Effects of Operational Variables on the Oxygen Transfer Performance of Ceramic Diffusers

Controlled changes in the operation of two side-by-side activated sludge pilot plants investigated the effect of operational variables on the performance of ceramic (fine bubble) diffusers. The experimental plan manipulated the following operational variables: aeration system volumetric organic loading rate, sludge age, recycle rate, and the addition of an upstream aerobic or anoxic biological selector. Each pilot plant consisted of a full depth aeration column, aerated using a single ceramic dome diffuser, and a dedicated clarifier for sludge acclimation. The aeration influent channel at a large municipal wastewater treatment plant provided the influent wastewater to the pilot plants. An off-gas method determined aeration system oxygen transfer efficiencies. Fine bubble oxygen transfer efficiencies fluctuated throughout the study. A nitrifying aeration system, operated at an 8-day sludge age, exhibited significantly higher αF values compared with a nonnitrifying system operated at a 2-day sludge age. Differences in organic loading, over the range investigated (0.50 to 1.2 kg biochemical oxygen demand$[{\rm BOD}]_{5}/{\rm m}^{3}\cdot {\rm d}$), did not show an effect on fine bubble αF performance. At a short sludge age (2 days) and a high volumetric organic loading rate, a pilot system operated at a higher recycle rate (100% versus 33%) exhibited improved αF values. The addition of either an aerobic or an anoxic upstream biological selector (13-minute hydraulic retention time) improved ceramic diffuser oxygen transfer performance at high volumetric organic loading rates. For the site-specific wastewater, ceramic diffuser oxygen transfer performance improved as the extent of nitrification increased. Ceramic αF values also increased as the soluble organic content of the mixed liquor decreased, although it was not possible to predict an αF value based solely on the mixed liquor soluble chemical oxygen demand level or the extent of nitrification. Because of the higher αF values in nitrified versus nonnitrified mixed liquors, a much smaller than expected increase in process airflows was required to meet the increased oxygen demand of nitrification, lowering process energy consumption, and aeration system emissions.