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We've just been installing an IKEA kitchen. The sink came with a trap
with 2 appliance waste connections, so I connected the dishwasher and
washing machine to these.
It looks like fluff from the washing machine is ending up on the dishes
- maybe some waste is flowing up the dishwasher waste when it empties.
(I haven't noticed food on the clothes yet!)

This seems to the expected way to connect, so I didn't think there
should be a problem. Previously they were connected to the waste pipe
with fittings that cut into the pipe and incorporated non-return
valves.

Screwfix and local hardware shop don't seem to have non-return valves
to fit in line.

Anyone else encountered this problem?
The problem is syphonage. To stop it you have to provide a way for air to
get in to each of the waste pipes. The usual way is to have a vertical piece
of plastic waste pipe. This has a u-trap at the bottom and is open at the
top. You can buy them readymade at merchants. You then poke the end of the
waste pipe from the dw into it. Its a good idea to secure it perhaps with a
cup-hook in the wall onto which you push the hole you normally get on the
crook in the waste pipe. Never happened to me but I'm told that the pipes
can jump out. Yuk! Do the same with the wash machine. Hey presto, no
syphoning. BTW best not to run the machines at the same time. A long waste
pipe can be overloaded and back up. Again yuk! This gets worse after a year
or two when the pipe starts to get clagged up.

Peter Scott