Our dishwasher is dual drawer, and hence two waste pipes. They are fed into the top of a standing pipe in the kitchen, more or less the same height as the top of the dishwasher. That pipe goes directly into the (more or less) horizontal waste pipe. The question is, should the vertical pipe have a u-bend (or s-bend I guess) in it somewhere ? It gets a bit smelly and as the horizontal pipe also drains the sink, when I empty the sink it can force foam out of the top of the dishwasher drain pipe (I tend to overdo the dishwashing liquid somewhat).