Ferric chloride and lime conditioning of activated sludges: An electron microscopic study on resin-embedded samples

Ferric chloride and lime conditioning of sewage sludge is usually interpret ed in terms of enhanced aggregation of sludge components. In this study, tr ansmission electron microscopy was used to investigate the conditioning mec hanism at a submicronic scale. Samples were collected from two municipal wa stewater treatment plants at different stages of the process. embedded in a n epoxy resin, and sectioned with an ultramicrotome. Transmission electron microscopy examination of thin sections revealed that clumps of crystalloid s develop on external surfaces of flocs after the application of both condi tioners. This precipitate creates a rigid structure around the flocs which, upon mechanical dewatering. transmits the stresses applied to the inner pa rts of flues. The porous structure of the precipitate may also participate to the withdrawal of water as a draining media. Energy dispersive X-ray spe ctroscopy revealed that Fe, P, and Ca are the dominant elements in the prec ipitate with Fe/P and Fe/Ca elemental ratios close to 2.8-3 and 1.1-3, resp ectively. (C) 2001 Elsevier Science Ltd. All rights reserved.