It is now,as far as I know,illegal to put your sewerage ( Poo) drainage down the same drain-system as your "grey " ,viz. shower /bath/ washing up system.( But not necessarily "vice versa ")-- Here in Henley,we still have houses within the Thames Valley area which /who/ are not on mains drainage when it comes to poo-poos.So it is quite common to see a big blue tanker ,with the logo of an Elephant with its trunk down a manhole-cover,running around and sucking up (as it were) to their clients.Telephone no. available if required !!