Effect of retention time and temperature on the efficiency stabilization ponds in treatment of petroleum wastewater

Biological treatment method is one of the best and most cost-effective methods for the treatment of contaminated wastewaters. The purpose of this study was to evaluate the effect of retention time and temperature on the efficiency of stabilization ponds in removing organic compounds, N-N 3 and PO4 from oil refinery wastewater. The method used in the present study was of an experimental type, in which an anaerobic stabilization pond with dimensions of 1 × 0.2× 1 m and a laboratory scale facultative pond with the capacity of 400 L were designed, set up and operated, using fiberglass sheet with a thickness of 6 mm. In this study, the hydraulic retention time of 2 and 5 days for anaerobic pond and 5 and 10 days for facultative pond were set in both hot and cold temperatures. Then, N-NH3 and PO4 parameters were measured at the wavelengths of 425 and 690 nm respectively. Also, TCOD, SCOD, TBOD, SBOD and pH of samples were measured according to the standard methods for water and wastewater tests. These results indicated that the retention time and temperature had a significant effect on the efficiency of stabilization ponds, so that the efficiency of anaerobic and facultative stabilization ponds in the removal of organic compounds was significantly increased with the increase of retention time and temperature (p<0.05). The maximum removal efficiency for COD, BOD, N-NH3 and PO4 was obtained to be 93.31, 93.66, 88.15 and 73.17 % respectively (at the retention time of days of anaerobic and 10 days of facultative and hot temperature), and the minimum removal efficiency for the same parameters was obtained to be 41.47, 40.55, 34.26 and 22.46% respectively (at the retention time of 2 days of anaerobic and 5 days of facultative and hot temperature). Also, the single alga available in the facultative pond was formidiom and it was able to grow in high concentrations of sulfur. The results of this study showed that the pilot-scale stabilization ponds in hot temperature, if operated proprly, have the ability to remove organic compounds with a high efficiency. However, in cold weather, its efficiency is low but in acceptable level.