Anaerobic digestion requires that feed material be less than 17% solids by weight. Typically, manure collected on a dry lot has a much higher solids content than 17%. Microorganisms that convert organic materials into methane are very sensitive, requiring a pH near 7 and temperatures, around 95 degrees Farenheit (35 degrees Celcius). For each 20 degrees Farenheit (11 degrees Celcius) decrease, gas production will be cut approximately one half, or will take twice as long. Nearly 50 percent of the biogas generated by anaerobic digestion may be required for maintaining the necessary temperature of the digester. Anaerobic digesters will require more heat input during winter months than summer months, but the rate of methane generation will not change as long as a temperature of 95 degrees Farenheit (35 degrees Celcius) is maintained.

Biogas can be directly supplied to a generator and converted into electricity. Electricity can then be used on-site or excess electricity can be sold to a utility. Biogas generated from anaerobic digestion of animal waste contains hydrogen sulfide, which is corrosive to generator parts. Successful generator operation depends on removal of hydrogen sulfide from biogas. This can be done by passing biogas through iron filings prior to introduction to the generator.

Biogas can be purified and compressed to be injected into natural gas lines. This requires an extensive cleanup of the biogas resulting in gas containing 93-99% methane. Carbon dioxide, water, and hydrogen sulfide must be removed from the biogas. Methane can be directly injected into natural gas pipelines (with quality control) or transported to an injection facility.

After biogas is purified to compressed natural gas, it can be converted to liquid natural gas rather than injected into natural gas pipelines. Compressed natural gas is cooled to -260 degrees Fahrenheit and stored as a cryogenic liquid in insulated storage vessels at 50-150 psi. Liquid natural gas can be used to fuel vehicles. It can also be transported offsite to supply liquid natural gas supply stations.

Anaerobic digestion is not a good fit for all animal feeding operations. Care should be taken to ensure that anaerobic digestion is feasible at an operation before installation. While typical management practices in the arid west do create challenges for installation of anaerobic digestion technology, there are technologies that can be a good fit. Anaerobic digestion technologies include covered lagoons, plug flow, complete mix, upflow sludge blanket, and fixed film reactors. Guidance is required to select appropriate technologies. After you have determined that anaerobic digestion may be both technically and economically feasible at your facility, you will need to become informed on types of anaerobic digester technologies and which of those may be the best fit at your site. It is recommended that you utilize the AD Feasibility Tool for guidance on selection of an appropriate technology for your facility.

Methane in a concentration of 6% to 15% with air is an explosive mixture. Since it is lighter than air, it will collect under rooftops and other enclosed areas. It is relatively odorless, and detection may be difficult. Extreme caution and special safety features are necessary in the digester design and storage tank, especially if the gas is compressed.

The biogas generated from anaerobic digesters contains highly corrosive hydrogen sulfide. Sulfides must be removed prior to supplying the biogas to a generator. A simple, low cost method for removal of sulfides from biogas is passage through iron particles. Sulfides attach to the solid surfaces and are removed from the gas. The iron particles must be replaced every six to twelve months.

In arid climates, collected animal wastes can have very high solids content. Dairies are typically thought to be a very good fit for installation of anaerobic digestion technology. However, waste management methods applied at dairies located in the arid west differ from other parts of the United States. As a result of water scarcity, water is not often utilized to flush dairy barns as is done in areas where water is plentiful. Instead, manure is often scraped from concrete floors or dry lots. While dairy waste has a solids content of 10-14% as excreted, solids content has been measured as high as 90% on dry lots in Colorado. For wastes containing more than 17% solids, substantial quantities of water may be required for anaerobic digestion. This can add to the cost of operating the digester. In addition, when clean water is added to an anaerobic digester, it will adsorb nutrients and pathogens and become a nuisance. Dilution of waste with water is most practical when there is an available source of wastewater (domestic or food processing) to utilize.

