Exam Study Guide: Plant Operating Condition and Transmission Line Flow Rate

Maintaining your education is important, especially in a career that demands licensing exams. Prove you’re an expert operator by answering these questions and others from our Exam Study Guide Series.
Exam Study Guide: Plant Operating Condition and Transmission Line Flow Rate

Welcome back to TPO magazine's new and improved Exam Study Guide Series, which offers a pair of water/wastewater study questions each week with in-depth explanations of the answers. We covered a set of wastewater and drinking water treatment questions last week on the topics of the MLE Process and Monochloramine Production. This week, you can test your knowledge about overall wastewater plant operating condition and water transmission main flow rate. Take a look at the multiple-choice sample questions and answer explanations below.

Sample Question No. 1:

The operator of the extended aeration activated sludge treatment facility running at a 10 day solids retention time (SRT) completes a monthly microscopic examination of a sample of mixed liquor suspended solids (MLSS) from the effluent end of the last aeration tank before the clarifiers. He looks at three slides and documents the following results:

Based on all the information given above, what is the best assumption of the current plant operating condition?

A) The facility is running optimally and no changes need to be made.

B) The MLSS and SRT are too high and the waste rate should be increased.

C) The F/M Ratio is too low and the waste rate should be increased.

D) The MLSS and SRT are too low and the waste rate should be decreased.

Answer: The answer is D, the facility MLSS and SRT are too low and the waste rate should be decreased. Remember that the question asks to base your answer on all the information described, including the stem of the question which states the extended aeration plant is operating at a 10 day SRT. This is on the low side of the normal operating range for extended aeration, which is between 20-30 days with an F/M ratio between 0.05-0.15 lbs BOD/lb. MLVSS/day. Typical MLSS values range between 2,000 to 6,000 mg/L.

A microscopic examination of typical extended aeration MLSS will normally reveal a predominance of rotifers, nematodes (round worms), stalked ciliates, with some free swimming ciliates, but few (if any) flagellates and blob (proteus) amoeba. There may be many shelled (testate) amoeba observed, and they resemble brown doughnuts. The sample results shown are more representative of a young sludge age commonly found in conventional or contact stabilization activated sludge facilities. The operator will need to decrease the waste rate to build the MLSS up, which raises the SRT and decreases the F/M ratio. On a side note, we recommend doing a micro exam of the MLSS more often than once per month! A few times per week is more acceptable.

Sample Question No. 2:

An 18-inch water transmission main is one mile in length. If the velocity in the main is 4 fps, what is the gallon per minute flow rate in gpm? Select the closest answer.

A) 52.9 gpm

B) 708.0 gpm

C) 3,177.6 gpm

D) 5,296.4 gpm

Answer: The answer is C, 3,177.6 gpm. Before I describe the steps I used to arrive at this answer, let me first say a few things about solving math questions. I recognize that there is usually more than one method to arrive at a correct, or at least a close to correct answer. In solving this word problem, I used the standard method of rounding (anything over five, round the preceding digit to next highest value) and I rounded out to the hundredth digit (two values to the right of the decimal place).

I also think it's valuable to identify how the distractors, or fake answers, are generated in many exam situations. As you work the math, you might recognize some of the numbers used to make the incorrect answer choices, but I hope you also recognize that the units shown with those answer choices are not correct. For example, answer choice B, 708.0 gpm, is actually the flow rate as cubic feet per second with the decimal in the wrong space. Watch out for traps along the way to solving the question that might draw you into picking an incorrect answer!

Calculation steps:

The answer uses the following formula, as adapted from State of Florida Drinking Water Operator and Water Distribution System Operator Math Formula Sheets: Flow Rate, gpm = (Area, sq. ft.)(Velocity, ft/sec)(7.48 gal/cu ft)(60 sec/min) or Q = V x A x 7.48 x 60

Convert diameter (D) inches to feet: 18-inch pipe opening ÷ 12 inches per foot = 1.5 feet. Then, calculate the cross-section area, in square feet, of the pipe: A = 0.785 x D x D. A = 0.785 x 1.5 x 1.5. Area = 1.77 square feet. (I used 0.785 x D x D instead of 3.14 x R x R, but you can use either method).

Next, calculate the cubic foot volume of just the 4 foot length given as velocity in ft/sec. The total pipe length of one mile is not needed! This question is about the how fast the 4-foot section of water in the pipe is moving, not the total gallons. Volume, V = A x length. V = 1.77 square feet x 4 feet. Volume, cu ft = 7.08. Note that this is actually 7.08 cubic feet per second, cfs, because the 4 foot number used was the velocity as 4 feet per second. The second units remain with the value as 7.08 cubic feet per second, cfs.

Now, calculate the gallons per second using the conversion value of 7.48 gal per cubic foot. Gallons per second = 7.08 cfs x 7.48 gallons per cubic foot. Gallons per second = 52.96.

Finally, convert to gpm by multiplying the gps by 60 seconds per minute: 52.96 gps x 60 seconds per min = 3,177.6 gpm.

That’s a lot of steps for one answer that may be only worth one point on an exam! But it may be the one extra point that rewards you with a 70 percent passing score. My advice to readers who struggle with math is to practice, practice and practice some more. Use the steps I described above, but change the initial numbers a bit to make it a new question (pipe diameter, velocity, etc.) then work it out again. State exams usually have math formula sheets, so practice with them to help you gain confidence in your math skills. You can do it!

About the author: Ron Trygar is the senior training specialist for water and wastewater programs at the University of Florida’s TREEO Center. Previously, he was the wastewater process control specialist at Hillsborough County Public Utilities in Tampa, Florida. He has worked in the wastewater industry for more than 30 years in a variety of locations and positions. Trygar became a Certified Environmental Trainer (CET) in 1998 and has since provided training for associations and regulatory agencies such as Florida Department of Environmental Protection (FDEP); Florida Water and Pollution Control Operators Association Short Schools; USABlueBook; Florida Water Environment Association sponsored training events; and local school environmental programs. Working alongside the FDEP Northeast District, Trygar helped begin the Florida Rural Water Association and FDEP joint operator certification review classes that are still given around the state today. He holds a Florida Class A wastewater treatment operator’s license and a Florida Class B drinking water operator’s license.


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