A comparative study of the flocculation behaviour and final properties of synthetic and activated sludge in wastewater treatment

In this study, we have investigated in a comparative fashion the flocculation behaviour and final properties of both synthetic and activated sludge. Synthetic sludge was prepared according to established procedures; activated sludge was produced in a lab-scale, continuous-flow reactor which was fed with live activated sludge from a wastewater treatment plant. The novelty of our approach lies in the attempt to use the former as a key to characterising the physical and chemical properties of the latter. The effects of calcium ion concentration on flocculation dynamics for both sludges were measured on-line using the photometric dispersion analyser; the effects of calcium, polyelectrolyte conditioner and shear rate on final sludge properties were determined, including settleability, turbidity of supernatant, sludge volume index and dewatering, sludge conditioning, and floc strength and structure. It was thus possible to link the flocculation dynamics to the final sludge properties. The results indicate that calcium ions play an important role in the floc formation and the final floc size for both types of sludge by the construction of calcium bridges, and hence played a significant role in determining the final properties of the sludge. A qualitative link exists between the flocculation dynamics and the final properties for both types of sludge, and between the floc dynamics and properties of synthetic sludge on the one hand and those of activated sludge on the other. The two types of sludge have very similar settling and dewatering characteristics after cationic polymer conditioning. However, there are quantitative differences in the calcium concentration required for flocculation, the supernatant turbidity, the sludge-volume index and the floc strength. This difference is believed to be due to the absence of filamentous material in the synthetic sludge which is present in the activated sludge. The stable and well-defined nature of synthetic sludge makes it useful as a non-complex analogue for the physical and chemical properties of activated sludge, but filamentous cellulose should be added to improve quantitative agreement with activated sludge.