mini digester...?

Digester design and construction info

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Shadow
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mini digester...?

Postby Shadow » Fri Oct 18, 2002 1:52 pm

Hello everyone, I am just looking for a little bit of info...For my science fair project I'm going to build a "mini" biodigester. I have a design, thanks to one of the posts that was on here, but I don't know how I should keep it maintained,as in stirring, keeping the temp., collecting the gas, etc. If anyone could suggest a site that I could find this info. on (hopefully with relatively little confusion, though that may be an oxymoron Image ) it would be greatly appreciated. Thankyou very much,
Shadow

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Bob
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Postby Bob » Sat Oct 19, 2002 10:17 am

Hi Shadow,

That's great, that you are building a digester for your science fair project. Can you say a little bit more about your proposed design? The answer(s) to your questions about stirring, temperature control, gas collection, etc. have a lot to do with exactly what it is that you are building. For example, are you building something that you could mix by turning upside down a couple of times a day or so?

Probably the most critical part of the whole process is startup -- what you use for a 'seed', buffering, etc. to keep it from going acid & killing the methanogens.

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Postby Shadow » Sun Oct 20, 2002 8:26 am

Well, that's precisely the kind of thing I need to know. I don't know about seed or buffering, and I don't know what would be the best way to stir it... here's the site that I got my design from http://www.cfe.cornell.edu/wmi/TrashGoesToSchool/ItsAGas.html
if that helps. Thankyou!

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Bob
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Postby Bob » Mon Oct 21, 2002 8:05 am

Yeah, I see what you mean. That's a good site as far as it goes, but there's not enough detailed info there. (Intended as a "lesson plan", it seems to be designed to generate as many questions as answers, questions to be answered by doing, in the classroom.)

As described, I don't think it will make much methane. Lots of CO2, but little or no methane. Depending on the 'vegetable matter' feedstock, without buffering or a seed inoculant of methanogens, it will more likely become so acidic that no methanogens can survive in it. Also, the third jug can't have a stopper in it, or at least will need a 2-hole stopper, to allow air to escape as the jug fills with water from jug 2. As described it's a closed system & pressure will build.

Give me a little time & I'll try to sketch up & post some possible designs. And anyone else monitoring this, feel free to jump in. Nathan?

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Bob
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It's a gas

Postby Bob » Mon Oct 21, 2002 4:29 pm

Image

Here is a sketch of the setup (if I understand it correctly) described in the ItsAGas website. Here's how it works: Gas pressure generated in the digester (jug #1) displaces water in jug #2 to flow into jug #3. The rising water level in jug #3 is equal to the volume of gas generated, and can be easily measured--because you previously calibrated it. Problems/features that I see include:

  1. The only thing you can learn from it is the volume of biogas produced. There doesn't appear to be any (easy) way to collect the gas or determine methane content.
  2. There is no (easy) way to monitor (or adjust) digester pH without taking it apart (and possibly letting air in).
  3. Difficult to heat in a water bath. (If you heat it in a warm water bath, you'll need to add enough weight to jugs 2 & 3 so that they don't float when empty.)
  4. Rigid tubing makes it difficult to agitate the digester without taking it apart. Replace with flexible hose connected to barbed fittings & plenty of slack.
  5. Delete stopper in jug #3 (or replace with 2-hole) for some place for air to escape. As shown, it will pop corks (or worse), when gas pressure builds sufficiently.
  6. No easy way to measure volume of gas greater than one jug full.
I can suggest some other ideas, but first I have a couple of questions. Have you come up with a hypothesis? Exactly what do you want to learn from your experiment? I'm also wondering something more about your venue. Methane experiments can be dangerous if not properly understood and properly handled. Do you have a teacher/parent/other supervisor or advisor?

