Development of Final Sedimentation Tank Maximum Operating Capacity Curves Using Vesilind Settling Parameters and Mathematical Modeling

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Courtesy of Water Environment Federation (WEF)

New advances in development of maximum operating capacity curves using final sedimentation tank (FST) computational fluid dynamic (CFD) package – High Accuracy Clarifier Model (HACM©) and Vesilind settling parameters (Vo and k) were successfully applied in a recent project at the Sai Kung Sewage Treatment Works of the Drainage Services Department (DSD) in Hong Kong. The advances consist of more accurate maximum capacity predictions of the FST for a range of Vo and k determined on-site, instead of their estimation using generalized sludge volume index/stirred sludge volume index (SVI/SSVI) values with empirical correlations to minimize any erroneous capacity predictions. A set of standard maximum operating capacity curves of the FST suitable for future usage under year round operation were developed using representative operating ranges of mixed liquor suspended solids levels and return activated sludge flowrates and settling characteristics in terms of Vo and k. To make it more user-friendly, the SVI/SSVI and Vo & k correlation can also be established for the operators to input SVI/SSVI data to predict FST capacity/performance, particularly when sludge settling characteristics deteriorate/improve under high/low loading conditions. However, Vo and k values and the SVI/SSVI and Vo & k correlation should be updated every three to six months, or whenever a major operational and/or configuration change occurs in the STW affecting the biological process.

Standard FST CFD packages such as HACM© Model can predict FST performance accurately using SVI/SSVI data to be within +/-10% of the actual capacity. It is recommended to conduct FST stress testing to confirm the improved accuracy, if any, when the frequently updated Vesilind settling parameters (Vo and k) and the empirical correlation of SSVI to Vo & k are used as inputs to the models.

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