Anaerobic digestion requires that feed material be of low solids content, less than 15% solids by weight. Typically, manure collected on a dry lot has a much higher solids content than 15%. The microorganisms that convert organic materials into methane are very sensitive, requiring a pH near 7. In addition, the organisms work best at high temperature, around 35°C (95°F). For each 11°C (20°F) decrease, gas production will be cut approximately one half or will take twice as long. While volume reduction of waste does not occur during the digestion process, 50-60% solids reduction can be expected and nutrients are conserved, adding value to the end product for crop use. An advantage of AD is that nearly 95% pathogen inactivation occurs. In the digester, organics are removed as they are converted to methane while nutrients (nitrogen and phosphorus) are conserved. The end product is a low odor, high nutrient, stabilized waste suitable for land application.
Methane in a concentration of 6 to 15 percent 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 hydrogen sulfide, and thus is highly corrosive. Sulfides must be removed prior to supplying the biogas to a generator due to their effect on generator components. A simple, low cost method for removal of sulfides from biogas is passage through iron particles. Here, sulfides attach to the solid surface and are removed from the gas. The iron particles require replacement every six to twelve months.
Dry Wastes in Colorado
Due to the arid climate in Colorado, animal wastes, as collected, can have a 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 Colorado dairies differ from other parts of the United States. Because water is so scarce in Colorado, water is not often utilized to flush dairy barns as is typically done elsewhere. 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 13% 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 groundwater is added to an anaerobic digester, it will adsorb nutrients and pathogens which may then become a nuisance. Dilution of waste with water is most practical when there is an available source of wastewater to utilize.
High Inorganic Content
When manure is collected from dry lots, the collected waste is often dry with high inorganic content consisting of rocks and soil particles. The rocks and soil particles cause major operational problems for anaerobic digesters and must be removed before the waste is processed. Sand in bedding can also be a problem for AD if it ends up in the waste material supplied to the system. Removal of rocks, soil, and sand typically involves addition of water to the waste and subsequent settling of the particles, thus adding complexity, capital cost, and additional maintenance for an AD system.
Anaerobic digestion is not a good fit for all animal feeding operations. Care should be taken to ensure that AD is feasible at an operation before installation. While Colorado conditions and typical management practices do create challenges for installation of anaerobic digestion technology, there are technologies that can be a good fit. Guidance is required to select appropriate technologies.
Also of note is that combining animal feeding operation wastes with wastewater generated onsite or by nearby facilities such as food processing plants or domestic wastewater treatment plants can be beneficial by both decreasing waste solids content and increasing methane production capacity. This is typically referred to as co-digestion and is gaining much popularity. The ability to combine manure with other wastes must be carefully evaluated prior to AD 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 AD reactor at all times.Last updated: April 24, 2018 at 17:53 pm