Morpho-chemistry in secondary sludge filtration cakes: a case study

Light Microscopy, Transmission Electron Microscopy (TEM, EDXS), and Fourier Transform Infra Red MicroSpectroscopy (FTIRMS) were used to describe the organization and chemical distribution of major constituents in a sludge filtration cake. Samples were obtained from a municipal wastewater treatment plant using conventional ferric chloride and lime sludge conditioning. Various samples collected at different stages of the process were embedded in an Epoxy resin after acetone-dehydration, and sectioned using an ultra-microtome. The thickness of the sections was adapted to the experimental techniques used. TEM showed that in the activated sludge, bacterial colonies, isolated bacteria and debris are trapped within a gel matrix of exocellular polymeric substances, whereas those same components are compacted and distorted in the filtration cake. Furthermore, conditioning chemicals appeared in the cake as amorphous aggregated colloids and acicular particles which do not form inside the colonies. A chemical mapping was obtained by determining and integrating FTIR bands characteristics of specific components of the cake. Preliminary results showed that the amounts of resin can be used to assess the relative compacity at different levels of the cake.