two environmental eng questions

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pinkpig

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1. Is BOD5 value always assumed to be at 20 degree, even if the water temperature given is at different tempature, say 25 degree?

2. If inflow from primary settlement tank to aeration tank is Q, and recirculation rate R, the 2nd clarifier surface loading rate is Q/A or (1+R)Q/A? A is surface area of the clarifier tank. CERM is giving this value as (1+R)Q/A, while Metcalf Eddy Book is saying this value is Q/A. Which one is the one I should use for PE exam?

Thanks.

 
1. Is BOD5 value always assumed to be at 20 degree, even if the water temperature given is at different tempature, say 25 degree?2. If inflow from primary settlement tank to aeration tank is Q, and recirculation rate R, the 2nd clarifier surface loading rate is Q/A or (1+R)Q/A? A is surface area of the clarifier tank. CERM is giving this value as (1+R)Q/A, while Metcalf Eddy Book is saying this value is Q/A. Which one is the one I should use for PE exam?

Thanks.
1. you may have to find the BODu at a temperature other than 20 degrees using the equation 28.29 from CERM. You can also use equation 28.31 to find BOD at a temp other than 20 degrees.

2. Use (1+r)Q/A when there is recirculation.

Please correct me if i am wrong.

 
Yeah the BOD rate coeffcients and all that are given for 20 degrees. If you need it at another temperature, you use a correction factor, which is generally given. You end up with something like:

kt = Theta^(t-20)

As for the second one, I honestly don't remember. WW calcs were never my strong suit.

 
I agree with Shree on the second one. You've got to use the recirculation rate. Actally, there is a workout on NCEES 2000, #1 If I remeber correctly since I don't have it with me now, in ENV depth that I taught is very helpfull in understanding the mass balance between different treatment tank flows.

 
Meltcalf and Eddy book says because RQ is withdrawn from the tank, although inflow is Q+RQ, the acutal upflow is Q. I googled this question around and found several different other sources are also using Q only. But I just do not believe CERM is wrong on this critical issue. Thanks for all the replies.

 
I agree with Shree on the second one. You've got to use the recirculation rate. Actally, there is a workout on NCEES 2000, #1 If I remeber correctly since I don't have it with me now, in ENV depth that I taught is very helpfull in understanding the mass balance between different treatment tank flows.
Are you talking about the trickling filter? for trickling filter, I do not have a problem with hydraulic loading rate (1+R)Q/A, but settling tank is a little bit different, that's why I got confused.

 
Are you talking about the trickling filter? for trickling filter, I do not have a problem with hydraulic loading rate (1+R)Q/A, but settling tank is a little bit different, that's why I got confused.
Can you post the question, if it is not a voilation of copy right? Better to see the whole picture.

 
Meltcalf and Eddy book says because RQ is withdrawn from the tank, although inflow is Q+RQ, the acutal upflow is Q. I googled this question around and found several different other sources are also using Q only. But I just do not believe CERM is wrong on this critical issue. Thanks for all the replies.
It all depends on what you do the mass balance around. I find it extremely helpful to draw a schematic of the process(s) described in the problem, then draw a box around the process (in your case, the sed. tank), then solve for what the problem wants. In = Out, always. I tend to agree with Tido. I would always use (1+R)Q/A. If the recirculation is not part of the components in and out of your box, then R=0 and the term disappears, leaving you with Q/A. Hope this helps. Good luck in 3 weeks, 2 days!

 
2. If inflow from primary settlement tank to aeration tank is Q, and recirculation rate R, the 2nd clarifier surface loading rate is Q/A or (1+R)Q/A? A is surface area of the clarifier tank. CERM is giving this value as (1+R)Q/A, while Metcalf Eddy Book is saying this value is Q/A. Which one is the one I should use for PE exam?
I am assuming the treatment schmatic would look something similar to the generic Primary Clarifier going to Aeration Chamber going to Secondary Clarifier with a recycle loop from secondary clarifier back to front end of aeration chamber. If the treatment train looks different, please advise.

In this case, I would say that the surface loading rate is going to be Q/A because you are evaluating the flow coming into the FRONT END of the Secondary Clarifier. My references say that it is Q and if M&E says it is Q then I believe that is the correct answer. My caveat to this response is to look at which Q is being discussed - I recommend drawing a mass balance diagram so you can be certain which Q is being used.

Be vary wary of mistakes in the CERM - there are CRTICAL errors in that reference. Also, make sure you are careful with nomenclature. Many people when conducting the mass balance analysis on the clarifier system make a habit of differentiating the flow streams into the various fractions (e.g. primary flow from return (waste) flow) so that it is very clear what flow you are utilizing.

I hope this helps. Please feel free to ask additional questions. :)

Best of luck on your exam!

JR

 
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