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The humic acids extracted from a compost of activated sludge at different stages of maturity were characterized by various chemical techniques. Elemental analysis showed the reduction of H, and the H/C and C/N ratios and an increase in the proportion of N and S. At the end of composting C% and O% presented the same values as initially, although they increased in the intermediate stage. Based on the ratios of FTIR absorbance it was shown that the end product was enriched in etherified and peptidic compounds absorbing at 1384, 1034 and 1544 cm(-1). The alkyl and other N-rich and oxidized recalcitrant structures compose the new humic polymers produced during composting. In principal components analysis, the first axis, PC1: 49.75% considers the variability between structures in decomposition from the other parameters that concern the stable new humic polymers formed after composting. PC2 (40.5%) shows a negative correlation between (aromatic carbon, FA level) and (aliphatic carbon, HA level) during composting.

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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