Impact of coagulant type and magnetic ion exchange (MIEX) pre-treatment on floc strength and structure at the Kluizen Water Treatment Plant, Belgium

Abstract

Drinking water supplied to the community should be free from Disinfection ByProducts (DPB). DPB occurs when organic matter is present in surface water and reacts with chlorine. Conventional coagulation and flocculation processes have been used in water treatment plants to remove organic matters from raw water. Recently, Magnetic Ion Exchange (MIEX) followed by coagulation and flocculation has started to attract attention since it improves natural organic matter removal and decreases coagulant demand. The efficiency of NOM removal and downstream processes not only depends on the prior removal process of organic matter but also on the structure , size and strength of flocs formed in the coagulation process as well. This study aims at comparing (1) floc structures (i.e. floc sizes, floc strength, and fractal dimensions) generated from coagulation of raw water and (2) water pre-treated with magnetic ion exchange obtained from Kluizen water treatment plant (Kluizen, East-Flanders, Belgium). The floc sizes during growth, breakage and re-growth were measured using (laser) light scattering (Malvern Mastersizer 2000), coupled with jar test equipment. The strength of flocs was evaluated by empirically relating the floc size to the increased shear rate applied. The results show that pre-treated water coagulated with all the coagulants produces flocs with larger sizes compared to the flocs generated from raw water. For both water types, FeCl3 gave better recoverability after breakage than PACl and AlCl3. After pre-treatment, the floc size was increased by 19%, 63% and 64% for
the coagulants such as PACl, AlCl3, and FeCl3, respectively. Coagulation process with PACl generated flocs with higher fractal dimension and lower strength constants which indicates that flocs generated is strong and compacted micro-flocs. The floc size measurement techniques and coagulation system were found to have an impact on the resulting floc sizes.