The effluent from the flotation plant, which is virtually free of solids, is collected in a storage tank from where it is passed to the biological treatment system, the membrane bioreactor. The nitrification tank, installed downstream of the denitrification stage, is aerated by membrane aerators mounted on the tank bottom. The effluent from the nitrification tank flows by gravity into the filtration tank with the membrane module where the permeate is discharged at intervals by a submerged rotation membrane (vacuum rotation membrane) with a membrane surface of 108 m². The rotating membranes are the core part of the MBR system. Their rotation, combined with scouring air being introduced from the rotation axis, ensures increased removal of solids from the membrane surface and reduces the energy required for scouring air as the air is blown in at half depth. The MBR plant achieves high clarification efficiency with continuously low concentrations of below 70 mg COD/l. Due to this high effluent quality the treated water is suitable for further treatment or reuse.