Mist washing generally takes 6 - 8 hours and with the set up shown left; the water should be periodically drained to prevent the tank from overflowing. The wash water drain valve can be cracked open to allow a continual outlet for the water, but care should be taken that the tank doesn’t drain to a point were the biodiesel is lost. From this point of view a ‘GL wash tank’ is a far better arrangement and is shown below.