Plug flow digester

Digester design and construction info

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bongo
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Plug flow digester

Post by bongo » Wed Dec 31, 2003 5:56 pm

We are planning to buy a small farm with 80 cows-calf operation. I heard about plug flow digester I even saw one in http://www.ias.unu.edu/proceedings/icib ... olydig.htm but they do not mix the slurry and I also read this " Some means of mixing the slurry in a digester is always desirable, though not absolutely essential. If left alone, the slurry tends to settle out in layers and its surface may be covered with a hard scum which hinders the release of gas. "
I wonder if someone in this forum is familiar with this kind of digester since I need answer to a few questions

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Bob
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Post by Bob » Thu Jan 01, 2004 12:28 pm

Hi Bongo,

I have seen that plug-flow digester design before. It is a wonderful minimum-cost, low-tech design, well thought out, beautiful in its simplicity.

Regarding your question about mixing, I think the issue is most applicable to tank-type digesters, rather than this horizontal plug flow design. (Of course you know that you will have to mix the incoming material with water before introducing it to the digester, right? -- as described in Step 7 of your link)

Of course, over time, some layering will occur in the plug flow digester also. Grit, sand and other non-biodegradable sediment will accumulate on the bottom, and scum will form on the top. I suppose that if the scum layer became problematic (hard & impermeable, preventing gas from passing through it), it could be broken up from time to time, if required, by poking (gently) the polyethylene bag. The rate of sediment buildup will largely depend on how much sand and dirt is included in the incoming mixture. If proper care is exercised, I wouldn't think it would require any maintenance besides replacing the poly bag every 10 years or so.

Please keep us posted on your project. Great idea.

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Ralph
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Post by Ralph » Wed Jan 07, 2004 8:44 am

Hi Bongo.

Are you planning to use the cattle manure to power the biogas generator? If so, how will you collect the manure? Are you planning to use a concrete feeding pad or confine the cattle to a loafing shed? Is this housing situation something you'll use only in the winter, or year around?

I'm curious because in the U.S., cow-calf operations are almost all based on pasture systems. Some folks use loafing sheds to feed hay in the winter, but the animals are allowed free movement to/from pastures. A confinement system wouldn't pay for itself via cow-calf in the U.S. I don't think. Too much labor and infrastructure costs. But given a situation with very high land costs and low labor costs, I see that it could work financially on paper.

Thanks,
Ralph
West Wind Farms
Organic & Grassfed Meats & Poultry
http://www.grassorganic.com

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Ralph
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Post by Ralph » Wed Jan 07, 2004 10:15 am

In the plug flow digester, what forces the digested waste (fuel) out the end of the digester? The link contains a picture of solids sort of oozing up and out of one end of the bag. Seems to me that the solids would accumulate near the feed end, with only low solids slurry weeping out the opposite end. Maybe the feed mixture is so fluid that it disperses into the reservoir? (I envision lots of hand-mixing.) The construction diagram details a flat trench floor, so there isn't an incline to the thing. Larger biogas operations use stirring pumps and multi-cell designs so they can completely pump out one cell before recharging with manure.
West Wind Farms
Organic & Grassfed Meats & Poultry
http://www.grassorganic.com

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Bob
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Post by Bob » Wed Jan 07, 2004 2:12 pm

Good questions, Ralph. Looking a little closer at the information on the linked site, I see that they say:
Daily, the biogas plant must be fed with 18 kilos of FRESH excrement (from cows, buffaloes, goats, pigs, etc.) blended with 90 kilos of water. This is approximately a mixture of 1 bucket of excrement with five buckets of water. This mixture is needed to replace the water lost during the process and keep a flowing material inside the biodigester.
Now that I think about it a little more carefully, that is the daily volume produced by one or two cows. This system is sized for a small rural household with a few chickens, a pig, a couple of goats, maybe a cow. And it is aimed at end-users who are now burning either dung or firewood for cooking fuel, able to generate about enough gas for a household -- not appropriate for an 80-head farm.

re the question about what forces the digested waste out the end of the digester, I see that they are recommending a very thin slurry. (Assuming 86% moisture content of fresh cow manure, if that is mixed 1:5 with water, that would yield about a 3% slurry.) So it looks like water flows through the system by simple displacement, with a slow gradual accumulation of undigestible sludge on the bottom.

