Pretreatment removes suspended solids from wastewater. Screens and rotary screens are most commonly used in the pretreatment stage. A typical wire mesh screen retains solid fractions sized 10–30 mm.

Rotary screens remove solids with a diameter greater than 0.5 mm. They also protect equipment from fouling, clogging, and jamming. During pretreatment, 60% of suspended solids and 30% of BOD are removed from the wastewater.

Other preliminary methods include catch basins, flotation, equalisation, and sedimentation.