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C

Cheese

I'm having trouble posting in Env. section, so I will try here.

Problem is from 6-min solutions - Environmental #54

Question involves finding MPN

I have tried to solve this problem using the appendix for MPN tables from Metcalf-Eddy.

I get 140/100ml (using 5,3,2 positives) - which is exactly answer c. The solution uses some formula I haven't seen and gets solution D.

The hint says to use 'easiest method to approximate' - I would consider the tables the easiest method :(

Am I doing something incorrectly here?

Would the exam have answer choices in such a tight grouping.

Thanks,

Cheese

 
Cheese --

I tend to agree with you, I would consult the tables (Metcalf & Eddy) to look this problem up. I checked the Poisson tables for 5-sample serial dilution with 10-mL, 1-mL, and 0.1-mL - it does come out to 140 counts/100-mL.

The discrepancy between using the Poisson table and the Thomas Equation is based on the 'inaccuracies' of the statistical methods. Using the tables referenced in Metcalf & Eddy was the way to do it based on Standard Methods for the Examination of Water and Wastewater, 15th edition (1980). Notice the age of the book - obviously other methods have been developed such as the Thomas Equation.

You will note in order to arrive at the correct answer using the Thomas Equation, you eliminate the 10-mL results and just use the ones for 1-mL, 0.1-mL, and 0.01-mL. I recall that this is the 'correct' approach, but I can't recall why - it has been a long time since I have 'done' this type of wastewater analysis.

FYI - I think of the Thomas Eq in the simpler form of MPN = (# Positive Samples)*100/SQRT(mL-Total * mL-Negative)

Bottom line, I think you should have some familiarity with MPN, but I wouldn't get too wound up over which method is 'correct'.

The good thing here is that you are thinking. :(

JR

 
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Check the introductory paragraph to the MPN tables in Metcalf & Eddy. It explains why you choose only the three highest dilutions - it comes from the Standard Methods procedures.

"In situations where more than three diltuions have been run, the following rule is applied to select the three dilutions to be used in determining the MPN value (Standard Methods, 1998): Choose the highest dilution that gives positive results in all five portions tested (no lower dilution giving any negative results) and the two next higher dilutions. Use the results at these three volumes in computing the MPN value..." And example table follows.

Defintely use the MPN tables. Don't worry about that equation. Or, at least that what my lab manager wife told me. :)

 
I'm having trouble posting in the enviro section too... so I'll post my question here.

I have a question about an equation in the sample "the other board" test, AM session. In the solution for the first problem, the following equation is listed for the reaction time:

theta = (Si - St)/(KXSt)

where:

Si = influent COD

St = effluent COD

K = substrate utilization rate constant

X = MLVSS

I cannot find this equation in the CERM. Can anyone let me know where this equation comes from?

 
That's an expression for the solids residence time (SRT, or theta_x).

There are several variations on that equation, but essentially it boils down to:

Mass of bugs in the reactor / Rate at which bugs leave the reactor

I've got a couple books describing this in better detail, but we're undergoing an office move and all my stuff is packed. I got all my stuff on this out of "Environmental Biotechnology" by Rittman and McCarty. They do a real good treatment on this.

 
gipper --

Keep in mind when you start talking about wastewater treatment processes, expecially activated sludge, it is based on mass balances. There are numerous derivations of equations for things like MLVSS depending on what 'stage' the sludge is at in the processing.

Having said that and looking at VTE's note, I wanted to point out that I noticed the biological kinetic term (k) in your equation. This lead me to believe we are talking about an equation that utilizes the biological 'decay' within the equation. The 'theta' VTE mentioned is commonly referred to as mean cell residence time and is based on primarily on the 'hydraulics' of the system or in other words the split in flow from was sludge flow rate and treated effluent flow rate. That equation is typically represented as:

theta = (Vol Tank * Conc VSS) / (flow waste sludge * conc VSS in waste sludge + flow treated effluent * conc Vss in treated effluent)

You will note the expression lends itself to a weighted average based on the flows and concetrations.

Now, your equation looks like it is a derivation of the food-to-microorganism ratio where the theta you are solving for is the hydraulic detention time of the aeration tank. Consider the following:

F/M = So/theta*X

U = (F/m)*E/100 = (So - S)/theta*X

I think if you twist these two equations around (from Metcalf & Eddy) you will obtain the expression you have.

Having said all of that - if you are NOT taking Env Depth then I don't think you will need to tinker with these Activated Sludge processes since that is not included in the specifications for the exam :true:

JR

 
Thanks for the answers VTE and JR! I'm very weak in the envio area - I haven't really studied that stuff since college :please: I'm taking the Structures Depth, so hopefully I won't run into activated sludge :true: It was on the "the other board" sample exam morning session, so I thought should spend some more study time on the subject.

I have a copy of Metcalf & Eddy, so I'll look into those two equations JR.

Thanks again - I really appreciate the help!

 
Can someone clarify what "X" is in the MCRT equation? I took a PE prep class last fall and they said that X = MLSS. However, I've been studying a great deal from my Environmental Engineering textbook from college (Davis & Cornwell) and it defines X = MLVSS. CERM also says that X = MLVSS. I'm confused and not sure which one to go with. Are there variations of MCRT equations that use X = MLSS? The equation that I have been using for MCRT = (Volume of aeration basin x MLVSS)/(WAS flow rate x Microorganism concentration in the return line (Xr)).

 
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