Analyzing the effects of different patterns of diffuser layouts on air distribution and mixing quality in an aeration tank using CFD

Abstract

A diffused aeration system in an activated sludge process is generally designed based on a limited number of typical diffuser patterns. There are instances where these patterns have shown various operational and process issues, resulting in poor process performance. To address these issues, bubble interactions and the mixing conditions must be factored into the design of alternative patterns. For this purpose, a validated full-scale CFD model was used to simulate five diffuser patterns: lateral, grid, line, stripe, and checkered patterns‎, the first two in operation, and the next three as designed patterns. The patterns were simulated for an aeration tank under identical functional conditions. The population balance method was incorporated into the model to analyze how various patterns affect bubble size distribution. It was found that changing the pattern of diffusers had a significant effect on bubble breakup/coalescence and the resulting bubble size distribution. The results showed a range of 6.1–22.9% for the coarse bubble percentage and 11.9–16% for the fine bubble percentage in the five simulated patterns. Moreover, evaluating flow circulation and velocity based on the average flow velocity data of the simulated patterns showed up to 30% variation, ranging from 0.29 to 0.375 m/s. It highlighted the great impact of diffuser patterns on bubble size and mixing quality as notable factors in oxygen transfer efficiency. Finally, the proposed patterns were compared and ranked based on air distribution, bubble size, and mixing quality criteria using the multi-criteria decision-making (MCDM) approach. It was concluded that the checkered pattern outperformed the proposed and existing ones.