The BIOCOS® activated sludge process is operated at a constant water level during the whole process cycle, and therefore represents a hybrid technology (Biological Combined System) comprising features of continuous flow and SBR systems. BIOCOS, developed at the University of Innsbruck, Austria, is a cyclic activated sludge system with an aeration tank hydraulically connected to two alternating sludge recycling and settling tanks. The effluent valve of one of the settling compartments is open allowing the feed flow to the aeration tank to push out the supernatant water from the corresponding settler. No RAS pump-station or any electromechanical equipment for mixing or recycling is required — all phases of the operation cycle are exclusively driven by pressurized air from the same blower-source saving a lot of effort in operation and maintenance.