Chemical and structural (2D) linkage between bacteria within activated sludge flocs

This study describes the complexity of the activated sludge floc structure using four methods: (i) microscopic observation of flocs in situ and after specific staining, (ii) optimization of floc dispersion by sonication of pure bacterial strains, (iii) analysis of polymers released from sonicated sludges, and (iv) floc size distributions after different sonication times. The sonication of activated sludge at 37 W for 60 s was found to be best for dispersing flocs and minimizing bacterial cell lysis. The polymers released from flocs were mainly proteins, with polysaccharides and DNA. Electron microscopy showed that a polysaccharide gel connected the cells together. Raw activated sludges give a continuous distribution of particle sizes (1.2–600 μm). The floc size distribution in sonicated sludge samples was used to build a model of floc structure showing that the predominating macroflocs (125 μm) are formed from 13 μm microfloc aggregates, which are made up of smaller particles (2.5 μm).