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Many dairy
producers are interested in innovative manure treatment to improve
nutrient recycling, reduce land-application costs, and provide useful
products (e.g., compost, electricity, hot water).
Should there be an innovative system in your future? That depends.
Each requires changes in manure handling and equipment. Each increases
operating costs. These costs must be weighed against expected benefits.
To make a change worthwhile, processes need to be cost effective,
labor efficient, environmentally friendly, and reliable year-round.
Mechanical
Solid-Liquid Separation
Mechanical
separators divide manure into solid and liquid fractions. The solid
fraction contains the fiber and some of the nitrogen and phosphorus.
With less water, solids can be transported at a lower cost per pound
of nutrient for land application. Solids can also be composted and
reused as bedding or sold as compost. The liquid can be recycled
as flush water for manure removal in freestall barns, or land applied
at rates based on crop needs. It's not clear whether separation
reduces odor.
Mechanical
separators include screens (inclined screens, rotating screens,
vibrating screens), belt and screw presses, and centrifuges. Capital
costs can range from as low as $5,000 for screens to more than $200,000
for a centrifuge. This type of equipment has long been employed
in municipal and industrial wastewater operations.
Performance
of mechanical separators varies widely. Total solids in the solid
fraction range from 5 percent with stationary screens up to 35 percent
with centrifuges. Separation efficiencies for total solids can vary
from less than 10 percent to about 60 percent. Presses and centrifuges
have higher separation efficiencies and produce drier solids than
screen separators.
The proportion
of nutrients remaining in the liquid fraction depends on the percent
solids in the manure, use of additives, and storage time before
separation. In general, more nutrients remain with the solids when
there are more solids in the manure being separated. Additives (e.g.,
lime, ferric chloride, and flocculants) can improve separation.
Nutrients and solids are generally more readily removed from fresh
manure than from stored manure. Both solid and liquid fractions
need proper handling or treatment to minimize odor and maximize
nutrient recycling on cropland. Both contain adequate moisture,
organic matter, and nutrients to pollute water and air if not handled,
treated, and stored properly.
Composting
Composting
turns dairy manure solids and organic bedding into a uniform, easily
handled, organically stable, soil-like material that can be land
applied and marketed as a soil amendment. Composting produces heat,
which drives off moisture and kills pathogens and weed seeds. It
also reduces the volume of material as much as 50 percent. Potential
benefits include improved manure handling, enhanced soil tilth and
fertility, and reduced environmental risk.
Composting
requires proper materials, equipment, space, and management. It
also requires oxygen, moisture, and an appropriate nutrient balance.
Material to be composted should have a moisture content between
40 and 65 percent and a carbon:nitrogen ratio of 20:1 to 40:1 (25:1
to 30:1 is preferable). For detailed instructions on composting,
see NRAES 54, On Farm Composting Handbook, available through the
University of Minnesota Department of Biosystems and Agricultural
Engineering (612/625-9733).
Compost sites
can generate offensive odors and gases if not properly managed.
The odors can originate in the ingredients if they have been stored
anaerobically for a week or more. It can also result from insufficient
aeration, improper nutrient balance, and excessive moisture. Odors
and gases appear to be more significant in the early stages of the
process and also during turning. Management is key for reducing
odors and gaseous emissions from composting operations.
Anaerobic
Digestion for Generating Biogas
An anaerobic
digester can transform dairy manure into a variety of end products,
including methane (CH4) and carbon dioxide (CO2).
This "biogas" can be burned for heat or used to fuel an
electric generator. The heat and electricity can be used on the
dairy or sold to others. The digester effluent, which contains all
of the nitrogen, phosphorus, potassium, and micronutrients in the
original manure, can be further processed or land applied. A complete
anaerobic digestion system can become quite complex.
One of the
most common anaerobic reactors used for the treatment of manure
is the plug-flow reactor. In this system, manure is added to one
end of a tank and effluent is removed from the other end into a
storage unit. Other types of anaerobic digesters include complete-mix,
contact, and upflow anaerobic sludge blanket digesters.
Digesters treat
manure best and make the most biogas when operated at temperatures
above 120 degrees
F. However,
this is not usually cost-effective because the energy required to
maintain this high temperature is greater than the energy gained
in the process. Anaerobic digesters are usually operated between
95 degrees F and 100 degrees F. There have also been some successful
applications in the 60 degrees F to 75 degrees F range, with lower
treatment efficiencies offset by higher retention times.
Anaerobic
digestion systems are fairly capital intensive. Components include
manure handling, digester tank (including heating and mixing equipment
if needed), gas handling, and electric generation equipment. Digesters
require good management and regular maintenance for good long-term
performance.
An anaerobic
digester at Haubenschild Farms Inc. near Princeton, Minnesota, is
generating more than 50,000 cubic feet of biogas and 2,000 kilowatts
of electricity per day plus hot water from the manure from 436 cows.
The unit, designed to handle the manure from 1,000 cows, is expected
to be operating near full capacity after an expansion. For more
information about the unit, go to the Minnesota Project web site
at http://www.misa.umn.edu/.
Information for determining whether an anaerobic digestion system
is a feasible choice for your dairy farm can be found at http://www.auri.org/.
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