Background& Objective: The advantage of Attached growth to the suspended growth systems Includes easier administering, high efficiency against shock loads, increasing concentration so contamination stand produce less sludge. For the above reasons the design of a system that benefits both suspended and attached growth system moving bed biofilm reactor may benefit (MBBR( was designed including acne that removing into the reactor and it sue is expanding day by day in the world. Objective in study Comparison between conventional activated sludge and MBBR system is the reduction of COD in sewage.

Materials and Methods: Studied at the Screener pilot wastewater treatment plant was installed in Rasht. In this project at first pilot on use of two reactor metal parallel (MBBR)chock- full of Acne duct shielded wire the percentage filling %45 to immerse and activated sludge reactor as a witness to beginning started. Sampling was conducted to checking efficiency removal COD of end each reactors in retention times 5,8,12 hours began and in every stage amount TSS, DO, MLSS, were determined. This Study purely was in environmental conditions and temperatures between 13 and 27ċ.        Results: Removal efficiency of COD in retention times of 5,8,12 hours, were %77.8, %88, %90 respectively. Biological mass and SRT in these retention times were 6153,9833,10533 mg/L 21, 46 and 69 days respectively. In state of optimum (MBBR) system, the efficiency was 20% more than conventional activated sludge.

Conclusion: It is concluded that COD removal efficiency of (MBBR) system is greater than conventional activated sludge in waste water treatment plant of city of Rasht.