Roles of coagulant species and mechanisms on floc characteristics and filterability

Abstract In full scale water treatment operation, the rapid filtration process, as the last step of solid–liquid separation, is largely influenced by floc characteristics. In this study, aluminium sulphate (alum) and nano-Al 13 were investigated to understand the influence of coagulant species on the formation and filterability of flocs. At neutral pH, it was found that nano-Al 13, a high MW polymer, showed better floc filterability than alum. This is because of the densely compacted and well-distributed size flocs from nano-Al 13 , even though floc sizes of alum were generally bigger. Al specie distributions of the two coagulants at different pH levels were compared by using electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) to further elucidate the reasons for the superiority of nano-Al 13 in floc filterability. Depolymerisation/re-polymerisation of nano-Al 13 occurred as pH changed, and Al species from nano-Al 13 were more abundant than that from alum, especially for the high molecular weight (MW) oligomers such as Al 11 , Al 12 , Al 13 and Al 14 . Under the charge neutralisation mechanism, higher MW Al species was found to improve coagulation performance and floc filterability. In addition, breakage resistance and regrowth ability of nano-Al 13 was better than alum, at weak acid condition. Flocs formed by the charge neutralisation mechanism readily regenerated after being thoroughly broken up. The floc regrowth ability of nano-Al 13  at high shear rates (200 rpm and 300 rpm) was much better than at low shear and better than any shear applied to alum., and the flocs after breakage at 200 rpm and 300 rpm also showed better filterability than other conditions.