Thursday, May 23, 2024

Do Iowa Counties Have Sufficient Crop Land to Meet Manure Management Plans Requirements Without Exporting Manure?

Iowa is known for its significant agricultural production, including crops like corn and soybeans and livestock such as hogs, cattle, and eggs. However, given the size of the Iowa livestock industry, a common question becomes whether there is too much manure. In large part, manure from these livestock operations is often used as a nutrient source for crops, so people are asking two questions at its core. 1. Is there enough crop ground to utilize produced manure and 2. Are farmers taking credit for this manure and reducing fertilizer purchases?

Whether Iowa counties have sufficient cropland to utilize all the nutrients from manure without exporting it depends on various factors, including:

Livestock Density: Areas with high concentrations of livestock will generate more manure and, therefore, have more manure nutrients.

Cropland Availability: The amount of available cropland, its proximity to livestock operations, and the productivity and nutrient need determine the feasibility of using all manure locally.

Manure Management Practices: Effective manure management practices can maintain more nutrients in the manure, but they require more land to use the manure nutrients. Manure practices a farmer picks could be influenced by the amount of manure they need. With that said, we want to pick practices that conserve manure nutrients because they increase the circularity of agricultural systems.

Within this work, we will look at these concerns in several ways. The first two are at a state level. This work is built off Andersen and Pepple's (2017) A County-Level Assessment of Manure Nutrient Availability Relative to Crop Nutrient Capacity in Iowa: Spatial and Temporal Trends. Within that work, I defined algorithms for using Census of Agriculture data to estimate livestock populations and from this both manure nutrient excretion and available manure nutrients for land application, with the former being an estimate of what is excreted by the livestock, and the latter being an estimate manure nutrient recovered for land application and corrected per Iowa State suggestions for nutrient availability and application losses. I also estimated a state level of nutrient needs using the Census of Agriculture production statistics and the USDA Crop Nutrient Removal Database for evaluating the nutrient content of the harvested material.

A summary of nitrogen and phosphorus comparisons between manure (excreted and available for crop use) and crop capacity is provided in Figures 1 and 2. Crop capacity focuses on corn, corn silage, soybean (for phosphorus only), hay and haylage (phosphorus only), and small grain. It represents the amount of nutrients estimated to be harvested and removed, not the amount to support the crop. As such, it is a low estimate of nutrient requirement. Pastureland nutrient needs were not considered, though, for animals estimated to be on pasture (beef cows), only a fraction of the manure was estimated to be recovered, with the remaining being on pasture at approximately nutrient need. Manure production system, nutrient excretion, and availability were estimated based on production practices standard from 2000-2025. As a result, manure estimates earlier in history (predating approximately 1990) may not be as representative as animals may have spent more time on pasture (especially dairy cows), or other production styles (open lot pigs) may have been more prevalent. 

Figure 1.  Comparison of crop nitrogen need and both manure nitrogen excretion and manure nitrogen retained and available to support crop production.

Figure 2.  Comparison of crop phosphorus need and both manure phosphorus excretion and manure phosphorus estimated to be retained and available to support crop production.

Results indicate nitrogen excretion with livestock manures has returned to levels last seen in 1970. Today, a more significant amount of this nitrogen is estimated to be retained and available to help support crop production. In at least part, this represents a shift from cattle systems (which, given the open lot nature, generally had higher ammonia volatilization losses than current swine systems and lower nitrogen availability due to differences in ration). Manure phosphorus levels have also returned to levels seen in the 1950s-1970s. Available phosphorus mirrors excretion due to limited means of nutrient loss during storage.

Over the same period, crop nutrient needs have significantly increased due to significant crop yields per acre increase.

Overall, while Iowa has a significant amount of cropland and livestock, the balance between manure production and cropland capacity varies by region and depends on the specific practices employed by farmers and regulators to manage nutrient cycling effectively. To understand these results and contextualize them, I look at two other variables: the percent of N or P that is excreted, recovered, and available to be used as a crop fertilizer. In general, this estimate has been trending up and I currently estimate it at around 60% of N and 80% of P. These estimates are slightly low, as they aren’t crediting N and P deposited on pasture in grazing systems where the nutrients could be used. The other factor I look at is what percent of nutrient need is supplied by livestock manures. In 2022, this is about 38% of the N and 30% of the P. Again, this represents the crop removal rate, not the nutrients required to support crop nutrient production.

