The manure handling system of any farm is made up of many different components, each with a different function and purpose. An anaerobic digester, although only one component of the system, can greatly improve the environmental performance and efficiency of the overall system. The main effect of anaerobic digestion is conversion of organic matter to biogas. This conversion has many potentially beneficial environmental and management side effects.
By removing organic matter, the digester reduces the organic matter-loading and associated oxygen demand on downstream manure handling components. This may allow the downstream components to be smaller, operate more efficiently and function with less environmental impact. Anaerobic pretreatment may be a more economical method of converting an anaerobic lagoon to an aerobic lagoon, compared to mechanical aeration. Digester effluent is more stable than raw manure. It contains more stable organic material and less volatile odorants. Thus, storage and land application of digester effluent greatly reduces odor nuisance compared to raw manure.
Manure solids are stabilized through anaerobic digestion. What was once reactive, partially digested material has been processed into stable microbial biomass and precipitated nutrients, although the majority of nutrients remain with the liquid. The potential to dry and transport digester solids is greatly improved over raw manure. The solids can be recycled and used for bedding or a soil amendment on the farm. The reduction in moisture content also increases the feasibility of selling the solids to farms that are greater distances away. In the right market conditions, composting the digested solids can result in a value-added product that can be sold to homeowners, gardeners or the landscape industry.
Plant nutrients are conserved and transformed during anaerobic digestion. Ammonium is created from manure proteins. This can be a benefit or a nuisance. If injected immediately into the soil, ammonium-rich effluent is highly available for plant growth. On the other hand, if digester effluent is stored under anaerobic conditions, ammonium will convert to ammonia gas and escape to the atmosphere. Since digesters are also a reducing environment, the potential exists for capture of ammonium and soluble phosphorus through precipitation as struvite.
Many metals are precipitated during anaerobic digestion. Sulfur is reduced to sulfide, which is generally a bad thing since it can escape as hydrogen sulfide gas. However, the digester environment can be manipulated so that sulfides are precipitated along with potentially harmful metals such as Ni and Zn.
Anaerobic digestion results in the reduced emission of greenhouse gases. This may seem ironic, since the methane contained in the resulting biogas is a powerful greenhouse gas. An anaerobic digester is a controlled environment that captures the methane. After capture, it is either flared or used to generate electricity and/or heat.
When flared, the carbon dioxide formed in the combustion has less heat trapping potential than the original methane, and it is essentially recycled atmospheric carbon. What is released to the atmosphere through combustion of methane was once plant material formed through photosynthesis from atmospheric carbon dioxide.
When used for energy generation, the biogas replaces power that might have otherwise been created through conversion of fossil fuel. Regardless, if the biogas is flared or used for energy generation, the farmer may be eligible for carbon credit payments.
With all of the potential benefits, one might wonder why relatively few farms utilize these systems. One major reason is that anaerobic digesters are expensive to install and operate. The economic benefits have, in the past, been limited to a reduction in electricity purchased by the farm, which is not enough to offset the costs of the system.
As the interest in renewable energy sources increases, farms are increasingly able to apply and receive carbon credits. Some farms also accept off-farm waste, collecting tipping fees, to co-digest with manure. In many states, more favorable net-metering laws have also made the economics more favorable. Power generated by the digester is valued at retail costs rather than wholesale costs.
The decision to install a digester is often driven by additional considerations, such as nuisance issues. A digester greatly reduces the odor potential of the manure, which also greatly reduces neighbors' complaints and the potential for lawsuits.
At the current time, anaerobic digestion is slowly but surely increasing as a manure treatment method in the United States. Additional information is available at: Economics of Anaerobic Digesters for Processing Animal Manure.
Contributors: Jill Heemstra, University of Nebraska
Reviewers: Mark Rice, North Carolina State University and Karl Vandevender, University of Arkansas