Haubenschild Farms Final Report - The Minnesota Project

The Minnesota Project August 2002
Comparing apples with apples: performance vs. design specifications
In looking for an explanation of the high biogas production of the Haubenschild digester,
an important factor to consider is the high manure production of Haubenschild’s cows.
More manure means more volatile solids that can be converted to biogas. Haubenschild
Farm’s cows are high milk producers, and since there is a direct relationship between milk
production and manure production, it makes sense that their cows would also produce more
manure than average.
An examination of Table 3 reveals that Haubenschild’s cows produce about 50 percent
more manure slurry than the design specification. 22 So it might be more appropriate to
compare biogas production per gallon of manure instead of per cow. The Haubenschild
digester, at 425 cows, had about a 40 percent higher biogas production per gallon of
manure than design specifications, while at 750 cows it is operating very near design
specifications per gallon of manure, not considering the biogas that is flared.
Reasons for high performance
The especially high performance of the digester in its first year of operation with 425 cows
may be due in part to the fact that the digester was operating at less than design capacity.
This resulted in the manure staying in the digester for about 30 days instead of 20 days, and
thus capturing more biogas. However, studies suggest that most of the potential biogas is
captured within the first 15 to 20 days of being in the digester, so this may not fully explain
the first year’s high production.
There are other factors influencing biogas production that may explain the high biogas
output of Haubenschild Farms digester (Refer to the box on page 23 – “What determines
how much biogas is produced?”). Manure is scraped and almost immediately fed into the
digester, resulting in higher methane capture. Dennis Haubenschild is an incredibly
knowledgeable and careful manager of the digester. He monitors the performance closely,
taking careful records and making adjustments as necessary, such as keeping the solids
content of the manure slurry above 10 percent.
22 Manure slurry is excreted manure plus wash water and bedding. Since the Haubenschilds are careful to
minimize water usage to keep solids content above 10 percent, it is likely that most or all of the 50 percent
increase in volume of the total manure slurry over design specifications is from manure excreted from the
cows.
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