The Mebane Bridge Wastewater Treatment Plant, an activated sludge wastewater system, serves Eden, N.C., a city of approximately 19,000 residents. The North basin is the main reactor basin, where aeration is supplied for mixing. It covers 1.74 surface acres with an operating depth of 13.5 ft. The system receives about 3 million gal per day, with a mix of 80% municipal wastewater, 10% industrial wastewater and 10% raw water. The average detention time in the basin is approximately four days.
The North basin used 240 hp of aeration to keep total suspended solids (TSS) in the 3,000 to 4,000 mg/L range, as required for this system. However, this amount of aeration also kept dissolved oxygen (DO) concentrations unnecessarily high at 6 to 7 mg/L, instead of more appropriate concentrations of around 1 to 2 mg/L. The oxygen transfer rate is almost zero at these levels, yet the aeration could not be turned down because TSS concentrations would drop too low. As a result, the city had very high energy costs.
In June 2009, one solar-powered SolarBee SB10000 v18 circulation unit was installed in the North basin, with the objective of reducing runtime for the high-horsepower aeration system. Once the unit was installed, aeration requirements were reduced 25% to 180 hp, resulting in a 60-hp savings while still maintaining the necessary high TSS levels and keeping DO in the 1 to 2 mg/L range. Because of the economic benefits and its ongoing effort to save energy, the city planned to install two additional SolarBee units in the North basin in fall 2010, with an anticipated total energy reduction of 120 hp, or an average of 40 hp per SolarBee. The savings are expected to result in a one- to two-year payback for the entire project. The city reported that it is happy with the energy savings achieved with the SolarBee, as well as with the ongoing after-sale technical support and customer service it receives.