> You’re all over this comments section with “biocides” but what the hell even is a biocide?

Biocides in terms of datacenter cooling is used to prevent moss and algae to take over your pipe systems and clog them. In inner loops, even though isolated, any small contamination creates serious clogging and flow issues (the pipes are pretty narrow to begin with).

Inside a typical DLC server: https://lenovopress.lenovo.com/assets/images/LP1602/SD650-I%...

In outer loops, coolant hygiene is equally important since any layers of algae or moss creates clogs and reduces thermal conductivity in heat exchangers.

> It sounds pretty scary (“life kill! Yikes!”)

Since biocides prevent "living organisms" being present in water, it basically sterilizes it. You can't use biocide contaminated water to grow anything or to feed anything and anyone. I mean, we use similar materials to kill weeds. If you're exposed to them, after a certain dosage you'll experience nasty illnesses.

> Add to that the fact that I’ve never heard it used in the context of pollution, let alone datacenter-based pollution

See: https://h2ocooling.com/closed-loop-vs-open-loop/#How_Open-Lo...

Excerpt: Maintaining water quality demands robust chemical treatment programs. Scale inhibitors, biocides, corrosion inhibitors, and pH adjusters require careful monitoring and frequent adjustment to prevent system damage.

Biocides specialized for open-loop cooling systems: https://www.dober.com/water-treatment/cooling-tower-biocides

> I’m inclined to believe you’re not being serious.

It's your call. I work at a team for two decades which maintains a couple of Top500 systems. We still handle everything from unpacking to cabling to installation and software side (i.e. Full Stack) for most of our servers. DLC systems need their specialized installation teams on the ground, so we let them do their work, but for conventional systems we still do everything.

> Also, what processing plants are supposed to be separating water from “biocides and other chemicals”?

Your water treatment plant removes many harmful things to create potable water, but not all chemicals can be removed by them. US tap water frequently tests positive for antidepressant and other drugs' traces. Also treatment plants generally use bacteria (i.e. bugs) to remove harmful stuff, not chemicals (except gases like ozone) to treat water IIRC.

> Are you saying that in an open loop system, none of the water makes it back into the water cycle?

What I say is, in an open loop system, you use much more water and poison your environment during the process. Closed loop systems doesn't waste much water and certainly doesn't leak nasty chemicals around. You forcefully remove heat from the water with drycoolers and/or chillers and you can use the heat for secondary purposes if you want/can, too.