Unified understanding of physico-chemical properties of activated sludge and fouling propensity.

A range of parameters affecting floc characteristics, sludge composition and filtration properties was investigated by analyzing 29 sludge samples from municipal and industrial conventional activated sludge systems and municipal membrane bioreactors (MBR). Samples were characterized by physico-chemical parameters, composition of ions and EPS, degree of flocculation, settling properties, dewatering properties, and filtration properties. By analyzing the interplay between various metrics instead of single parameters, a unified understanding of the influence of sludge composition and characteristics was developed. From this, a conceptual model was proposed to describe the interplay between sludge composition, characteristics, and filtration properties. The article shows three major results contributing to describe the interplay between sludge characteristics and fouling propensity: First, the degree of flocculation could be quantified by the ratio between floc size and residual turbidity and was a key parameter to assess fouling propensity. Second, extracted EPS to polyvalent cations ratio was used as an indicator of the flocculation. A high ratio combined with a high concentration of EPS resulted in large, loosely bound, and weak flocs that were easily deformed, hence giving compressible fouling layers. Finally, high amounts of carbohydrates in both total and extracted EPS resulted in more pronounced fouling, which may be explained by carbohydrates forming poorer flocs than humic substances and proteins. Accordingly, samples with high humic content showed lower specific resistance to filtration due to better floc structure. The amount of carbohydrates in EPS correlated positively to the influent COD/N ratio, which may explain why systems with high influent COD/N ratio demonstrated higher fouling propensity.