two-stage digester and plug-flow design ?

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ragadinks
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two-stage digester and plug-flow design ?

Post by ragadinks » Fri Dec 23, 2005 5:53 am

Hello,

Have just found your this site here and must say that it is really very interesting - great job !
It's the first time that I come across the two stage digester design where the acidogenic and the methanogenic phase are seperated.
I wonder if the same effect is achieved in a plug-flow digester with it's long and thin shape ?
Mean that there could be a acidogenic section at the beginning of the tube and a methanogenic section at the end with a kind of transition zone in between, or am I wrong ?

Another question regarding the digester design on this site (which I think is great) and the calculator:
How does the calculated amounth of produced biogas correlate with the achieved values of the digester in praxis ?

Another thing that came to my mind when I tried out the online calculator is the low
ambient temperature used. Can this low temperature be used in praxis and how does it affect HRT ?
(have seen that it does not have an affect on yield in the calculation, so it must have an affect on HRT?)

Also realized that the biogas yield in the standard calculation increases from 20.37 cf/day to 95.66 cf/day when increasing scrap paper from 1 Lb/day to 14 Lb/day but it decreases suddenly to .01.46 cf/day when adding a value of 15 Lb/day scrap paper ?

Anyway, should the scrap paper be chopped up before adding it to the digester - and if yes: how is that done ?

thanks and regards,
arno

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Bob
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Post by Bob » Mon Dec 26, 2005 1:57 pm

I wonder if the same effect is achieved in a plug-flow digester...?
That's a good question. One that I've wondered about, and have been fooling around with a new design for, but with some variations. My idea is to replace the acid phase reactor with plug flow (a measured length of 4" dia corrugated sewer pipe coiled around inside bottom of tank), but leave the methane-phase as a CSTR. Then modify the operation so that a portion of the discharge from the *end* of the coil is recirculated to the beginning, for continuous re-inoculation. (I hesitate to say more, until I can build one and see how it actually works. So far only an idea...)

In principle, it would operate differently, but it could still be as efficient -- though for a different set of reasons. And, more importantly, maybe simpler to build.

The 2-stage design is able to culture distinctly different populations in each stage solely by controlling the retention time in each. A 2-3 day retention time in the first stage prevents the slower-growing methanogens from ever becoming established there, as they will wash out faster than they can reproduce. In the proposed plug-flow design, (perhaps augmented with extensive surface area for colonization) this would not be the case. Methanogens will become established, but changing it from suspended media (free-swimming) CSTR to a fixed media (with extensive surface area to cling to) plug flow will change all the dynamics.

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Post by Bob » Mon Dec 26, 2005 2:40 pm

How does the calculated amount of produced biogas correlate with the achieved values of the digester in praxis?
I have only used it with human waste (augmented with kitchen wastes of varying quality and quantity), and have never performed detailed analysis of either the composition of the wastes or of the biogas. That said, with these feedstocks, total gas volume produced correlates well with the model, at times exceeding predicted production by as much as 50% (though I suspect at lower methane/CO2 ratio), at others less --i.e. when system is out of whack, needing adjustment (e.g. add buffer to adjust pH, reduce loading, etc.) to allow it to recover.

Also note that these measurements are in no way of laboratory precision or rigor. At best, I can only say that, when the digester is fully operating properly, it can produce between 3 and 4 volumes per volume per day.
Last edited by Bob on Mon Dec 26, 2005 3:12 pm, edited 1 time in total.

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Post by Bob » Mon Dec 26, 2005 2:49 pm

Another thing that came to my mind when I tried out the online calculator is the low ambient temperature used. Can this low temperature be used in praxis and how does it affect HRT? (have seen that it does not have an affect on yield in the calculation, so it must have an affect on HRT?)
I'm not sure of your question. The model assumes 95 degF reactor temperature, and that some percentage of the biogas generated will be used to maintain that temperature above ambient. If you will look under the section "Gas Usage", the item "Remaining Heat available for use" is the net available *after* subtracting the amount necessary to heat the reactor.

Hope that clarifies...

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Post by Bob » Mon Dec 26, 2005 3:07 pm

Also realized that the biogas yield in the standard calculation increases from 20.37 cf/day to 95.66 cf/day when increasing scrap paper from 1 Lb/day to 14 Lb/day but it decreases suddenly to .01.46 cf/day when adding a value of 15 Lb/day scrap paper ?
Hmmm...

