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Among the phthalic acid esters (PAEs) targeted by the United States Environmental Protection Agency (USEPA) as priority pollutants, diethyl-hexyl phthalate (DEHP) is the major pollutant identified at high concentration level in lagooning sludge (LS), at about 28.67 mg/kg, and in activated sludge (AS), at about 6.26 mg/kg. Other phthalic acid esters, such as di-butyl phthalate (DBP) and di-methyl phthalate (DMP) show very low concentrations. During sludge composting, after the stabilization phase, the subsequent appearance of DEP and then DMP occurred indicating that microbial metabolism begins by alkyl side-chain degradation before aromatic ring-cleavage. The appearance and accumulation of PAEs with a short alkyl side-chain in the last stages of AS and LS composting is suggested originating from the degradation of phthalates with a much long side-chain. The DEHP showed a rate of biodegradation that follows a first-order kinetic model during composting of both AS and LS. The calculated DEHP half-lives are 45.4 days for LS and 28.9 days for AS. The better DEHP biodegradation rate (2.4 Â 10 À2 day À1 ) have been observed in the case of AS composting compared to LS compost (1.53 Â 10 À2 day À1 ). The mono-ethylhexyl phthalates MEHP has been shown to follow the same order of biodegradation as DEHP indicating that the same mechanism is followed (hydrolysis or dealkylation of each DEHP side-chain). Composting could be suggested as a detoxification process for the removal of PAEs (mainly DEHP) from sludges after a sufficient time of treatment to provide a safe end product.

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The fate of 16 polycyclic aromatic hydrocarbons (PAHs), targeted by the USEPA agency, has been investigated during composting of lagooning sludge. Composting shows efficient decrease of the content and the bioavailability of each PAH. Biodegradation and sorption are suggested as the main mechanisms contributing to this decrease. During the stabilization phase of composting, extensive microbial degradation of PAHs, mainly those with a low number of aromatic rings, was achieved following development of intense thermophilic communities. However, partial sorption of PAH to non-accessible sites temporarily limits the mobility mainly of PAHs with a high number of aromatic rings plus acenaphthene and acenaphthylene, and allows them to escape microbial attack. During the maturation phase, the development of a mesophilic population could play an important role in the degradation of the remaining PAH. During this phase of composting, PAH sequestration and binding of their oxidative metabolites within new-formed humic substances might also explain PAH decrease at the end of composting. The tendency of change of content or bioavailability of various PAH compounds during composting is found to be strongly related to the number of their aromatic rings, their molecular weight and structure.

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The effect of composting on the chemical structure of humic acids extracted from a mixture of sewage sludge and straw at different times of treatment was investigated by elemental analysis, Fourier transform infrared and carbon 13-nuclear magnetic resonance. The starting material was composed of a heterogeneous mixture of etherified aromatic structures bearing peripheral long-chain lipidic, peptidic and carbohydrate structures. The results show a decrease in aliphatic compounds during composting, and an increase in aromatic structures-mainly those substituted by functional groups such as phenolic, methoxylic and carboxylic groups. These changes support the preferential oxidation of readily accessible moieties such as aliphatic side chains of lipidic and peptidic structures, leading to the production of highly functionalized aromatic macromolecules. The latter compounds are resistant to microbial degradation and are therefore preserved as part of the newly-formed humic acid structures. sewage sludge / composting / humic acids / chemical structure / humification Résumé-Caractérisation des acides humiques extraits au cours du compostage de boues résiduaires par analyse élémentaire, IRTF et 13 C-RMN. L'action du processus de compostage sur la structure chimique des acides humiques extraits de mélange de boues résiduaires de stations d'épuration et de paille a été suivie à différents stades du traitement par analyse élémentaire, spectroscopies infrarouge à transformée de Fourrier et résonance magnétique nucléaire du carbon 13. Le matériel initial est composé d'un mélange hétérogène de structures aromatiques éthérifiées sur lesquelles sont liées de longues chaînes lipidiques et des structures peptidiques et carbohydrates. Les résultats obtenus montrent une diminution, durant le processus de compostage, des composés aliphatiques, avec une augmentation relative des structures aromatiques principalement celles substituées par des groupements fonctionnels tels que phénols, méthoxyles, carboxyles. Ces transformations mettent en évidence l'oxydation préférentielle des composés facilement accessibles, comme, par exemple, les chaînes aliphatiques périphériques des structures lipidiques et peptidiques, ce qui conduit à une production de structures aromatiques riches en groupements fonctionnels. Ces dernières sont résistantes à la décomposition microbienne et sont donc préservées pour constituer une partie de la structure d'acides humiques néoformés. boues résiduaires / compostage / acides humiques / structure chimique / humification