Question

A FWS constructed wetland is being designed to polish grey water from a school building in Indiana. The grey water characteristics are 80 mg/L BOD, 20 mg/L TSS and 10 mg/L nitrogen (as ammonia nitrogen). They would like their constructed wetland to reduce their BOD to 15 mg/L before the water exits the wetland. No treatment objectives are required for TSS, nitrogen or pathogens in the wetland. Decaying plant matter provides a background BOD concentration (C*) of 7 mg/L. Assume a temperature in the wetland of 12ºC, and a kA,20ºC of 117 m/yr for BOD. The other design parameters are:  Q = 80 m3 /d,  Safety factor multiple = 1.2  Aspect ratio = 8:1  Average water depth (d) = 0.5 m  Bed slope = 1% (0.01 m/m)  Manning’s resistance factor (a) for sparse vegetation of 0.487 Design the wetland and calculate the head loss for the FWS constructed wetland. Is the head loss too great for the design to work? (Note: You do not need to design the individual zones 1-3 for this assignment. You may stop at number 4) below). Five Steps to Design FWS Constructed Wetlands 1) Determine the limiting effluent requirements for BOD, nitrogen, or pathogens 2) Calculate the surface area for BOD, nitrogen, and pathogens (the largest area will control the design); increase the area by a safety factor of 20% (multiply by 1.2) 3) Select the aspect ratio (AR) based on site constraints. Calculate the surface dimensions. 4) Check the head loss to ensure that it is smaller than the elevation difference between the inflow and outflow points. This allows continuous flow. 5) Design zones 1 through 3 based on hydraulic residence time (HRT), volume, flow rate, and calculated length and width.

A FWS constructed wetland is being designed to polish grey water from a school building in Indiana. The grey water characteristics are 80 mg/L BOD, 20 mg/L TSS and 10 mg/L nitrogen (as ammonia nitrogen). They would like their constructed wetland to reduce their BOD to 15 mg/L before the water exits the wetland. No treatment objectives are required for TSS, nitrogen or pathogens in the wetland. Decaying plant matter provides a background BOD concentration (C*) of 7 mg/L Assume a temperature in the wetland of 129C, and a kA,20°C of 117 m/yr for BOD. The other design parameters are: . Q 80 m2/d, .Safety factor multiple 1.2 Aspect ratio 8:1 Average water depth (d) 0.5 m Bed slope: 1% (0.01 m/m) Manning's resistance factor (a) for sparse vegetation of 0.487 . Design the wetland and calculate the head loss for the FWS constructed wetland. Is the head loss too great for the design to work? (Note: You do not need to design the individual zones 1-3 for this assignment. You may stop at number 4) below). Five Steps to Design FWS Constructed Wetlands Determine the limiting effluent requirements for BOD, nitrogen, or pathogens Calculate the surface area for BOD, nitrogen, and pathogens (the largest area will control the design); increase the area by a safety factor of 20% (multiply by 1.2) Select the aspect ratio (AR) based on site constraints. Calculate the surface dimensions. Check the head loss to ensure that it is smaller than the elevation difference between the inflow and outflow points. This allows continuous flow. Design zones 1 through 3 based on hydraulic residence time (HRT), volume, flow rate, and calculated length and width. 1) 2) 3) 4) 5)

Answer 1

G.T Manning's coefficient(n) is a function of the density of the vegetation and the flow - depth: where, n = a/1/d^2_w where a = resistance - factor, s.ft^t/6 Head loss in a IWS wetland can be calculated using the following modified - eq^n. i.e v - 1/n (d^2/3_w)(N^1/2) where: - v = liquid flow velocity, ft/s n = Manning's coefficient s/f't^13 d_w = depth of water in wetland, ft s = hydraulic gradient or slope of the water surface ft/ft