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sketch

Postby Bob » Tue Oct 22, 2002 10:08 am

Image
Here's another idea:

  1. Jug #1 is the same as in the previous sketch, but jugs 2&3 are replaced with a 'gasholder' -- a water-filled container with a floating cover.
  2. Measure and mark (calibrate) the floating cover as described in the ItsAGas site, so you can record daily gas collection.
  3. Cover should be transparent so you can see the gas/water level inside.
  4. As gas collects under the cover it will rise. Take daily readings as described in ItsAGas site.
  5. Replace rigid tubing with flexible hoses. Add shutoff valves (can be inexpensive clamps that pinch the hose) so you can, e.g. turn the digester over to agitate. (To be even fancier, provide two clamps with a coupling in between, in case you want to take it apart without spillage/leakage.)
  6. Place it all inside a cheap styrofoam cooler. Fill the cooler with water (lower than the level of the water in the containers, though, so they don't float).
  7. Install a thermostatically controlled aquarium heater to maintain water temperature at 95degF (35degC).
  8. Add some weight to the top of the floating cover to provide a minimum amount of pressure. When the draw-off valve is opened, gas collected under the cover is released (to a bunsen burner?), and the floating cover will drop. Measure and record difference in level.
  9. US Plastic Corp is a good source for tubing, fittings and valves, and containers. Also, check out local pet stores and/or garden supply houses. Both typically have a selection of tubing & fittings(used for aquarium air systems, and drip irrigation systems, respectively).
  10. One idea that works well, for attaching tubing & fittings to containers, is to buy polyethylene bottles and caps (e.g. from source above), drill a hole in the cap, and push a barbed tube fitting into the hole. Then silicone around the hole. (Quality of connection will depend on thickness of material, size of hole drilled (relative to fitting).

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nathan_lamothe
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Postby nathan_lamothe » Wed Oct 23, 2002 4:45 pm

Bob / Shadow,

Like the second design better. It addresses the issues raised in the first that I was going to mention when I forgot my password yesterday.. :-(

The new design will experience some error due t evaporation in the second jug, especially when the water is warmed. There is however, nothing preventing you from placing the second jug outside the warm water bath and keeping it in a cool, even refrigerated environment... so long as you keep it at the same temperature throughout your experiment.

Also, the scince teacher in me has to say, run more than one! it is a research project and not a science experiment unless you run more than one. I would look for a decent hypothesis ("Biogas is generated better at 98 degreed F than at 58 degrees F" or "biogas is generated faster when the digester is agitated" or my personal fav..."which of these crop residues produce the most biogas, and why?") and then run a series of tests, remembering to not only experiment, but also run control tests. (easy enough to keep all the digesters at the same temp... just put em all in the same styrofoam cooler and put the collection bottles outside it)

I guarantee you will get a better mark out of a decent experiment than an awesome research project...at least if this is a science fair....

also, your teacher is likely a great source for supplies. They have access to a number of educational science supply comany catalogues (Boreal, Northwest, etc)... just ask soon for what you need as teachers tend to be busy.

will keep thinking. Image
Nathn

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Bob
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Postby Bob » Thu Oct 24, 2002 9:51 am

Good points, Nathan.

"The new design will experience some error due to evaporation in the second jug"

True if you measure the height of the floating cover above the top of the stationary container rather than to the standing water level. But if you measure it to the water level, it shouldn't matter. (To do that easily though, the bottles should probably be transparent.)

There is however, nothing preventing you from placing the second jug outside the warm water bath and keeping it in a cool, even refrigerated environment so long as you keep it at the same temperature throughout your experiment.

Good point. In fact, putting it in a cooler environment will improve the quality of the biogas by condensing water vapor out of it.

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Postby Shadow » Fri Nov 22, 2002 8:19 am

Hello again, all. Sorry I haven't posted in a while, I been busy with school and such. I think I have a pretty basic understanding now. I also have figured out an engeneering goal, I think. I'm going to see which pre-treatment of two helps to produce more biogas.

