Improved biodegradability of hardly-decomposable wastewaters from petrochemical industry through photo-Fenton method and determination of optimum operational conditions by response surface methodology

BACKGROUND: Petrochemical wastewaters are highly polluting due to having various destructive materials such as aromatic hydrocarbons and heavy metal ions. Therefore, they need to be treated before disposal to the environment. However, due to low biodegradability, applying common treatment methods such as activated sludge is not feasible for these wastewaters. METHODS: Photo-Fenton is an advanced oxidation process which was applied to promote the biodegradability of hardly-decomposable petrochemical wastewaters. The wastewater samples were provided by Maroon and Karoon petrochemical plants, located in Mahshahr, Iran. To design the experiments and analyze the experimental results, response surface method with four variables (input COD and TDS concentrations and injected a dosage of H2O2 and Fe2+) and four fixed parameters (temperature, pH, retention time, and UV power) were used. RESULTS: The ranges of input COD, H2O2, Fe2+ and TDS were 1000 to 2500 mg L− 1, 1000 to 4000 mg L− 1, 500 to 3000 mg L− 1, and 4500 to 11,500 mg L− 1, respectively. Average input BOD5/COD ratio was 0.09. These ranges and values were determined according to the quality of the raw wastewater and experimental design. Output BOD5/COD ratio was varying between 0.3 and 0.6, which declined with an increase of input COD. The results showed that the biodegradability of the industrial wastewater was promoted upon application of higher H2O2 and Fe2+ concentrations. Meanwhile, TDS concentration had no significant effect on biodegradability of this wastewater. The following optimum conditions were resulted by evaluating the maximum efficiency of the reactor in enhancing the biodegradability of the wastewater: 1000 mg L− 1 input COD, 2668 mg L− 1 H2O2, 1655 mg L− 1 Fe2+, 8000 mg L− 1 TDS, 0.6 output BOD5/COD, 852 mg L− 1 output BOD5 and 939 mg L− 1 output COD. CONCLUSION: Photo-Fenton method is highly efficient for increasing the biodegradability of petrochemical wastewaters before applying biological wastewater treatment.