Raw water enters the central draft tube above the re-circulator impeller where it is mixed with treatment chemicals and re-circulated precipitates. Precipitate re-circulation is accomplished by a variable speed impeller which acts as an air lift pump. The mixture of raw water and sludge rises through the central draft tube and is discharged into the flocculation compartment.
This mixture undergoes gentle hydraulic turbulence as it passes through this zone.
Part of the water (equivalent to the instantaneous influent flow rate) enters the clarified water zone and rises toward the effluent collector.
Settled precipitates (sludge) are moved continuously along the floor toward the centre of the unit by means of a slowly rotating scraper, which covers the entire floor area. The accumulated sludge is transferred to the sludge pit at the centre of the unit, where sludge-thickening pickets concentrate the sludge, reducing the total amount of blowdown.
Excess sludge is removed by a blowdown system. Backflushing with water under pressure clears the blow-off line. Opening of the blow-off valve permits the sludge to flow to waste. These operations are either manual or automatic. Most of the water and suspended precipitates enter the lower end of the draft tube to be re-circulated, providing positive solid contact regardless of sludge inventory level. Clear water rises and is uniformly collected.