Comparative analysis on floc growth behaviors during ballasted flocculation by using aluminum sulphate (AS) and polyaluminum chloride (PACl) as coagulants

For enhancing the understanding of ballasted flocculation (BF) mechanisms, a series of BF tests were performed to explore the coagulant dosage and mixing intensity effects on floc growth behaviors and corresponding particle settling by using coagulants of aluminum sulphate (AS) or polyaluminum chloride (PACT). As expected, different coagulant dosages brought about distinct surface charge characteristics of micro-floc aggregates after either Al-based coagulant addition, thereby affecting the development of ballasted floc size. More specifically, the higher the coagulant dosage was, the more the cationic hydrolyzed-Al species were produced, leading to a requirement of more binding sites on the injected polymer for bridging between micro-flocs and ballasting agent (BA) particles, and accordingly, the degree to which BA particles had been incorporated into the aggregate matrix seemed to largely account for ballasted floc formation. Besides, floc aggregates formed after breakage appeared larger than those after maturation, especially at higher coagulant dosages, possibly caused by a stretched conformation of long polymer chains providing more binding sites available within a higher-shear suspension. In an operational sense, poor incorporation should be maximally eliminated, since this undesirable behavior could produce a greater number of resultant flocs with fewer or even no BA particles, thus worsening the settling performance. This research would be helpful for thoroughly understanding ballasted floc formation mechanisms in either water or wastewater treatment operations.