en تخصصي Special Original articles <div style="text-align: justify;"><strong><em>Introduction:</em></strong>&nbsp;Extended aeration activated sludge system has a lower sludge production than other activated sludge processes. Due to its high hydraulic retention time (HRT), the tolerance of this process is higher than the shocks caused by increased organic loading rate. The main objective of this study is to evaluate the performance of the aeration system in removing physicochemical and microbial parameters from the wastewater of Nowshahr, Iran.<br> <strong><em>Materials and Methods:</em></strong>&nbsp;This is a descriptive-analytical study that was carried out in the wastewater treatment plant of Nowshahr during 6 months. The parameters BOD₅, COD, TSS, total coliform (TC) and fical coliform (FC) were measured, and also the MLSS, F/M ratio, SVI, HRT and &theta;c were measured in aeration basin. Data were analyzed using the Excel software and SPSS (Pearson correlation test and one-sample <em>t</em>-test), and <em>P</em> < 0.05 was considered significance level.<br> <strong><em>Results:</em></strong>&nbsp;The average removal efficiency of BOD₅, COD, TSS, TC and FC was 57.7%, 61.4%, 70.8%, 84.6% and 84.3% respectively. The &theta;c, HRT, SVI, F/M and MLSS in the aeration basin were obtained, respectively, 5.64 day, 25 h, 48.83 ml/g, 0.28 day^-1 and 180 mg/L. In addition, the average output of parameters in the hot months of the year was higher than those in the cold months.<br> <strong><em>Conclusion:</em> </strong>According to the results, the Nowshahr wastewater treatment plant has the adequate efficiency to produce effluent in accordance with environmental standards for discharge into surface water and consumption in agriculture.</div> Activated Sludge, Extended Aeration, Wastewater Treatment, Nowshahr City. 509 517 http://jehsd.ssu.ac.ir/browse.php?a_code=A-10-1-80&slc_lang=en&sid=1 Mohammad Sadegh Nikmanesh No Health Care Network of Chalus, Deputy of Health, Mazandaran University of Medical Sciences, Mazandaran, Iran. Hadi Eslami No Environmental Science and Technology Research Center, Department of Environmental Health Engineering, School of Public Health, Shahid Sadoughi University of Medical Sciences, Yazd, Iran. Seyed Mojtaba Momtaz No Environmental Science and Technology Research Center, Department of Environmental Health Engineering, School of Public Health, Shahid Sadoughi University of Medical Sciences, Yazd, Iran. Rahmatollah Biabani No Health care Network of Chalus, Mazandaran University of Medical Science, Mazandaran, Iran. Amir Mohammadi No Environmental Science and Technology Research Center, Department of Environmental Health Engineering, School of Public Health, Shahid Sadoughi University of Medical Sciences, Yazd, Iran. Babak Shiravand No Department of disaster medicine, School of Public Health, Shahid Sadoughi University of Medical Sciences, Yazd, Iran. Tahereh Zarei Mahmoudabadi Yes Environmental Science and Technology Research Center, Department of Environmental Health Engineering, School of Public Health, Shahid Sadoughi University of Medical Sciences, Yazd, Iran.