Characterization of activated sludge flocs structure

Activated sludge is the most widely used process for wastewater treatment. However basic properties such as floc structure or settling properties, still remain unknown. In order to describe activated sludge floc structure, four investigation methods were used: (i) size distribution analysis associated with sonication, (ii) transmission electron microscopy (iii) laser scanning confocal microscopy and fractal dimension analysis, and (iv) three dimensional modelling. The evolution of floc size distribution in dispersed samples was used to build a model of floc showing that the predominating microflocs (125 μm) are formed from 13 μm aggregates, which are made up of smaller particles (2.5 μm). By transmission electron microscopy it was assumed that the 2.5 μm sub-units could correspond to microorganisms and that exopolymers form a gel-like matrix which holds the structure of the aggregates together. The microfloc (13 μm) structure was investigated by confocal scanning light microscopy and image processing. The analysis of the mass distribution pointed out that the microfloc is a fractal object (fractal dimension around 3). The total number of cells in these aggregates was found to be 2n number. These two results suggest that division of microorganisms explained the formation of 13 μm units which are microcolonies. A mass fractal dimension 2.5 ± 0.15 was measured for the microfloc. Its external surface was reconstructed by using interpolating software (GOCAD). It appeared that the surface is also a fractal object with a dimension 2.3 ± 0.1. According to the fractal theory, it seems that diffusion limited aggregation via monomer-cluster collision is the mechanism which describes the formation of macroflocs.