Effect of pH modification on bilge waste biodegradation by a native microbial community

During laboratory biodegradation of the oily liquid residue generated in normal ship operations, bilge waste, by an adapted native microbial community, bioemulsifier production and decrease in pH were observed. When the pH of the medium was modified by adding NaOH or buffering with Tris–HCl (pH 7.8), the greatest hydrocarbon reduction and biodegradation were found with NaOH treatment. With this treatment, total hydrocarbon removal and biodegradation efficiencies of, respectively, 97% and 86% for n-alkanes, 40% and 30% for total aliphatic hydrocarbons and 25% and 17% for total aromatic hydrocarbons were recorded after culture for 10 days. The emulsifying activity (quantified as emulsion index E24) was highest (60%) for neutralized treatment. For the three treatments the emulsifier activity was optimal at pH 6.5–7.5. Therefore, the culture medium should be neutralized to increase the rate of biodegradation and to minimize the residence time of the waste in a treatment unit.