The role of polymer in improving floc strength for filtration.

Dosing polymer to improve floc characteristics is a widely practiced method in water treatment to improve floc strength, and there is strong operational evidence showing the benefit of polymer dosing. However, there is a paucity of information on how polymer operates in terms of quantifying the resulting floc size and strength over different size scales. A dual particle sizing approach was used to monitor large floc that contain most of the sludge volume and small floc that can cause downstream treatability problems for systems with and without polymer dosing. The polymer investigated was a slightly anionic polyacrylamide dosed in water collected post dissolved air flotation at concentrations of 0-0.03 mg L(-1). With increasing polymer dose, median floc size increased from 228 to 325 microm. Floc responses to increased shear rate showed that polymer dosing increased resistance to floc break-up. While all of the flocs showed high potential to regrow, regrowth was greatest in polymer-dosed systems, where flocs exceeded the size that they had reached previously. Increasing the dose of polymer showed increased removal of small particles (<8 microm) showing that polymer was able to effectively remove particles that are liable to cause downstream problems.