The dissolved oxygen (DO) in a stream that receive WWTP effluent cannot drop below 5 mg/L, or the fish will die. The flow rate in the stream and WWTP are 20 ft3/s and 2 MGD, respectively. The DO in the stream is 8 mg/L before mixing with the effluent, while the DO in the effluent is 2 mg/L.

a. Estimate dissolved oxygen concentration (DO) in a stream after it has received effluent from a WWTP. Will the effluent cause the DO to drop below the 5 mg/L limit?

b. If the effluent has a BOD of 20 mg/L, and the BOD in the stream before mixing is negligible, estimate the BOD in the stream after if mixes with the WWTP effluent.

c. Assuming the BOD consumes the oxygen in the stream, and no re-aeration, will the DO drop below 5 mg/L?