Physical characteristics of flocs in water treatment processes
Aluminium sulphate, calcium nitrate, and two cationic polymers have been used to coagulate and flocculate dilute kaolin and latex suspensions (< 100 mg/1) in a stirred vessel. The effects of the different destabilising agents on the resulting floes have been monitored using a simple continuous optical technique based on turbidity fluctuations and the behaviour of these floes under shear conditions and the possibility of subsequent floe re-formation have been investigated. The results showed vast differences in behaviour regarding to floe breakage and re formation between the systems. For a low molecular weight, high charge polymer (A) and calcium nitrate, floe breakage was almost completely reversible. However, for aluminium sulphate, limited floe re-formation was observed on restoring previous shear conditions after breakage. For a high molecular weight polymer (B), limited floe breakage was observed. Moreover, a new automated monitoring technique based on cake and vacuum filtration has been developed to assess the filterability of floes. The technique is reliable and provides reproducible results. The results showed that polymer A was more effective in enhancing filterability than polymer B for both kaolin and latex suspensions. However, polymers do not have a significant influence on the filterability of floes if the primary particles involved have high sphericity.