A system for treating wastewater includes a basin to contain wastewater, an aerator provided in the basin to mix the wastewater, and a cover system to cover a surface of the wastewater contained in the basin. The cover system includes a plurality of hollow bodies disposed on the surface of the wastewater. Each hollow body is adapted to float on the wastewater's surface, and has a contiguous outer surface and a sufficient mass so that 30 to 70 percent of the hollow body's outer surface is exposed to atmosphere during use. A method of treating wastewater is further disclosed.