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Abstract
The anaerobic removal of 13 Polycyclic Aromatic Hydrocarbons (PAHs) was measured in
five continuous anaerobic digestors with different feed sludge, in which abiotic losses
were neglected. These feeds were chosen to generate different levels of PAH bioavailability
and cometabolism within the reactors. Based on the accurate modelling of PAH sorption
in sludge, the aqueous fraction (including free and sorbed-to-dissolved-and-colloidal-matter
PAHs) was demonstrated to be bioavailable, which validated a widespread assumption
about micropollutants bioavailability in sludge. It was also demonstrated that bioavailability
is not the only influencing factor. Indeed, PAHs biodegradation resulted from a combination
of bioavailability and cometabolism. An equation adapted from Criddle (1993, The Kinetics
of Cometabolism. Biotechnology and Bioengineering 41, 1048-1056) that takes into account
both mechanisms was shown to fit the experimental data, with dry matter removal rate
identified as the criteria for cometabolism. The existence of a threshold of dry matter
cometabolism was suggested, below which PAHs removal would not be possible. The parameters
of the Criddle equation were demonstrated to depend on PAH molecular structure, and
the results suggest that they would also be influenced by substrate composition and
microbial population. This research provided original outcomes for the assessment
of micropollutants fate. Indeed, the understanding of the driving mechanisms was improved,
which has implications for the optimization of micropollutants removal.