Why is Manure Anaerobic?

Manure is stored with one surface in contact with the atmosphere. That atmosphere contains oxygen. Shouldn’t this oxygen diffuse into the manure and make it aerobic? And the truth is it does, but often the diffusion rate is low compared to the rate at which oxygen in the manure is consumed. As a result, the manure will still be anaerobic. In this month’s article, we’ll look at oxygen diffusion into the manure and discuss the impact manure strength and the amount of organic matter in the manure has on how much of the manure is aerobic.

This analysis is simplified because it doesn’t account for surface disturbance from wind or mixing in the manure that might spread oxygen around. That is to say, it is primarily for illustrative purposes and to get a feel for anaerobic manure storage conditions.

This analysis works down to two parts understanding oxygen movement through the manure and the destruction, or use of that oxygen, to consume organic material. The first part relies on Fick’s law to understand how oxygen moves in manure. It should be noted that this is based on no mixing. Mixing drastically changes oxygen movement as it allows for advection, mixing with the movement of the manure, and diffusion. To get an estimate of just diffusion-driven oxygen movement in manure, though, I did have to make a couple of assumptions. The first is that there is a small layer of manure at the surface with an oxygen content always in equilibrium with the atmosphere. The second is Fick’s Law and the oxygen diffusion rates in water, which describe the movement of oxygen in manure.

The oxygen content of the surface water is estimated using Henry’s Law and assuming Henry’s constant of K_H = 4.34 x 10⁴ atm @ 77°F (25°C). Accepting a partial pressure of 0.21, based on the 21% oxygen content of the atmosphere, gives, 4.84 x 10^-6 moles of oxygen in a mole of water or 4.84 x 10^-6 moles of oxygen in 18 g. And after a little rearranging and a unit conversion, this amounts to, 4.3 mg/L. .

Using this information, Fick’s Law can be used to estimate diffusion. The oxygen diffusion rate in water at 77°F (25°C) is 2.42 x 10^-9 m²/s. The rest of the equation represents the change in concentration over distance. The change in concentration is 4.3 mg/L since we know the concentration is at a maximum at the surface and goes to zero, but we aren’t sure over what distance this will occur yet. To help get an estimate of the distance, we need to think about how quickly the oxygen will be used.

Chemical oxygen demand is often measured on different wastewater materials, including manures, to help understand the strength of the material. The typical COD of deep pit swine manure is around 100,000 mg/L. About 25% of this will often be BOD₅ (the amount of oxygen bacteria will consume in five days as they break down organic matter) or 25,000 mg/L. Assuming that 1/5^th of the oxygen will be broken down daily, 5,000 mg/L of oxygen will be consumed.

The solution becomes iterative as we try to guess the depth that causes all the oxygen to be depleted. To start, assume a 1 m x 1 m x 1 m deep chunk of liquid manure. The diffusion of oxygen can then be calculated  as 0.9 mg/m²-day.

How does this compare with the oxygen demand of the manure? That 1 m x 1 m x 1 m chunk of manure has a 1-day BOD of 5,000,000 mg. Suppose you take the 0.9 mg/m²day/ 5,000,000 mg/m³day= 0.00000018 m. This gives us our first guess of the depth. The smaller depth will cause a much higher diffusion rate and lower oxygen demand. We want to keep iterating this depth process until we find a consistent depth.

If we iterate this solution, we will find that the depth to which oxygen diffuses is 5.6 mm, or

Now iterate the depth you selected in the diffusion equation until it matches the depth of extinction

The diffusion depth of oxygen into manure is 0.42 mm or about 0.02 inches. More than that, that oxygen consumed a tiny amount, 0.00002%, of the oxygen demand.

In lagoon systems, often the manure is diluted to help it flow better, but this dilution also helps the lagoon be more aerobic, at least at the surface. Let’s see what happens if the manure is diluted by a factor of 5. In this case, about 1 mm would have oxygen or 0.04 inches. Even in this case, diffusion alone has a limited impact on making the manure aerobic. To help with this, lagoons often have residual material left in them to help further dilute the manure and to help improve oxygen transfer by providing a larger surface area. In addition, wave action from wind or surface aerators or often necessary to make the lagoon aerobic.

So manure is anaerobic because it has a high capacity to consume oxygen relative to how quickly diffusion allows oxygen to move through the manure.

