DUODRAIN operates based on the principles of gravity drainage and mechanical pressure filtration. Polymers are added to the feed sludge via the ROEDOS flocculant dosing system and are rapidly mixed using inline mixers. The flocculated sludge then flows into the flocculation reactor, where under laminar conditions, floc formation occurs. From there, the flocculated sludge enters the ROEFILT drum thickener inlet, where a sludge roll begins to form.

Water in the sludge is separated through gravity and drained through the filter mesh. The clean filtrate is reused as spray water to clean the filter fabric. As sludge progresses through the drum, a helical screw conveys the increasingly thickened sludge toward the outlet. It is then discharged either to a belt press or via a sludge pump. The ROEPRESS belt filter press dewaters the sludge in three phases: (1) gravity drainage zone between upper and lower belts; (2) wedge zone with high-pressure rollers applying strong compression and shearing force; and (3) S-zone where residual water is removed under alternating tension, producing a high-solids cake ready for discharge.