OK, I understand the second design and how it works, so now I'm trying to figure out sizes. Um, I don't know how big the containers should be. I think we will be feeding the digester approx. 1/2 lb. of chicken "waste" a day. Also, what kind of container would the gas collection bottle be?
Thanks much.

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Postby Bob » Fri Nov 22, 2002 11:39 am

"I'm going to see which pre-treatment of two helps to produce more biogas."

That sounds like a worthy goal. How are you going to pretreat it?

"I think we will be feeding the digester approx. 1/2 lb. of chicken "waste" a day."

The designs described above are suited for a batch feed process rather than continuous feed. That is, you fill it once only, rather than continuing to add to it daily. If you want to do a continous feed experiment, you'll need something a bit more complicated.

"Also, what kind of container would the gas collection bottle be?"

Give me a little time, I'll sketch something up for you...

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nathan_lamothe
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Postby nathan_lamothe » Sat Nov 23, 2002 4:54 pm

Now it is sounding like a science fair project!

I was going to make some comment about using inner tubes... but I am sure Bob has something better up his sleeve.

How long do you have to do the experiment? are we talking 10 days, 1 month, till June? The size will depend on the retention time you are working with (how long you leave it in there) and the designs get more complicated as you get more fussy about that...

[This message has been edited by nathan_lamothe (edited 11-23-2002).]

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Postby Bob » Tue Nov 26, 2002 5:53 pm

ok, here are some pictures of a simple gas collector. The pieces are:
  • 1- 4" PVC coupling
  • 1- 2-liter soda bottle (7-Up not necessary)
  • 1- 6" length of 3" ABS sewer pipe
  • 1- 3-1/2" diameter disk cut from 1/4" PVC sheet material
  • 1/8" ID (1/4" OD) polyethylene tube & fittings and/or vinyl tube and fittings as required to connect to your digester.

    Image
  • Cut the top off the bottle. (I cut it just above where it starts to curve in, slightly bigger than the OD of than the 3" pipe).
  • Cut the 1/4" PVC disk (cap for the floating gas collector) with a jig saw. 1/4" is what I had on hand. 1/8" would have been better. Drill a hole through the center to accept tube fitting.
  • Cut a 6" length of 3" sewer pipe & glue cap to it. 3" sewer pipe (thin wall white PVC) would have been better than the ABS. But that's what I had on hand. Glue the cap to the pipe w/ PVC solvent weld glue. Measure & mark volumes on side of collector.

    Image
  • Drill hole in the bottom of the bottle to accept fittings, and the side of the coupling to slip tube through.

    Image
  • Insert fitting into hole through bottom of bottle and attach tube.
  • Drill a hole through gas collector cap & attach discharge tube. Note this tube is soft flexible, so can use pinch clamp. Attach balloon to end of tube.

    Image
  • Here's a view from the bottom.

    Image
  • Here's the assembled unit.

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Postby Shadow » Fri Nov 29, 2002 1:06 pm

OK, um, so far as how long I'm going to do the experiment, I have been told that it sometimes takes a very long time to get gas production going, and so I was thinking that we'd probably have to have it going for at least a month, if not more. What would you suggest?

Bob, thank you VERY much for all that info., it's greatly appreciated and was very helpful. Also, my mom wants me to tell you that you're awesome, and thanks you for going to the trouble.
Another thing, about how big should the tubing for the main part of the digester be, and about how big should the containers be. I'm trying to gather up materials but I'm not sure of sizes. It's probably not to terribly important, I mean your idea probably won't differ far from what I've got already, I would just like to be sure.
What types of pretreatments? One acid and one alkaline, but I'm still reading up on the specifics.

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startup

Postby Bob » Fri Nov 29, 2002 3:22 pm

Shadow, the amount of time it will take to get gas production going will depend on how many live bugs you have to start with. There is some discussion of 'seeding' or 'inoculation' in this conference

Worst case would be that it never produces any gas at all. This could happen if you load up your digester with food (bug food, that is), and the acid-producing bacteria take off and thrive, but the slower growing methane-producing bacteria (that feed on the acid-producing bacteria's waste) can't keep up, which would cause more acid to accumulate, dropping the pH further, which will kill even more of the methanogens, and so on, until the are all dead and it crashes.