Anaerobic Digestion – Covered Manure Storages

 Impermeable covers minimize odor and limit ammonia emissions. Covers prevent rainfall from mixing with manure, making manure production volumes more consistent from year to year, reducing the chance of overflow, and providing the potential for methane capture, reducing the farm's environmental footprint. These opportunities indicate that covered manure storage should be favored, yet incorporating impermeable covers has been minimal. Many resources suggest the potential benefits of impermeable covers, but a thorough economic evaluation of benefits is lacking, with the existing tools predating the development of RIN and LCFS Carbon Credit Markets. These markets are game-changing in the potential opportunities they offer. With this in mind, we developed a model to estimate how covers would impact the spreading costs, fertilizer value, biogas production, and carbon credits a farm receives and evaluate implications for Iowa livestock farms.

With that in mind, I've been putting together some estimates of the value covers and potential costs. Many assumptions are involved in these estimates, and hopefully, a future extension publication will walk through some of the calculations. Here we provide a comprehensive economic exploration of the value impermeable covers offer, focused on illustrating both perceived values and those that are the definitive economic drivers in the current marketplace. We looked at three example farms, a 4800-head swine finishing farm starting with deep pit manure storage, a 4800-head swine finishing farm starting with a drain pit and out-of-barn slurry storage, and a 500-head dairy with out-of-barn manure storage. We calculated the cost of modifying the facility to add a cover at each farm and the potential cost savings provided with current economic and carbon credit-based incentives. Evaluations assumed a 5-year or 10-year life with interest set at 8%.

Results are shown in table 1. In general, when all potential values are considered, both the 4800 head swine and the 500 head dairy using lagoon, earthen basin, or out-of-barn manure storage were cost feasible within five years and had net annual incomes of $50,000-$70,000 within five years. Economics didn't appear as favorable at deep pit facilities as construction costs for manure storage modifications increased cost. Combined with a higher carbon intensity score on methane, this produced limited payback opportunities.

Table 1. Estimated cost-benefit ratio for covers at livestock facilities creating CNG.

	5-year life			10-year life
Facility Type	4800 swine	4800 swine	500 head dairy	4800 swine	4800 swine	500 head dairy
	deep pit	lagoon		deep it	lagoon
Methane Value	$44,661	$44,661	$44,030	$44,661	$44,661	$44,030
LCFS Value	$199,352	$420,744	$414,804	$199,352	$420,744	$414,804
RIN Value	$283,311	$283,311	$279,312	$283,311	$283,311	$279,312
Nitrogen Value Savings	$5,635	$4,827	$2,401	$5,635	$4,827	$2,401
Odor Reduction	$6,900	$6,900	$884	$6,900	$6,900	$884
Storage Construction Savings	$0	$1,226	$2,427	$0	$729	$1,444
Manure Application Costs	($2,674)	$398	$2,845	($2,674)	$398	$2,845
Carbon Credits from N2O	($770)	($2,690)	$554	($770)	($2,690)	$554
Pipeline Injection Point	$250,457	$250,457	$250,457	$149,029	$149,029	$149,029
Biogas Cleaning & Compression Equipment	$186,254	$186,254	$183,624	$110,827	$110,827	$109,262
Biogas Cleaning & Compression Maintenance	$74,366	$74,366	$73,316	$74,366	$74,366	$73,316
Biogas Cleaning Operation	$11,018	$11,018	$10,862	$11,018	$11,018	$10,862
Biogas Management Employee	$80,000	$80,000	$80,000	$80,000	$80,000	$80,000
LCFS/RIN Sales Fees (10%)	$48,266	$70,405	$69,412	$48,266	$70,405	$69,412
Manure Storage Construction	$102,391	$0	$0	$60,926	$0	$0
Annualized Cover Installation Cost	$13,948	$13,948	$22,126	$8,300	$8,300	$13,166
Cover Maintenance Cost	$5,569	$5,569	$8,834	$5,569	$5,569	$8,834
Rainfall Removal	$14	$14	$28	$14	$14	$28
Annual Benefit	$536,415	$759,377	$747,257	$536,415	$758,880	$746,274
Annual Expense	$772,283	$692,031	$698,659	$548,315	$509,528	$513,909
Net Benefit	($235,868)	$67,346	$48,598	($11,900)	$249,352	$232,365

 

These results are highly dependent on a farm scale and assumed covered manure storage, not the implementation of an anaerobic digester. Next month we'll look closely at the role farm size plays. In November we'll look at how heated digester systems compare to impermeable covers.

