 The drum is pre-coated with a filter aid, typically of diatomaceous earth (DE) or Perlite.After pre-coat has been applied, the liquid to be filtered is sent to the tub below the drum.The drum rotates through the liquid and the vacuum sucks liquid and solids onto the drum pre-coat surface, the liquid portion is "sucked" by the vacuum through the filter media to the internal portion of the drum, and the filtrate pumped away.