Abstract <p>Wastewater usually is treated by using a variety of physical, chemical and biological methods to remove phosphorus and nitrogen from it. This paper examined an experimental investigation to assess the removal of nutrients from synthetic wastewater using a lab-scale Bardenpho process. This study aims to address the impact of hydraulic retention time (HRT) on the nutrients removal efficiency at various internal recycle ratio by using enhanced Bardenpho process. A modified Bardenpho process consists of five reactors (anaerobic, first anoxic, first aerobic, second anoxic, second aerobic) with a secondary settling tank to separate the biomass before to discharge in order to accomplish significant nitrogen and phosphorus removal. The nitrate was recycled into the first anoxic chamber (IR 1), first aerobic chamber (IR 2), and second anoxic chamber (IR 3) in that order after oxidizing in the aeration chamber. Hydraulic retention time of 9.5, 13.5 and 17.5 h were shown to have an effect on the biological reduction of nitrogen and phosphorus using enhanced Bardenpho process. Inlet chemical oxygen demand (COD), total nitrogen (TN), and PO₄ concentrations of 413, 35, and 15 mg/L, respectively, were used to operate at each hydraulic retention time. The HRT of 17.5 h and IR 1 attended maximum removal efficiency of 77.86% for TN at IR 1 and aerobic/anoxic ratio ratio of 1.65, and 68.33% for TP at IR 1 and aerobic/anoxic ratio of 2. While the highest elimination efficiency for COD was 94.92% at IR 1 and total HRT of 17.5 h when the aerobic/anoxic ratio ratio is 2.</p>