Impact of Manure Timing on Effective Nitrogen Supply

 Nitrogen use efficiency (NUE) is the dry mass productivity per unit N taken up from the soil. The term can be used in many complex ways to illustrate different factors, like the amount of nitrogen supplied from the soil.

I looked at applied nitrogen (from liquid swine manure) as a function of manure application timing and crop yield response. At Nashua, Iowa, we have four years of data looking at manure application timing. Treatments were early fall (October), late fall (November), and spring. We’ve presented this data as the difference in yield. Here, I look at it a different way; how effective was the application in supplying nitrogen?

The process to estimate how effective the nitrogen was at supplying the crop was:

1. Use the difference in yield between the treatments to estimate the percent of maximum yield.

2. Use the Iowa State Maximum Return to Nitrogen nitrogen response curve to estimate effective nitrogen supply.

The results are plotted in figure 1.

So what does this mean? The spring manure achieved 99% of the maximum yield. It has the same yield as the spring applied UAN. The late fall manure yielded around 85% of the maximum yield. The early fall manure yielded 65% of the maximum yield. Reading from the x-axis provides an effective nitrogen supply.

The spring application, by default, supplied approximately 150 lb N/acre (as it is used as the baseline). The late fall application had a yield equivalent to 75 lb N/acre in the spring. The early fall manure had a yield equivalent to if only 25 lb N/acre had been applied. This means that the late fall manure was only taking advantage of about 50% of the nitrogen applied. The early fall manure was only 17% effective. These numbers illustrate the important role application timing plays in using manure

Figure 1. Effective nitrogen application rate from EFM (Early Fall Manure), LFM (Late Fall Manure), and SM (Spring Manure) when all were applied at 150 lb available N/acre at the time of application.

Carbon Footprint and the Frequency of Manure Removal

Carbon footprints have become important topics for sustainability. In pursuit of carbon neutrality, many technologies are promoted. Some require large capital investment or changes to a farm’s infrastructure. For example, adding aeration to existing manure storage. Or adding capturing biogas capture and cleaning to make renewable methane. But not all practices that reduce a farm’s greenhouse gas emissions must be so complicated. More frequent manure applications can also reduce greenhouse gas emissions.

 Manure methane production is proportional to methane potential and percent conversion during storage. The potential depends on the ration fed and completeness of digestion in the animal. The percent conversion is based on the storage length and temperature. Higher temperatures, or longer storage times, lead to greater conversion.

 In 2015 I wrote a paper exploring methane production from deep pit manure storage. I examined how temperature, volume, and properties influenced biological activity. Using this information, I put together a model to estimate how much methane a pig space produced per year. I use this model to explore how different application strategies impact methane emissions.

 Assume the base case will be a swine finishing operation that land applies manure once per year in the fall. Let the initial manure depth be 30.5 cm in November. Each pig has 0.9 m² of floor space and generates 4.9 L/headspace-day of manure at 9% solids. The methane production rate would be estimated at 0.0996 L CH₄/L-day. The annual methane emission is 19 kg CH₄/animal space-year.

 How would switching to twice a year manure application alter this estimate? Using the same assumption, adding manure removal in May would reduce the estimated methane emission to 10 kg CH₄/animal space-year. This is a 46% reduction.

 These calculations assume no change in nitrogen use efficiency. Making synthetic nitrogen is energy intensive. Thus, our ability to use manure as a fertilizer is important to our greenhouse gas footprint. Our work with spring manure has shown that it can be a method to increase nitrogen use efficiency.

 New technologies, like in-season manure application, may further reduce methane emissions. This model suggests that applying in spring, summer, and fall might result in a 70% reduction.

 So, what does this mean? California offers a carbon offset market at approximately $30 a ton per CO₂ equivalent. At this price, going from once a year to twice a year manure application would reduce emissions by 225 kg CO₂ per pig space. This is worth about $6 a pig space. Similarly, going to an in-season application would be worth about $10 per pig space.

 At 5,000 pig sites, around $50,000 a year in potential revenue. This revenue could help buy new equipment or pay labor to make such changes achievable.

Carbon Corner – Manure Aeration

 

Carbon has become an essential topic in agriculture through the Renewable Fuels Standard, Low Carbon Fuel Standard, and, more recently, through voluntary carbon markets and sustainability pledges within agriculture. These efforts allow either marketing of products as sustainable or the sale of carbon offsets through trading markets. While most agricultural carbon markets have focused on carbon storage within the soil, at some point, manure management that limits greenhouse gas emissions may be able to generate these credits.

