Characterising sedimentation velocity of primary waste water solids and effluents

Sedimentation in waste water is a heavily studied topic, but mainly focused on hindered and compression settling in secondary sludge, a largely monodispersed solids, where bulk sedimentation velocity is effectively described by functions such as double Vesilind (Takacs). However, many waste water solids, including primary sludge and anaerobic digester effluent are polydispersed, for which application of velocity functions is not well understood. These systems are also subject to large concentration gradients, and poor availability of settling velocity functions has limited design and computational fluid dynamic (CFD) analysis of these units. In this work, we assess the use of various sedimentation functions in single and multi-dimensional domains, comparing model results against multiple batch settling tests at a range of high and low concentrations. Both solids concentration and sludge bed height (interface) over time are measured and compared. The method incorporates uncertainty analysis using Monte Carlo regression, DIRECT (dividing rectangles), and Newton optimisation. It was identified that a double Vesilind (Takacs) model was most effective in the dilute regime (<1%v/v), but could not effectively fit high solids concentrations (>1%v/v) without a substantial (50%) decrease in effective maximum sedimentation velocity (V₀). Other parameters (R_h, R_p) did not change. A power law velocity model (Diehl) was significantly less predictive at low concentrations, and not significantly better at higher concentrations. The optimised model (with reduction in V₀) was tested vs a standard (optimised) double Vesilind velocity model in a simple primary sedimentation unit, and resulted in deviation from -12% to +18% in solids capture prediction from underload to overload (washout) conditions, indicating that the effect is important in CFD based analysis of these systems.