Modifications were made in 1986 to the surface aeration sewage treatment plant at Blackburn Meadows in the United Kingdom to improve aeration efficiency. The four original lanes were each powered by four identically sized 37 kW aerators. The aerators in three of the lanes were replaced with a tapered system designed to match the decreasing demand for oxygen through the aeration lanes. In addition, dissolved oxygen (DO) control systems were installed to reduce over-aeration at periods of low loading. The control options selected were: Lane 1 -Outlet weir level control; Lane 2 -Outlet weir level control + on /off switching of aerators; Lane 3 - Variable speed control + on/off switching; Lane 4 -No control. The modified plant has operated very successfully. The most significant savings, compared with unmodified plant, were made in Lanes 2 and 3. Energy savings in these Lanes were 40% and 33% respectively, and annual cost savings 46,000 pounds/year and 37,900 pounds/year respectively. Other benefits area: 40% increase in throughput from 61 M1/day to 85.2 M1/day and improved effluent quality. (author).