Question

You are tasked with conducting BOD studies for a waste treatment facility and its discharge to a nearby river. You are told that the facility removes 93% of the BOD in total with a discharge rate of 1.0 m3 /s. you sample the receiving water and find that it has a 0.5 mg/L deficit from saturation concentration (saturation is at 9.7 mg/L) and a BOD5 content of 10 mg/L. The river flows at a speed of 0.25 m/s and has a flow rate of 9.0 m3 /s. The reaeration constant is known to be 0.49 d-1 , and the deoxygenation constant for the river is 0.20 d-1 .

a. In a 5-day BOD test of untreated wastewater in a 300 mL bottle, you measure an initial DO of 9.7 mg/L and a final DO of 3.4 mg/L. If the dilution factor was 0.025, what is the BOD concentration in the influent wastewater?

b. What is the largest volume of treated wastewater that you can use to test the BOD in the effluent, if you must have at least 2.0 mg/L of DO in the bottle after 5 days?

c. Assuming complete and instantaneous mixing, estimate the ultimate BOD of the river just downstream of the outfall.

d. If the stream has a cross-section that is approximately constant, flowing at a fixed speed, estimate the BOD remaining in the stream at a distance of 25 km downstream.

e. Estimate the oxygen deficit in the river just downstream of the outfall (assume the wastewater is aerated to saturation).

f. How far downstream will the DO reach a minimum? g. What is the largest DO deficit that can be expected in the stream?

Answer 1