A wastewater treatment plant (WWTP) discharges treated wastewater to a river. Th

Question

A wastewater treatment plant (WWTP) discharges treated wastewater to a river. The wastewater effluent characteristics are: flow = 1.5 m^3/s, BOD = 30 mg/L, DO = 0 mg/L, and temperature = 25 degree C. The river characteristics just upstream of the wastewater discharge are: flow = 6.0 m^3/s, BOD = 5.5 mg/L, DO = saturation, and temperature = 25 degree C. For this water, k_d = 0.30 d^-1 and k_r = 0.35 d^-1. Look up the DO saturation constant at 25 degree C and calculate the DO in the river just upstream of the discharge point. Find the BOD (= L_0), DO, and oxygen deficit (= D_0) just after the wastewater effluent and river mix. Remember: This is a simple mixing problem with no reaction. Plot the DO from one day upstream to the point of discharge to 15 days downstream of the discharge point as a function of travel time. (Remember that the equations give you the deficit (D) and here you are asked to plot the DO.) (EXTRA CREDIT) Fish hatcheries required more than about 4.5 mg/L of dissolved oxygen. If the river flows at 2 km/day, where in the river should you avoid siting a fish hatchery? Express your answer as a range in km downstream from the discharge point. (This problem is asking you to calculate where along the river the DO is less than 4.5 mg/L.)

Explanation / Answer

1. Flow Q1 = 1.5 m3/s, L1 = 20 mg/L River characteristics after flow of waste water; Q2 = 6.0 m3/s, L2 - 5.5 mg/L

The BOD Lo = Q2xL2 + Q1xL1 / Q1+Q2 = 6 X 5.5 + 1.5 X 20 / 1.5+6 = 9.69 mg/L.

The DOsat - 10.08 mg/L

The initial Do deficit - DOsat - DO = 10.08 - 9.69 = 0.39 mg/L.

2. D15 days = KdLo/Kr-Kd (e-kdt-e-kr t) + Doe-krt

Given Kd - 0.3/day and kr - 0.35/day

= 0.3 X 9.69 / 0.35 - 0.3 (e-0.3x15 - e-0.35 x 15) + 0.39 e-0.35x15

= 2.907/0.05 (e-4.5 - e-5.25) + 0.39 x e-5.25

= 58.14 (1.5 - 1.6) + 0.39 x1.65

= 58.14 (0.1) + 0.64

D 15 = 6.454 mg/L

Dsat = 10.08 mg/l, Do - 0.39, D15 - 6.454. By plotting three points we get the graph with time.

c. Fish hatcheries can be set after 30 km since it travels 2 km/day after 15 days is 6.45 mg/L. Since it requires 4.5 mg/L we can set after 30 km.

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