On mine (using both Firefox and IE), it increases from 95.66 to 101.46 cf/day. I'm going to guess that the problem is with font size or browser display. What browser are you using?
Anyway, should the scrap paper be chopped up before adding it to the digester - and if yes: how is that done ?
I have not added scrap paper to mine, other than toilet tissue. (The calculations are based on data from the literature). But the main idea would be to get the paper to a condition so that it can flow through a pipe or pump orifice without clogging. A paper shredder, maybe? Pre-soaking and stirring?

ragadinks
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Post by ragadinks » Wed Dec 28, 2005 5:17 am

Bob wrote: I have only used it with human waste (augmented with kitchen wastes of varying quality and quantity), and have never performed detailed analysis of either the composition of the wastes or of the biogas. That said, with these feedstocks, total gas volume produced correlates well with the model, at times exceeding predicted production by as much as 50% (though I suspect at lower methane/CO2 ratio), at others less --i.e. when system is out of whack, needing adjustment (e.g. add buffer to adjust pH, reduce loading, etc.) to allow it to recover.

Also note that these measurements are in no way of laboratory precision or rigor. At best, I can only say that, when the digester is fully operating properly, it can produce between 3 and 4 volumes per volume per day.
That sounds really good to me and means that the calculations can be used for an estimation of biogas yield !
Bob wrote: I have only used it with human waste (augmented with kitchen wastes of varying quality and quantity)
How does the human waste enter the digester ? I mean is the raw substrate somhow mixed/stirred before entering beeing pumped into the digester ?

ragadinks
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Post by ragadinks » Wed Dec 28, 2005 5:19 am

Bob wrote: I'm not sure of your question. The model assumes 95 degF reactor temperature, and that some percentage of the biogas generated will be used to maintain that temperature above ambient. If you will look under the section "Gas Usage", the item "Remaining Heat available for use" is the net available *after* subtracting the amount necessary to heat the reactor.

Hope that clarifies...
Ok, thanks - now I understand: I missed that the model assumes 95 degF reactor temperature.

ragadinks
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Post by ragadinks » Wed Dec 28, 2005 5:47 am

Bob wrote: On mine (using both Firefox and IE), it increases from 95.66 to 101.46 cf/day. I'm going to guess that the problem is with font size or browser display. What browser are you using?
Guess you are right - I use firefox on linux and it seems that the first digit (01.46) is cut off due to the larger font.

I have not added scrap paper to mine, other than toilet tissue. (The calculations are based on data from the literature). But the main idea would be to get the paper to a condition so that it can flow through a pipe or pump orifice without clogging. A paper shredder, maybe? Pre-soaking and stirring?
Once I have done some paper making and therefore soaked old newspaper in a big bucket and stirred it with the kind of stirrers you can put onto a cheap electronic drill:
Image
This worked very well - the consistency whas like that of a thick soup. But I think for other substrates like straw or grass there should be kind of blades on the tool so that it cuts the fibers ...

Another tool that probably could be used would be a mincer like this:
Image
There are very strong ones that even crush bones - I am sure that they would work well for opening up the cells of the organic substances for further decomposition.
But I am not sure how well they would mix the substrate with the water ?

ragadinks
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Post by ragadinks » Wed Dec 28, 2005 6:06 am

Bob wrote:but leave the methane-phase as a CSTR
Sorry for the question, but what does CSTR mean ? I am not familiar with this term ...
The 2-stage design is able to culture distinctly different populations in each stage solely by controlling the retention time in each. A 2-3 day retention time in the first stage prevents the slower-growing methanogens from ever becoming established there, as they will wash out faster than they can reproduce. In the proposed plug-flow design, (perhaps augmented with extensive surface area for colonization) this would not be the case. Methanogens will become established, but changing it from suspended media (free-swimming) CSTR to a fixed media (with extensive surface area to cling to) plug flow will change all the dynamics.
Not sure if I understand you properly here:
You mean that you would try to eliminate all surface area where free -swimming suspended material and methanogeous bacteria could acumulate and mix with the fresh substrate and rather pump the fresh material through a small pipe with extensive surface area where the methanogens cannot establish due to short retention time ? And the extensive surface area of the corrugated sewer pipe would allow acidogenous bacteria to cling to and inoculate the fresh material ?
Probably the lack of older substrate will prevent the methanogens from establishing anyway due to the lack of apropriate nutrients. Wouldn't they first have to be produced by the acidogens ?

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Post by Bob » Wed Dec 28, 2005 10:16 pm

Sorry for the question, but what does CSTR mean ? I am not familiar with this term ...
Sorry bout that. CSTR = Continuous Stirred Tank Reactor (as opposed to plug flow)
Not sure if I understand you properly here: You mean that you would try to eliminate all surface area
No, there would be no effort to minimize surface area, just don't go to the expense and trouble of *adding* more. Without a lot of surface area to cling to, the methanogens just wash out faster than they can reproduce.