In the area of manure, several factors can impact our carbon footprint. Generally, methane and nitrous oxide emissions are the driving factors. Carbon dioxide (CO₂) released from manure is biogenic. Recently, atmospheric carbon was transformed to plant material and then into atmospheric CO_2, where it stays a part of the active carbon cycle. Therefore, it is not included as a greenhouse gas.

One option currently receiving attention is the aeration of liquid manure. Typically, liquid manures create anaerobic conditions as they have high biological oxygen demand that consumes the oxygen at a rate greater than it can diffuse from the surface. In these anaerobic conditions, as the bacteria eat the organic matter within the manure, they make methane, carbon dioxide, ammonia, hydrogen sulfide, and numerous volatile organic compounds. These volatile organic compounds are typically responsible for the odors we associate with manure. In aeration systems, we try to supply enough oxygen to maintain aerobic conditions. In aerobic conditions, the decomposition of organic matter will result in the formation of carbon dioxide, water, and microbial cells.

A deep pit swine manure storage will emit around 12 kg CH₄ per headspace per year under Iowa conditions and 36.2 kg of CO₂. In this system, 341 kg CO₂ equivalents per pig space per year are emitted. Aeration systems can reduce or eliminate this methane emission and instead emit it as CO₂. Assuming the same amount of organic matter would be degraded under the aerobic system, only 70 kg CO₂ would be emitted, or a savings of 270 kg CO₂ per headspace per year. To estimate the value of a current carbon market, the California Carbon Allowance has a carbon price of around $30 a metric ton. This means the value of a carbon credit would be approximately $8.14 per pig space per year minus whatever energy is used to aerate the manure.

Aeration can be achieved from either natural aeration or mechanical aeration. Natural aeration requires large, shallow storage that generally has some dilution water or residual storage volume to help reduce the concentrations of the wastewater to help match oxygen diffusion to oxygen demand for the liquid. In some cases, it may not be possible for the entire storage to be aerobic, but the design should facilitate aerobic conditions near the surface, which effectively reduces odor emissions. This option has the advantage of needing little energy but often requires large areas of land. Alternatively, aeration can be done mechanically by blowing bubbles into the air or causing surface splashing to encourage oxygen diffusion into the manure.

To eliminate methane production from the manure, full aeration is probably required. Partial aeration can provide mixing and some reduction in methane production, but more research is required to determine how aeration amount reduces methane production. A finishing pig produces about 0.27 kg oxygen demand per day for complete aeration (on average, it will vary with pig size and diet-fed), or 100 kg per year per pig. A typical oxygen transfer rate for mechanical aerators is around 1.2 to 2.1 kg oxygen per kilowatt-hour of energy used. Assuming we’ll get 1.65 kg oxygen per kilowatt-hour of energy used, we’d need 60 kW-hrs of energy per pig space per year. At an electricity price of $0.10 per kWh this amounts to $6 per pig a year. On average, US electricity emits 0.85 lb of CO₂ per kWh consumed. Adding this added electricity use into our carbon budget, the aerated manure would release 93 kg of CO_2, as compared to the 341 kg from the stored manure. Assuming a spot market of around $30 per ton of CO2, this would be an annual net asset to the farm of $1.50 per pig space. This would have to pay for the initial equipment.

This illustrates that moving forward, carbon markets are and will continue to be a key driver in how manure management systems evolve.

Manure Management and Biosecurity

 

The prevention of disease transfers due to manure handling equipment moving from one farm to the next is essential for your operation. Biosecurity advice often revolves around owning your pumping equipment dedicated to your farm. It also ensures manure equipment was washed, disinfected, and in some cases, had a “downtime” before moving to the following site. These techniques still hold today, but new ideas and innovations have made biosecurity more convenient.

 Owning your manure pumping and handling equipment still provides the most biosecure option. It gives complete control over what sites the equipment has been on, how recently it was cleaned and disinfected, and the amount of downtime before it is used.

 When working with custom manure haulers, make them aware of any biosecurity requirements for your farm upfront. Please take a moment to inspect their equipment as they arrive to make sure it is clean. Try to work with them to provide cleaning and disinfecting options as they finish your site—review lines of separation and routes through the farm site to maintain biosecurity. Be clear about the health status of a facility. As custom applicators organize their jobs, they can ensure proper biosecurity is maintained. Work from farms with the highest to lowest biosecurity requirements and current health status. Switch the feces species you are hauling as the risk of disease transfer between different livestock is typically lower; when possible, add a cattle job between pig sites.

