Restrictions in water treatment by conventional processes (coagulation, flocculation, and sand-filtration) following scenarios of dam failure

Dam failure events can result in a high suspended solids concentration in surface waters. To understand the impact of high suspended solids concentration on the treatment process of water treatment plants, this study investigated coagulation-flocculation stages using different coagulants (ferric chloride and a polymeric coagulant - CAT-FLOC 8799 Plus) and solids concentrations (2000-46,000 mg/L), and how they impacted on the mean diameter (d50), density (rho s), and settling velocity (vs) of the agglomerates formed. After the sedimentation pro-cess, the turbidity values varied between 201-26 NTU when FeCl3 was used as a coagulant, and 36-11 NTU when using the organic coagulant. The study reinforces the effect of particle diameter on sedimentation velocity, which is lower for cases involving smaller average diameters. Even so, solids removal was high regardless of the initial conditions and characteristics of the agglomerated particles, starting early in flocculators with removals >80 %. The results point to the need for frequent maintenance of these units to remove the sludge settled, which could compromise the operation of water treatment plants. In this sense, two alternatives were proposed as pre-and post-treatment. Under high total solids concentration (>20,000 mg/L), pre-treatment with thickeners becomes mandatory, and ultrafiltration comes as an additional barrier that will prevent high turbidity values in drinking water. In this study, the use of ultrafiltration membranes in partial or total replacement of sand filters showed stable performance (permeate flow: 130-136 L/m2h) with no tendency to fouling and guaranteeing a residual turbidity value <0.3 NTU. Future studies should prove the technical and operational feasibility of the strategies proposed, which until then showed promising results to guarantee the operation of water treatment plants in adverse scenarios.

Automated summary

It is important to understand the impact of high suspended solids concentration on water treatment plants. This study investigated the coagulation-flocculation stages and the settling velocity of agglomerates formed using different coagulants and solids concentrations. The results showed that particle diameter and initial conditions did not significantly affect solids removal, but frequent maintenance is needed to remove settled sludge. Pre-treatment with thickeners and ultrafiltration can be effective in maintaining water quality in adverse scenarios.