I estimate this is sufficient nitrogen to supply between 4.1 and 4.8 million acres of corn production (if all the manure was applied to the ground for corn production). In 2022, Iowa had about 12.9 million acres planted to corn, so manure should account for 32-38% of all nitrogen fertilizer use in Iowa. In the fall of 2021, the National Agricultural Statistics Service collected nitrogen fertilizer use for corn in the Agricultural Resource Management Survey. They reported 87% of Iowa corn acres received fertilizer (presumably, the other 13% were manure only). Manure should be about 1/3 of the nitrogen fertilizer use; 13% sounds too low. But many acres would get some of their fertility from manure and be supplemented with commercial fertilizer, so that data doesn’t tell the whole story. The survey estimated the total commercial N fertilizer applied to corn at 834,650 tons of N. I estimated 393,000 tons of N from manure. Based on these figures, manure was at 32% of the nitrogen fertilizer supplied by the state. For phosphorus, the ARM survey reported that 49% of corn acres received phosphorus fertilizer (presumably, the other 51% were either manure only or had high-testing soils that didn’t need additional P applied). In this case, the estimate is that there were 201,500 tons of P from fertilizer. I estimated 89,000 tons of P from manure, making manure about 30% of the P applied in the state, in agreement with the phosphorus budget proposed earlier.

I also like to look at this data on a county level. Again, I’ll be using my estimate of crop nutrient removal and comparing that against the amount of manure I estimate to be produced, retained, and available for crop production within that county. While the work assumes no manure is moved from one county to another and gives a low level of crop nutrient need, but still serves as a helpful indicator of nutrient budgets. In general, we see a continuation of the trends we’ve been seeing; some counties are getting more manure-rich, and others continue to get a smaller fraction of their nitrogen and phosphorus needs from manures. Again, this figure shouldn’t indicate whether we have sufficient land for manure but more an indicator of potential areas where giving a closer look makes sense. In particular, you could question plenty of assumptions – the percent of manure I’m collecting on cow-calf farms and the type of storage. While probably reasonable for the state, I think these assumptions might have outsized effects in this area. Specifically, the southern region of Iowa may be more likely to choose lagoon manure storage because of both location and associated lagoon performance (a warmer area of the state), and many of the more extensive swine facilities in this area may be related to gestation-farrowing operations or nursery farms.

Figure 3. County level comparisons of crop nitrogen removal with harvest (excluding legumes and hay) as compared to amount of manure estimated to be available to support crop production in the county. Example, the dark green counties indicate that less than 10% of the nitrogen removed in the harvested fraction of crops could be supplied by livestock manures.

Figure 4. County level comparisons of crop phosphorus removal with harvest as compared to amount of manure estimated to be available to support crop production in the county. Example, the dark green counties indicate that less than 10% of the phosphorus removed in the harvested fraction of crops could be supplied by livestock manures.

I also wanted to look at this another way: if we use my estimates of manure production and available for land application, would it be possible for all the manure within a county to be used within that county in compliance with manure management plans (and for ease I’m going to assume all manure would require manure management plans). To do this, I obtained corn, corn silage, soybean, and alfalfa acres from the USDA Census of Ag at the county level. I got hay production numbers from the Census of Ag and divided them by hay acres to get a yield value. I then used Appendix A from the Iowa Manure Management Plan form to get estimated yields of corn and estimated corn silage yield (Assumed 70% moisture and a harvest index of 0.5). The Nitrogen Use Factor for corn (weighted based on the county being considered), corn silage, and hay were obtained. Only three counties, Lyon, Washington, and Clark, couldn’t use all the manure produced on the corn acres available in their count, but each had sufficient acres if land was considered.

Several factors could be contributing to this:

1. I could be making flawed assumptions about the types of manure systems used (more lagoon systems instead of deep pits, for example).

2. Some manure is being applied to pasture land, which I didn’t consider in the analysis.

3. Some manure is going to hay or soybean.

These three considerations alone would take care of any issues. However, it could also be that some of the manure within these counties is exported from the county.

The final piece of the puzzle is understanding how farmers are valuing the manure and trying to breakdown the commercial fertilizer and manure at a county level. Unfortunately, at least for today I’m out of space, and as of yet, having trouble finding county level nitrogen fertilizer data.



People, Pigs, and Poop

 

Recently, there was a little exercise for how much swine poop there is in Iowa and turning it into a pyramid. In that exercise, they (Raygun – I didn’t fall out of my chair or roll my eyes. I was excited, conversations about manure are welcomed) calculated about 85 billion pounds of pig poop per year – I won’t dispute that number though I calculate a slightly higher amount. I’ll even add in cattle and poultry and estimate 85 million tons of livestock poop annually.