 However, we have also seen equipment changes help make manure withdrawal more biosecure. One challenge with maintaining biosecurity of manure removal is removing a pump-out cover from the barn and dropping an agitator into the manure. The pump-out opening modifies the ventilation system. It often serves as an air inlet; in filtered barns, this weakens biosecurity as  air isn’t forced through the filter before entering the pig space. As a result, some farms have begun to switch to a straw or a “mass agitation” system. Rather than opening a pump out cover, the manure pump is hooked directly to the straws built into the pump-out lid to recycle manure back into the barn and one port for manure removal.

 In addition to often being the weak link in terms of maintaining the integrity of the barn ventilation system, the drop-in agitation pump is often the hardest to clean and disinfect. These systems, with manure straws, allow pump trailers to be pulled alongside the barn and speed cleaning and disinfecting as because they are not lowered into the manure, they stay cleaner.

 An alternative option is to use a pump dedicated to the site to move the load station further away from the building, making maintaining lines of separation much more straightforward. In this way, tractors and tanks have greater maneuverability. There is less risk of manure haulers and farmworkers crossing paths and inadvertently contaminating the clean side of a clean/dirty line. Similarly, the farm site provides the lead pump and a short distance of hose dedicated to the facility when using the dragline application. In this case, it is vital to ensure that the pumping capacity of the supplied lead pump and the rest of the pump system align.

 

Figure 1. The load station is located away from the facility and in the field to help maintain lines of separation.

Higher Fertilizer Prices, Manure Opportunities

 

Last fall, fertilizer prices were trending higher (Figure 1), and with supply change issues and soaring fuel prices, that trend has continued into the spring. Even with potentially higher crop prices, this offers challenges to finding ways to increase or maintain farm profitability. However, one available option is to explore the use of manure on your farm. Many livestock owners have long known the value manure has to offer. With skyrocketing commercial fertilizer prices, this offers the opportunity to make even better use of the manure resources.

 

Figure 1. Weekly anhydrous ammonia prices.

Farmers who historically have relied on synthetic fertilizer, and even those who have used manure, but have limited experience with spring application, should consider using some manure this spring. While there is never a guarantee of what fertilizer prices will do in the future, current prices are unprecedented. Until 2021, most years showed a lower average price in the fall than in the spring. Even if your manure resources are limited and applying some manure this spring means you won't be able to cover as much ground next fall, it still represents an opportunity to save some money now since fertilizer prices haven’t stabilized.

ISU research evaluating the impact of swine manure timing on the yield of corn has consistently shown moving manure application closer to the growing season has a yield benefit. In 2021, with a dry fall and spring nitrogen loss via leaching and denitrification was at a minimum. The yields between fall and spring-applied manure were similar. We also compared spring UAN fertilizer to spring manure application and found, on average, the spring manure to corn (following soybean the previous growing season) out yielded the spring UAN application by 13 bushels per acre.

Figure 2. Average corn yields for 2021 at the Nashua site.

 

Similarly, this may be a good time to reexamine the nitrogen application rates you are selecting. Manure management plans typically utilize the yield goal method to set nitrogen application rate maximums, intended for environmental protection, not to maximize profit. Rate selection tools, like Maximum Return to Nitrogen, can be used to determine rates that will help you maximize your manure fertility value and typically will help you stretch the manure across more acres.

 

Farms that have not historically used manure in the past may be more interested in purchasing manure due to either inability to obtain other fertilizers or because of the high prices. If you are working with someone new to using manure, you can do a few things to help facilitate the exchange.

 

1. Set a price that works for both sides. Manure has value, and the value moves with the price of other fertilizer sources. Can selling some manure now potentially help you obtain acres for manure application in the future? If you are selling manure, look for fields that can utilize the N, P, and K to maximize value and price.
2. Know your regulations. Suppose the manure is coming from a confinement animal feeding operation. In that case, it needs to be applied by a certified applicator. Unless it is sold under Chapter 200A, through an independent manure broker, the field needs to be in a manure management plan (and have appropriate soil tests and erosion assessment).
3. Share the experiences you had in what helps you get the most value and best benefits of your manure. Discuss improvements to soil health all the nitrogen won't be available right away, and then state how the injection units will leave the field or best practices you've found to make planting a success.
   
   

Higher fertilizer prices make getting the most from your manure essential. Spring application has consistently shown similar or improved yields. Moreover, with the high fertilizer prices, getting manure nutrients to the right field at the right time makes manure more valuable than ever.