When manure is collected from dry lots, the collected waste is often dry with high inorganic content consisting of rocks and soil particles. As noted within the On-Farm Anaerobic Digester Checklist, rocks and soil particles cause major operational problems for anaerobic digesters and must be removed before the waste is processed. This has been one of the most prominent causes for failure in on-farm anaerobic digesters and thus is very important to consider. Sand in bedding can also be a problem for anaerobic digestion if it ends up in the waste material supplied to the system. Removal of rocks, soil, and sand is possible, but typically involves addition of water to the waste and subsequent settling of the particles. Such processes add complexity, capital cost, and additional maintenance for an anaerobic digestion system.

Combining animal feeding operation wastes with wastewater generated onsite or by nearby facilities such as food processing plants or municipal wastewater treatment plants can be beneficial by both increasing water content and increasing methane production capacity. This is typically referred to as co-digestion and is gaining popularity. The ability to combine manure with other wastes must be carefully evaluated prior to anaerobic digester installation/operation. In particular, it is recommended that wastestreams are not varied seasonally or daily, but rather that a consistent waste is supplied to the anaerobic digester at all times. The microorganisms in an anaerobic digester are very sensitive and when the waste source is changed, it can take a long time, up to three months, for them to adjust and begin producing methane. Therefore, when the wastestream is changed on a daily or seasonal basis, the organisms do not have enough time to recover. If you are considering adding a waste in addition to manure into the anaerobic digester, you need to make sure that the waste will be available on a daily basis throughout the year to add into your system.

Anaerobic digestion effluent must be handled properly. Anaerobic digester effluent is a slurry, containing 1-15% solids, depending on the solids content of the waste which is input to the system. This effluent is a stabilized product low in pathogens and high in nutrients and is suitable for land application. The processed material containing solids can be applied by a honey wagon, or solids can be separated for land application separate from liquids. When solids are separated, they can be composted and land applied by a manure spreader. Solids separation in combination with composting can result in a lower weight product which can be transported at lower costs compared to a slurry for land application. The weight of the processed slurry material containing liquid and solids (5-15% solids) may be too expensive to transport over large distances. Utilizing the nutrient rich liquid component for irrigation is referred to as fertigation or chemigation and is regulated in most states. When fertigation systems are connected to a freshwater source, appropriate measures must be taken to avoid contamination of the freshwater source such as inclusion of a backflow preventer and shutoff valve. Fertigation systems must adhere to state and local regulations. If land application is not an option, you will need to find another method for anaerobic digester effluent storage or treatment on site.

Covered lagoons are considered the cheapest and simplest anaerobic digestion technology available. Anaerobic digestion and subsequent production of methane takes place naturally in lagoons which contain animal wastewater. A synthetic cover, typically high density plastics or rubber is used to trap and store the biogas. Covered lagoons are difficult to heat and they are only recommended in warm climates where freezing temperatures are rarely observed. Often times too little methane is generated by covered lagoons during cold winter months to justify installation of biogas capture and use equipment. Covered lagoons have the lowest operations and maintenance of the technologies described here, but also have the lowest energy yields. Covered lagoons can have a low capital cost, but are greatly affected by economics of scale and are not practical for very small operations. You should consider installing a covered lagoon if complaints of odor or greenhouse gas reduction are the major factors driving interest in AD technology. Carbon credits can be obtained through reduction in methane release from the lagoon compared to an uncovered lagoon, but the carbon market is volatile. Installing a covered lagoon can also add complexity to the required cleaning and maintenance of existing lagoons due to issues with removing covers. Covered lagoons have low volatile solids destruction rate compared to other technologies, meaning that there will be a larger amount of solid end product to handle when this technology is applied compared to others.

