A municipality has asked you to evaluate the expected BOD removal performance of

Question

A municipality has asked you to evaluate the expected BOD removal performance of a FWS wetland design, proposed by another engineering firm, to make sure it is adequate. The proposed wetland is to have 4 cells of 4 acres each, and to receive an inflow of 0.7 MGD with BOD of 80 mg/L. The water depth is to be 1.0 ft, k20 will be 0.8 d-1, plant porosity is estimated to be 0.7, average BOD from plant decay is expected to be 5 mg/L, temperature coefficient will be 1.02, and 10% of the inflow is expected to be lost to evaporation as the water passes through the wetland. The municipality has a very stringent NPDES permit that mandates a discharge BOD of 15 mg/L or less, and wishes to know what effluent BOD the wetland can be expected to achieve during the winter when the water temperature is 10ºC. What would you tell your client?

Explanation / Answer

effluent BOD the wetland can be expected to achieve = 0.7 * 80 + 20 * 0.8 + 0.7 * 5 + 1.02 * 0.1

                                                                                      = 75.602   mg/L

permit that mandates a discharge BOD of 15 mg/L

=>   the expected BOD removal performance =    (15 mg/L) / (75.602 mg/L)

                                                                           =    0.1984

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