Yes. In Silicon Valley, if you go to Bedwell Bayfront Park, which is behind Meta/Facebook HQ, there is, on the bay side, a small sewerage treatment plant. There's a fenced concrete-lined pond, usually empty. That is a sewerage overflow containment pond. It's next to a hiking trail, so it's easily visible to the public. That whole park, by the way, is a recycled garbage dump. So is the bay side park behind Google HQ.

Wastewater plants have other ponds and tanks which are part of the process, and they're usually full, with liquid moving in and out, accompanied by stirring, air, and chemical injection. A big empty one is a backup system.

Real engineering.[1]

[1] https://www.epa.gov/sites/default/files/2014-09/documents/la...

> BTW this likely means quite a bit of land used up by such a reservoir which is ideally never needed, but must be present

This is enough to earn chemical plants a spot on a future "BANNED in California 2" article, because it's "clearly" overregulation.