Development and application of an on-line floc sizer device aiming at the optimization of coagulation/flocculation and filtration

Abstract

This work will develop an on-line floc sizer device in Brazil and investigate the integrity of flocs when they are exposed to shear associated with passage through a Rapid Gravity Filter (RGF). The floc sizer device is based on the principle of turbidity fluctuations and was largely developed by Professor John Gregory at University College London (UCL). Here, we propose: to develop a device on the same basis but adapted for particular conditions; to establish a relation between the generated signal and floc size distribution; and to apply to drinking water treatment plants in Brazil. After the steps mentioned above, the device will be applied in order to provide a better understanding of the relationship between the nature and concentration of additives (metal coagulants and polymeric flocculants) and the properties of flocs, especially size, breakage and re-formation under controlled shear conditions. These findings will be related to measurements of floc penetration into filter media under laboratory conditions. The results of this work may allow floc properties to be adjusted to give optimum performance for various types of filtration processes, providing an on-line automation method. In outline, the program of work will be as follows: to develop the monitoring equipment in Brazil; to establish relation between the generated signal and flocs size distribution; to establish, by the modified jar test procedure, coupled to the continuous monitoring, floc size, floc strength and reversibility of floc breakage for different combinations of metal coagulants and polymeric flocculants, typical of those used in water treatment. Subsequently the flocculated suspensions will be passed through a model filter in the laboratory and filtration parameters such as headloss, impurities penetration and filter run-time will be monitored. Special attention will be paid to the effect of the filtration rate (flow rate per unit surface area) on these parameters, since this can be related to the effective shear rate in the filter pores. The findings from this stage will be interpreted in terms of the floc properties derived earlier for the same coagulant/flocculant combinations.This work should provide a valuable addition to knowledge in the area of water filtration. The development of a method to optimise the type and amount of coagulant and flocculant used in terms of the floc integrity and hence on filter performance will be proposed. This will provide a new and simple method for water treatment plant (WTP) automation. (AU)