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 CO2, 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 CO2, 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.