Anaerobic Digestion ABC's
Anaerobic digestion is a process in which bacteria consume organic matter in the absence of oxygen. This process is completely natural and can be found widely in nature. Examples include the rotting of wood in a forest, production of “swamp gas” in bogs, and the decay of any organic matter. This same process has been used indirectly for years in municipal and food-processing wastewater treatment systems as a means to conveniently reduce organic waste. Now, anaerobic digesters can not only reduce organic wastes, but also produce a renewable, low- to medium-grade fuel gas, or biogas that can be converted to power and heat.
An anaerobic digester is simply an enclosed container that holds the waste material, the microorganisms, and the environment they live in while excluding oxygen. The digester can be of almost any size, from only several hundred to several million gallons in volume. The digester is really a system of equipment components including solids handling equipment, liquids handling equipment, and gas handling equipment. Key properties used to describe the particular attributes of an anaerobic digester are volume, hydraulic residence time (HRT) and solids retention time (SRT). The HRT is the numerical value, usually expressed in days a volume of water will reside in the digester. Similarly, the SRT describes the numerical value, again, usually in days, that solids will reside in the digester. The HRT and SRT characteristics of the digester are set to achieve the desired treatment of a water stream and simultaneously achieve optimal conversion of the organic matter to biogas.
Digesters must be maintained in order to keep the digester functioning at design level. This entails feeding the organisms; removing materials indigestible by the microorganisms; and maintaining appropriate temperature, water chemistry and nutrient levels.
Oftentimes, the food must be presorted in order to remove items that would either interfere with digester operation or poison the broth. Undesired items include objects containing poisonous materials, such as batteries, herbicides, disinfectants and other toxic chemicals. Additionally, metal items such as nails and small pieces of scrap metal should likewise be removed as they may damage pumps and other mechanical components in the system.
The biogas produced in such a system has many uses. Frequently, the biogas is utilized to operate a modified diesel generator set. In this way, a waste material is conveniently transformed to readily usable electrical power. Over the past several years, federal and state incentives have stimulated the use of anaerobic digestion systems at dairy farms, where a large proportion of the electrical power used in production of dairy milk is self-generated from digestion of manure produced by the herd.
The Energy & Environmental Research Center is working on new and exciting uses for biogas. This includes generation of both renewable hydrogen and renewable ammonia from biogas. The EERC is presently working on moving recent laboratory-scale advances into distributed-scale production units for both hydrogen and ammonia. This effort will result in production of future renewable fuels and fertilizers from common waste materials.
Author: Paul Pansegrau
Research Scientist, EERC