Towards optimal design parameters for around-the-end hydraulic flocculators

Abstract

Hydraulic flocculators are principally characterized by their volume (which determines the time of flocculation) and the water level difference between inlet and outlet (which determines the average energy dissipation). Within these constraints, however, the designer has many degrees of freedom, such as the average water depth, the number and spacing of baffles, the length of the gap at the baffle ends, and the degree to which adjoining baffles overlap. In an earlier paper (Haarhoff 1998), these variables were systematically reduced to a number of critical ratios, and a comprehensive mathematical framework was presented whereby hydraulic flocculators can be designed once these ratios are fixed. In this paper, these ratios are further investigated by computational fluid dynamics (CFD) to find their optimal values. The validity of CFD modelling is first verified by comparing experimentally measured velocities in two flocculators against modelled velocities for similar hydraulic conditions. CFD is then used to systematically optimize the three critical design ratios for flocculator design. The optimal ratios are considered to be those that cause the least deviation from the average G -value; addressing the concern that there may be large G -value differences at different points in the flocculator.