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The efficiency of activated sludge as inoculum and municipal wastewater as diluent for the biodegradation of refinery sludge was investigated. At a laboratory scale, biodegradation experiments carried out in an aerobic batch reactor showed that toxic oily sludge was biodegradable after dilution. Compared with freshwater, the use of municipal wastewater for dilution clearly enhanced the biodegradation of this toxic pollutant. Using municipal wastewater, experiments showed that, starting from an initial total hydrocarbon concentration (THC) of 750 mg L-1, we could reach a final THC of 5 mg L-1 and a COD removal of 97% in only 19 days. In addition, we found that bacterial dominance was affected by hydrocarbon load. An absolute dominance of Gram-positive strains with high hydrocarbon load (7500 mg L-1) was found. However, with a hydrocarbon load of 750 mg L-1, some Gram-negative strains were also active. This bacterial dominance also changed in the course of treatment. We conclude that municipal wastewater and activated sludge, less often used nowadays in oily sludge biodegradation processes, contained bacterial strains that are efficient and able to work in a community. Municipal wastewater is also strongly recommended as a diluent as it offers an effective low-cost solution

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