But how much human poop is there? Iowa has about 3.2 million people in it. A person poops about 175 grams per day or 0.38 lbs. This is about what was assumed when they compared humans to pigs, but unfortunately, that’s not the pig number they were using; the pig number includes urine and wash water, too. As it should, because, in the name of water quality and manure management, we would also manage that component. Humans generally make about 0.37 gallons of urine daily, so another 3 pounds of material. So, humans are up to 3.4 lbs of “manure” a day, not that 0.38 lbs.

I mean, if we are going to talk “poop,” we probably want an apples-to-apples, or a poop-to-poop comparison, don’t we? But here is where it gets complicated – for my livestock manure numbers, I include wash water volumes – because we manage it like manure, and we should view this wash water like manure. I’m glad we do. But does that mean for humans we should include our “wash water” as well? That would include the water you use when you flush a toilet, shower, or do dishes. In developed countries like the US, the average person generates about 80-100 gallons daily. Let’s go with 90 gallons a day, or 750 pounds a day. Extrapolate this to a year, and you get 438 million tons of human wastewater! Or about five times what we generate from livestock production.

So, let’s play the game of how much poop is there?

So, for a pig, we have about 10 pounds a day, about 10% of this is solids material, and I’m going to say that fecal material is about 50% moisture, so a pig excretes about 2 pounds of feces a day, 8 pounds of urine. Throw in wash water used at the site, and we are at 10.8 lb/day. So, what’s the comparison now?

Table 1. Comparison of human and pig related “manure” and wastewater generation.

 

Human

Pig

 

Feces

0.38

2

lb/day

Poop (urine + feces)

3.4

10

lb/day

Wastewater

750

10.8

lb/day

Population

3,200,000

30,500,000

million

Feces

221,920

11,132,500

tons/yr

Poop (urine + feces)

1,985,600

55,662,500

tons/yr

Wastewater

438,000,000

60,115,500

tons/yr

 How we choose to manage wastewater greatly influences the question of what characteristics are important for me to know about that wastewater. Alternatively, the characteristics of the wastewater greatly affect how I’d choose to manage the wastewater. All that to say, a simple volume comparison isn’t enough; we have to dig deeper. What does this tell us? Humans send more “manure” to wastewater treatment systems in Iowa than livestock would, but the animal manure would have more feces in it. But this, at its heart, is why we choose to manage human and livestock manures so differently. If you have a lot of water, not much stuff in it, and are far from cropland, treatment and discharge makes sense. If you are managing volume to be smaller and get higher nutrient concentrations in it, then making decisions to use that material to replace fertilizer makes more sense.

So, how should we think about wastewater and characterize it? There is more to it than this, but if we want to keep it simple, we should start with four parameters.

Total volume, chemical oxygen demand (COD), nitrogen, and phosphorus. Why these four? Because, at their core, they tell me a lot about how poop could impact the environment. How much are we dealing with, what’s the immediate impact to water (chemical oxygen demand), and what is the potential for eutrophication (nitrogen and phosphorus).

Alright, let’s look at chemical oxygen demand. For untreated municipal wastewater the COD/five-day biochemical oxygen demand (BOD5) ratio is about 2. Why am I using this ratio? BOD5 is a much more common measure of wastewater strength (great history to this measurement, and it comes from London and the Thames River – basically because it took five days for the sewage they dumped in the river to make it to the ocean). So, what’s the BOD5 of municipal wastewater? It depends, but a good average number is around 220 mg/L. I’ve also included N and P in human wastewater and what I estimate is excreted by a pig for comparison (Table 2).

Table 2. Estimated COD, N, and P in human and swine wastewaters.

Human

Pig

Wastewater

750

10.8

lb/day

COD Concentration

440

84,500

mg/L

N Concentration

40

8400

mg/L

P Concentration

8

1360

mg/L

COD Mass

192,165

5,065,142

tons/yr

N Mass

17,470

503,517

tons/yr

P Mass

3,494

81,522

tons/yr

 How do we try to turn this into water quality impacts? Quantifying impacts is difficult, it requires us to make assumptions about how treatment and utilization impacts COD, N, and P movement and losses to water quality. With municipal wastewater, we typically treat and then discharge. To quantify what may be making it to a stream, we have to estimate the percent removal with treatment and then quantify where it ends up. For COD, hopefully, around 90% will be removed, and this will be mostly converting material into CO2 (70%) and municipal solids (20%).  The municipal solids would then be land applied. However, land application is highly effective at COD removal and preventing it from entering water, so we’ll say 0.05% is lost from the land applied fraction.

