Waste water draining from the sink
Plumbers, for speed and their convenience
will often connect the washing machine drain hose
directly to the trap under the sink. This can sometimes cause the
waste water draining from the sink to flow into
the washing machine rather than going to the main drain.
Raise the height of the waste
Correct the problem by simply raising the height of
the washing machines waste hose at some point so that it is higher than the
outlet pipe from the sink.