Gravity dewatering begins as the bed is filling. The clear liquid, separated from the flocculated solids flows down through the porous media, through the support plenum and out of the bed structure.

After the bed is filled to maximum liquid level, the conditioned sludge feed is shut off. The vacuum pump is started creating a vacuum in the plenum and media causing a uniform pressure on top of the cake. Motorised filtrate drain valves automatically controlled by level sensors discharge the filtrate from the plenum of the VSDB. Typically, filtrate suspend solids are less than 30 mg/l, indicating very high solids capture in the dewatered sludge.

As the sludge continues to consolidate and shrink, the resulting cake will start to crack. This will continue until the bed is uniformly cracked and the vacuum gradually lost. As the plenum area loses vacuum the vacuum pumps shut down.The stop-gates can now be removed to allow a front-end loader access to the bed to remove the dewatered sludge.

The surface is then washed down with high pressure, low volume water. This washdown water is discharged to the sewer or directed back to the head of the plant.

Following cleaning, the stop-gates are placed beck into position and the facility is ready for another dewatering cycle. A complete dewatering cycle is typically 24 hours.