Dougherty, B.; Gray, M.; Johnson, M.; Kleber, M. (2017). Can Biochar Covers Reduce Emissions from Manure Lagoons While Capturing Nutrients? Journal of Environmental Quality, 46 (3), 659-666, DOI: 10.2134/jeq2016.12.0478
What are Manure Lagoons?
Ever wonder where the waste from all the cows on a dairy farm goes? Or how about a hog farm? Well, in some locations it is stored in outdoor basins known as manure lagoons (Figure 2). These lagoons allow the waste to be separated into solids and liquids. The solids sink to the bottom and are occasionally removed and used as fertilizer, while the liquid is transferred to other lagoons for further treatment. As you can imagine, these lagoons can produce quite a stench, but it is actually the gases you can’t smell that have serious consequences. In addition to odor-causing gases (ammonia -NH3 and hydrogen sulfide -H2S), the lagoons can be sources of greenhouse gases that contribute to climate change. This study examined a new method for reducing odor and greenhouse gas emissions from manure lagoons.
What can we use to reduce gas emissions from manure lagoons?
One way to reduce gas emissions from manure lagoons is to place floating covers on the surface. When the floating cover is made of biologically based materials such as straw, corn, and wood chips it is called a biocover. Unfortunately, biocovers are not widely used, possibly because they can be expensive and difficult to apply and maintain. Biochar, which resembles charcoal, is another biologically-based, long-lasting material that researchers believe could be used as a cover. Biochar covers not only have the potential to reduce odor and greenhouse gas emissions, but also to absorb valuable nutrients. The combination of the ability to absorb nutrients and a long life span make it possible for the floating cover to be recovered and applied as a fertilizer. Dougherty and his fellow researchers wanted to determine how well biochar could perform as a lagoon cover compared to another type of biocover (straw) by answering the specific questions below.
Questions 1: Do biochar covers reduce odor causing gases?
To answer this question the researchers collected manure from two different locations and placed it in pails in a temperature-regulated greenhouse. They tested four coverings: two types of biochar, straw (biocover), and no cover. Once every 14 days the pails were sealed with a lid to allow sampling of odor-causing gases (ammonia and hydrogen sulfide) in the space above the manure and covers.
It turns out that the concentration of hydrogen sulfide in most of the pails was so little it could not be measured. Great news! However, this was not the case for the other odor-causing gas, ammonia which was found in all the pails. Both the straw biocover and one of the biochar floating covers reduced the ammonia concentration compared to the pail with no cover.
Question 2: Do biochar covers absorb nutrients?
The biochar cover was successful at reducing the emission of odor-causing gas, but can the cover also absorb nutrients? Remember, if the cover will eventually be used as fertilizer, then it is important that it absorbs nutrients. After 56 days the floating covers described above were removed from the pails and analyzed to determine nutrient uptake. While the biochar covers did absorb nutrients, the straw covers absorbed more nitrogen and phosphorous, two of the most important nutrients required for fertilizer.
Question 3: Do biochar covers mitigate odor?
The researchers already determined that the biochar cover helped reduce the emission of one odor-causing gas (ammonia), but did this translate into less of a stink from the pails? To answer this question, the researchers used a highly intricate machine: the human nose. Some unlucky individuals were enlisted to evaluate odors from pails containing manure and either biochar, straw, or no cover. Four different types of biochar covers were tested. The judges ranked the offensiveness of the odor from -10 (extremely unpleasant) to +10 (extremely pleasant). Three of the biochar covers tested reduced odor offensiveness compared to the control, while the straw biocover and fourth type of biochar did not.
What does this all mean?
In the end, did the biochar covers perform better than the straw covers? Dougherty and his colleagues determined that there was no clear advantage to using the biochar cover rather than the straw. The biochar covers did absorb nutrients and reduce ammonia emissions, but so did the straw covers. The only test in which the biochar performed better was in the reduction of odor as judged by humans. This test was tricky as odor is known to be subjective.
Dougherty is not giving up hope on the use of biochar covers yet. He points out that these experiments were short. Remember one potential advantage of biochar is its long life span. It is believed that biochar covers could last longer than alternative biocovers such as straw. If biochar covers can absorb nutrients and reduce ammonia emissions just as well as straw, but for longer periods of time, then biochar covers may be preferred. Dougherty suggests that future experiments should go on longer to determine if this is the case.
As long as we continue to consume large amounts of meat and dairy in the U.S. this animal poop problem will continue to exist. Gas emissions from manure lagoons not only include ammonia and hydrogen sulfide which create quite a stench for neighbors, but also greenhouse gasses that contribute to climate change. While these researchers did not directly measure greenhouse gases, they concluded that since the covers trapped odor-causing gases, they trapped greenhouse gases too. Future research will be needed to confirm this.