The Fourche Creek Treatment Plant in Little Rock, Arkansas, produces its own electricity. So the staff cannot afford to waste it. This 16 MGD plant runs two 800 horsepower generators powered by a combination of digester gas and purchased natural gas to supply all of their electrical needs.
Since oxygen transfer accounts for most of the operating energy cost, fine bubble diffusion was chosen by Greeley and Hanson for the plant expansion in 1989. To maintain the efficiency of the system, Clyde Frost, plant superintendent, tracks the air usage in CFM/#BOD. Figure 1 shows the effectiveness of the aeration over the past several years.
The transfer efficiency shows significant improvement (reduction in air usage per pound of BOD) in November ’97 and again in September ’98. In the fall each year, when the sewage temperatures drop, the oxygen transfer rate is much better. Sewage temperatures vary from 14°C in the winter to 24°C in the summer.
With the start up of the activated sludge process in 1989, the plant ran into a problem. The pH fell below the effluent limit of 6.0, which caused a violation. The plant had begun to nitrify, which suppressed the pH. Lowering the SRT might help. It was time to consider the trade-offs inherent in the activated sludge process.
The plant was using data management software by OPS Systems in Milwaukee for reports including the DMR. The software has a process modeling utility that can show the amount of oxygen saved by lowering the SRT. This tactic would not only prevent nitrification but also save energy. The model also can show the effect on sludge production of a reduction in SRT. In this case, we would expect a significant increase in solids production. But Fourche Creek uses these solids to produce energy. Further, they have plenty of efficient solids handling capacity with four 175,000 gallon holding/thickening tanks, eight 1,000,000 gallon anaerobic digesters, and four 15,000,000 gallon sludge storage basins. The solids are then land applied at a concentration of 7% to 10% solids. The cost is around $70 per dry ton. This analysis resulted into a goal of 1.5 days SRT.
To maintain an SRT of 1.5 requires tight control of the wasting. Fortunately, the OPS software contains a function for determining the recommended waste flow. Figure 2 shows a typical report.
Using this daily report, the plant has been maintaining an SRT of 1.503 with a standard deviation of 0.5374. Figure 3 shows the results of this practice.
With only a few excursions over this two-year period, the plant has met its objectives. It has truly controlled its energy use to a bare minimum. The pH has been maintained at 6.7 to 6.9.