The airlift pump discharges into the center of the washbox where the waste solids and media are separated. Solids removal is accomplished as a small portion of filtered water flows up into the washbox in an attempt to equalize the weir controlled differential water level between the filter tank and washbox.

The upward velocity of the water keeps the low density solids in suspension so they can be discharged over the reject weir, while the higher density silica sand settles through the counter-current flow to the top of the sand bed.

For most applications, the reject rate will range from 2% to 10% of the total flow.