Impact of extended versus typical rapid mixing HRTs on continuous-flow coagulation-ultrafiltration

Although bench-scale coagulation-ultrafiltration (UF) studies are typically conducted using batch systems, continuous-flow systems allow operational parameters to be more readily evaluated and optimized over consecutive permeation cycles. When simulating coagulation-UF at bench-scale using continuous-flow systems, relevant reactor volumes require flowrates >= 1 L/min in order to achieve hydraulic retention times (HRTs) typical of full-scale rapid mixing (30 s to several minutes). In the present study, HRTs of 2 and 20 min were evaluated to determine their impact on particle properties (size, structure, and concentration) as well as UF performance (fouling resistance, reduction of natural organic matter) over a range of alum dosages. While not practical at full-scale, a 20 min HRT has been previously applied at bench-scale to reduce the overall volume of water required to conduct continuous-flow coagulation-UF studies. However, potential impacts on steady-state particle properties and subsequent UF performance not been clearly elucidated. Results from this study suggest that increasing HRT significantly increases steady-state particle size and concentration, as well as reduces total/hydraulically irre-versible UF fouling resistance and improves hydraulic cleaning efficiency. Despite reduced fouling when considering a longer HRT, significant differences in particle properties and UF performance suggest that HRT values equivalent to those typically applied during full-scale rapid mixing (30 s to several minutes) must be considered in order to produce results relevant to full-scale.