Li Guibai

Flocculation and sedimentation of high-turbidity waters

Abstract High-turbidity waters, containing up to a few percent of suspended solids, occur quite widely and present considerable treatment problems, especially because of low sedimentation rates. Such waters are normally treated by sedimentation and the rate can be greatly increased by the use of polymeric flocculants. The degree of flocculation achieved and the improvement in settling rate can be markedly affected by the dosing and mixing conditions. These effects have been studied with stirred clay suspensions, using a simple, flow-through monitor which gives a rapid indication of the degree of flocculation. The same monitor has been used to derive sedimentation rates, which show good correlation with the flocculation response. For high solids concentrations and relatively low polymer doses, flocculation occurs rapidly, but the flocs are not stable and can be broken at moderate stirring rates. This leads to a maximum floc size and a corresponding maximum settling rate soon after dosing the suspension with polymer. This peak is most pronounced with the most concentrated suspensions. By reducing the stirring rate after a short period, the flocs can be maintained at their peak size, without break-up. In this way, appreciable flocculation can be achieved with much lower polymer dosages (about 100 μg/g, based on the weight of solids) than are usually reported.

EFFECTS OF DOSING AND MIXING CONDITIONS ON POLYMER FLOCCULATION OF CONCENTRATED SUSPENSIONS

A simple monitoring technique has been used to derive information on Hoc growth and break-up in agitated clay suspensions of quite high solids content (1 and 3% w/v), under the influence of a high molecular weight cationic polymer. For fairly low polymer dosages and high mixing speeds floe growth in the 3% suspensions can be very fast initially, but the floes rapidly break up and do not re-form, even when the stirring speed is subsequently reduced. By reducing the mixing speed very soon after polymer dosing (typically after a few seconds), the break-up can be prevented and floes maintained at larger sizes. This effect is much less apparent for 1% solids suspensions, where longer periods of rapid mixing can be tolerated without significant floe break-up. For the higher solids case, great improvements in flocculation can be achieved by modifying the polymer dosing procedure—either by adding the solution over an extended period or by dosing a more dilute solution. These changes have much less effect at the l...