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Water Science & Technology Vol 53 No 7 pp 77–85 © IWA Publishing 2006

The effect of polymeric flocculants on floc strength and filter performance

M.M. Rawlings*, C.S.B. Fitzpatrick**, J. Gregory*** and A. Wetherill****

* Department of Civil and Environmental Engineering, University College London, Gower Street, London, WC1E 6BT, UK, (E-mail: m.rawlings@ucl.ac.uk)
** Department of Civil and Environmental Engineering, University College London, Gower Street, London, WC1E 6BT, UK, (E-mail: m.rawlings@ucl.ac.uk)
*** Department of Civil and Environmental Engineering, University College London, Gower Street, London, WC1E 6BT, UK, (E-mail: m.rawlings@ucl.ac.uk)
**** Water Quality, Yorkshire Water, George Street, Bradford, UK


ABSTRACT
Polymeric flocculants are widely used throughout the water industry as flocculant aids, they are known to increase floc density and aid settlement in the clarification stage of the water treatment process. In this research, polymeric flocculants were used to improve floc strength prior to filtration on a dissolved air flotation (DAF) plant in an attempt to prevent filter breakthrough. A modified jar test procedure using a PDA (photometric dispersion analyser) optical flocculation monitor was developed in order to evaluate the system floc strength. Filtration trials were carried out on a pilot filter rig situated on a surface water treatment works in Yorkshire. The filter feed originated from the main plant filter channel. Filter performance was assessed by continuous online monitoring of effluent particle counts, turbidity and headloss over the period of the filter run. Results indicated that low doses of polymeric flocculants had a beneficial effect on filtered water quality, as measured by particle counts, turbidity, UV254 absorption and dissolved organic carbon (DOC). Polymeric flocculants also had the effect of extending filter run length. The modified jar test results indicated that the flocculants used improved the floc strength and enhanced reflocculation of the micro flocs present after the flotation process.

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