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Anonymous Poster #1

Activated Sludge Question

01/22/2016 2:09 PM

Hello...Can anyone here prove that the retention time in aeration tank of activated sludge system is equal to V/Q without considering the Qr? Thanks

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#1

Re: Activated Sludge Question

01/22/2016 3:25 PM
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#2

Re: Activated Sludge Question

01/22/2016 3:33 PM

Qr is assumed...

Ref..

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#19
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Re: Activated Sludge Question

01/23/2016 9:26 AM

But what is the idea behind it at first place?

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#22
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Re: Activated Sludge Question

01/23/2016 10:11 AM

Healthy bacteria injection, perpetuating the culture...

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#3

Re: Activated Sludge Question

01/22/2016 5:21 PM

Lyn and SE, you guys really know your . . . .'it.

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#5
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Re: Activated Sludge Question

01/22/2016 6:10 PM

Actually, I don't know ...'it.

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#15
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Re: Activated Sludge Question

01/23/2016 7:59 AM
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#8
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Re: Activated Sludge Question

01/22/2016 6:56 PM

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#9
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Re: Activated Sludge Question

01/22/2016 7:16 PM

Among the different types, do you notice any variation in the taste?

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#12
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Re: Activated Sludge Question

01/22/2016 8:35 PM

Ha ha we're more sheit kickers than turd biters down 'ere...

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#4

Re: Activated Sludge Question

01/22/2016 5:41 PM

time is a factor, yes but without knowing your BOD you can not calculate time

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#6
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Re: Activated Sludge Question

01/22/2016 6:11 PM

The phrase "You can't get 6 pounds of poop in a five pound bag" is applicable here.

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#16
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Re: Activated Sludge Question

01/23/2016 8:05 AM

In Rednek Lingo we call that a Blivet,except with 10 pounds in a five pound bag.

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#26
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Re: Activated Sludge Question

01/25/2016 12:24 PM

But he didn't ask that. Presumably he already knows what retention time he wants, but not the aeration volume to achieve it.

In any case, retention time isn't the best way to design an AS plant, F/M, the ratio of incoming BOD to (effective) mass of biomass in aeration, is more fundamental. For a given F/M can then choose low MLSS, giving a long retention, or higher MLSS, giving lower retention but maybe larger surface area of the final clarifier. An optimum is needed.

Also if there is an ammonia spec to meet there are other criteria.

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#27
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Re: Activated Sludge Question

01/25/2016 9:50 PM

But can you prove it?

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#28
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Re: Activated Sludge Question

01/26/2016 3:54 AM

Prove what? That retention time = V/Q, not V/(Qr)? If so, yes, see my #24.

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#29
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Re: Activated Sludge Question

01/26/2016 8:18 AM

The OP asks for a proof of V/Q= retention time. #24 does has only one equation repeated twice. That's not a proof.

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#30
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Re: Activated Sludge Question

01/26/2016 9:34 AM

#24 does has only one equation repeated twice. Doesn't make a lot of sense. Typo?

If you treat the aeration tank and the return sludge system as a "black box", with volume V, the average retention time must = V/Q. There's nowhere else for the incoming sewage to go, and the recycle rate is irrelevant. To me, that's a satisfactory proof (I thought it was worth a GA or two ). I'd be interested to know what other posters think.

I do know a bit about mathematical philosophy, though no expert.

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#31
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Re: Activated Sludge Question

02/09/2016 9:34 AM

If everybody hasn't lost interest, I tried to do a calc.

If V/Q = T, and the RAS ratio = r, some of the incoming flow sees retention T/(1+r). But some of it is recycled for another pass through aeration. That part sees retention T*r/(1+r)2, and that gets split again, giving T*r2/(1+r)3, and so on, so total = T*(1/(1+r) + r/(1+r)2 + r2/(1+r)3..............) = T*Σ1/r*rn/(1+r)n for n 1 to ∞. That's a GP and sums to T, for any value of r (and Mathcad confirms). That's my take on it anyway. I still think my earlier approach is OK (and a neat proof, if I may say so!).

This reminds me of the problem of a guy getting off a train with his dog, and setting out to walk home 2 miles away at 2 mph. His dog runs home at 10 mph to tell missus he's on his way, then runs back to his master, then back again and again till the man reaches home. How far does the dog run? The thought is that a mathematician sums an infinite series, but a scientist (or engineer) notes that the man takes 2 hours to get home, so the dog runs 20 miles in that time.

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#7

Re: Activated Sludge Question

01/22/2016 6:49 PM

All I know is that when Montezuma inflicts his revenge, retention time quickly approaches zero. I cannot prove this and I regret every demonstration.

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#17
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Re: Activated Sludge Question

01/23/2016 8:09 AM

Whole kernel corn has been used for centuries to time throughput.

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#10

Re: Activated Sludge Question

01/22/2016 8:19 PM

See the Cheech and Chong routine where, after smelling, touching, and tasting it, you'd better not step in it.

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#11
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Re: Activated Sludge Question

01/22/2016 8:21 PM

Or Dilbert's "Don't Step in the Leadership"

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#13

Re: Activated Sludge Question

01/23/2016 7:17 AM

lyn and SolarEagle had good answers. I voted Good Answers for both.

However, you must know what type of Activated Sludge process you are employing, ie, Plug Flow vs. Complete Mix.

There is a DIFFERENCE! The OP needs to go back and learn the fundamentals and how to perform the Activated Sludge Unit Processes calculations......

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#20
In reply to #13

Re: Activated Sludge Question

01/23/2016 9:28 AM

Complete mix system..plus I need to know the bases to not including the Qr in the HRT calculations. Can you help me with this? Thanks

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#21
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Re: Activated Sludge Question

01/23/2016 9:49 AM

Now this is starting to sound like either homework or a job interview question where you are required to show your work, and you don't know s*it from shinola!

That's why your shoes don't shine.

<Beam up Scottie>

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#14

Re: Activated Sludge Question

01/23/2016 7:22 AM

BTW, you must consider Qr, the Return flow, ie "Recycled Waste Flow", from the Secondary Clarifier which is returned to headworks of aeration basin.

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#18
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Re: Activated Sludge Question

01/23/2016 9:25 AM

99% of references state that I should not and I do not know why.

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#23
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Re: Activated Sludge Question

01/23/2016 12:57 PM

Because it's not part of the incoming influent stream, it's a part of the process....

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#25
In reply to #14

Re: Activated Sludge Question

01/24/2016 1:33 PM

No, return activated sludge (RAS) flow does not affect the average retention time. And you might confuse him by calling it Recycled Waste Flow. Waste or surplus activated sludge (SAS) is something different. Both shown in SE's #22.

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#24

Re: Activated Sludge Question

01/24/2016 1:25 PM

Yes. Think of the aeration tank and the return sludge system as a "black box". Then it's clear that the average retention time = V/Q, whether plug flow, completely mixed or what. Of course some incoming molecules see shorter time, others longer, but average = V/Q.

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