CHARACTERIZATION OF ACTIVATED-SLUDGE FLOCS STRUCTURE

Activated sludge is the most widely used process for wastewater treatm ent. However basic properties such as flee structure or settling prope rties, still remain unknown. In order to describe activated sludge fle e structure, four investigation methods were used: (i) size distributi on analysis associated with sonication, (ii) transmission electron mic roscopy (iii) laser scanning confocal microscopy and fractal dimension analysis, and (iv) three dimensional modelling.The evolution of flee size distribution in dispersed samples was used to build a model of fl oc showing that the predominating microflocs (125 mu m) are formed fro m 13 mu m aggregates, which are made up of smaller particles (2.5 mu m ). By transmission electron microscopy it was assumed that the 2.5 mu m sub-units could correspond to microorganisms and that exopolymers fo rm a gel-like matrix which holds the structure of the aggregates toget her. The microfloc (13 mu m) structure was investigated by confocal sc anning light microscopy and image processing. The analysis of the mass distribution pointed out that the microfloc is a fractal object (frac tal dimension around 3). The total number of cells in these aggregates was found to be 2(n) number. These two results suggest that division of microorganisms explained the formation of 13 mu m units which are m icrocolonies. A mass fractal dimension 2.5 +/- 0.15 was measured for t he microfloc. Its external surface was reconstructed by using interpol ating 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 colli sion is the mechanism which describes the formation of macroflocs. (C) 1997 IAWQ. Published by Elsevier Science Ltd.