Performance Evaluation of Petrochemical Wastewater Treatment System with Respect to Improve the Efficiency Using Mass Balance Analysis

Abstract

Introduction: Tabriz petrochemical wastewater treatment plant employing extended aeration activated sludge system and designed to treat the wastewater generated from different units of petrochemical complex with reuse purposes of treated effluent. Wastewater treatment system consists of a screening unit, an API, an equalization basin, coagulation and flocculation, DAF system, aeration tanks, primary and secondary clarifier and filtration. In this study, mass balance calculation and data reconciliation were applied to improve petrochemical wastewater treatment plant data and the resulted data were used to evaluate the performance of treatment plant. Microbial structure of activated sludge was also evaluated. Methods: 12-h composite samples were carried out in 4 days within 6 months and were analyzed for quantitative and qualitative parameters in accordance to standard methods. Daily average values for 209 days from the inlet and outlet of the plant obtained from WWTP documentation center along with the results of four sampling runs in this work were used for data reconciliation and performance evaluation of the plant. Biochemical tests and PCR- based amplification of 16S rRNA using universal primers and sequencing were applied for identification of bacterial structure. Analysis of fungal communities was performed using morphological characters. Results: The results show that standard deviation and relative errors in the balanced data of each measurement decreased, especially for the process wastewater from 24.5 to 8.6% for flow and 24.5 to 1.5% for COD. The errors of measured data of –137 m3/d (– 4.41%) for flow and 281 kg/d (7.92%) for COD were observed in the results. Based on the results, the calculated BOD5/COD ratio was about 0.73. Overal performance of WWTP in elimination of BOD5, COD, oil and grease, TSS, phenol and benzene were 93, 91.1, 66, 98, 99.8 and 99.6%, respectively. According to the balanced data, the removal rates of COD and BOD5 through the aeration unit were 37 and 46% respectively. In addition, the COD and BOD5 concentrations are reduced by about 61.9% (2137 kg/d) and 78.1% (1976 kg/d) respectively, prior to biologicalII reactor. At the same time, the removal rates of benzene, toluene and styrene were 56, 38 and 69% respectively. The results revealed that about 40% of influent benzene (75.5 kg/d) is emitted to the ambient air above the equalization basin. Based on microbial analysis, the genera of Sphingopyxis, Schlegelella, Pseudomonas, Bacillus and Cloacibacterium were identified as dominant bacterial genus, and also Geotrichum sp., Chrysosporium sp. and Cladosporium sp. were identified as a predominant fungal genus in activated sludge. Conclusion: Based on results obtained from data reconciliation and mass balance calculations, overal performance of WWTP was insufficient in elimination of petrochemical pollutants, and it is concluded that the volatilization of organic compounds is the main mechanism for the removal of volatile organic compounds (VOCs) and it corresponds to the main part of total COD removal from the WWTP. Results showed that increasing in concentration of pollutants in influent wastewater leads to increasing their final effluent value above national guidelines for surface water discharge. Microbial analysis results show that the most of isolated bacterial and fungal strains in this study have been documented as capable degraders of pollutants present in petrochemical wastewaters