Hello Bob,
I have been playing around with designing my own prototype digester, and stumbled upon your design posted on this site. (fortunate for me!) A question first on your design. The gas vent from the first stage portion of the digester, does that connect to some device to keep a positive pressure in the second stage, and thus, a "gas pocket" in the first stage, much as there is in the second stage? Secondly, you state that the majority of the gas produced by the first stage is CO2. Do you have, or know of a place to get data on what the typical composition of that gas is?
I am working on a design now to digest dairy cattle slurry primarily, but, in the future I want to seperate the solids from a portion of the material exiting the digester in order to hydrate poultry litter with the effluent, and send it through as well. From memory, I believe that dairy manure is considered to be around 12% solids or so, and I would hope dilute the poultry litter to a simmilar level. Do you think that the HRT's that you use would work at this level of solids? (I believe your digester is designed for around 6% or so?) Currently I have designed for a HRT of 5 days for the first stage, and 15 for the second, but this is subject to change, as the prototype is far from finalized.
Bah! I had some other questions, but failed to write them down, and now since it is 10pm, my brain has shutdown for the day. I'll post them when I remember them. I know I should not reinvent the wheel, but most of the dairy digesters that I have seen are single stage plug flows, and to me, a two stage deal would just seem to be more effecient. Thanks for your website, BTW!
Ian
questions on the design of your digester
Moderator:Bob
"A question first on your design. The gas vent from the first stage portion of the digester, does that connect to some device to keep a positive pressure in the second stage, and thus, a "gas pocket" in the first stage..."
Yes, you could, but offhand I can't think of how to do it without adding unnecessary complication. Or you could terminate it high enough that the gas pressure in the tank would not force liquid out. Did you see this construction detail?
"Secondly, you state that the majority of the gas produced by the first stage is CO2. Do you have, or know of a place to get data on what the typical composition of that gas is?"
I think the only reliable source would be direct experiment and analysis, as it will depend, first of all, on the feedstock, but also on pH, retention time, temp, etc.
"Do you think that the HRT's that you use would work at this level of solids? (I believe your digester is designed for around 6% or so?"
Yes. The primary reason is ease of material handling & pumping. But if you have a manure pump capable of handling a 12% slurry, it should work. With the higher loading rate, though, you will digest a smaller % of the waste. Probably best bet would be to just experiment until you find the best compromise.
[This message has been edited by Bob (edited 04-19-2003).]
Yes, you could, but offhand I can't think of how to do it without adding unnecessary complication. Or you could terminate it high enough that the gas pressure in the tank would not force liquid out. Did you see this construction detail?
"Secondly, you state that the majority of the gas produced by the first stage is CO2. Do you have, or know of a place to get data on what the typical composition of that gas is?"
I think the only reliable source would be direct experiment and analysis, as it will depend, first of all, on the feedstock, but also on pH, retention time, temp, etc.
"Do you think that the HRT's that you use would work at this level of solids? (I believe your digester is designed for around 6% or so?"
Yes. The primary reason is ease of material handling & pumping. But if you have a manure pump capable of handling a 12% slurry, it should work. With the higher loading rate, though, you will digest a smaller % of the waste. Probably best bet would be to just experiment until you find the best compromise.
[This message has been edited by Bob (edited 04-19-2003).]