In terms of nutrients, it gets a little more complicated and depends on the treatment system being used. For phosphorus, hopefully 50% of the P ends up in the municipal solids (which are land applied) and 50% are discharged after treatment. Of those land applied, again it depends on the management practices used, but assuming good phosphorus management, probably only 0.5% of the P land applied moves with water from the land application area. For animal manures, we will use the 0.5% for all phosphorus as it should all be land applied. In the case of nitrogen, ultimate fate is again harder because it is very much dependent on if the wastewater treatment method employed. Still, for a working version of what is happening, we’ll go with 30% is denitrified, 40% is nitrified and discharged, and 30% is recovered in the wastewater sludge and land applied (assume 20% of N is lost during storage before land application). Assuming that it is land applied as a fertilizer, we’ll go with 20% of the nitrogen is lost after land application. With manure, I’m going to assume 20% is volatilized during manure storage and lost to the environment and that, again, 20% of the nitrogen that is land applied is lost. I’m providing these results in Table 3 to show an estimated N loss.

Table 3. Estimated impact on the environment from human and pig manure after treatment for human wastewater and land application as a fertilizer for pig manure.

 

Human

Pig

 

COD

19,236

2,533

ton/yr

N

8,875

181,266

ton/yr

P

1,756

408

ton/yr

 Where does that leave us? Swine manure probably is having more impact on the environment than human wastewater in Iowa. At least in part this is due to the vast differences in populations of pigs and people. I’ll give you one more table, COD, N, and P estimated to be released to the environment, but on a per person and per pig basis.

Table 4. Estimated impact on the environment per person or per pig after treatment for human wastewater and after land application as a fertilizer for pig manure.

 

Human

Pig

 

COD

12

0.2

lb/person(pig space)-year

N

6

12

lb/person(pig space)-year

P

1

0.03

lb/person(pig space)-year

 I want to do this one more time (table 5). What happens if we say that where we were applying manure would have received fertilizer anyway. Well, assuming the manure is being managed like a fertilizer, nitrogen and phosphorus losses from that acre would be similar. That is, the losses are driven by land use, and not directly by manure (and we can, and should in the future have a discussion on if manure is being managed as well as commercial fertilizer, and how to continue to improve our management of both).

Table 4. Estimated impact on the environment per person or per pig after treatment for human wastewater and after land application as a fertilizer for pig manure.

 

Human

Pig

 

COD

12

0.2

lb/person(pig space)-year

N

5

12

lb/person(pig space)-year

P

1

0.03

lb/person(pig space)-year

 Each method, municipal treatment for human wastewater and storage and land application of manure for livestock, has its pros and cons. If we were to treat pig manure like human waste, does water quality get better? For COD and P, I don’t think so; in fact, we probably add more of each to Iowa water ways using this method. If we treat N like human waste – it’s complicated and depends greatly on the amount of N that goes into denitrification, but unless there was a land use change associated with no longer having manure as fertilizer, we’d still get some of the losses with the use of commercial fertilizer on those crop acres that we get right now when we use manure.

The system is complicated. We need to continue to innovate to reduce N volatilization losses from storage. Specifically, these volatilization losses, are what make nitrogen losses from manure greater per pig than per person. We need to continue to develop improved nitrogen utilization practices and nitrogen fertilizer recommendations tailored to each year, location, and growing season so we can do better utilizing manure nutrients and lessen impact on water quality. The conversation is difficult, and hopefully, that comes through, that it is more than a pyramid of poop.

Thursday, January 25, 2024

Optimizing Anaerobic Digestion Efficiency: Calculating Biogas Potential

 

 Introduction:

 

Anaerobic digestion is a process that harnesses the power of microorganisms to break down organic materials without oxygen, producing biogas as a valuable byproduct. This process has gained significant attention as a means to manage organic waste and generate renewable energy. This blog will delve into the intricacies of optimizing anaerobic digestion efficiency, focusing on factors influencing different digester types and substrates – specifically, livestock manures and crop residues.

 

Factors Influencing Anaerobic Digestion Efficiency:

 

Temperature and pH:

Anaerobic digestion is a temperature-sensitive process with optimal efficiency and stability within specific temperature ranges. Additionally, maintaining an appropriate pH level is crucial for the activity of microorganisms involved in digestion. Different digester types may require adjustments in temperature and pH to maximize efficiency. Generally, designed heated digesters for agriculture are maintained at around 95-100ºF. As a general rule of thumb, microbial activity doubles for every 20ºF, so heating a digester allows significantly shorter retention times. While heating above 100ºF can further increase reaction rates, it also makes the process less stable as different bacteria and archaea populations respond differently to temperature.