The trick to getting it started is to (1) have a viable population of methanogens (which you can get from the muck at bottom of a swamp, or from septic tank sludge, or other places where you can find well aged organic sludge), and (2) to monitor the pH to keep it from getting too acid. Methanogens like it about neutral (pH 7.0) If it drops much below that (say, below 6?) add a small amount sodium bicarbonate (baking soda). For the fastest start (almost immediately, within 3-4 days...), mix them (bugs & food) 50:50.

NOTE: If you need to add buffer, add it s-l-o-w-l-y! If your digester has gone acid and you add a bunch of bicarbonate and shake it up, it will produce mass quantities of CO2. The results will not be pleasant. (Imagine shaking up a bottle of pop, and then removing the cap. Then imagine that it isn't pop in there but chicken shit. You get the picture... :-)

The tubing for your digester can be the same as for the gas collector. 1/8 (1/4" O.D.) should work ok. A 1-gallon plastic jug should work.

wrt pretreatment, how about trying different 'recipes' instead? Maybe vary the C/N ratio of the mixture instead. As noted above, I'd suggest starting with both digesters near neutral, and making sure they are well buffered to keep them that way.

You (and your mom) are welcome. You can pay me back (forward?) by telling us how it turns out. email me some pictures, and I'll post 'em here.

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Postby Shadow » Sat Dec 07, 2002 12:37 pm

Hey Bob,
I have a question about your design. The sewer pipe is supposed to fit into the bottle preety well, right? I got the materials you listed, but I can't see how the pipe you used fit so well into a 2-liter bottle, because the sewer pipe I have is REALLY small compared to the 2-liter WE got. What I mean is, I can't understand why, with the measurements given, our equipment seems not to fit at all. Sorry if I took a long time to ask a simple question. Thanks,
Shadow

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Postby Bob » Sat Dec 07, 2002 4:46 pm

Hmmm... don't know what to think. The O.D. of my 3" pipe is about 3-1/2". The I.D. of the bottle at the widest point fits snugly into a 4" diameter PVC coupling which measures about 4-1/4". I cut the top off the bottle just above where it starts to curve in, so the I.D. is about 3-5/8" -- just big enough for the inner pipe.

If that's what you did, then the only thing I can think is that either you don't have a 3-1/2" O.D. piece of pipe, or the pop bottles in your area are different shape than they are here.

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Postby nathan_lamothe » Sat Dec 07, 2002 7:09 pm

Bob, trying something new here. Counldn't figure out how to actually put it here, so it is linked in from my lame site...

Image

[This message has been edited by nathan_lamothe (edited 12-07-2002).]

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Bob
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Postby Bob » Sun Dec 08, 2002 6:14 am

Cool, Nathan,

How did you do that? Capture the images and write over them in Photoshop (or whatever) with a tablet stylus?

wrt uploading images, I do have anonymous ftp, but I see it isn't accepting uploads at the moment. will fix.

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nathan_lamothe
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Postby nathan_lamothe » Sun Dec 08, 2002 12:06 pm

Yah. I have a graphics tablet (Wacom Graphire 2) for a calculus course I am taking at the U. All of us students are full time teachers, so we do all our questioning etc online using the tablest in Netmeeting's whiteboard app.

So I just dropped the pics into corel painter classic and wrote on top.

hard to write with, but easier than trying to describe which piece of pipe I am talking about...

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Postby Shadow » Sat Dec 28, 2002 6:09 am

I understand how things fit now, thankyou, but my step-dad is trying to help me and can't seem to find the fittings for the tubes that come from the collector. Could you tell me what they are called and where they might be found. He has searched everywhere but cannot seem to find them. Thanx much.


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