During the past years, due to scarcity of water resources, the trend has been to move from the conventional open systems to the high density and highly productive land-based recirculating systems. To achieve these goals, 60 recirculating aquaculture systems were constructed and used for rearing rainbow trout. Unfortunately, so far many of these systems have failed their mission because of poor design, inferior management, or inconsistent economics. Hence, in this study, a low-cost closed recirculating system designed and its performance was evaluated. The present system was consisted of three 0.3 m 3 circular fish holding tanks, drum filter, trash submerged biofilter, and UV disinfection unit. The system was designed in a way that only 3-5 percent of the system volume would be replaced by fresh water on a daily basis. Acclimation was accomplished by seeding the filter media with nitrifying bacteria (available from two different sources) and using ammonia excreted by the fish. Following the acclimation period, a comparative analysis on biofilter performance was conducted. During the acclimation period, environmental conditions including: temperature, pH, dissolved oxygen level, hardness, alkalinity, flow rate, and hydrological load were adjusted within the optimum range for bacterial growth and performance. Water quality parameters including; temperature, pH, ammonium, and nitrite concentration in influent and effluent water to and from the biofilter were tested daily. Ammonium and nitrite levels increased to a peak prior to decreasing to a steady state conditions. Ammonium concentration peaked (2.86mg/L) in day 9. Similar dynamics occurred in NO 2 concentration, where the peak (0.7mg/L) was observed in day 29. The complete activation of biofilter was achieved in day 31. Results obtained during 16 days period of system performance, showed a significant decrease (p 0.01) in ammonium and nitrite concentrations of effluent from biofilter compared to influent. Hence, maintaining concentration these items below the accepted levels. The efficiency of ammonium removal through the biofilter unit was within the range of 12 to 52 percent, indicating the high efficiency of present system in nitrogenous waste removal. Based on these results, the good performance of fish (condition factor = 1.52), and the low cost, the designed system can not only be suggested for an intensive culture, but also it could be used as an acceptable effluent treatment unit for minimizing the organic loads of aquaculture waste to receiving waters. Keywords: recirculating aquaculture system, acclimation, nitrifying bacteria, submerged biofilter, nitrite, ammonium, waste.
