Help needed for EnvE practice problem (Wastewater)

[GTjoy]

Sorry if this is a stupid question.

In the Lindeburg Practice Problems book (the companion to the Lindeburg ENVRM), problem 25-1, the question is as follows:

1. A town of 10,000 people (125 gpcd) has its own primary treatment plant.

(a) What mass of of total solids should the treatment plant expect?

The solution says to assume 125 gpcd for average flow, and 800 mg/L total solids.

Where does the assumption for 800 mg/L total solids come from? Is it referenced in ENVRM, and if so, where? Do you know of another source that gives this as a typical value? Table 25-4 in ENVRM gives a range of values 350 to 1200 mg/L for total solids in domestic wastewater.

I hope someone can help!

 

[IlPadrino]

Where does the assumption for 800 mg/L total solids come from? Is it referenced in ENVRM, and if so, where? Do you know of another source that gives this as a typical value? Table 25-4 in ENVRM gives a range of values 350 to 1200 mg/L for total solids in domestic wastewater.

You've got to admit... 800 mg/L is awful close to the average of 350-1200 mg/L. The CERM gives the same "strong=1200 mg/L, weak-350 mg/L" numbers. But I've got one reference (McGraw's Introduction to Environmental Engineering) that says they range from "strong=350 mg/L, weak=100 mg/L". Still, order of magnitude is usually enough to get the right answer on the exam - or worst case, factor of two.

 

[GTjoy]

You've got to admit... 800 mg/L is awful close to the average of 350-1200 mg/L. The CERM gives the same "strong=1200 mg/L, weak-350 mg/L" numbers. But I've got one reference (McGraw's Introduction to Environmental Engineering) that says they range from "strong=350 mg/L, weak=100 mg/L". Still, order of magnitude is usually enough to get the right answer on the exam - or worst case, factor of two.

Thanks for the response. Yeah, it is close to the average but I was not sure if it would be enough to make that assumption, and plus I am just curious if this is something that is standard knowledge.

Thanks also for the tip regarding order of magnitude on the exam. I'll try to keep that in mind from now on, if I get stuck in a similar situation.

 

[IlPadrino]

Thanks for the response. Yeah, it is close to the average but I was not sure if it would be enough to make that assumption, and plus I am just curious if this is something that is standard knowledge.
Thanks also for the tip regarding order of magnitude on the exam. I'll try to keep that in mind from now on, if I get stuck in a similar situation.

What are the four choices given? Would the difference between assumptions have given you a different answer?

 

[Dleg]

I checked my notes on that problem, and I had just simply said "use the average."

However, I wouldn't get too stressed out thinking about what constants to use in the actual exam. In my experience, the vast majority of problems supply you with the constants, probably to avoid just such an issue. After all, in real life, there are many "correct" (i.e. justifiable) selections of assumed constants. A room full of experienced engineers, left to choose their own constants, would likely come up with a wide range of justifiably "correct" answers, which would be a real nightmare for NCEES. Plus, in real life, you would usually have a lot more resources available to you (like real data from the community or a neighboring community) than just blindly picking out a number from a textbook. I think the NCEES problem developers know this and make sure their test questions are "fair" in this regard and easy/noncontroversial to grade.

On the other hand, if you do find yourself needing to pull some wastewater constant out of thin air during the exam, go straight for the Ten State Standards, first. That's by far the most realistic source of data that a "real" engineer would use in the workplace. But as IlPadrino said, the values in the ENVRM will probably get you close enough to be within the correct order of magnitude for the exam.

 

[GTjoy]

What are the four choices given? Would the difference between assumptions have given you a different answer?

in lbm/day:

A. 57

B. 740

C. 3800

D. 8300

The correct answer is D (from 8345 lbm/day). This is based on the 800 mg/L assumption. The latter two values are of the same magnitude, so on test day that might have tripped me up. For example, if I used 350 mg/L, the answer would have been 3651 lbm/day.

