> The additives used by datacenters in some cases can't be filtered or processed out practically or in economically viable ways, so the water returns back as unusable, esp. if they get water from a body of water and return it with added chemicals the process gets very damaging.
Why would any water be returned?
Datacenters (and literally every industrial process that requires cooling) use water for evaporative cooling. The water evaporates, then returns as rain elsewhere.
The only water being "returned" I can think of would be from the intermittent or continuous blowdown systems to maintain TDS levels; but blowdown is a small fraction of the water consumed and is typically returned back via the city sewer system.
Roughly 80% of the water evaporates into the atmosphere. The remaining 20% is flushed out of the system to remove mineral build-ups and sent to local treatment plants.
It's complicated, and depends on the hardness of the water entering the system from the water supply as well, since blowdown is ultimately to control the level of dissolved solids in the water so it does not contrate enough to start coming out of solution. Rarely if ever is true demineralized water used in open loop cooling systems; it's simply too expensive to produce to then "throw away" into evaporation.
The previously stated 80/20 isn't a bad general estimate, but it varies depending on the specific site. 10%-25% of makeup water being lost to blowdown is a reasonable range.
As for papers about it, I was only able to find a few in my brief search, but they don't land on a single number since it depends on the ratio of dissolved minerals/solids in the circulating cooling-tower water compared with the incoming makeup water, which varies due to acceptability for the process and due to the incoming water quality.
This short PDF on "Mechanical Systems Water Efficiency Management" from the EPA goes over the concepts pretty well though
This document from the US DOD (UFM 3-230-13, Industrial Water Treatment Operation and Maintenance (2026), Chapter 5) says that most cooling towers operate at 3-10 CoC, with 5-7 CoC identified as a cost-effective range. Where CoC is Cycles of concentration described in the previous PDF, and a higher CoC means less blowdown and better water efficiency, with the trade-off of a greater risk of scaling, corrosion, and biological/water-chemistry problems.
Why would any water be returned? Datacenters (and literally every industrial process that requires cooling) use water for evaporative cooling. The water evaporates, then returns as rain elsewhere.
The only water being "returned" I can think of would be from the intermittent or continuous blowdown systems to maintain TDS levels; but blowdown is a small fraction of the water consumed and is typically returned back via the city sewer system.