Effects of chemical amendments on aquatic floc structure, settling and strength

Abstract

Using a shear-cell/flow-cell combination integrated with an inverted microscope, the behaviour of Hamilton Harbour sediments was studied mixed with three different amendments: alum, chitosan (both coagulants) and a polyacrylamide (a flocculant). Samples from the shear cell were drawn into the flow cell, where floc structure and size were assessed throughout the floc formation and breakage stages using computer image analysis. Settling velocity, density and porosity were also assessed, with results suggesting that amendment addition may be an effective method for the management of high-turbidity environments, provided there are no toxicological effects. In an assessment of performance, it was found that the polyacrylamide flocculant showed the greatest promise in reducing turbidity levels as it produced the largest flocs with the highest settling velocity. Although more prone to break-up, these flocs still remained larger than those formed with alum or chitosan at the same shear. All flocs, regardless of amendment, broke up due to a fracture mechanism rather than by microscale erosion. By improving our understanding of how these amendments may influence floc properties and behaviours, more effective management tools may be developed for the remediation and control of high-turbidity aquatic environments.