Kim, Jinho; (2002) Floc properties in stirred suspensions. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The present work was carried out in order to introduce a newly modified light transmission technique and to measure the properties of floes in a test using this technique. The changes in solution and hydraulic conditions have been made in a systematic manner to show how these factors play a role in the flocculation process under typical water treatment conditions. The experimental results presented suggest that, following a pre-test routine under standard conditions, the procedure can characterise the flocculation behaviour and give information on floe density changes, depending on the solution or hydraulic conditions. The principle of turbidity fluctuations, which is well known for simple experimental procedure, is employed for monitoring. Basically, it measures the intensity and fluctuations of light occurring when suspension flows through a light beam. The data obtained directly from the monitoring have been used to derive parameters representing the characteristics of flocculation and the floe properties. It allows a convenient means of real time determination of floe properties. Previously used commercial PDA (Photometric Dispersion Analyser) has been modified to reduce forward scattered light, by using a better collimated light beam. The tests have been carried out in stirred suspensions of latex, kaolin and two different kinds of silica particles with selected polymers and electrolytes. To investigate the influence of solution conditions, the standardised pre-test routine has been performed to observe the basic flocculation characters shown in polymer concentration distribution. During the pre-tests, the relationship between polymer concentration and the flocculation rate has been shown. During the main tests, it is found that (1) cationic polymer with high molecular weight leads to extremely high degree of flocculation while the speed of the flocculation depends greatly on the concentration and charge density of polymers. (2) the charge density of polymer also plays a role in determining the optimum polymer dosage, although its effect on the floe structure, which corresponds to conformational changes during the adsorption/flocculation process under shear, has not been observed. (5) Adding electrolyte to flocculating suspension could change the effective polymer concentration range. The electrolyte concentration below the c.c.c. (critical coagulation concentration) could enhance latex flocculation in terms of flocculation rate. This trend becomes more significant in the case of polymer with low charge density. As hydraulic condition is used as a variable, the following are observed. (1) Monitoring flocculation could assess the reversibility of flocculation process which depends on mode of action of the polymer. (2) Continuous aggregation and break-up in the later stage resulted in denser floes and the limiting size which is related to the floe strength. (3) Shear rate plays an important role in governing the rate of the process. Although the current monitoring method provides useful information on the rate of flocculation as well as the development of floe structure, the complementary use of other measurement techniques such as particle counting or image analysis will give information on more complicated floc properties.

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