Clarifier CFD modeling is relatively common, however, most secondary clarifier studies are conducted in an activated sludge application. Trickling filter effluent (TFE) has different characteristics than conventional activated sludge. To the knowledge of the authors, a similar case study to the one presented here for the evaluation and optimization of TF clarifiers has not been presented before. This study is unique and presents a detailed analysis of settling and flocculation properties of TFE and the application of secondary clarifier CFD modeling to establish clarifier capacity and identify optimization strategies.
The City of Salem’s Willow Lake Water Pollution Control Facility (WLWPCF) has a permitted capacity of 35-mgd average dry and 155-mgd design peak wet weather flow. The City budgeted for clarifier rehabilitation due to ageing mechanical equipment but desired to understand clarifier capacity limitations and evaluate whether modifications could expand existing available capacity.
Hazen conducted stress tests and developed calibrated CFD models for the secondary clarifiers. Model calibration was based on an extensive clarifier testing protocol to simulate peak clarifier loadings, characterize sludge settleability and flocculation properties, and evaluate performance. This work included field testing, zone settling, flocculation and dispersed solids testing.
After development of the CFD models, the clarifiers were evaluated to determine available capacity with the current geometry and mechanism type. Optimization strategies such as the addition of energy dissipating inlet wells, modifications to the flocculation well sizing, and the addition of baffling were evaluated. The City used these results to tailor capital planning for clarifier rehab projects and re-evaluate wet weather capacity and operating strategies at the WLWPCF. Based this work, improvements were identified to potentially expand the combined clarifier peak flow capacity from approximately 105 mgd to over 140 mgd.