SWUSA: Swine USA Anaerobic Digester
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Tour Handout, October 2000, Dr. Jeffery
Lorimor, Dept. of Ag. & Biosystems Engineering, 200A Davidson
HallAmes, Iowa 50011, ph 515-294-9806, FAX 515-294-9973, E-mail jclorimo@iastate.edu
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Swine USA Anaerobic Digester
Swine USA operates
a 5000 sow gestation-farrowing facility in Southern Iowa.
It uses shallow pull plug gutters that drain to a reception pit.
Raw manure is pumped from there to a 70’ X 90’ X 16’
heated, complete mix anaerobic digester operating since July, 1999.
The methane is burned in a Caterpillar engine to drive an 80 Kw
generator. The electricity is used on the farm. To date the digester system has been able to provide
approximately 35% of the farm’s energy needs.
When excess methane is produced (more than the engine can use) it
is flared off to the atmosphere. The
digester started operation August, 1999.
The
digester was designed by Mark Moser of RCMDigesters, Inc. as part of the
AG Star program. Total construction cost was approximately $500,000, or
$100 per sow. Government
cost sharing for the project was provided by the Iowa Department of
Natural Resources (IDNR), and the Natural Resources Conservation Service
(NRCS) for approximately half of the total cost.
ISU has monitored the digester and storage system
by sampling at 4 locations since October 8, 1999.
Samples were initially taken at two week intervals, and are now
taken monthly. Sample
locations are 1) the reception/transfer pit, 2) outlet of the digester
(overflow liquid), 3) north storage tank, 4) south storage tank.
The following charts show data summaries for 17 sample dates
taken from Oct. 8, 1999 through Sept 2, 2000.
Loading rate has
been 1.46 Kg/M3-day of COD, or 1.23 Kg/M3-day of
VS.
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SOLIDS
AND NUTRIENT DATA
Total
and volatile solids
Figure 1 shows the means of 17 samples for Total Solids (dark
bars), Volatile Solids (light bars), with the VS/TS percentage written
above the bar. One would
expect the TS percentage to decline through the digester.
The increase along with the large decrease in TS indicates the
likelihood of some TS settling within the digester.
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Figure 2.
Solids concentrations at four locations |
Chemical
Oxygen Demand provides a gauge of how complete the digestion process
is. Figure 3 shows before
and after COD of the manure. COD
leaving the digester has averaged 37.7% of the incoming COD.
The lower COD in the storage tanks is the result of a combination
of dilution, settling, and continued biological stabilization in the
tanks.
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Figure 3. COD at four locations |
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Nitrogen:
TKN and ammonia are tested at the four sampling points.
Figure 4 shows results. As
expected, ammonia concentration increases through the system relative to
TKN. TKN concentrations
were 10% less coming out of the digester than going in.
The decrease could be due to either ammonia loss in the gas (pH
has averaged 7.9), or to solids settling.
Lower TKN concentrations in the storage tanks may result from
either ammonia volatilization, settling, and/or dilution from
precipitation.
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Figure 4. Nitrogen
concentrations at four locations |
Phosphorus
concentrations decrease significantly as the manure passes through the
digester. Phosphorus does
not volatilize and is generally quite insoluble, so it is associated
with the solids portion of the manure.
The loss of phosphorus means that solids are settling in the
digester.
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Figure 5. Elemental phosphorus at four locations.
(to get P2O5, multiply by 2.29).
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Potassium
is expected to change little as the manure passes through the digester.
Sampling has confirmed it. Although
Figure 6 looks like the storage tanks are significantly higher in
potassium than the raw or digested manure, the scale only spans
2 lb/1000 gallons, so almost no difference in
concentration is shown (1.1 lb/1000 gal).
The lack of a difference says that little outside water such as
rainfall diluted to the storage tanks
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Figure 6. Elemental
potassium at four locations. (to get K2O, multiply by 1.2) |
Odors
Odors were tested in Dr. Dwaine Bundy’s pit additive
columns at ISU. Figure 7
shows that the gases from above the digested manure has very much
reduced H2S and odor threshold as compared to the undigested
manure gases.
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Figure 7. Hydrogen
sulfide and odor threshold in gases from digested and
undigested manure.
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OPERATIONAL
DATA
In general the digester has operated reliably
since startup. The 80 Kw
generator has produced approximately 460,000 Kw-hrs of electricity
through June, 2000, worth $30,400 to swine USA at 6.6 cent/Kwh.
The following operational data averages have been documented:
Average gallons fed daily
14,730 gallons/day
Overall generator run time since startup
84%
Biogas production:
~950,000 ft3/mo
=
190 ft3/sow-mo (6.3
ft3/sow-day)
CO2 measured concentration
26%
Methane production (assumes 74%) = 140 ft3/sow-mo
(4.7 ft3/sow-day)
Elec. Produced per month
42,290 Kw-hr/mo
Per day
1375 Kw-hr/day
Per sow per month
8.5 Kw-hr/sow-mo
Value ($0.066/Kw-hr)
$2790/mo
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Figure 8. Total
energy distribution. |
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Labor
requirements: Digesters
require management input. Normal
labor requirements for this system have been approximately 1.5 hours per
day. This includes manually
starting feed pumps, taking temperatures, changing engine oil, etc.
Some problems have occurred which have required additional
management input. They are
mentioned here not to emphasize problems, but to point out that complex
biological/mechanical system do require additional monitoring and
management attention compared to simply storing the manure.
Liquid
Spill An elbow within the
digester came apart, releasing some manure to the nearby drainage ditch.
The potentially serious spill was contained before reaching the
river approximately 1 mile away.
Engine
failure The engine failed
resulting in an 11 day shutdown before being repaired/replaced. The engine was replaced at no cost to the operator, but
resulted in down time and management input requirement.
Blown
fuse Although a seemingly
insignificant failure, a large fuse blew and required replacement.
Several days elapsed before a replacement could be shipped in,
resulting in engine/generator down time and management input
requirement.
It should be noted that the labor requirements
discussed above are somewhat elevated by additional data recording
requirements imposed by the Iowa Department of Natural Resources in
return for grant monies for construction.
Overall
Operation
Overall the digester has functioned well and
resulted in significant electrical savings for Swine USA.
Methane generated electricity has replaced approximately 35% of
their energy demand since startup.
It replaces up to ½ of their demand during the warm season, and
less during the cold months when the demands for heat of both the
buildings and the digester are higher.
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