If care is taken to avoid sand and grit in the collected waste as much as possible, the buildup of undigestable solids in the digester should be quite slow, taking years to accumulate. Assuming that, say, 30% of the manure solids are not digested (converted to water, CO2 and CH4), that would leave 18 kilo x .03 x .3 = .16 kilo / day solids accumulation, or less than 1/50,000 of the volume (if I did my arithmetic correctly).

(I'm handicapped by thinking in English units and having to convert to/from S-I :oops: )

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Bob
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Post by Bob » Wed Jan 07, 2004 2:55 pm

Bongo, here are a couple of links to some resources that may be more appropriate to your interests:

http://www.epa.gov/agstar/library/index.htm
(In addition to a lot of useful information, Agstar has software you can download for free, to help you determine the economic feasibility of a biogas system.)

http://www.mnproject.org/index-biogas.html
(Includes links to case studies, existing biogas projects.)

bongo
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Plug flow digester

Post by bongo » Fri Jan 23, 2004 12:33 pm

Hi Ralph.

I am not sure yet if I can generate enough biogas to power a generator.
But about your questions as how I am planning to collect the manure.
The farm has an old loafing barn with a free movement to a corral which I will use to feed the cows in the winter. In spring I will take the cows to pasture. I am planning to finish steer during spring, summer and fall in order to keep the digester alive.

I will very much appreciate your comments about it.

Thanks,

Bongo
Last edited by bongo on Sat Jan 24, 2004 9:13 am, edited 1 time in total.

bongo
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Plug flow digester

Post by bongo » Fri Jan 23, 2004 2:54 pm

Hi Bob

Look like I am back to square one.
I was trying to avoid the high cost of the digester tank, but good result is what is important isn?t it?
We live in a very cold place (Alberta, Canada) our plan is to use biogas to heat greenhouses which we may have to buy some of your technology (Controlled Environment Growth Chamber)

I have used your Anaerobic Digester Calculator and have visited lots site about the subject, but cost vs. effectiveness is my most important concern since I am a neophyte about the biogas


Bongo

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nathan_lamothe
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Post by nathan_lamothe » Sat Jan 24, 2004 8:34 pm

Hey Bongo...

Where in Alberta?

Me too...

bongo
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plug flow digester

Post by bongo » Sun Jan 25, 2004 12:16 am

Hi nathan_lamothe,

I from Calgary but will move to Hay Lake 38 km north of Camrose, 30 min southeast of Edmonton ....
to the farm. This winter I am working 70 km north of Red earth,
which I think is not far from you... I may be wrong though
Have a nice day

Bongo

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Bob
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Post by Bob » Sun Jan 25, 2004 3:46 pm

Hi Bongo,
I was trying to avoid the high cost of the digester tank, but good result is what is important isn?t it?
Yes, of course assuming you have, or can get, the funding to build it in the first place. I know that in the U.S. there are some sources of public money (e.g. matching grants) for such projects. I just finished doing an economic analysis for a farmer in interior Alaska, as part of a grant proposal to be submitted to the Natural Resources Conservation Service (NRCS). Does Canada have similar programs?
We live in a very cold place (Alberta, Canada) our plan is to use biogas to heat greenhouses which we may have to buy some of your technology (Controlled Environment Growth Chamber)
I don't have a "technology" to sell, per se, but rather, some expertise in designing integrated whole systems that yield surprising synergies. If you email me your address, I'll send you a copy of a paper I wrote (published in this book), describing an integrated system that uses biogas to produce electricity, which is then used to light a heated, insulated growth chamber, with the waste heat and CO2 from the engine and lights used to heat the processes. (Disclaimer: the project was never built, but engineering calculations and methodology sufficient to evaluate it on its merits are provided.)

