Sciencemadness Discussion Board - Biogas(Methane from Organic Waste)

Biogas(Methane from Organic Waste)

Manifest - 25-7-2014 at 18:23

I'm surprised that I can't seem to find any discussion on Biogas on this forum. It would certainly make for a fun experiment.

The only discussion I found was here on removing Hydrogen Sulfide from it. http://www.sciencemadness.org/talk/viewthread.php?tid=7631
So, have any of you experimented with it?

WGTR - 26-7-2014 at 09:45

Yeah, I've got a sealed Erlenmeyer full of fermented grass sitting on the bench right now. I think it's a few years old now. The blades of grass are still green.

It's pretty easy. You just take grass clippings, add water, remove all oxygen from the container, and watch the pH drop over a matter of hours. It's probably a mix of lactic and acetic acids. It's not very concentrated, though, the pH is about 4.

Some of this acidic liquor can be separated from the solids, and adjusted with magnesium hydroxide to a pH of 7-8. A little bit of dirt is added to it, and then this is suctioned into a large syringe, which is then sealed.

Incubating the syringe at about 35°C, over a period of several days gas bubbles form in the syringe. Eventually the stopper gets pushed out, and gas production slows. It seemed that sometimes production was faster if I pulled gently on the syringe from time to time, keeping a slight vacuum in the syringe.

After several days, the gas can be transferred to another syringe, and a bit of sodium hydroxide solution can be suctioned into the syringe. This absorbs the carbon dioxide, leaving methane/hydrogen behind. The syringe is chilled down in an attempt to condense moisture out of the gas, and then the gas is passed into another dry syringe.

A small stainless needle can be attached to the syringe, and the gas can be lit with a match, while applying slight pressure to the syringe.

I've made probably half a dozen 60 c.c. syringes full of methane this way, but it wasn't more than a curiosity. It is a slow process, and not spectacularly effective. If I remember correctly, a lawn bag full of green clippings would get you a lawn bag full of gas. That's pretty much how I calculated it, although it may be an underestimation.

Hey, now I'm a National Hazard!

[Edited on 7-26-2014 by WGTR]

Manifest - 26-7-2014 at 11:42

It's been a few years and it's still fermenting? I would imagine grass clippings would be much slower than left over food waste(banana peels, etc)
Do you not want your Erlenmeyer back?

That's very efficient considering when I mow the lawn I end up needing to empty the bag many times.

WGTR - 26-7-2014 at 14:11

Quote: Originally posted by Manifest

It's been a few years and it's still fermenting? I would imagine grass clippings would be much slower than left over food waste(banana peels, etc)
Do you not want your Erlenmeyer back?

That's very efficient considering when I mow the lawn I end up needing to empty the bag many times.

Once the pH gets low enough, fermentation stops. This happens pretty quickly, within hours to days, as long as the mix is not buffered. As long as the pH remains low, no further decomposition takes place. This is what happens in those large silos that you see on a farm. With the right amount of moisture the pH will fall rapidly in the grain, and it can be stored for a very long time. The longer the acid production takes, the more likely you are to get some butyric acid in there as well. If I remember correctly, if the pH is buffered to about 6 then butyric fermentation can predominate (depending on your particular brand of dirt), and hydrogen is produced.

Technically if the grass soup is mildly buffered, then after acid production it will proceed to methane production, but it is much slower to do it this way. It is quicker to do an acid fermentation in an unbuffered mix first, then adjust the pH upward for methane production.

The guy in the lab downstairs hasn't asked for his Erlenmeyer back yet, so there it still sits.

[Edited on 7-26-2014 by WGTR]

hyfalcon - 26-7-2014 at 14:26

With grass clippings, you have plenty of nitrogen. Add some crushed dead leaves, about 40% by volume with fresh grass clippings. You don't need all that water, just a little. You will be surprised at the amount of heat this mix will give off after it begins fermenting. Shake it every day and it will be done to compost within 14-21 days.

Manifest - 26-7-2014 at 15:52

I'm sorry but your explanation is way over my head. What do you mean by buffered? Do you mean adding something to balance out the PH?
My knowledge is as follows: I have seen videos on youtube where they add food waste and water to a tank and leave it and it forms methane, they didn't adjust PH, they simply added food and water.
Is dirt needed? In the videos I seen no need for dirt.
I can give you a source if needed.

WGTR - 26-7-2014 at 21:44

Quote: Originally posted by Manifest

I'm sorry but your explanation is way over my head. What do you mean by buffered? Do you mean adding something to balance out the PH?
My knowledge is as follows: I have seen videos on youtube where they add food waste and water to a tank and leave it and it forms methane, they didn't adjust PH, they simply added food and water.
Is dirt needed? In the videos I seen no need for dirt.
I can give you a source if needed.

This following reference explains buffering much better than I can, so I'll post it here:

Attachment: CAS05_KungBiologyOfSilage.pdf (80kB)
This file has been downloaded 688 times

The article approaches fermentation from the perspective of trying to preserve the nutritional value of feed, not methane production. For this reason they are trying to avoid clostridial fermentation, as it produces poor quality feed.

In the following article, however, they are trying to produce hydrogen, which occurs during a clostridial fermentation (along with butryate). The chemistry is the same, but the focus is completely opposite. They outline a two-stage process (similar to what I mentioned earlier), where acidogenenic hydrogenesis is carried out in a separate reactor than the methanogenesis. Methane is also made in this process, during the second stage.

Attachment: Pilot-scale two-stage process-a combination of acidogenic hydrogenesis and methanogenesis.pdf (361kB)
This file has been downloaded 749 times

Clostridial fermentations are generally slow compared to the lactic and acetic-forming varieties. This is why slow fermentations generally produce increasing pecentages of butyric acid and hydrogen. The selectivity of one fermentation over another is partially pH dependent, as you can see from the references.

I usually add a small bit of fresh dirt before the methane-producing step, as I want to be sure there are some methanogenic bacteria in there. It may not be necessary.

Even if the pH is not adjusted during a fermentation, it should be monitored somehow, to ensure that the fermentation is proceeding properly.

Manifest - 31-7-2014 at 09:14

Thank you for the references, I put a couple of slices of bread into a empty milk container along with some banana peels and half filled it with water, closed the lid and left it out in the sun.
It was cold for a couple of days but yesterday the weather cleared up and I checked it today and it's giving off some gas and it's fizzing.
Might be fermenting from the yeast in the bread, hopefully the Ethanol will get oxidised to Acetic Acid.

Basically I took bits off about 4 slices of bread and put them in there, then I got a load of banana peels and put them in the blender and blended them, it turned brown and mushy so I diluted it with water and added these to the milk jug.

[Edited on 31-7-2014 by Manifest]