Introduction to Anaerobic Digestion
Anaerobic digestion is a biochemical conversion process that is used for treatment and reduction of organic wastes such as organic sludge or concentrated organic industrial waste which contain solids. In this process, microorganisms break down biodegradable material in the absence of oxygen. Anaerobic digestion is used for industrial or domestic purposes to manage waste or to produce fuels (renewable natural gas). Much of the fermentation used industrially to produce food and drink products, as well as home fermentation, uses anaerobic digestion.
Anaerobic digestion technology refers to the development of systems that consume organic materials, often thought of as waste, using natural microorganisms in an environment where oxygen is unavailable to make energy, and other useful solid and liquid by-products.
Anaerobic Digestion Process
The process of anaerobic digestion takes place in three steps. The first step is to liquefy solid feedstock material using hydrolysis. The second step is to digest the soluble solids that resulted from hydrolysis. This step is carried out at the molecular level by acid-producing anaerobic bacteria (primarily acetic, propionic, and butyric acid). The microbes involved in this stage are organisms that can use oxygen, but also have anaerobic methods of energy production. These organisms can function in a wide pH range.
The last and slowest step is gasification. The organic acid produced in the second step is used by certain microbes as a substrate where methane and carbon dioxide gases are produced as a result. This step is called methanogenesis as it leads to the production of methane. Biogas is typically 60% methane and 40% carbon dioxide, with trace amounts of hydrogen sulfide and other gases. Once the biogas is produced it is sent through various scrubbers and upgrade equipment to remove the trace amounts of greenhouse gases. Typical gas upgrading technologies are PSA (pressure swing adsorption) units or membrane filters which are used to upgrade the gas to pipeline quality specifications and remove greenhouse gases.
Anaerobic digesters are airtight tanks operated at either a mesophilic (95°F to 105°F) or thermophilic (135°F to 145°F) temperature. Common types of anaerobic digesters are covered lagoon, plug flow, and complete mix. Less common digester types include fixed film, induced blanket, and two-phase. Manure characteristics, handling, and the use of bedding dictate which technology is appropriate. Depending on the digester technology manure may be co-digested.
In conventional digesters, mixing is intermittent and holding times can vary from 30 to 60 days which can result in stratification of the sludge and reduced gas production. Modern digesters have higher turnover and digestion rates due to continuous mixing and a more efficient sludge feeding and removal system. The holding times in modern digesters can be as low as 15 days or less.
Throughput Products and Byproducts
Anaerobic digestion can produce many value-added products. Biogas can be used to generate electricity or fuel a boiler, It can also be upgraded to remove impurities to renewable natural gas standards. Upgraded biogas can be sold into the natural gas pipeline or be used on farms as vehicle fuel. Digestate can be land applied as a source of nutrients or be separated into a liquid fraction (filtrate) and a solid fraction (fiber) via a solid-liquid separator. The following table provides biogas yields for various animals.
1. Gould, M., Cow Powered Farm, Extention Bulletin E-3080, Page 2, September 2009
2. ASABE. 2003. Manure Production and Characteristics. ASAE D384.1.
3. USDA NRCS. 2007. An Analysis of Energy Production Costs from Anaerobic Digestion Systems on U.S. Livestock Production Facilities.
4. Assumes biogas is 60% methane and 1 ft3 of methane contains 1,000 Btu.
The filtrate can be used as a fertilizer or be further treated to meet discharge quality standards. Liquid digestate (filtrate) from a dairy farm mix digester can produce approximately 30 pounds of nitrogen, 10 pounds of phosphate, and 30 pounds of potash per 1,000 gallons. The nutrient analysis will differ depending on the management of the digester, feedstocks used, and other factors. Fiber from the solid fraction can be used as a soil amendment, cattle bedding or compost feedstock, or to make medium-density fiberboard. These represent some of the possibilities of the value-added options available with a digester.
Key Project Considerations
Feedstock is the key to consistent operations and our experience is that understanding up front how this will impact operations and gas yields is of critical importance to AD facilities.
- Is adequate testing of expected feedstock completed?
- How are feedstock contaminants to be managed?
- Is the feedstock content consistent?
- Is the feedstock rate reasonable for the projected biogas yield?
Every facility is different and understanding the key design considerations and projected facility life cycle provides the backup necessary to validate pro-forma assumptions.
- Does the design adequately pre-treat planned feedstock before anaerobic digestion, in consideration of the process being employed (covered lagoon, CSTR, plug flow, or solid AD)?
- Size of tanks/system, retention time, temperature, controls, and monitoring including inline gas testing.
- The planned life cycle of the facility.
The strength of an AD project is in its supply chain and offtakes requiring a thorough understanding of potential project risk.
- Feedstock supply terms
- Offtake, interconnect, and utility service agreements
- O&M agreement (as applicable for 3rd party)
- EPC and OEM agreements
Anaerobic Digestion allows for the processing of typical waste streams into facility feedstocks such as excess sludge from wastewater treatment plants or waste slurries from agriculture manure or dairy industry. Energy crops may also be added to increase the gas yield. There are many different types of large-scale anaerobic digesters and technology available. Most anaerobic digestion technology requires expert construction, operation, and maintenance experience. The biogas produced has the potential to reduce greenhouse gas emissions. Due to increasing fuel prices, incentive programs, and climate change, biogas generation from wastes to energy large-scale projects is gaining significant interest and commercialization.