Seyed Mahdi ghasemi - Expert at Process Group, Toossab Consulting Engineers Company, Mashhad, Iran. Assistant supervisor, Research Center for Membrane and Separation Processes, Ferdowsi University of Mashhad, Mashhad, Iran
Mahsa Sadeghi Vazin - M. Sc. Student, Department of Civil Engineering, Faculty of Engineering, University of Ferdowsi, Mashhad,Iran

According to the standard rules of effluent discharge to surface waters and the importance of nutrient removal in municipal and industrial wastewater treatment processes, EBPR processes have attracted lots of attention due to the low cost compared to the chemical methods, and the reduction of environmental risks. In most WWTPs, particularly medium and low capacity treatment plants, engineers tend to use extended aeration processes to stabilize the sludge in the liquid line, and as a result, to reduce the costs of the wastewater treatment and sludge processing. Studies indicate that increasing solid retention time leads to better sludge stabilization, improve BOD removal, and complete the nitrification reaction. In addition, the denitrification process would not be a problem. However, increasing SRT reduces the rate of EBPR reactions, and consequently, lowers the quality of the effluent. In this paper, the effect of SRT on EBPR reactions was investigated. Three scenarios were analyzed to improve effluent quality at high SRT and complete sludge stabilization. The results show that in high SRT, the desired effluent quality can be achieved by removing the primary settling tank, increasing the retention time of the anaerobic tank, and reducing the nitrate concentration in the returned sludge stream.

Solid Retention Time, EBPR, A۲O, Extended Aeration, BNR