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Transmission electron microscopy (TEM) coupled with electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDXS) was used to investigate the coagulation of natural organic matter with a ferric salt. Jar-test experiments were first conducted with a reconstituted water containing either synthetic or natural extracts of humic substances, and then with a raw water from Moselle River (France). The characterization of the freeze-dried coagulated sediment by EELS in the 250-450 eV range, showed that Fe-coagulant species predominantly associate with the carboxylic groups of organic matter, and that this interaction is accompanied by a release of previously complexed calcium ions. The variation of Fe/C elemental ratio with iron concentration provides insightful information into the coagulation mechanism of humic substances. At acid pH, Fe/C remains close to 3 over the whole range of iron concentrations investigated, while a much lower atomic ratio is expected from the value of optimal coagulant dosage. This suggests that a charge neutralization/complexation mechanism is responsible for the removal of humic colloids, the aggregates being formed with both iron-coagulated and proton-neutralized organic compounds. At pH 8, the decrease in Fe/C around optimal coagulant concentration is interpreted as a bridging of stretched humic macromolecules by Fe-hydrolyzed species. Aggregation would then result from a competition between reconformation of humic chains around coagulant species and collision of destabilized humic material. EELS also enabled a fingerpriting of natural organic substances contained in the iron-coagulated surface water, N/C elemental analyses revealing that humic colloids are removed prior to proteinic compounds.

The nature of trace element carriers contained in sewage and combined sewer overflow (CSO) was investigated by TEM-EDX-Electron diffraction and SEM-EDX. During dry weather, chalcophile elements were found to accumulate in sewer sediments as early diagenetic sulfide phases. The sulfurization of some metal alloys was also evidenced. Other heavy metal carriers detected in sewage include metal alloys, some iron oxihydroxide phases and neoformed phosphate minerals such as anapaite. During rain events, the detailed characterization of individual mineral species allowed to differentiate the contributions from various specific sources. Metal plating particles, barite from automobile brake, or rare earth oxides from catalytic exhaust pipes, originate from road runoff, whereas PbSn alloys and lead carbonates are attributed to zinc-works from roofs and paint from building siding. Soil contribution can be traced by the presence of clay minerals, iron oxihydroxides, zircons and rare earth phosphates. However, the most abundant heavy metal carriers in CSO samples were the sulfide particles eroded from sewer sediments. The evolution of relative abundances of trace element carriers during a single storm event, suggests that the pollution due to the "first flush" effect principally results from the sewer stock of sulfides and previously deposited metal alloys, rather than from urban surface runoff.

The nature of coagulant species formed in the system ferric chloride/municipal sewage was explored with Transmission Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy (TEM-EDXS) and Fe K-edge X-ray Absorption spectroscopy. Jar-test data combined with chemical analysis of supernatant (dissolved organic carbon, iron, and phosphorus) and Fourier-Transform-Infrared spectroscopy (FTIR) of freeze-dried sediment, provided a detailed description of sewage clarification. The results showed that the nature of coagulant species evolves with Fe concentration. Up to the optimum turbidity removal, mainly iron dimers linked with one phosphate anion are detected. At higher dosages, polymers of hydrolyzed Fe appear even though PO(4) still participates in the formation of coagulant species. TEM observation of freeze-dried sediments corroborates such an evolution of Fe speciation. EDXS analyses reveal that minute amounts of sulfur, silicon, aluminum, and calcium, are associated with the coagulant species. Even though the coagulant species change with Fe concentration, the destabilization mechanism, inferred from electrophoretic mobility of aggregates and the evolution of floc size under cyclic changes of stirring conditions, is equivalent with a charge neutralization of sewage colloids in the whole range of coagulant concentration.

The aggregation behavior of activated sludge flocs was investigated by monitoring the size distribution of flocs and transfer of mineral particles between flocs, under various conditions of agitation and dilution. The results showed that (i) the shape of the floc size distribution can be fitted with a gamma function, (ii) a steady-state mean floc size is reached for a given stirring rate, (iii) this stable floc size is shifted towards floc growth as sludge concentration is increased, (iv) under cycled-shear conditions, microbial aggregates break up and re-form in an almost reversible manner, (v) blending of raw sludge and sludge spiked with Aquatal mineral particles results in particle exchange between flocs and (vi) the detailed study of exchange kinetics indicates that some flocs do not participate to the aggregation dynamics. These experimental results suggest that the activated sludge floc size is governed by a flocculation/deflocculation balance, implying an exchange of floc constituents between microbial aggregates.

Ferric chloride and lime conditioning of sewage sludge is usually interpreted in terms of enhanced aggregation of sludge components. In this study, transmission electron microscopy was used to investigate the conditioning mechanism at a submicronic scale. Samples were collected from two municipal wastewater treatment plants at different stages of the process, embedded in an epoxy resin, and sectioned with an ultramicrotome. Transmission electron microscopy examination of thin sections revealed that clumps of crystalloids develop on external surfaces of flocs after the application of both conditioners. This precipitate creates a rigid structure around the flocs which, upon mechanical dewatering, transmits the stresses applied to the inner parts of flocs. The porous structure of the precipitate may also participate to the withdrawal of water as a draining media. Energy dispersive X-ray spectroscopy revealed that Fe, P, and Ca are the dominant elements in the precipitate with Fe/P and Fe/Ca elemental ratios close to 2.8-3 and 1.1-3, respectively.

Fluvial suspended particulates collected from the Rhine River were investigated in terms of composition, structure and size distribution. Elemental analysis and Diffuse Reflectance Infrared Spectroscopy reveal that most particulate organic matter is formed from material derived from microorganisms. Transmission Electron Microscopy observations on resin-embedded samples and structural characterization from break-up experiments, show that fluvial particulate matter should be viewed as fractal aggregates organized by bacterial exopolymeric substances. The shape of particulate size distribution suggests that the formation and dynamics of suspended particulate matter are controlled mainly by physical processes. Finally, particulate growth and structure are consistent with a cluster-cluster aggregation scheme.