It looks like the current scheme for tagging “water-containing features” has certain significant inconsistency. This scheme originates from this proposal made by @Zverik back in 2011. I completely support the idea of having some uniform scheme for more or less natural water bodies, but this proposal also has an effect of deprecation of several tags, related to artificial, man-made or even industrial objects, where water is in use.

This issue has been addressed in original proposal discussion by @G0ldfish, but @Zverik didn’t answer his question at all.

After approval of this proposal, the whole landuse=reservoir scheme has been labeled as deprecated, because supposedly, we can now use natural=water water=reservoir to tag artificial water bodies instead of using landuse=reservoir. But it’s only partially true. Indeed, water storage reservoirs are often not that different from natural water bodies. But there still are objects such as cooling basins, technical water reservoirs, wastewater reservoirs etc. Important tag reservoir_type= is deprecated as part of the whole old scheme.

Please, keep in mind, that when we have to tag a cooling basin of an industrial facility, water itself is just a part of this object. Ideally, proper tag (something like former landuse=reservoir reservoir_type=cooling) for it should indicate that there could be a concrete basin with sprinklers and other equipment. It can even use some sort of cooling liquid, which is most likely water-based, but it is absolutely not necessarily a water.

Same situation with sewage and wastewater reservoirs. It’s quite hard to call its content “a water” - water usually is a product of long filtering process, and aeration in sewage reservoirs is just a part of it. So, initially, concentrated sewage liquid can not be called “water” by any standards.

So, here we have the case, where a proposal has not been discussed and tested in every detail, which leads to the situation, where artificial objects were forcibly brought into the same class with more or less natural objects. Another result of it is that now we have less detailed options for tagging artificial objects. Fortunately, depreciation doesn’t mean automatic re-tagging or prohibition to use this scheme (as it is mentioned in proposal text), so it is still possible to use reservoir_type=, for example.

Personally, I think there is certain sense in reviving man_made=reservoir tag for obviously artificial objects such as sewage treatment tanks, cooling basins and so on, where the whole vessel is formed by concrete or steel walls, since these objects have nothing in common with natural water bodies at all, being artificial and not containing a water.