The curtain biological filter includes immobilized biomass also called a biofilm, developed on the vertical surface of a loosely falling partition made of plastic, favorably in the form of ropes ( 1 ). The ropes ( 1 ) are fiber ropes of plastic, favorably of polypropylene and/or nylon and/or polyethylene with gauge from 5 to 50 mm. The ropes ( 1 ) are arranged at a distance from 1 d to 10 d, where d means the rope's diameter. The rope ( 1 ) used for making a curtain may have the form of a twisted or braided rope. The curtain for settlement of immobilized biomass is placed in a steel frame ( 2 ) of acid-resistant steel. In turn the steel frame ( 2 ) is placed in guides ( 3 ) resistant to corrosion caused by water or wastewater fixed inside the tank ( 4 ) for treatment of water or wastewater.