricity use, especially in hot weather. |
| Direct liquid cooling | Liquid captures heat close to chips or racks and transfers it to a coolant-distribution unit and heat-rejection equipment. | Can use a recirculating coolant loop; another part of the plant may still evaporate water. | Rack design, fluid compatibility, maintenance and heat-rejection architecture. |
| Closed-loop direct-to-chip design | Liquid circulates continuously between servers and chillers in a sealed circuit. | Can avoid ongoing cooling-water evaporation in the announced design. | Higher or different energy requirements, initial installation and site-specific heat rejection. |
Outside-air and air-cooled options are not automatically better in every location. A design that saves onsite water may consume more electricity, and electricity generation can have its own indirect water footprint. Climate, grid conditions, heat density, reliability requirements and water availability must be evaluated together.
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What direct liquid cooling changes—and what it does not
Direct liquid cooling places a heat-transfer fluid near high-temperature components instead of relying only on room air. DOE FEMP describes arrangements in which a closed loop carries heat to a coolant-distribution unit and then to the facility’s heat-rejection system. That last stage may be dry, evaporative or hybrid, so “liquid cooled” does not by itself answer whether a site consumes water.
In August 2024, Microsoft announced a direct-to-chip design that is filled during construction and then continually recirculates water without a fresh cooling-water supply for the cooling loop. Microsoft says the design is intended to avoid evaporative cooling-water consumption and estimates savings of more than 125 million litres per data centre per year. That figure is the company’s design estimate, not an independently measured benchmark for all liquid-cooled facilities. Microsoft also states a nominal annual energy-use increase compared with its evaporative designs, illustrating why water and energy cannot be optimized in isolation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to compare a data centre’s water performance
Water Usage Effectiveness (WUE)
DOE FEMP defines site-based Water Usage Effectiveness as annual site water use in litres divided by annual IT-equipment energy use in kilowatt-hours:
WUE = annual site water use (litres) ÷ annual IT energy use (kWh)
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| Reported value | Scope and qualification |
|---|---|
| 0.30 L/kWh, FY24 | Microsoft’s figure for data centres it fully owns and controls that had been operational for 12 months at the time of calculation. |
| 0.27 L/kWh, FY25 | Microsoft’s same stated dataset qualification; FY25 covered July 1, 2024 through June 30, 2025. |
| 0.49 L/kWh, 2021 | Microsoft’s previously reported global average, cited in a December 2024 comparison with its later figure and subject to the company’s stated scope. |
A lower WUE indicates less reported site water per unit of IT energy within that boundary. It does not describe the water intensity of a specific application, user request or machine-learning workload by itself.
Why a workload’s water footprint varies so widely
A 2025 review by Lei, Lu, Shehabi and Masanet reports more than a 10,000-fold variation across the analysed workload cases. The result does not mean every pair of data centres differs by that amount; it shows that water per unit of digital work can change dramatically under different assumptions and operating conditions.
The review identifies several determinants:
- Server efficiency and the proportion of time servers are doing useful work.
- Electricity-grid water-consumption factors, which represent indirect water associated with power generation.
- Cooling-system type and facility infrastructure efficiency.
- Climate zone and the resulting need for mechanical cooling or humidification.
- The share of inactive servers and the timing of hardware refresh cycles.
Direct facility water and indirect electricity-related water should be reported separately or clearly combined with an explained boundary. A WUE number cannot, on its own, answer the total water footprint of a workload.
A practical framework for evaluating a cooling system
- Map the heat path. Identify where heat leaves the chip, rack, room, chiller and final heat-rejection equipment.
- Separate water flows. Record withdrawals, makeup water, evaporation, blowdown, drift, leaks, recirculation and wastewater discharge independently.
- Check local conditions. Assess climate, water scarcity, source-water quality, wastewater capacity and permit limits.
- Compare energy and water together. Include the electricity consequences of dry cooling, economizers, pumps, fans and chillers.
- Define the metric boundary. State which buildings, months, water sources and IT loads are included before comparing WUE values.
- Plan for abnormal operation. Include drought restrictions, poor water quality, leaks, sensor failures, tower treatment problems and backup cooling modes.
There is no single cooling recipe that minimizes water in every location. The best outcome usually combines server and facility efficiency, suitable heat-rejection equipment, high utilization, careful water chemistry and a power supply with a transparent water boundary.
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