# Critical Oxygen Deficit Calculation



## Take1 (Apr 7, 2008)

Chapter 25, problem 3. ©. in the 2nd edition of the practice problems for the Environmental Engineering PE Exam (Lindeburg), Critical oxygen deficit is calculated using equation 25.37. Yet, for problem 2.4 on pg 58, (solutions) in the 2nd Edition of Environmental Engineering Solved problems (Schneiter), equation 25.34 is applied adding on the initial Oxygen deficit. Which equation applies to calculate the critical deficit and why?


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## Dleg (Apr 7, 2008)

Well, it's been a year and a half for me on these problems, but after looking through my notes and re-trying the problem, I get the same answer for Schneiter's problem 2.4 using either equation. So, I would guess that they are equivalent.

The thing to keep in mind (and something that I wrote as notes in my ENVRM beside these equations) is to make sure you substitute base-e for base-10 if the "river constants" are given to as base-e. Don't get unduly confused as to how to tell ( as in lower case k = base-e, and upper case K = base-10) - I think you can count on NCEES telling you if the constants are base-e or base-10 in the problem statement, if you see this type fo problem on the real exam.

Using Schneiter's nomenclature, his "k1" = "Kd" in the ENVRM, and his "k2" = "Kr".

I got 5.46 using Schneiter's equation and my own answers to the other parts of the question, which isn't what he got, but it was still within "most correct" range. Using the simpler equation 25.37, I got 5.77, which is also within range. I wouldn't worry about it - use the equation you are most comfortable with, or even the one that will give you the answer fastest. I think when there are two or more different equations that are accepted, you can count on the range of possible answers to be broad enough to accomodate any (accepted) method.

Sorry I can't give any "real" guidance on which to use. (I've never dealt with any of this on the job - I do ocean outfalls and use the visual plumes to model it.)


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## RIP - VTEnviro (Apr 8, 2008)

It's been two years since I've done this, so the specifics are vague, but we covered this thoroughly in undergrad and grad classes I took, so I remember the concept. I believe this is the general procedure:

1. Find the initial O2 deficit, D0. (O2sat - O2 at discharge point)

2. Find the critical deficit, Dcr

3. Find O2min, (D0-Dcr)

4. Find the travel time at which the O2min occurs, tcr

5. Find the distance donstream at which O2min occurs

This link may help.


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## Guest (Apr 8, 2008)

I don't have the EERM (I took the Civil PE), so I don't know which two equations to which you refer. However, the general equation for finding Oxygen Deficit is based on the Streeter-Phelps Equation. I believe you can represent it in different forms - most likely the two equations ARE equivalent but just presented differently and maybe even have different parameter inputs based on substituation.

If you still have questions, please feel free to post a follow-up and include the problem since I don't have the reference.

JR


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## Take1 (Apr 8, 2008)

All,

thanks so much for the help. MUCH Appreciated!!


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