Impact of MIEX pre-treatment on floc strength and structure

In drinking water treatment regulation standards for natural organic matter (NOM) are tightening owing the fact that NOM leads to formation of carcinogenic disinfection by-products which can be harmful for human health. To further remove NOM from water, advanced NOM removal processes are developed such as the magnetic ion exchange (MIEX) resin process. MIEX followed by coagulation has been shown to improve the dissolved organic carbon removal in comparison with coagulation alone. However knowledge is lacking about how the magnetic resin influences the floc structure which is of major importance for the efficiency of the downstream processes. The present thesis aimed to evaluate and understand the impacts of MIEX pre-treatment on floc properties. To meet these aims, a literature review was carried out in order to provide sufficient background on floc size, floc strength, re-growth potential and fractal dimension concepts. It showed that almost all of factors influencing these properties act on the floc structural properties via a modification of either the nature of the primary particles, or the conditions of coagulation process, or the hydrodynamic conditions of the surrounding water. De facto, these pre-cited properties are all linked together. It is especially recommended to interpret with care the floc strength concept. The pre-defined objective was to compare properties of flocs generated by coagulation alone and combined treatment (MIEX pre-treatment followed by coagulation). Coagulation steps were operated with three different coagulants - FeCl3, AlCl3, PACl - at optimum pH and dose conditions on jar test bench. A light scattering instrument measured floc size and fractal dimension during growth, breakage and regrowth periods. Additional fractal dimension measurements for ferric flocs were gathered by image analysis using a microscope. MIEX pre-treated flocs were larger but withstood less well increases in shear rate than conventional flocs. This reduced robustness to increases in shear rate was thought to be due to the initial larger size of the pre-treated flocs. Light scattering and image analysis gave different fractal dimension values for the same floc system. It is probably because they are applicable for different size ranges: light scattering for the small primary particles and image analysis for the overall flocs. Of more operational interest, image analysis results showed that MIEX pre-treated flocs were more compact. Combined treatment gave an overall improvement in floc structure quality, producing flocs with better filterability and settleability.