Hi

I have a dishwasher which puts it's waste out into an open stand pipe.

This stand pipe then connects to the sink waste and flows down a long
40mm pipe across under my kitchen floor (concrete and tiled) and to an
outside drain.

I have a problem in that when soapy water is drained down the sink, i
can see bubbles and a small amount of water flowing out of the top of
the stand pipe. I have made the stand pipe as high as possible, but
the problem still happens.

I've checked for any blockages, and tried clearing the pipe just to be
sure with Caustic soda etc.

I'm pretty sure that because the fall of the waste pipe isn't that big,
water is favouring the easiest route and partially going out of the
stand pipe.

Is there any way that I can seal the top of the stand pipe that the
dishwasher outputs into, or does anyone else have a suggestion that
will save me having to dig up the concreted kitchen floor!
Many thanks

Charles