For secondary clarifiers that require rapid sludge removal, spiral-shaped scraper blades that have an angle of attack of 30 degrees and positioned to effectively transport sludge to a centrally located hopper or sludge manifold may be applied.  Spiral flights vary from a minimum at the tank periphery to a maximum depth at the opposite end near the center.  The maximum depth of the flight is determined by sludge transport calculations.

For greater flows and the resultant larger basins, higher torque capacities, a spur gear driven pier-supported collector is best applied. Walker Process pier-supported collectors are built around a center pier which supports the entire rotating portion of the machine plus half of the access bridge load. In addition to providing support, the pier resists the torque developed by the collector's sludge plowing action.

Bridge-supported collectors have as their basic support a bridge that spans the entire clarifier basin diameter.  The entire rotating mechanism is supported mid-span by the bridge that must be designed to limit deflection to less than ½ inch to prevent the scraper arms from contacting the floor.

Bridge supported collector mechanisms may be designed with Segmented Plow type scrapers, Spiral Rakes, Tapered Suction Headers, or SightWell type sludge removal arrangements.