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Documents, Tools and Resources
Anaerobic Digestion at Dairy Farms
The need to upgrade dairy waste management practices to overcome
pollution problems is leading more farmers to seek solutions
with anaerobic digestion technology. Two recent examples of
this trend are underway at California sites. The Cal Poly Dairy
is located adjacent to the California Polytechnic State University
campus in San Luis Obispo. The dairy milks 180 cows with a
total population of over 350 animals, including heifers and
calves. Most of the herd is housed in freestall barns. About
90 percent of the manure is deposited on concrete and flushed
with fresh or recycled water to a single-cell lagoon. The remaining
ten percent is deposited in the corrals and collected only
seasonally. Solids are separated from the flushed wastewater
prior to storage in the lagoon, which has a volume of 19,000
m3, translating to 50 to 90 days of storage, depending upon
the water used by the dairy.
A methane recovery system has been constructed at the dairy.
The initial design was based upon the present and anticipated
herd size. The new lagoon, which has a liquid volume of 14,000
m3, was constructed next to the existing one. The first lagoon
is being used for storage and the second for methane recovery.
The new lagoon was covered with a flexible membrane incorporating
buoyant material so that the cover floats on the surface, and
a gas collection system was attached. The project will reduce
odors, keep greenhouse gas out of the atmosphere, and cut down
on water pollution through reduction of organic matter in the
lagoon.
Covered Lagoon Design
Design of a constant volume methane-producing lagoon must
consider all of the volume flowing into the lagoon to avoid
hydraulic washout of bacteria. The inclined screen will capture
15 percent of the manure volatile solids from the  liquids
flowing to the lagoon. The estimated daily wastewater is 350,000
liters/day at 0.3 percent total solids and 0.25 percent volatile
solids (VS).
The predicted output of the lagoon will average 320 m3/day
of biogas. As the gas bubbles form in the lagoon, they are
channeled along 23-cm diameter flotation blocks under the cover
to the gas manifold at the bank of the digester. This manifold
exits the cover via a 7.5-cm flange, which is connected to
a 7.5-cm diameter PVC buried pipe. The pipe then connects to
the rest of the gas handling system. Eventually, the gas system
will be piped to a gas handling/utilization building containing
the electric generation system. Methane production from the
covered lagoon is adequate to produce 20 to 25 kW on a continuous
basis from the present animal population.
Costs and Savings
A biogas-fired microturbine with 25 kW maximum electrical
output will operate in parallel with the utility system at
a constant level of output controlled by the biogas supply.
The microturbine will be supplied at no cost by Reflective
Technologies.
The annual electrical use by the dairy operation is approximately
234,000 kWh with a value of $21,000, averaging almost $0.09/kWh.
Benefits from using biogas to produce electricity are based
upon an average of up to 23 kW, which is the estimated electrical
generation. The completed methane recovery system will produce
170,000 kWh of electricity and 77,000 kJ of hot water annually,
worth approximately $16,000. Total project cost is expected
to be $200,000 with a simple payback of 13 years, not including
the environmental benefits of odor control and water pollution
prevention. Although there is no capital cost of electric generation
for this project, the cost of the state-bid lagoon construction
was very high - $60,000, compared with typical farm construction
of under $25,000. Since the estimated cost of a 25 kW engine-generator
is over $30,000, the higher lagoon cost offsets the absence
of an electric generation capital cost for the Cal Poly system.
Preliminary gas measurements taken this summer indicate approximately
200 m3 of biogas produced daily from a partial cover of less
than 50 percent of the total lagoon surface area. The biogas
is being continuously flared and is maintaining a self-supporting
flame. Preliminary gas analysis has indicated a methane percentage
of 75 percent, with the balance of 25 percent being carbon
dioxide. The biogas quantity and quality will continue to be
monitored, and the microturbine generator will be incorporated
sometime in 2000. Cover installation will be completed this
year, at which point electricity output will be at its maximum.
Organic Dairy
As "the first organic dairy west of the Mississippi," implementing
a sustainable manure management system is important for the
six-year-old Straus Family Creamery in Marshall, California.
The dairy barn is cleaned and washed three times daily with
each milking. The freestall barn, where the cows rest and eat
after milking, also is flushed. The waste water goes into several
holding ponds for separation of solids and liquids. The solids
are then composted.
Straus is experimenting with new manure management methods.
Its first project was development of a system using bacterial
inoculation to break down waste from the barn and reduce solid
matter and toxins. The system helps increase absorption by
cover crops and stop harmful contamination from runoff. Some
of the treated water is recycled to flush the freestalls.
Advanced Integrated Pond System
Last August, the dairy was selected as a site by the Marin
County Resources Conservation District for the application
of an innovative process, Advanced Integrated Pond System (AIPS)
with a methane digester. Dairies in the Tomales Bay watershed
are potential sources of nopoint pollution. The AIPS process
reduces pollution loads before disposal on land or surface
waters of the basin. The operation will demonstrate low-cost
treatment technology and provide opportunities for replication
at other locations. Funding has been made available by the
Clean Water Act, and Swanson International Engineering, Inc.
of Martinez, California is the technical consultant.
Five existing ponds will form a system of anaerobic and aerobic
processes to reduce wastewater organics, solids, nutrients,
coliform bacteria, etc. Due to limited funds, modifications
will be confined to Ponds 2 and 3. Ponds 1, 4 and 5 will be
operated as part of the system, but no work will be included
under the project. The effluent disposal system will be used
without an upgrade.
In Ponds 2 and 3, dikes, inlet structures and pipes/valves
will be modified or installed as needed. The ponds will be
deepened for anaerobic digestion chambers. In Pond 2, a cover
will be installed to capture methane and a meter installed
to measure gas production and for atmosphere venting. The inlet
for creamery wastewater will be modified for loading into Pond
3. Outlet structures and a pump station will be modified as
needed for pumping wastewater to Pond 4.
The system will be operated and monitored for a six-month
period. A report will be prepared on the monitoring, evaluation,
findings and conclusions of the technology demonstration. At
the completion of the study, the facilities will revert to
the owners.
Information for the Cal Poly project was adapted from a paper
written by D. W. Williams, BioResource and Agricultural Engineering
Department, California Polytechnic State University, San Luis
Obispo, California; M.A. Moser, Resource Conservation Management,
Oakland, California; and G. Norris, USDA Natural Resource Conservation
Service, Templeton, California. Dr. Williams can be contacted
at (805) 756-6153. |
