Is an aerobic process for treating biodegradable wastewater to a high quality effluent and plants can be designed for simple operation and reduced attendance.

The displaced mixed liquor passes through to a separate settlement tank where the biomass is settled out and returned to the aeration tank to provide the acclimatized activated sludge for continuing treatment. Clarified treated effluent is discharged to a watercourse (subject to approval from regulating authorities) re-used for another non-human consumption e.g. irrigation or recycle for toilet flushing after disinfection.