Batch or Continous Flow...Please Help!

Hi All, I've been starting a lot of topics lately. I'm sorry if that is not too cool. Anyway, I just have q's and am searching for answers. So, here goes.

I want to build a methane digester in my barn to use bedding and manure from my goats and chickens. I use straw and sawdust as bedding and since goat poo is pretty dry (and small) I plan on feeding the digester with this bedding mix (sawdust, straw, droppings).

There is so much info on the web about bio digesters and some of it seems outright contradictory. So, I'm looking for some experienced advice.

I do not see how anything containing sawdust, stray or hay could be digested in a continuous flow digester because is will block the flow and or settle on the bottom (hay and straw block, sawdust settle). Some claim digesting woody material is IMPOSSIBLE, some clai it works WONDERFULLY. Some claim that undigested vegetable matter (hay) will take far too long, others say it takes a bit longer but produces considerably more gas.

I can get several large 1m3 (about 260 gal) square tanks (you know the kind on pallets in metal cages) for next to nothing. I was thinking of buying four of them in my barn (insulating them very well before burying them) and plumbing in a heat exchanger from a jean pain pile. I would fill one, seal it, hook it to my gas collectors. I would then wait 3 weeks and fill the next one and son so that in twelve weeks I'd have all of them flu and producing. Then I'd empty the first one and start over. Is there some reason this will not work?

Also, does vegetable mater (sawdust and straw) break down enough in 12 weeks to be pumped out with a sludge pump or will I need to shovel it out?

And what is a good, authoritative source of info on this topic. A book?

Thanks All!

Hello Justus,

It seems like we have the same projects going. I am currently building a small scale biogas digestor. I plan to put pictures up later in another thread, I had posted in a while back.

Returning to your questions. I am building my/our digestor using a build similar to the UN biogas digestor specifications. This is because I can get the steel 55 gallon barrels for next to nothing. Try searching the internet for "Better farming series 32 - Biogas 2 building a better biogas digestor unit" The design in that PDF from the UN is what I am building at the moment.

From what I have read, and my preliminary experiments. Straw, horse manure, and other cellulosic material takes a longer time to digest than less fibrous material. Maintaining the digestor temperature will be very important. I've got a 'stuck' fermentation when the temperature dropped below 70 Fahrenheit (21 Celsius). In the stuck fermentation none of the fibrous material digested and very little methane was produced.

I think using the plastic totes for the digestor is a good idea. How are you planning to store the gas?

Brett, Sounds good. I have the Better Farms 2nd Biogas plant. Is it the Same as the UN plant?

I am planning, if I do settle on a batch design, to use this plan but to use the square 260 gal containers instead of 50 gal drums. I'm having a hard time projecting my digester.

Currently I use about 17,138 buts worth of propane per day.

It seems like I need to load the batch with 92% water and 8% solids (by weight). So i'll put in 176 lbs of feedstock (sawdust, straw, hay, manure) and that should give me a total of about 320,000 btus (1,818 btus per lb). Does this all lok right so far??

The problem I'm having is figuring out how much time will be needed. I'm looking at batch times differing from 30 days to 100 days!! At 30 days one cube (batch container) would give me about 10.667 btus per day. So with two cubes I could get over 20,000 btus worth of methane. BUT if my batch time needs to be 100 days then I would only be getting about 3,200 btus worth of methane per day. I could scale up to four cubes but even that would only give me 12,800 btus worth of methane per day. Sooooooo...What gives??

I really want to do methane. BUT the bad numbers look really bad and the good numbers look well, good! If it is a situation where I could get 80% of the methane in 40 days and then reload the batch and forget about the other 20%, that would be fine too. but, i'd like to know. Cause with my 80% model (if it is even realistic) I'd get about 256,000 btus in 40 days from one batch that gives me about 6,400 btus per day, so I could run 3-4 batches and that would give me my needed btu amount per day. I would start a new batch every 10 days. That is not that much work and would work well with my cleaning schedule.

Any thoughts?? I'd love to see photos once you've got them up.

I will use a parallel connected string of inter tubes as my collector and then run an underground pipe to my house. IF I can find the answers to my qs and IF those answers make me happy then I will do this and will also place photos.

How are you keeping the your batches warm? How many batches will be in your system?

