Size and structure effects on centrifugal dewatering of digested sewage sludge

The application of light scattering over small angles for the determination of digested sludge floc size and structure and its relationship with dewat erability is investigated. It appears that improved dewatering corresponds with lower floc fractal dimension (a more open structure) and a smaller pro portion of fine particles. The initial increase in fractal dimension with i ncreasing polymer dose for the digested sludge is most likely due to more e fficient aggregation of the finer particles and the resulting formation of denser particle aggregates. A large colloidal fractal of the digested sludg e (< 10 mum) appears to be less negative than the bulk digested sludge. Thi s suggests that the fine particles will react differently and possibly less aggressively to the cationic polymer than the larger and more negative par ticles. The higher negative charge associated with the larger particles mig ht be related to greater levels of highly negatively charged extracellular polysaccharides (EPS) adsorbed to the floes or could result from the associ ation of FeS with the finer fraction. The appearance of much greater levels of fine particles after digestion suggests that the floes have undergone d isintegration. Whether this is due to reduced biological efficiency within the digestor or iron reduction under the anaerobic conditions is not known for certain, although no indication of prolonged stress in the digesters co uld be found from plant performance data.