Properties of Activated Sludge Flocs

Doctoral thesis, 1999

The quality of the effluent from activated sludge treatment plants is highly dependent on the efficiency of the solid-liquid separation process. The aim of this thesis was to study (i) the effects of dissolved oxygen (DO) concentration on effluent turbidity and flocculation, as well as on the settling properties of activated sludge, for both short term and long term scales, and (ii) the influence of aerobic microbial activity on the stability of activated sludge flocs. Relatively short periods of oxygen limitation (a few hours) caused increased turbidities of the effluent from both a pilot plant and a full scale plant. In addition, the adsorption of colloidal material onto the sludge flocs decreased under anaerobic conditions. The variations in activated sludge floc structure, size and size distribution were studied for DO concentrations of 0.5 to 5 mg/l, and sludge ages of 1.25 to 5 days, in pilot scale, completely mixed reactors. There was a trend towards larger flocs at higher DO concentrations. The size distribution by volume of flocs within the range of 10 - 1100 µm fitted well to log-normal distribution functions, while the size distribution by number of the small flocs (1 - 100 µm) in the supernatant after settling fitted best to power functions. Lower DO concentrations produced sludge with poorer settling properties and higher turbidities of the effluent than higher DO concentrations. Activated sludge deflocculated when it was subjected to anaerobic conditions and shear. The extent of deflocculation varied with the length of the anaerobic period. Activated sludge of a low sludge age deflocculated more than older sludge. To restore the deflocculated sludge, oxygen was added; thereafter, in less than an hour, most of the deflocculated matter was reflocculated. However, part of the deflocculated sludge did not reflocculate. The deflocculated material was composed mainly of bacteria and extracellular polymeric substances (EPS) attached to cells or floc fragments. Particles deflocculated under anaerobic conditions included more bacteria and protein than there were the composition of the total sludge. Greater floc strength was obtained when the aerobic microbial activity was stimulated, while deflocculation increased when the aerobic microbial activity was inhibited. Not only inhibition of the aerobic microbial activity increased the deflocculation, but the deflocculation under anaerobic conditions could also be increased by stimulating the anaerobic microbial activity, e.g. Fe(III)-reduction. Field experiments showed seasonal variation in floc strength with the largest proportion of weak flocs present in the winter: there was a temperature effect on the degree of deflocculation. The degree of reflocculation could be increased by stimulating the aerobic microbial activity.