Optimizing water treatment with two-stage coagulation

Drinking water utilities are facing increasing regulatory pressure to optimize treatment processes, particularly the filtration process that is relied on as a physical barrier to pathogen contamination. In the presence of high concentrations of natural organic matter, traditional coagulation processes can be inadequate to meet these future standards. In this study, a natural water with a high total organic carbon concentration (>6 mg/L) was used to compare one- and two-stage alum coagulation processes. The two-stage coagulation process involved the application of alum at two locations separated by approximately 60 s of detention time. Bench-scale experiments were conducted with various water quality conditions and verified at the pilot scale. The settled-water turbidity was 25% less using the two-stage process, and the steady-state filter effluent particle counts for the two-stage process were approximately 50% lower than for the single-stage process. More importantly, the two-stage process did not exhibit particle breakthrough during a typical filter run, whereas the one-stage process broke through 3 h before the hydraulic completion of the run. Additional bench-scale experiments allowed a mechanism for the improved process performance to be proposed.