You should check no taps are on or dripping and no overflows are active, if all is ok do your pipes from the macerator run away and up a wall or down or just along a wall, if your macerator is pumping waste upwards at any point then maybe the none return valve is defective, but much more likely its a faulty pressure switch not detecting that the unit is empty, best advice call a engineer, try this company at the time of writting this they had a fixed price of just £85.00 http://www.advancecare.co.uk/saniflo1.html if that address is not clickable just copy and paste it into your url
David Davis
March 2009