Plug flow digesters are a low tech anaerobic digestion technology for treatment of high solids content waste. The thick, high solids content waste travels down the digester in a "plug," as a continuous mass. Plug flow digesters can be a good fit with the high solids content waste generated by animal feeding operations int eh arid west. Some of the biggest advantage of plug flow digesters are that they have low probability of upset, low maintenance, and require lower energy input compared to other types of digesters. Plug flow digesters are the most trusted and tested digester on teh market, with roughly 55% of all digester implementations containing either a plug flow or modified plug flow design. Plug flow systems require a larger foot print than other digesters and can have issues with consistent gas production. Given the simple nature of plug flow AD systems, most on-farm installations can be monitored and maintained by a committed staff.

Plug Flow Anaerobic Digestor
(figures developed by Lucas Loetscher, Colorado State University)

Also known as continuously stirred tank reactors (CSTR), complete mix AD reactors are large, often cylindrical, tanks which have a mechanism to keep the reactor completely stirred. The stirring mechanism can be injected biogas, or a motorized paddle. Mixing produces an ideal environment for anaerobic microorganisms by spreading the nutrients evenly throughout the reactor, while sumultaneously helping to dampen shock loads of toxins which may enter the system since influent is instantaneously diluted through mixing. Complete mix reactors operate best when solids content is between 5-10%. Because solids content of waste produced at most intermountain west cattle feeding operations is higher than 5-10%, complete mix reactors are often not a good fit unless an external source of water or wastewater is readily available. Complete mix reactors may also have problems with inorganic materials such as rocks and sand which can clog the system hindering mixing capacity. Sometimes, methane bubbles result in flotation of solid particles to the top of the system resulting in performance issues. Complete mixed AD systems can often times require higher maintenance compared to plug flow systems due to moving parts and pumping requirements. Complete Mix AD systems produce a comparitively large amount of biogas per volume of reactor ocmpared to other technologies. Given the relative reliability of complete mixed reactors, it is possible to monitor and maintain operations without expert technicians, however it is strongly advised to have expert help during initial start-up.

Complete Mix Anaerobic Digestor
(figures developed by Lucas Loetscher, Colorado State University)

Upflow sludge blanket reactors are similar in design to a complete mix reacctor, except that there is no integrated mechanism for homogenizing the waste. Instead, settling of solids is encouraged so that a sludge blanket is formed, maintaining biomass within the system, thus reducing the required holding time. THese reactors are highly efficient and have been successfully up-scaled to the commercial scale. Sludge blankets are considered an emerging technology and are still in development phases. In general, waste generated at intermoutnain west animal feeding operations is too high in solids for application of an upflow sludge blanket reactor. Upflow sludge blanket systems are capable of producing very large amounts of methane biogas per volume of reactor compared to other technologies. They also have a very low retention time requirement, resulting in decreased foot-print. However, whent reating manure they can have longer start-up times, often taking six to eight months to form a proper sludge blanket. This is critical when considering the variability of manure streams and that a single upset event could shut down operations for the better part of a year. Given the sensitive nature of bacterial cultueral it is important that an upflow digester be properly monitored by technical experts.

Upflow Sludge Blanket Anaerobic Digestor
(figures developed by Lucas Loetscher, Colorado State University)

In a fixed film digester, baccteria colonize a provided support structure within the reactor. This support structure is a high surface area material suitable for colinization, such as PVC pipe or shredded plastic. Fixed film reactors have successfully been implemented with low solids content (<3%) dairy manure wastewaters in Florida, but are not liekly to be a good fit with wastes produced in teh arid west unless a large quantity of wastewater is available from a nearby source. Fixed films digesters have low retention times (comparable to the upflow sludge blanket system) while also having the capacity to generate large amount of methan biogas. Fixed film digesters do not require solids removal as all excess bacteria is passed through the system and have lower startup times than upflow sludge blanket or complete mix reactors. Capital cost for fixed film systems can be offset with grants and other funding mechanisms, however costs remain higher than other technologies. The oeprational complexity of fixed film digesters can often lead to potential variations in gas production or improper substrate removal rates and it is recommended that these systems are monitored by technical experts.

Fixed Film Anaerobic Digestor
(figures developed by Lucas Loetscher, Colorado State University)