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Post by Bob » Fri Dec 30, 2005 5:51 pm

How does the human waste enter the digester ? I mean is the raw substrate somhow mixed/stirred before entering beeing pumped into the digester ?
Right now, I have a 1-1/2 pint/flush marine toilet emptying into a small holding tank (holds about 1-1/2 gallons) under the floor. I pump this out every other day or so (depending on usage) with a hand pump. The pump macerates and mixes the feces and toilet paper, and the toilet flush water provides just about the right amount of water required for a pumpable slurry without having to add more.

Here is a description of a similar system (the difference is that it shows pumping into a composter instead of a digester). (Scroll down about 1/3 the way down the page to photo and link to installation diagram.

Also, when I was first experimenting with this, (before the more permanent installation in the house, connected to the toilet), I mounted a large single bowl kitchen sink with garbage disposer on a 2x4 frame over a 30 gallon drum that I could pump from.

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Post by ragadinks » Sat Dec 31, 2005 7:57 am

Bob wrote:
How does the human waste enter the digester ? I mean is the raw substrate somhow mixed/stirred before entering beeing pumped into the digester ?
Right now, I have a 1-1/2 pint/flush marine toilet emptying into a small holding tank (holds about 1-1/2 gallons) under the floor. I pump this out every other day or so (depending on usage) with a hand pump. The pump macerates and mixes the feces and toilet paper, and the toilet flush water provides just about the right amount of water required for a pumpable slurry without having to add more.
Usage: Pump the handle 5-6 strokes just before flushing, to develop a vacuum in the holding tank. When the toilet is flushed, the vacuum will rapidly evacuate the bowl and tank, macerating wastes in the process.
This seems a very good way to get rid of the human waste without using electrical energy.
I just wonder how the macertion process works and if it also can handle substrate like kitchen waste, grass clippings, straw etc. ?


cheers,
arno

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Post by Bob » Sat Dec 31, 2005 10:53 am

I just wonder how the macertion process works and if it also can handle substrate like kitchen waste, grass clippings, straw etc.?
It won't handle fibrous waste like straw, etc, without chopping it up further. (Using a garbage disposer, maybe?)

Also, I should update the bit about creating a vacuum with the hand pump. In fact, now, the flush valve in the Sealand toilet leaks, so that it won't hold a vacuum. But it doesn't matter. Just pumping the waste at high velocity through the check valves of the pump breaks it up ok.

ragadinks
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Post by ragadinks » Tue Jan 03, 2006 7:34 am

Bob wrote:I just wonder how the macertion process works and if it also can handle substrate like kitchen waste, grass clippings, straw etc.?
It won't handle fibrous waste like straw, etc, without chopping it up further. (Using a garbage disposer, maybe?)
A garbage disposer seems to be well suited for this ( though it's the first time I have heard of such a thing :-) )
Maybe worth trying it out. Wonder how much power it consumes ? Are there some that work with human power only ?

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Post by ragadinks » Tue Jan 03, 2006 9:40 am

http://www.keidel.com/design/select/disposers.htm:
Bio-Charge? is a fresh, citrus-scented solution featuring natural microorganisms that help break down food waste.More than 300 million microorganisms start to digest food waste instantly - even before waste reaches the septic tank. (each cartridge lasts 3 to 4 months with average use, and is easy to replace.)
I wonder what kind of microorganisms these are ...
Maybe this would be a good way of inoculating the fresh substrate with metanogenous bacterias ?


Arno

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Post by Bob » Tue Jan 03, 2006 10:10 am

I wonder what kind of microorganisms these are ...
Maybe this would be a good way of inoculating the fresh substrate with metanogenous bacterias?
They are most likely aerobes, or facultative anaerobes (can go either way), but I don't think they could possibly be methanogens -- which are quite finicky, slow growing organisms. These can be collected from fresh manure or swamp sludge, and require care in the collection process.

ragadinks
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Post by ragadinks » Tue Jan 03, 2006 10:33 am

Hi,

have found an interesting article called:
"LIFE CYCLE COMPARISON OF FIVE ENGINEERED
SYSTEMS FOR MANAGING FOOD WASTE"

here: http://www.insinkerator.com/pdf/uwstudy.pdf

cheers,
Arno

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Post by Bob » Thu Jan 05, 2006 9:06 pm

Thanks for the link. Looks interesting.

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