What’s the best tag to use please (if there is one) to indicate sewage storm overflows? (These are places where, in times of high rainwater volume that is beyond the capacity of the sewage system, raw sewage is discharged into bodies of water - rivers, lakes, the sea).
Edit: I have at last found the following:
man_made=outfall
substance=sewage
outlet=bell_mouth
Or
outlet=direct
in the UK at present natural=water, water=river 
More seriously a suitable outfall/outlet tag.
Ok thanks, I think I have it sorted now, see my edit to my original message. However, I have a query about outlets that have a one-way valve. Usually, a hinged flap that only opens when there is pressure in the sewage pipe. It’s to prevent river water entering the sewage system in times of flood: should I use the valve outlet value, even though this explicitly suggests that there is a means of controlling the outflow, which there isn’t? Or direct? Or neither? We really want a one-way value for the outlet key?
Edit: The official designation is “flap valve”, according to the vendors of these devices.
I could have sworn I’d uploaded photos of some either to the OSM wiki or Wikimedia but can’t find them. Virtually, all the outfalls/outlets on the culverted section of my local river are of this form. I think most discharge grey water rather than sewage, but have no idea where the drains run. This is one example:
I discover I neither tried to add the information they had flaps, nor the reference numbers painted next to them on the walls of the culvert. 
I’m not convinced by “no subtance on outfall” from the wiki: often the outfalls are the only part of the network visible enough to be mapped. The recommended, rarely used tag is outlet=valve, but with only 19 uses I see no problem is using flap_valve instead. I suspect the overall tagging of this type of infrastructure will need revision once more have been mapped, but the key thing is to capture the basic data now.
Thanks. Some are known sewage outlets by virtue of being on the water company maps labelled as such, and some by being photographed in action, but yes, some are just rainwater runoff.
