[Digestion] sludge gas yields

[Warren Weisman]

Another gas yield question. Please check my math, as I
am the worst and I apologize to our European members,
but if I tried to convert to metric it would only make
it worse: 

An engineer friend recently told me that gas yields
decrease with increase in scale. 

In my small demonstration batch digesters with a good
1 part nitrogen (chicken guano), 4 parts carbon
(apples and vegetable scraps) and the rest water I can
get about 11 cubic feet/lb. volatile solids over a
7-10 day retention period at 80 F. 

Presently, I know for a fact our three, 1.1 million
gallon floating lid, plug and flow wastewater
digesters here in town (the ones Paul Harris and I
stood on top of during his visit) each produce average
of 120,000 cubic feet per day at 100 F each. They use
a 14 day retention cycle. They use gravity belt
thickeners and I am trying to determine what water
content they end up with. 

They are not co-digesting the sludge with anything.
The 120,000 ft. per day should give them 1.68
million/14 days. Assuming 6 cubic foot/lb. volatile
solids, how many lbs. of solids to they have in all
that water? I've read it could be 6 lbs./gallon, but
find it difficult to believe there's only 46,000
gallons of waste in such a large digester. 

The reason I ask is I'm trying to calculate the value
of adding dry crop waste compared to GTW or food
waste.

Warren Weisman  


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