bongo
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plug flow digester

Post by bongo » Mon Jan 26, 2004 9:45 am

Hi bob
http://www.energy.state.or.us/biomass/d ... #Farmhouse
http://dspace.dial.pipex.com/town/terra ... hor1559679
http://www.epa.gov/agstar/library/biocycle3.htm
Leo and Linda Langerwerf said:
?A plug flow digester was selected because the manure solids could be more easily removed and land application of the liquid manure can be incorporated as part of the irrigation system. The digester itself is installed in a greenhouse that protects it from the elements.?
Those entire hyperlinks and many other take you to plug flow digester my question is how are they made inside that is different from this one http://www.ias.unu.edu/proceedings/icib ... olydig.htm

Bongo

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Bob
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Post by Bob » Mon Jan 26, 2004 3:11 pm

I think the basic principles are essentially the same, but the primary difference is one of scale (and different climate). The rural Ecuador system is a simple low-tech design sized to handle 18 kilos of manure (mixed with 90 kilos of water) per day.

But the daily production of manure from 80 head of cattle will be more like 5,000 kilos/day, not mixed with water. There is just no (feasible) way to handle that kind of volume with a simple polyethylene bag in a hand-dug trench.
Last edited by Bob on Tue Jan 27, 2004 9:34 am, edited 1 time in total.

bongo
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plug flow digester

Post by bongo » Mon Jan 26, 2004 11:53 pm

Hi bob,
I see your point now and I agree with you
I have never seen a digester before. That link show me almost everything. But I needed the opinion of people who really know about it.
The digester I have in mind have a capability of 8000 gls made of heated concrete with a waste liner inside maybe from http://www.westernliner.com/farmanimal.html, but following the priciple of that digester of low-tech design

bongo

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Bob
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Post by Bob » Tue Jan 27, 2004 11:09 am

Using the Haubenschild dairy info as a basis, I come up with something more like:

80 head x 17.5 gal/cow/day x 20 days = 28,000 gallons or about 3,743 cu ft. (106 M3)

With a preferred length to width ratio of about 5:1 (for a plugflow digester), and a ratio of width to depth of 2.5:1, the approximate dimensions for this digester would be 65' Long x 12' Wide x 4.8' Deep (19.81M x 3.66 M x 1.46 M)

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Bob
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Post by Bob » Tue Jan 27, 2004 12:05 pm

Further thoughts (whipping out calculator :)):

Gas production from this digester should be in the neighborhood of 3.5 - 4 cu ft/gallon (assuming proper temperature, loading rate, etc), producing about 5,000 cu ft/day.

If you enlarge the digester to 30 days retention time instead of 20, gas production should rise to about 5 cu ft/gallon, producing 7,000 cu ft/day. Approximate dimensions for this digester would be 70'L x 14'W x 5.6'D.

Of course which way to go will depend on the local cost of fuel gas vs the cost of construction. When amortized out over the life of the digester, will the value of the additional gas offset the higher initial cost of construction? Of course this will depend on how you use the gas.

bongo
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plug flow digester

Post by bongo » Tue Jan 27, 2004 1:08 pm

I when back to the Anaerobic Digester Calculator and found out I was forgeting to enter the amount of water.
That will increase the cost. I was thinking to use a waste liner because what I read in this site...
http://www.manure.mb.ca/projects/completed/01-01-03.htm
"The challenge of reinforced concrete is its long-term stability, which controls the so-called durability or service life of concrete structures. Due to the hostile service environment associated with manure storage, corrosion rates of the steel reinforcement are potentially high. These deterioration rates could lead to a significant mechanical weakening and finally to a relatively short service life of the tanks."
should I avoid using the linner

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Bob
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Post by Bob » Tue Jan 27, 2004 1:33 pm

Heh. I went back to the Digester Calculator too. And discovered that I need to add some caveats about its use. It is intended only for much smaller scale digesters of the specific design described -- not suitable for processing manure from 80 head of cattle.

That is an interesting link, about concrete deterioration & need for liners. I know I have seen reports of existing digesters opened after many years of operation, that reported only minimal deterioration of the concrete. But it certainly merits further investigation. Perhaps, before paying for a liner, there are other factors to consider: quality of concrete mix & additives, type and cost of liner, etc. Good question.

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Post by jed turtle » Thu Sep 02, 2004 8:56 pm

there are many brands of epoxy type paint also that could be painted on the concrete surface to provide a barrier between the slurry/ gas and concrete.

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