The relationship between cation ions and polysaccharide on the floc formation of synthetic and activated sludge

This study has investigated in a comparative fashion the effect of cation ions and polysaccharide on the floc formation 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 cation ions and polysaccharide could influence sludge floc formation. The relationship between polysaccharide concentration and cation ions concentration was examined at laboratory scale during flocculation with both synthetic and activated sludge; an increase in feed cation ions concentration led to a decrease in final polysaccharide concentration in the supernatant. An increase in cation ions 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. Synthetic and activated sludge behave very similarly in terms of their characteristics 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 cation ions and polysaccharide 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 ions concentration fed to the reactor. The presence of cation ions decreases the polysaccharide concentration in the supernatant. An increase in cation ions concentration was also associated with an increase in the bound biopolymer concentration. The cation ions and polysaccharides play an important role in the flocculation 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 cation ions contribute significantly to the binding of the sludge flocs in both synthetic and activated sludge; this binding appears stronger for a ‘slowly fed’ sludge.