Strange connection on sink bottle trap

It's not a connection. It's an anti-syphon trap. Underneath is a small one-way air inlet valve. The cap on the top is simply for cleaning it out if necessary.

Owain

Thank you. I looked up bottle traps on Wikipedia and various plumbing sites, and didn't see any reference to that sort of anti-syphon valve.

I know about the problem of syphoning and also of a build-up of *positive* pressure downstream: the bathroom washbasin and bath (which have been in this configuration for about 20 years) started interacting recently, so letting water out of (for example) washbasin would blow the water out of the bath U trap and also start to fill the bath since its plughole was lower. Both would begin to drain quickly then after a minute or so the flow would reduce to almost zero. We'd assumed that it was a blockage downstream of the bathroom, but it turned out to be air pressure building up in the pipe, which was solved by inserting an upward vent to the atmosphere (like a smaller version of the vent pipe on a toilet soil pipe).

The plumber wanted to drain the bath into the soil pipe rather than the communal rainwater soakaway for the village, until I asked whether it was a good idea to drain *large* amounts of water into a septic tank, and that maybe this was why the bathwater (and kitchen sink) drain into the soakaway. Having improved the flow of bath water through the pipe, it now eventually builds up in the underground soakaway pipe (probably a wide earthenware pipe) but it only trickles out of the hopper and can drain through gravel into the earth by the house - it's not so bad that it flows onto the paving stones, so it probably can't *quite* cope with the peak flow,

I presume a *good* soakaway (which ours evidently isn't!) is normal practice for bath/kitchen "grey water" if a house has a septic tank, to limit the septic tank to just toilet waste.