Performance comparison of complete mix reactor and static mixer in ballasted flocculation

Abstract

In this study, the performance of complete mix reactors and modern instant mixing device (static mixer) is evaluated for ballasted flocculation application using jar test to replicate complete mix reactor, and a lab scale static mixer designed for this study. The advanced magnetite ballasts of two varied sizes and specific gravity at varied applied concentrations are employed in this study for reliable results. The results explicated that static mixer aided ballasted flocculation aggregated uniform flocs with higher ballast embedment leading to high-rate clarification and superficial sedimentation for both types of employed ballasts. Moreover, the advanced high specific gravity ballast B (Size: 33um/specific gravity: 5.5) exhibited higher settling performance (97 mh−1) and turbidity removal (98 % in 900s settling interval) despite its lower embedded fraction. This revealed that ballast specific gravity drives the settling mechanism and differential sedimentation of these aggregated flocs leads to high-rate clarification. Furthermore, the investigation of embedded fraction of ballast in the floc volume and fraction of unballasted aggregates in overall floc distribution under various conditions revealed that jar test replicating complete mix reactors in this study is uncapable to suspend complete fraction of employed ballast material due to limitation of lower G-value operation (up to 500s−1) and uneven agitation in various areas of tank due to paddle-based mixing. This leads to poor ballast embedment (up to 1 % of overall floc volume) in aggregated flocs, compromising sedimentation performance. The higher fraction of unballasted aggregates (2.5 % in overall floc distribution) due to this poor ballast suspension and uneven agitation amplifies the relatively poor sedimentation phenomenon (achieving settling velocity of 77 mh−1) of jar test aggregated flocs. On the other hand, a static mixer exhibited compact flocs with a higher and relatively uniform fraction of embedded ballast in floc volume (up to 2.4 % of overall floc volume), and lower percentage of unballasted flocs (1.3 %) in overall aggregated floc distribution due to employed homogenous agitation (achieved due to design of static mixer) and complete suspension of incorporated ballasts. This leads to relatively less ballast requirement as optimized dose during aggregation of flocs, less GT (indirect energy input scale) and space requirements in ballasted flocculation operation.