The VSB treatment system is a high rate activated sludge process in which the shallow aeration basins of 3 to 10 m (9 to 30 ft) are replaced with deep subsurface reactors of 90 to 250 m (270 to 760 ft). In addition, the North American version of the VSB process utilizes dissolved air flotation for final clarification of mixed liquor suspended solids. According to an EPA report (4), the most suitable equivalent technology for comparison purposes is the enriched oxygen process (pure oxygen). Aside from other similarities, the pure oxygen system is usually designed to operate with high mixed liquor suspended solids (4,000 to 6,000 mg/L) and with a high dissolved oxygen concentration (5 to 7 mg/L). These design features allow the bioreactor to operate under high organic loadings (F/M) end with reduced aeration volume similar to those achievable in the VSB process.