Which device is used to physically separate solids from liquid in sludge, typically as a dewatering step?

• A. Belt press
• B. Dissolved air flotation
• C. Screens
• D. Plate press

Answer: (D)

Dewatering sludge means squeezing out as much water as possible to produce a drier solid cake. A plate press accomplishes this by using a stack of filter plates and filter cloths. Sludge is fed between the plates, which are pressed together, forcing liquid to pass through the filter media while the solids are retained and compacted. The result is a much drier sludge cake that’s easier and cheaper to handle, dispose of, or dispose of, or send to incineration.

This device is typically used after thickening to reduce volume and improve handling. Other options serve different purposes: flotation devices pull solids to the surface for removal rather than squeezing water out, screens remove large solids before treatment rather than dewatering, and belt presses do dewater but rely more on belt drainage and may yield a wetter cake compared to a plate press. The plate press’s emphasis on high-pressure filtration is what makes it the preferred dewatering step in many plants.