Non-Return Valve for Drain?

[John C]

I have the ground floor flat in a four-storey block. Occasionally the kitchen sink fills with suds which seem to be caused when someone else's washing machine empties. There is very little water just suds.

The sink drain and the buildings drains have all been cleared and when the contractors pour gallons of water into the drainage system nothing comes back up into the sink. It has been suggested that the design of the drainage is allowing soapy water to fall down the pipe and then hit a bend where the suds are being formed.

Does anyone know if it is possible to buy a one-way or non-return valve for the sink drain (after the U-Bend) that would stop this happening?

Thanks

John C

[Palindr☻me]

IANA plumber but it sounds like they did the wrong test - it is not so much whether water came back into the sink, it is whether a temporary suction was formed which emptied your sink trap. Once empty, of course suds in the pipe will find their way into the sink as there is nothing to stop them.

With the U bend full of water, which it should be at all times, no suds should be able to appear.

If that is the case, they should either be looking to avoid creating the suction in the first place, by correct design and venting of the down pipe or should be fitting an anti-syphon valve to your drain (and not a non-return valve). These are cheap and easy to fit and basically let air into the pipe if a vacuum is forming - but don't let gases(ie smells) out if there is a positive pressure instead.

[John C]

Sue

That makes sense! I had assumed that there was enough 'back pressure' to force the water back up from the U-bend as there is enough pressure in the suds to lift the stainless steel "crumb catcher" off the drain and sometimes there is some water forced into my sink.

The flats are about 40 years old although this is a fairly new problem so I am stuck with the current design. I will try the anti-syphon valve and see if that fixes it.

Thanks for the quick response.

John C