Activated sludge flocculation: direct determination of the effect of calcium ions

The effect of calcium on activated sludge flocculation dynamics is investig ated using a unique experimental technique. The technique allows on-line an alysis of the size of activated sludge flocs during flocculation and provid es valuable insight into the mechanisms of flocculation. Activated sludge s amples were firstly sonicated for 3 minutes at 50W and then stirred at 100 rpm. The floc size was subsequently measured on-line using a Malvern Master sizer/E. For concentrations of calcium less than 4 meq/L no significant inc rease in final floc size was observed even though an increase in the initia l rate of change of floc size could be seen. Addition of calcium greater th an 4 meq/L resulted in a dramatic increase in floc size. Results from this investigation support the theory that cations are involved in flocculation through cationic bridging, and will be used in ongoing investigations to mo del the flocculation process.