Wednesday, December 20, 2023

Maximizing Efficiency: Aeration for Manure Treatment in Livestock Barns


 Managing manure is a critical aspect of modern livestock farming, and as the agriculture industry evolves, so do the techniques for handling and treating manure. One innovative method gaining popularity is aeration, particularly in deep pit manure storages for swine finishing, dairy cattle, and beef barns. This article delves into the science behind aeration, its impact on manure solids breakdown, nutrient content, ammonia and greenhouse gas emissions, and its influence on biological oxygen demand (BOD).

 

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.


Figure 1. Aeration lines outside the barn that actuate in zones to aerate the manure.


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