With regard to water crisis in recent years and significance of conserving water resources and reducing of water shortages, the use of recycled wastewater is unavoidable. On the other hand, wastewater treatment is essential to ensure public health. Some of the most important applications of the recycled wastewater are irigation, groundwater recharge, fish farming, and recreational uses. Use of treated wastewater for irrigation is not only a water source but also a nutrient-rich fertilizer for plant growth. Therefore, treatment and reuse of wastewater in agriculture is one of the most efficient methods for maximizing of the water resources usage and minimizing of the fertilizer costs. On the other, Construction of wastewater treatment plant for small and scattered societies is not feasible due to economical and technical reasons. On-site wastewater treatment system which is less expensive and needs less expert operators that is a solution to the above mentioned problem. Simultaneous removal of organic, nitrogen and phosphorus compounds in a single bioreactor is of important issue in items of reactor volume and energy consumption. In this study, an up-flow anaerobic /anoxic /aerobic bioreactor under one feeding regime and continuous-feed, were evaluated for the treatment of an industrial wastewater with low BOD 5 /COD ratio. The process of performance in each regime was compared, Two numerical variables (total retention time and aeration time) were selected to analyze, model and optimize the process. The region of exploration for the process was taken as the area enclosed by retention time (22-42 hr) and aeration time (4-12 hr) boundaries. The experiments were conducted based on a central composite design and analyzed using response surface methodology. In order to analyze the process, twelve dependent parameters a were studied the process responses. As a result, retention time showed a decreasing impact on the responses in the hydraulic regimes, AAO. The AAO showed better performance in removal of COD, TN, PO 4 and TSS. This study showed that the COD ratio was a key factor affecting the systems performance removing the nutrients. In the continuous system, at hydraulic retention time (HRT) of 30 hr, the removal efficiency for BOD, COD, TN, PO 4 and TSS were achieved 90, 95, 82, 90 and 91.3 % respectively.