Determination of the effect of cations and cationic polyelectrolytes on the characteristics and final properties of synthetic and activated sludge

This study has investigated in a comparative fashion the effect of cations and polyelectrolytes on the characteristics 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 results from experiments indicate that cations and polyelectrolytes could influence sludge floc characteristics as they relate to sludge conditioning. The relationship between polysaccharide concentration and cation concentration was examined at laboratory scale during flocculation with both synthetic and activated sludge; an increase in feed cation concentration led to a decrease in final polysaccharide concentration. An increase in cation concentration in the feed to the reactors was also associated with an increase in the bound biopolymer concentration. The changes in the bound biopolymer were explained according to the cation bridging model. The effects of a polyelectrolyte conditioner on sludge conditioning were determined. A qualitative link exists between polyelectrolyte conditioner dosage and sludge conditioning for both types of sludge. The two types of sludge also have very similar sludge conditioning after cationic polymer is added to the reactor. Thus synthetic and activated sludge behave very similarly in terms of their characteristics and sludge conditioning, and synthetic sludge can be used as a surrogate in activated sludge studies. The results of this work also indicate that the formation of cation-polymer complexes and polymer gelation are amongst the most important mechanisms for sludge coagulation–flocculation, and offer a means for optimisation of the activated sludge process.

• The strong relationship between cations and polysaccharides for both synthetic and activated sludge has been explored through the measurement of the concentration of polysaccharide in the supernatant with the changes of cation concentration fed to the reactor. The presence of cations decreases the polysaccharide concentration in the supernatant. An increase in cation concentration was also associated with an increase in the bound biopolymer concentration. • The cations and polysaccharides play an important role in the formation of both synthetic and activated sludge. Calcium ions contribute to the floc formation by constructing calcium ion bridges between polysaccharides or biopolymer gel adsorbed on individual particles/ bacteria; aluminium ions also seem to assist in floc binding and the flocculation of other organic material. The cations contribute significantly to the binding of the sludge flocs in both synthetic and activated sludge; this binding appears stronger for a ‘slowly fed’ sludge. • The effects of calcium and cationic polyelectrolytes on the resulting final properties of sludge, such as sludge conditioning, have also been measured and characterized for both synthetic and activated sludge. An optimum concentration exists for polyelectrolyte conditioner; higher concentrations of Ca2+ in the sludge contributed to a significant improvement in sludge dewaterability and lowered the optimum conditioner concentration.