Carbon cycles can be as complex as we make them. In this case, I’m only going to focus on the manure portion of the carbon cycle; that is to say, I’m not looking at how diet and the crops are grown impact the carbon footprint, but only the manure management choices we make.
In terms of carbon, if we assume that the average weight of our pigs is 70 pounds and they excrete manure at a rate of the ASABE standard (0.375 kg VS/animal-day), with approximately 58% of the volatile solids being carbon, then every pig space will generate 80 kg of carbon in the manure every year. All we can do is control the form this carbon takes as we move it around.
Although this system is minimally used in practice, it still gives us something to compare against. One quick note, I won’t be accounting for the amount of energy used to hauling the manure, only where that 80 kg of the carbon ends up. Most research suggests about 13% of the carbon in manure is stabilized in soil, which amounts to 10 kg. The other 70kg of carbon is converted into carbon dioxide, so this system generates about 257 kg CO2 per pig.
Deep Pit Manure Storage
In a deep pit manure storage, the average is approximately 12.2 kg CH4 generated per animal space per year. This is approximately 9.1 kg of carbon, but as methane, it is 25 times stronger greenhouse gas than carbon dioxide, which works out to 305 kg CO2 equivalents. While the manure is stored in the pit, CO2 is also generated and released; most manure-based biogas is approximately 60% methane and 40% carbon dioxide. Assuming this ratio, we will generate another 8 kg of CO2, accounting for 2 kg of C.
We started with 80 kg of carbon and have converted 17 kg into gasses before application, leaving 63 kg of carbon in the manure. Again, 13% of this carbon will be stabilized in the soil, so approximately 8.2 kg, while the remaining 54.8 kg of carbon gets converted into CO2, another 201 kg of CO2.
Thus, in this system, 508 kg of CO2 equivalents are generated per pig space per year. This is about double what we saw from the daily haul system because some carbon is converted into methane, a potent greenhouse gas. This provides our first insight into how to minimize our carbon footprint; reducing or eliminating methane emissions is critical.
Anaerobic Digestion of Manure
In this case, we will again, not be accounting for any energy that goes into moving manure for land application or getting it into the digester, but I will account for energy used in heating the anaerobic digester and for cleaning and compression of the generated methane to put it on in a natural gas pipeline.
Starting with our previous assumptions, pigs are 70 pounds. They excrete manure at a rate of the ASABE standard (0.375 kg VS/animal-day), with approximately 58% of the volatile solids being carbon giving the 80 kg of carbon per pig space per year. But in this case, we also need an estimated methane production potential, which I will estimate as 0.4 m3 CH4/kg VS.
Anytime manure is stored anaerobically, some of the organic matter will break down and release methane and carbon dioxide. In an anaerobic digestion system, we want to minimize this time so more of the methane is captured in the digester. I assumed methane generated before manure collection and movement to a digester as 0.05 m3 CH4/kg VS. This amounts to 4 kg CH4 (3 kg of carbon, 75 kg CO2 equivalents). Manure decomposition will also generate 2.6 kg CO2 (0.7 kg C).
This leaves 0.35 m3 CH4/kg VS of potential; I assumed the digester would be 75% efficient at converting potential into production. In the digester, we would hope to generate 21 kg CH4 (16 kg C), which will all be combusted into CO2 for power (58 kg CO2); however, this means we don’t need to combust a fossil fuel for power, saving that CO2 from being emitted, making this a negative emission of 58 kg CO2. During digestion, we will generate 14 kg CO2 (4 kg C).
The effluent from the digester needs to be stored, and as it is stored, more methane and CO2 will be emitted. I assume that 10% of the remaining potential will be converted to methane. This is 0.7 kg CH4 (0.5 kg C, 2 kg CO2 equivalents) and 0.45 kg CO2 (0.1 kg C).
These emissions leave us with 55 kg C in manure. Again, assuming 13% will stabilize in the soil (7 kg C) and the rest will become CO2 (48 kg C, 176 kg CO2).
Doing some math, we are at 212 kg CO2 per pig space per year. I still need the energy to heat the digester and compress biogas. How much heat is needed is dependent on location, digester design, insulation value, and operation scale. As a best guess, I estimate this as 0.144 MMBtu per pig, with each MMBtu time 53 kg CO2/MMbtu giving 7.6 kg CO2. Cleaning and compressing the biogas from a pig space would take approximately 30 kWh, with every kWh generating about 0.38 kg of CO2. Compression and cleaning of the biogas take 11.6 kg CO2 gives a carbon balance of 231 kg CO2 equivalents per pig space per year.
What does this tell us? If we can find a way to encourage the adoption of anaerobic digestion systems, we can save around 277 kg CO2 equivalents per pig space per year, a reduction of 55% compared to our baseline, and even slightly lower than the daily haul system. More importantly, we get that without wasting as much fertilizer value the manure would offer as daily haul systems will typically result in large amounts of nitrogen loss (next Scoop, we’ll look at energy in fertilizers and what that means for different manure systems).