PE Exam Practice Problem #32: Water Resources | Environmental Engineering - Mass Balance

Welcome to SolvedIn6: Free practice problems for the Professional Engineering Exam! Each question is styled after those created by the NCEES, and is usually solved in under 6 minutes, which is the average time you will have to solve each question on the Professional Engineering Exam.
Today's problem is another mass balance problem that you might have learned about in an undergraduate environmental engineering class. The hardest part of this problem may be just recognizing that. The reference manual isn't really needed here, but you could look up the equation for activated sludge treatment referenced in the video if you think it will help.
Step 1: Solve for the current dissolved oxygen concentration downstream of the mixing point.
Step 2: Use the 5mg/L min allowable concentration to find how much lower the flow in the river can drop. Replace the original flow in the river with the term Q1 on both sides of the equation.
Step 3: Subtract Q1 old - Q1 new.
If you would like more Water Resources practice problems, please check out the rest of the playlist which this video is part of.
Please note that newer versions of the reference manual may have been released since this video was recorded. The search terms are usually still correct, but the page numbers listed in the video will be different.