Sawdust as feedstock discussion from Facebook Group October 31 2014

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Thomas H. Culha...
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Sawdust as feedstock discussion from Facebook Group October 31 2014
Sawdust discussion

Can someone tell me if the introduction of saw dust to a manure digester will effect the production of gas or will it break down sufficiently. The sawdust is utilized as a covering in humanure composting setups for odor control.

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  • Dominic Kahumbu Wanjihia 1. Sawdust is very boyant and will cause heavy scum formation.

    2. Sawdust breaks down so slowly it will extend the retention time of whatever you're mixing it with.
    Considering it will be floating only worsens the situation.

    3. Sawdust doesnt have much food value so will probably not contribute much to methane gas formation.

    I mean, would it do anything for you if you are it? 
  • Warren Weisman Sawdust actually sinks. But it is mostly undigestible. What is volatile solids is straight carbon, and can lead to low pH. For humanure buckets it would be better to use ashes, leaves or moss.
  • Warren Weisman I guess I should add - like grasses - it needs to decompose aerobically a little bit before sinking. To break down the hard outer coating. Fresh log will float in water, rotten log will sink in water.
  • Lyndon Bradshaw Ashes, leaves and moss aren't typically something you can buy off the store shelf and here in the desert none of that grows lol. So it sounds like I would be better off having regular flush toilets that enter my digester or come up with a way to separate the sawdust from the manure. Maybe in the slurry process? Our digester isn't going to utilize any vegetation as that all has other uses on our farm. It will have to run completely on human/animal waste.
  • Warren Weisman Absolutely. Dominic Kahumbu Wanjihia does a very nice flushing toilet where solids and liquids are separated with a low-tech screen.
  • Lyndon Bradshaw Would love to hear more about that. Where does the separation take place and where do the respective parts, solid/water, end up?
  • Warren Weisman Hopefully he can post an image. I will look to see if it is in the files here.
  • Hatem Mohamed Elgamal In such situation I would recommend dry fermentation as sawdust actually will float making a thick scum
  • Warren Weisman Didn't see it in the files. It was basically something like this. Only there is a certain angle it needs to be. I believe like 15-degrees, but don't quote me.
    Warren Weisman's photo.
  • Lyndon Bradshaw Thanks! Sort of what I had in mind when you mentioned it. So I assume the solids would then continue to the digester and the fluids would go to our septic system?
  • Warren Weisman Keep in mind, toilet waste makes very little gas. It's just a waste disposal system.
  • Lyndon Bradshaw Yes but it would add to our gas production from the animal waste would it not? We would also end up with a high grade compost while decreasing the solids in our septic tank. My ultimate goal is to eliminate the septic system completely and have a 100% renewable system.
  • Warren Weisman Sure, yep. The rule of thumb is 2 cubic feet of gas per person "on the line" per day.
  • Warren Weisman Here's a nice set up from India I would use for a home wastewater treatment. Digester followed by slow sand filter, followed by carbon filter and UV tertiary treatment.
  • David William House Adding to what has been said, one of the better ways to handle sawdust is to inoculate it with fungi. (The proper species of fungi will depend on the species of tree from which the sawdust will come. See, "Biochemical Methane Potential of Biologically and Chemically
    Pretreated Sawdust and Straw"

    And I might mention that the reference provided by another poster has it correct: the feces and urine provided by the average adult will provide one cubic foot (30 liters, pg. 4) of biogas per day.
  • Lyndon Bradshaw Thanks guys the info is much appreciated. I think I will pursue a way of separating the sawdust from the waste. Possibly make in to a slurry and if the sawdust will accumulate on top then it should be relatively simple to put the slurry through a sieve of some sort that traps the sawdust. This could then be removed and composted in the same way we already compost our humanure. We live in the desert and do not have a dedicated water supply so we have to haul it in. Finding ways to reduce our water usage like not having flush toilets is imperative.
  • Warren Weisman If you had the money, you could add a reverse osmosis system and do toilet-to-tap.
  • Lyndon Bradshaw Yeah that would be awesome but those run in the thousands of moolahs. Most of what we do has to be done with limited funds and i almost always have to redneck our way through it. Being completely off grid poses a whole new set of challenges also.
  • Warren Weisman I feel you. I lived off the grid in Alaska for two years.
  • Harpal Singh Grewal no saw dust for gas
  • Jacob Douenias Lyndon Bradshaw To advocate a bit for sawdust here – it can be used in low volumes as a carbon additive for nitrogen heavy waste, like food waste. I do think that premixing the sawdust into the feed slurry and agitating the digester slightly to prevent all of the wood material from either sinking or floating.
  • Warren Weisman True dat, Jake. However, paper, cardboard and brown leaves will give you carbon without the risk of tannins.