Assessment of aeration basin performance efficiency: sampling methods and tank coverage

A program of oxygen transfer efficiency (OTE) measurements using offgas analysis at the Tillman Water Reclamation Plant (TWRP) in Los Angeles has provided thorough documentation of the performance of fine-pore diffusers in an activated sludge plant. From 1991 to 1993, measurements were made in a uniform pattern that avoided both the ends of the aeration grids and the main pipes along the grid centerlines, which were more likely to be the locations of leaks that could bias the overall average efficiency estimates toward low values. This pattern was chosen in part because of the limited time available during these measurement sessions. When measurements were resumed in 1997 with additional resources, it was decided that several alternatives in sampling should be investigated. First, to assess the significance of the contributions of areas that previously had been avoided, a much more thorough coverage was performed. Second, a new “longitudinal” sampling scheme was tested, as an alternative to the “transverse” approach that had been used previously. This method not only allows making larger numbers of measurements within a similar time, but provides separate results for the left and right sides of a tank. Measurements were also made with the hood in fixed positions for periods of several hours, to check measurement stability and diurnal variation. In addition to the offgas measurements, data on plant operation were recorded from the field instruments and the control room. The data show enhanced efficiencies near the upstream and downstream ends of the grids, and in the areas beyond the ends of the grids. This is at least partly the result of known variations in diffuser fouling, resulting from air flows in the distribution pipes. The longitudinal results show an apparent left–right asymmetry, but this turns out to be an artifact, caused by changing air flows during the measurement session, as shown by the control room data. In the fixed position measurements, variability of several percentage points was observed on relatively short time scales. The plant data also corroborated most of the air flux from the offgas instrument. These observations show the results of going beyond the EPA recommendation to collect offgas from at least 2% of the surface of an aeration basin, particularly when the measurements are planned according to the structure of the air distribution system of the tank. They are steps toward enhancing the importance of OTE monitoring as an input to decisions to clean or repair aeration systems.