I will be running a heat exchanger from a giant (70 cubic yard) compost pile to each cube batch and using my wood fired hot water heater I'll be putting in water that is hot so the heat exchanger only ever has to keep the batches warm. The batches will be situated under the hallway of my barn, between the stalls, and will be very well insulated and then reburied. So, should be good.

Hope to hear from you, and others.

Justus Walker wrote:Brett, Sounds good. I have the Better Farms 2nd Biogas plant. Is it the Same as the UN plant?

Yes, it looks to be the same. I am not able to view the book it looks like it has more information than the freely available PDF.

Justus Walker wrote:

It seems like I need to load the batch with 92% water and 8% solids (by weight). So i'll put in 176 lbs of feedstock (sawdust, straw, hay, manure) and that should give me a total of about 320,000 btus (1,818 btus per lb). Does this all lok right so far??

Your calculation looks to be in the average of all the values I've used. I've only done theoretical calculations at the moment. I was planning to do 90% water 10% solids just because it was easier to remember when I am shoveling manure.

Once it is running you will probably have to adjust depending on temperature, manure pre-digestion amount, and water quality. Is the water you are using well water or urban water with chlorine? Try to avoid the urban water with chlorine, fluoride, and anti-algae chemicals (naturally).

Justus Walker wrote: The problem I'm having is figuring out how much time will be needed.

Time is a good question. I wish I had my plant running to give you some actual numbers. I can say from my small test batches that it is highly temperature dependent. If the digestion gets to be 140 Fahrenheit (60 Celsius) you might be looking at 30 days for complete digestion. If it is 70 Fahrenheit (21 Celsius) it might take 100 days for digestion. If the temperature drops below 70 Fahrenheit your digestion will stop.

Justus Walker wrote: Any thoughts??

One thought is the production rate of biogas. Since it is a bacterial digestion it will follow a asymptotic production curve. For example:


Thus, you will keep getting gas until a point when the rate reaches a maximum. The digestion may not be complete but your rate of production will have maxed.

Justus Walker wrote:
How are you keeping the your batches warm? How many batches will be in your system?

The reactor is in a green house, the temperature is around 21 Celsius during the winter, hotter in summer. The water will be well water that is probably going to be colder than 21 Celsius so we may have to let the water preheat before we add the manure slurry.

We are planning to start small at the moment, just one 55 gallon reactor. Then once we have a system going then scaling up.

It sounds like you have a pretty good heating system setup. Well thought out and lots of available heat.

Here is the link to the other thread where with my picture:
Homescale biogas
The picture is the very last post (at the moment)

Why did you decide to go with a batch reactor other than a continuous flow reactor?

Your greenhouse stays that warm all winter? what are your outdoor temps like? If you have clear weather a simple solar collector should give you enough added BTUS to keep the reactor nice and toasty even in the winter and that would affect the greenhouse temps positively as well.

So are you up and running now?

Thanks for posting the photo!

You probably need a little more back story on the biogas, it will help answer your questions. The biogas setup is not on my farm or property. I volunteer once a week on a local farm run by a permaculture instructor. So while a lot of my projects are in suburban settings this one is on a farm many miles from my house. The farmer is in his late 60's having had back injuries, so I try to to all the heavy lifting.

So I only work on the biogas once a week, so progress is slow. Sometimes we plant trees, make berms, plant crops rather than working on the digestor. Yet, we are trying to finish it before his next permaculture class starts in the spring.

I decided to do batch because I am not there everyday. The batch is easier for he and his students to understand. We wanted to keep it has simple as possible so as many student could understand it.

The greenhouse stays about 15 to 20 Celsius depending on cloud cover. We are in a desert so the sun is always shining. We live at 1,500 meters so the sun in very intense. The winter temperatures can be -10 Celsius to 0 Celsius with very strong winds. Yet, keep in mind that the sun is still shining as bright although it was summer though the temperature is below freezing. In the winter the sun rises at 7 am in the winter sets around 5 pm. It is a very different climate than Siberia. I know from experience having lived in Germany for a year. My German winter was gray, wet, cold, and cloudy with the sun rising at 10 am and setting at 3 pm. The Germany winter was probably warmer than what you experience.

Yes, a solar collector would be more than enough, perhaps even too much. Some of my solar concentration experiments have reached 300 to 400 Celsius even in the winter!

go to this web site http://csanr.wsu.edu/anaerobic-digestion/small-scale-biogas-technology/ and see the innovative way they have dealt with cellulose and manure timing in there digester for small scale production. you may change your mind on how your going to do this project.