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  Background : Kinetic models in natural wastewater treatment systems design can have an important role to help designing other parts from aspects of physical, hydraulic and process in real scale, and to determine the strengths and weaknesses of system. So, the initial aim of wetlands design is removal of organic matter is , BOD₅ and COD, so. Determining the kinetic coefficients is necessary.

  Method: In the present study, reaction kinetics of biological removal of organic substances in wastewater in subsurface reed beds have been investigated. For this purpose, samples from three sites in the lagoon were taken monthly during 6 months and the removal of organic materials, sBOD5 and sCOD was calculated. Afterward, the results were analyzed using the first order kinetic models, second order, Stover - Keane canon and statistical parameters.

  Results: Results showed the average concentration output of sBOD5 and sCOD in control of wetlands and reed, were 101.49, 56.92 and 81.78 and 46.18 mg/L, respectively. In the predicted behavior of organic matter removal, Stover - Keane Canon model presented closest connection between theoretical predictions and data of field reed beds. And the maximum coefficient of organic matter (U_Max) in the wetlands with reed sBOD5 sCOD and control were 120.48, 142.85 and 33.3, 31.25 mg/L, respectively.

  Conclusion: According to the results, due to atmospheric conditions and design of Yazd wastewater treatment plant, the efficiency of this plant in removal of organic matter was high Thereby provides sufficient reducing pollution and efficiency to meet the effluent outlet standards.