| The Challenges and Results
of 25 years use of Digester Systems on Mason-Dixon Farms (MDF) |
|
Rather than describe
all the learning steps to develop a working digester at MDF I will give
our success formula principles for a working system.
1.
The goal for dairy cattle manure is to
scrape the manure and urine from alleys on a daily basis into the digester.
If you are using a flush system or sand bedding in free stalls
forget even trying. The economics of flush water manure is too diluted
and sand-laden manure will require frequent sand removal from digester.
2.
Build digester above the water table so
that ground water doesn’t rob heat from digester.
3.
A round digester only has 60% of wall
surface of a rectangular design. Also wall construction cost for rebar and cubic
yards of concrete are reduced by a similar amount.
4.
Install a divider wall through the center
of the circle extending it to within six feet of the other side.
From this wall install 3-inch black iron heating pipes at least
6 inches away from the wall attached to channel iron embedded in the
divider wall. Remember to use
stainless steel pipe nipples through the outside wall.
5.
Concrete lid is poured on top of walls
to carry the weight. For example,
a 68 feet diameter tank by 14 feet will accommodate 800 cows.
6.
Load tank three feet from top on one side
of central wall and discharge from opposite side.
7.
Mound ground over digester for insulation.
8.
If possible locate digester so input slurry
will enter by gravity.
9.
A 1200-pound cow unit will produce about
2.2 kW per day.
10.
We maintain 105 degrees F in digester.
11.
We are now building our 4th
digester to handle the manure from 2500 cows.
12.
The digester system at MDF just completed
25 years of successful operation.
13.
17 % of energy sold to power grid at grid
price, which is variable according to time of day or night price.
14.
All manure solids are separated from spent
slurry and composted to destroy weed seeds and pathogens before using
for bedding and etc.
15.
The liquid part is spread by center pivot
irrigation by a four mile buried 8-inch PVC pipe to center pivots or
for storage during winter months.
16.
Be sure to design for low maintenance
without a lot of bells and whistles, which reduce profitability |
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