Manure - What’s the Concern About its Carbon
Adapted by Deanne Meyer, Livestock Waste Management Specialist, UC Davis
When most people think about manure management they think about acre feet of liquid manure that will be irrigated onto a crop to provide nutrients for the growing crop. Or they think about solid manure and the loads of material that will be removed from corrals or separators/basins and hauled to fields. Let’s take a step back and think about the manure cycle in order to identify ways to enhance any decision making needs on individual farms.
Remember in junior high school when you first heard about photosynthesis? In the presence of light, plants convert carbon dioxide and water into carbohydrates and oxygen. And so the carbon cycle used in dairy farms begins. These carbohydrates are used by plants to produce plant roots, stems, leaves, flowers, fruits and seeds. The plant, or some component of it, is harvested and finds its way to dairy diets. Plant parts are incorporated into animal diets and the carbon ends up in milk, a growing fetus, body mass (animal weight gain), manure, or is emitted to the atmosphere. In the process of eating and ruminating, animals emit carbon dioxide (just like we do when we breathe), methane and other compounds. The methane is produced by rumen microbes as they manage free hydrogen in the rumen. Manure, once excreted is handled either in a wet form through a liquid/slurry system or a drier form (solids from corrals or a separator/basin). Liquids/slurry storage systems in CA are predominantly anaerobic (oxygen is absent) and behave similarly to animal rumens where carbon dioxide and methane are primary end products. Some intermediate products of metabolism include volatile organic compounds (VOC) and these may or may not be released from liquid storage systems.
Carbon dioxide losses to the atmosphere can be incorporated back into plant material through photosynthesis. Methane emissions are classified as a greenhouse gas and State legislation has us on target to reduce these emissions. Volatile organic compounds may react with oxides of nitrogen (emitted from combustion engines) in the presence of sunshine to form ozone. Ozone is one of our criteria pollutants at ground level (in the air we breathe) with health effects.
Getting and maintaining manure in a dry form reduces both methane and VOC emissions. However, getting it to the dry form may result in losses of ammonium (plant available nitrogen) and other compounds, including VOC.
Use of manure treatment technologies.
First, identify what you expect the technology to accomplish (job description) before you ask any questions about the technology. There are many management practices identified by the San Joaquin Valley Air Pollution Control District that theoretically reduce the emissions of VOC when used appropriately. If you want a technology that removes solids from a liquid waste stream there are many different types and they all function a bit differently. If this is your focus---carefully evaluate your bedding source, amount used and particle size length. Experience shows us that particle length of different bedding sources varies, making separators function markedly differently. Alternatively, if you want a technology that reduces the amount of carbon you emit to the atmosphere from your manure treatment/storage area, then perhaps you’re considering an anaerobic digester or a pyrolysis unit. Anaerobic digesters (microbial) and pyrolysis units (thermal) decompose manure carbon in the absence of oxygen to methane and carbon dioxide (anaerobic digestion) and biochar or gases (pyrolysis). Keeping either of these technologies functional long term on commercial dairy/feedlot operations has had varying successes.
Carefully identify the job description and expectations (manure function, employee labor, etc.) of any new management practice or technology you consider for your facility. Do your due diligence with air and water regulatory agencies before considering purchase and installation.
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