Wet air oxidation enables a severity window for purple phototrophic bacteria valorization of real refinery oily sludge mixtures.

Refinery oily sludge is typically managed via the API–DAF–biological treatment train, generating residual sludges rich in hydrocarbons and nutrients that are often disposed of with limited valorization. In this work, a photobiorefinery concept coupling wet air oxidation (WAO) with purple phototrophic bacteria (PPB) is evaluated using a realistic mixture of API, DAF and biological sludges at their production ratio. WAO effluents obtained at different severities were screened for biodegradability and further investigated under selected conditions. Effluent generated at 200 °C supported robust PPB growth, with WAO liquors at 33% (v/v) providing the best compromise between substrate availability and inhibition. In kinetic batch experiments, PPB exhibited similar maximum growth rates when treating WAO effluents obtained at 200 ºC and 1–3 h, with soluble COD removals of 1453–1545 mg/L (45–48%) together with 26–37% of NH₄⁺, but achieving higher biomass yields and faster carboxylic acid removal at 1 h. In contrast, ammonium and soluble COD reached partial plateaus, consistent with a persistent soluble fraction enriched in oxidized aromatic and N-heterocyclic compounds. Amplicon sequencing revealed a stable PPB core ( Cereibacter , Rhodopseudomonas and Rhodocista ), complemented by heterotrophic taxa associated with degradation of complex organics. Overall, these results delineate a WAO operating window that maximizes PPB-treatable carbon and support integrating WAO and PPB for valorization of real refinery sludge mixtures. • WAO treats real mixed refinery sludges (API, DAF, BIO) in their production ratio. • WAO severity shapes soluble COD into readily degradable and recalcitrant fractions. • 200 °C 1 h WAO effluent (33% concentrated) maximizes PPB growth and organic matter removal. • PPB kinetics and metabolism reveal carbon-limited NH₄⁺ removal and redox regulation. • GC–MS, HPLC and 16S rRNA link recalcitrant COD to N-heterocycles and complex aromatics.