Developing a model to determine the settling velocity of ballasted flocs. Issue 6 (December 2020) - Qasim, Muhammad
Graphical abstract: Highlights: Settling velocity cannot be observed directly for each individual floc. Existing settling velocity models were inadequately correlated with floc size. Ballast induction produces denser flocs with much higher size. Floc density is major driving factor in ballasted floc settling phenomenon. A new empirical model is proposed to effectively predict floc settling velocity. Abstract: Ballasted flocculation is an advanced clarification process capable to treat exceed of colloidal and suspended particles in natural water resources caused by climate change. The settling velocity of aggregated flocs is the most important design parameter in BF facilities. The ballasted flocs exhibit hybrid properties due to solid (ballast) and porous (coagulated suspended particles) fractions of internal structure. Therefore, the transition of ballasted flocs in fluid medium exhibits different phenomenon compared to conventional flocs. Therefore, it is imperative to investigate and model the transition of ballasted flocs in fluid medium using real time camera observations. During the past decade, many attempts have been made to develop models exhibiting settling phenomenon of ballasted flocs. However, the developed models underestimate the settling velocity due to a lack of real-time observations of the settling behavior of ballasted flocs. In this study, real-time observations of the settling velocity of individual floc were conducted by adopting an image analysis method. The observed experimental velocities were used to compare and modify the existing floc density and settling velocity models. The modified velocity models based on existing correlations insufficiently predicted the settling velocity variations of ballasted flocs. Therefore, we propose a model that extends the widely used Stokes' law with the modified Tambo and Watanabe model to precisely predict the settling velocity of ballasted flocs. Statistical analyses (70% correlation) illustrated that the proposed model is the most suitable approach for predicting the settling velocity of ballasted flocs.