Effect of extraction method on the structure and bioactivity of polysaccharides from activated sludge

Resource recovery is a pivotal facet of waste activated sludge treatment, particularly within the framework of carbon neutrality and the circular economy. Polysaccharides are emerging as a valuable resource from waste activated sludge, and the choice of extraction method affects the properties of the polysaccharides, which is of utmost importance for subsequent application. This investigation examined the effects of six extraction methods (i.e., acidic, alkaline, ultrasonication, hot-water, microwave, and electric treatments) on the yield, chemical composition, structural characteristics, and bioactivities of polysaccharides extracted from sludge. For each extraction method, two operational parameters, namely the treatment time and strength (e.g., the acid and alkali concentration), were initially optimized in terms of the polysaccharide yield. The polysaccharide yield varied from 1.03 ± 0.12 % to 5.34 ± 0.10 % adopting the extraction methods under optimized conditions, and the alkaline extraction method had the highest yield of polysaccharides with a treatment time of 120 min and NaOH concentration of 1 %. At least one polysaccharide fraction was successfully purified from the crude polysaccharide of each extraction method. The compositions and structures of these fractions, including carbohydrate, protein, sulfate, uronic acid contents, and monosaccharide compositions, were determined. Carbohydrate was the dominant component, with the hot-water-2 fraction having the highest carbohydrate content (77.90 % ± 2.02 %). Monosaccharides in the polysaccharides were measured, with mannose, rhamnose, glucose, and xylose being found in all fractions, whereas ribose was exclusively found in the acid-1 fraction. The molecular weights of these fractions ranged between 1.60 × 10⁴ Da and 7.11 × 10⁶ Da. Furthermore, the bioactivities of the polysaccharides, encompassing five anti-oxidant and three anti-coagulant properties, were assessed, with the ultrasonication-1 fraction having superior performance in seven of the assays. Finally, the association among the fractions in terms of composition and bioactivity was assessed adopting cluster analysis and regression methods. The findings underscore the effect of the extraction method on the properties of polysaccharides extracted from sludge, thereby providing valuable insights for the prospective applications of polysaccharides.