High purity oxygen supplied either as a liquid delivered to site, or as a gas generated on site, can be applied effectively to aerobic biological waste water treatment processes to allow the Dissolved Oxygen content of the plant to remain at optimum levels, even during peaks of BOD, COD and ammonia. Air Products' Oxy-Dep range of technologies provide efficient oxygenation (usually >90% Oxygen Transfer Efficiency), where high loads exist.

Oxy-Dep also provides a reduction in odours, surface foaming, aerosols and VOCs, as well as improved sludge settling and dewatering, either when retrofitted, or in a purpose-built activated sludge plant. Oxygen is often successfully used in combination with aeration.