Oxygen transfer comparison of jets and coarse bubble aeration in concentrated sludge

The South Truckee Meadows Water Reclamation Facility (STMWRF) in Washoe County, Nevada, commissioned a biosolids facility with jet aerated aerobic digestion. The jet aerators were not performing as designed, so they were tested on-site in the new tanks in both clean and process water according to ASCE standards. The aerators failed by substantial margins. To make up for the aeration shortfall, Washoe County elected to replace the jet aerators with coarse bubble aerators and to add the additional blower capacity necessary to meet the oxygen requirements in this configuration. After partial replacement, with one basin containing coarse bubble diffusers and the other still containing jets, the efficiency of both systems was tested in process water. The jets and coarse bubble diffusers had similar results for OTE and αSOTE, whereas the coarse bubble diffusers had substantially higher aeration efficiency after accounting for the jet pump power draw. Overall, the project demonstrated the deleterious effects of highly concentrated non-Newtonian sludge on the coalescence of fine bubbles and ultimately jet aeration efficiency, confirming the incompatibility of fine bubbles and thick sludge. Our results can be extended to other bioreactors operating at MLSS concentration above 1%. PRACTITIONER POINTS: Fine bubble systems perform worse than coarse bubbles in thick sludge due to bubble coalescence. The alpha factor is a function of sludge thickness; hence, claims of constant alpha are unrealistic. Polymers and additives for sludge thickening should be carefully evaluated because they can affect adversely oxygen transfer. Independent verification of aeration system manufacturer claims prevents design issues and subsequent disputes.