 

Retention Time:

The duration for which organic materials are retained in the digester, known as retention time, plays a vital role in achieving optimal biogas production. Longer retention times generally lead to higher gas yields, but striking the right balance is essential to prevent process inhibition and to balance the initial cost of the digester against the potential yield for the substrate. For example, holding materials for an additional 30 days to make 5% more methane often can’t be justified.

 

Substrate Characteristics:

The type and composition of substrates significantly impact anaerobic digestion efficiency. Livestock manures, such as those from cattle, poultry, and swine, vary in nutrient content and organic composition. Crop residues, including straw and stalks, also introduce diverse characteristics to the digestion process. Understanding these variations is crucial for efficient biogas production and material handling considerations. Different tests, biochemical methane potential, and anaerobic toxicity assays are often used to characterize how desirable different substrates may be and if there could be issues with inhibition from chemical compounds. Physical properties are often characterized for solids content and particle size, with viscosity and settling rate sometimes characterized.

 

Digester Types and Their Influence:

 

Batch Digesters:

Batch digesters are characterized by loading organic materials in batches and allowing them to ferment for a specific period. These digesters are suitable for smaller-scale operations and can handle various substrates. However, optimizing efficiency in batch digesters requires careful consideration of loading frequency and substrate characteristics. In practice, few of these digesters exist, though more use has been seen with “high-solids” digestion.

 

Continuous Stirred-Tank Reactors (CSTR):

CSTRs maintain a constant flow of organic material into the digester, ensuring a continuous process. These systems are efficient for managing large quantities of waste. Factors such as temperature control, stirring mechanisms, and substrate consistency play key roles in optimizing CSTR performance.

 

Plug Flow Digesters:

Plug-flow digesters facilitate a unidirectional flow of organic material through the digester, promoting better mixing and higher gas yields. Achieving optimal performance in plug flow digesters involves careful design considerations and monitoring of substrate characteristics. Essentially, it needs a high enough solids content to act as a plug but not so high that it won’t flow through the digester.

 

Substrates: Livestock Manures and Crop Residues:

 

Livestock Manures:

Different livestock manures present unique challenges and opportunities for anaerobic digestion. Cattle manure, for instance, is rich in volatile solids, while poultry manure has a higher nitrogen content. Understanding the nutrient profiles of various manures is essential for tailoring digester conditions and maximizing biogas potential. In general, liquid manures are often preferred as little modification is needed to make them amenable to use in a digester. Many manures have higher nitrogen contents, which can make ammonia toxicity a potential concern.

 

Crop Residues:

Crop residues, such as straw and stalks, contribute to the diversity of anaerobic digestion substrates. These materials often have a higher lignocellulosic content, requiring special attention to enhance breakdown and gas production. Exploring pre-treatment methods can improve the digestibility of crop residues in anaerobic digesters. Particle size and maceration considerations, as well as the overall moisture content of the mix, are important to make these materials function in a digester.

 

Simple Calculation for Estimating Biogas Production:

 

A simple calculation for estimating methane production is based on the volatile solids content (VS), biochemical methane yield potential (BMP), and digester efficiency. The formula for biogas production (BP) is given by:

BP=VS×BMP×DE

 

Where:

VS is the volatile solids content of the substrate.

BP is the methane yield potential, representing the volume of methane produced per unit of volatile solids.

DE is the digester efficiency, accounting for the proportion of methane produced compared to the maximum potential.

 

The complicating factor is getting good values for each of these parameters. The volatile solids and BMP vary based on diet, manure holding time, weather conditions, and other factors, making estimating for a specific farm difficult.

 

For lagoons, this can be complicated, as the temperatures and storage times vary regionally. In the figure below, the blue color represents lagoon digester efficiency if a yearly retention time is used, the red if manure is applied twice per year, and the purple the loss of efficacy from more frequent manure removal. Estimated lagoon efficacies reported in the EPA Ag Star database are provided to the right of the figure. Green dots represent states where reporting lagoon digesters are located (the majority in California). Heated digester efficiency is generally closer to 75%, with temperature, substrate, and retention time all impacting reported efficiency.



Figure 1. Estimated lagoon efficacies for livestock manures. Blue represents efficacy of annual application, red represents a twice a year application strategy, and the purple represents the loss in efficiency from twice a year application instead of annual application.

Conclusion:

Optimizing anaerobic digestion efficiency is a multidimensional task that requires careful consideration of factors influencing both digester performance and substrate characteristics. By understanding the nuances of different digester types and the diverse nature of livestock manures and crop residues, we can pave the way for sustainable waste management and renewable energy production. The future of anaerobic digestion lies in the synergy between scientific understanding and practical application, offering a promising avenue for addressing environmental challenges while harnessing the full potential of biogas production.





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.