Performance evaluation of Tabriz petrochemical wastewater treatment plant

Introduction: Petrochemical industry is one of the major industries playing significant role in the economy of Iran. In general, petrochemical effluents contain various contaminants including suspended solids, organic matters, oil and grease, metal salts, sulfide, ammonia, hydrocarbons, cyanides, volatile organic compounds (VOCs) and other toxic substances. In most of petrochemical complexes, wastewaters are treated by activated sludge process along with the oil/water separation systems as a pretreatment. Since the performance evaluation of wastewater treatment plant is required to assess the effluent quality, to meet higher treatment requirement and to know the feasibility of handling higher hydraulic and/or organic loadings, this study was conducted to evaluate the performance of a full-scale petrochemical wastewater treatment plant.
Materials and methods: 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. The treatment plant was designed to treat the wastewater generated from different units of petrochemical complex with reuse purposes of treated effluent. To evaluate the performance of the treatment plant, 12-h composite flow weighted samples were carried out in 4 days within 6 month and were analyzed for COD, BOD5, TDS, TSS, phenol, cyanide, oil, ammonia and TKN in accordance to standard methods. Microbial structure of activated sludge was also evaluated. Overall performance of the plant and the performance efficiency of each unit were calculated.
Results: According to the results, the actual average influent flow was significantly lower than the average design flow based on the long-term data and our measurements. This increases the hydraulic retention time (HRT) in all units in the WWTP. Based on the results, the values of COD, BOD5 and TSS in the influent and effluent were 1319±230, 967±491 and 227±174 mg/l and 73.6±19.6, 33.6±25.9 and 6.4±5.9 mg/l respectively. The ratio of BOD5/COD was calculated about 0.58 indicates a good potential for biodegradability of the wastewater. The results also revealed that 68.5% removal of COD and 81.9% removal of BOD5 have been occurred in preliminary and primary units while; these units are usually designed for the elimination of oil and suspended solids as well as the equalization of quantitative and qualitative parameters of the influent. At an organic loading of 0.48±0.15 kg/m3.d the removal efficiencies for COD and BOD5 in biological unit were calculated about 37 and 46% respectively. The mean value of MLSS within the biological reactor was 1463 mg/l. In addition, the mean plus standard deviation values of MLSS measured in returned activated sludge (RAS) were 2323 ± 1080 mg/l. According to the daily average concentrations of COD in the effluent of the WWTP, in some cases within the study period, the COD values were exceeded from national discharge standards. Microbial analysis showed that among the isolated bacterial strains, the genus belonging to Alcaligenes, Pseudomonas, Bacillus and Moraxella as heterotrophic nitrifying bacteria was identified as predominant strains in biological unit.
Conclusion: Based on the results, in 64% of our measurements (135 days from 209 days of study period), the daily average effluent COD values were above national guidelines for surface water discharge (60 mg/l). In 3% of the time during the study period, the daily average values of COD in the effluent were more than 200 mg/l. The results also indicated that the average effluent concentration of oil was significantly higher than the expected design value.