 

[GTjoy]

I checked my notes on that problem, and I had just simply said "use the average."
However, I wouldn't get too stressed out thinking about what constants to use in the actual exam. In my experience, the vast majority of problems supply you with the constants, probably to avoid just such an issue. After all, in real life, there are many "correct" (i.e. justifiable) selections of assumed constants. A room full of experienced engineers, left to choose their own constants, would likely come up with a wide range of justifiably "correct" answers, which would be a real nightmare for NCEES. Plus, in real life, you would usually have a lot more resources available to you (like real data from the community or a neighboring community) than just blindly picking out a number from a textbook. I think the NCEES problem developers know this and make sure their test questions are "fair" in this regard and easy/noncontroversial to grade.

On the other hand, if you do find yourself needing to pull some wastewater constant out of thin air during the exam, go straight for the Ten State Standards, first. That's by far the most realistic source of data that a "real" engineer would use in the workplace. But as IlPadrino said, the values in the ENVRM will probably get you close enough to be within the correct order of magnitude for the exam.

Dleg, thanks for your advice. I think I am just really weak in wastewater stuff, so perhaps I am overanalyzing everything. Also, I've never even heard of the Ten State Standards until now (but I did just find a copy online if anyone needs it). So thanks for the tip.

 

[IlPadrino]

in lbm/day:
A. 57

B. 740

C. 3800

D. 8300

The correct answer is D (from 8345 lbm/day). This is based on the 800 mg/L assumption. The latter two values are of the same magnitude, so on test day that might have tripped me up. For example, if I used 350 mg/L, the answer would have been 3651 lbm/day.

Yeah... that would have been a problem. Still, I think Dleg is also spot on when he says you usually won't have to guess an assumption like that - it'll be given explicitly.

 

[FLBuff PE]

^^That's what I recall from my recent attempts (that they provide the values)

 

[snickerd3]

in lbm/day:
A. 57

B. 740

C. 3800

D. 8300

The correct answer is D (from 8345 lbm/day). This is based on the 800 mg/L assumption. The latter two values are of the same magnitude, so on test day that might have tripped me up. For example, if I used 350 mg/L, the answer would have been 3651 lbm/day.

This could show up on the exam as one of those did you read the problem statement correctly problems. They may include extra information to trick into choosing the wrong answer. Very important to double check what they are really asking.

 

[RIP - VTEnviro]

I took the Envl exam 2 years ago. When I was prepping for it, I also encountered some of this practice problems with assume or implied values that affect the result of the solution. I freaked out a bit.

On the real exam, there is no guess work. Everything you need (and often stuff you don't) is given in the problem statement. There are too many reasonable assumptions an engineer could make that will alter the solution - state and local codes, personal experience, reference material, etc.

I had one practice problem on I&I in a sewer. I used the value in the state design manual, they used a different value in the solution and had a totally different answer.

 

[Guest]

in lbm/day:
A. 57

B. 740

C. 3800

D. 8300

The correct answer is D (from 8345 lbm/day). This is based on the 800 mg/L assumption. The latter two values are of the same magnitude, so on test day that might have tripped me up. For example, if I used 350 mg/L, the answer would have been 3651 lbm/day.

I checked several of my references and could not find where 800 mg/L was derived. Most likely this is a 'middle of the road' value arbitrarily selected by the author OR someone has a chart of 'average' TSS values based on volume WW collected per day even though I haven't found anything between AWWA, M&E, and other selected references.

My thought would have been to use 720 mg/L (M&E value) and chug through calc - 1.25 MGD * 720 mg/L * 8.34 = 7,506 lbm/d ~ 8,300 lbm/d (D.).

The key here is recognizing that you should select something in the MIDDLE of the range (~ 800 mg/L) rather than the BEGINNING of the range (~ 350 mg/L).

Please keep in mind that you will encounter this situation on the exam - you will calculate a value that doesn't match up exactly. You will need to be able to use your judgement to be able to select the value that MOST represents the correct answer. So this problem does serve as a means for getting used to addressing problems of that kind.

Good luck!

JR