Background: Moving bed biofilm reactor (MBBR) is one of the new hybrids biological systems that are used in treatment of wastewater contaminated by organic and nitrogen pollutant. In this study, the continuously operated laboratory scale Kaldnes(k1) moving bed biofilm reactors (MBBRs) was investigated.

  Methods: In this system, two series as Anoxic reactor (R1) and aerobic (R2) have a size 3.5 and 10 liters, respectively. This system was operated at temperature 28.5 ^oC , pH=7-8, DO= 1-1.5 mg/l and internal recycle ratio in continuously. All tests based on the standard methods were performed. The Anoxic and Aerobic reactors were filled to 40 and 50 %( v/v) to attach and retain biomass with k1 biofilm carriers, respectively. Statistical analysis software was SPSS and EXCEL.

  Results: During operation, the average dissolved oxygen in the aerobic reactor was 1-1.5 mg/l. The average removal efficiency of soluble organic carbon (SCOD) was obtained over 95% at different load organic matter (300-2000 mg/l) in the anoxic reactor. The average removal efficiency of total nitrogen (TN) occurred in the range of 80-99.4% at different load nitrogen (25-250 mg/l) and maximum removal efficiency based on biofilm surface area occurred close 92% at loading rate of 0.127-0.181 mg-N/m2.day. The results show that nitrification rate was increased to increase loading rate of nitrogen compounds but the rate of nitrification was decreased at high loading rate. Probably this was due to an excessive accumulation of nitrite ion in the aerobic reactor.

  Conclusions: This investigation showed that the process of removal organic and nitrogen compounds in the MBBRs system under pre-denitrification without recycle sludge and external carbon source has an acceptable performance for treatment of wastewater with high load organic carbon and organic nitrogen compounds.