Aeration
vs. Anaerobic Decomposition:
Traditionally,
liquid manure management has relied on anaerobic decomposition, which occurs
without oxygen. While this method is effective, it focuses most effort on
storage, especially for deep pit facilities, with anaerobic decomposition
occurring as a side effect of organic matter in the manure being held. In
anaerobic situations, organic carbon molecules break down in cascading
reactions towards carbon dioxide and methane. However, as little energy is
released in these reactions, reaction rates are typically slower and can result
in the loss of some partially degraded organic compounds that result in odor.
Similarly, as these reactions are low energy they often are slow, which can
mean slower solids decomposition.
Aeration
introduces oxygen into the manure, fostering an aerobic environment. Aerobic
reactions release more energy and, as a result, encourage greater microbial
activity and faster reaction rates. Because reaction rates are faster, the
breakdown of solids is encouraged. Carbon processing in aerobic conditions
flows from organic carbon towards carbon dioxide, and while it is again a
series of cascading reactions, the higher microbial activity limits the
accumulation of carbon breakdown products and the potential for odor emissions.
Manure
Solids Breakdown:
Why do
elephants breathe in oxygen? I'll often start a lecture this way when
discussing manure because it illustrates the concept of energy flows well. If
you want to get big you need to be making energy in the reactions. Aeration
enhances the breakdown of manure solids through microbial activity.
Microorganisms thrive in the presence of oxygen, accelerating the decomposition
process. This results in a more homogeneous and liquid manure, making it easier
to handle and apply to fields as fertilizer. However, the extent of the change
depends – on how complete was brake down under anaerobic conditions and how
complete it is under aerobic conditions. Aeration can potentially increase
solids breakdown and generally will, but the extent matters in these
conversations about what the result will be.
The air
flux through the manure also has the potential to suspend and mix manure solid
particles back into the manure, making it more uniform. However, limited work
has shown the impact at different air flow rates. Several years ago, experience
with foaming manure indicated that higher methane fluxes lead to more uniform
manure composition; similar impacts would be expected for aeration mixing, but
results will vary based on changes in solids content, particle size, and the
intensity the aeration mixing provides.
Nutrient
Content and Availability:
Aerobic
conditions promote the conversion of organic matter into free ions. Less
organic matter (i.e., greater solids breakdown can free nitrogen and phosphorus
making it more rapidly available in aerated manure. For phosphorus, this has
little impact on overall fertility, as the phosphorus is held reasonably well
in soil, and we can implement fertility management planning to handle
availability. For nitrogen, the answer is more complex. For a manure like
swine, where all the nitrogen is already first year, it is more immediately
available at application, but it doesn't change our ability to take advantage
of the nitrogen. With cattle manures where typically a multiyear mineralization
sequence is used, it will push more of the nitrogen being available into the
first year, where we have more opportunity to account for it and credit the
manure appropriately.
Ammonia,
Odor, and Methane Emissions:
Aeration
can either increase or decrease ammonia emissions from manure, with no clear
answer yet available, and the answer is at least somewhat dependent on the
aeration system used. Aeration provides a relatively clear decrease in odor
emissions for the reasons discussed previously. Additionally, methane emissions
are diminished under aerobic conditions – with low levels of aeration generally
as effective as methanogenesis, performing microorganisms are relatively easily
disrupted.
Biological
Oxygen Demand (BOD):
How
much oxygen or air is required? It depends – what are the goals of the aeration
system, and what are we hoping to achieve? However, in all cases, we are generally
trying to aerate based on the biological oxygen demand of the manure. The
biological oxygen demand, or biochemical oxygen demand, is the amount of
dissolved oxygen aerobic organisms need to break down organic material in a
given water sample at a certain temperature over a specific period.
Essentially, it is a measure of the wastewater, or manure strength, with higher
biological oxygen demand, meaning greater wastewater strength.
Historically,
the recommendation has been approximately supplying enough oxygen to satisfy
twice the biological oxygen demand in the manure. This recommendation is based
on stabilizing all decomposable organic matter in the manure and assuming
oxygen transfer efficiencies. However, low-rate (0.3-0.5x BOD) aeration has proven
successful for odor reduction and methane emission mitigation, with lower
potential benefits.
Full
Rate Aeration vs. Low Rate Aeration:
The
choice between full-rate aeration and low-rate aeration depends on various
factors, including the type of livestock, barn size, and climate conditions.
Full-rate aeration involves high airflow, facilitating rapid manure
decomposition and more complete decomposition. On the other hand, low-rate
aeration is a more energy-efficient option that may provide many benefits
(reduced odor, reduced methane emissions) while reducing energy requirements
for system operation.
Example:
The
ASABE manure production standard provides estimated BOD excretion for different
animals, with finishing cattle estimated at 1 lb BOD per day, dairy cattle at 3
lb BOD per day, and finishing swine at 0.3 lb BOD per day.
Looking
at swine, let's do an example where we want to supply 2X the BOD for aeration
and another at 0.3x.
We can
make a rough estimate of the energy needed to achieve the oxygen requirement.
While there is variation in oxygen transfer efficiencies, 3-5 lb O2/kWhr are
typical.
Assuming
an electrical cost of $0.10/kWhr allows operating costs to be estimated.
Full Aeration
0.3 lb
BOD/pig- day x 2 lb O2/lb BOD = 0.6 lb O2/pig-day
0.6 lb
O2/pig-day / 4 lb O2/kWhr = 0.15 kWhr/pig-day
0.15
kWhr/pig-day x $0.10/kWhr x 365 days = $5.50 /pig space-year
Partial Aeration
0.3 lb
BOD/pig-day x 0.3 lb O2/lb BOD = 0.09 lb O2/pig-day
0.6 lb
O2/pig-day / 4 lb O2/kWhr = 0.0225 kWhr/pig-day
0.025
kWhr/pig-day x $0.10/kWhr x 365 days = $0.82 /pig space-year
Further
work is needed to quantify the difference in potential benefits of each system
to understand how different aeration rates impact solids suspension, breakdown,
and mixing.
Conclusion:
Aeration
is revolutionizing manure management in livestock farming, offering a
sustainable and environmentally friendly alternative to traditional anaerobic
methods. The benefits, from improved nutrient content and availability to
reduced ammonia and greenhouse gas emissions, make it a valuable tool for
modern agriculture. Farmers should carefully assess their needs and consider
factors such as BOD levels, livestock type, and barn size when implementing
aeration systems, ultimately contributing to a more efficient and sustainable
farming future.
In next
month's article, we will use this information to size a blower system and the
airlines for an example aeration system.
