Introduction to Anaerobic Digestion

Anaerobic microbes evolved as some of the earliest organisms on earth – before photosynthetic plants had “poisoned” the atmosphere with free oxygen. As faster-metabolizing oxygen-powered organisms, including aerobic decomposers, have largely taken over all of the ecosystems they could, anaerobes have migrated over hundreds of millions of years to more water-logged, oxygen-free environments. They now occur naturally in swamps, peat bogs and water-logged soils, as well as deeper portions of bodies of water -- any place with large quantities of organic residues and highly restricted access to oxygen. This includes the lower digestive tracts of large animals. Indeed, humans already have a very intimate symbiotic relationship with anaerobic microbes.

AD systems may be simply understood as extensions to the digestion processes and digestive tracts of large animals. Thus, anaerobic decomposition is most widely known as digestion and the units within which this occurs are digesters. Anaerobic digesters are basically vessels in which the anaerobic decomposition process is contained and controlled. Their shape and configuration can vary tremendously and their size can vary from laboratory-scale to multi-million-gallon tanks and lagoons. Biochemical science has refined and deepened our understanding of the digestion process at various levels of complexity.

Due largely to the generation of combustible biogas which can be used much like natural gas, AD systems have received much attention over the past 50 years – and especially where very large quantities of organic residues are accumulated. However, in many areas around the world, the biogas is more like a very convenient by-product when compared to the other benefits from controlled exploitation of the AD process. Perhaps most important is the sanitary treatment of organic residues fed into the digester. When allowed to go toward completion, AD results in total destruction of many/most disease vectors which may have been present in the feed materials; nearly total destruction of most of those remaining; and very significant destruction of the most recalcitrant (including ascaris and other eggs). Pathogen destruction reported through various levels of anaerobic digestion is delineated in this review and shows that AD offers significant sanitation benefits. After AD, the remaining effluents and solids contain all nutrients originally present in the digester feed. These materials provide readily available nutrients for plant growth and soil-building humus which, when added to soils, increases nutrient and water-holding capacity. This, of course, greatly reduces the need for chemical and other fertilizers. When biogas is used to replace biomass or coal as a cooking fuel, indoor air pollution and related health problems are greatly reduced. Indeed, AD offers perhaps the least polluting way to sustainably utilize biomass energy. Methane is more than twenty times stronger than carbon dioxide in trapping heat in the atmosphere. Thus attending to collection and burning the biogas emitted by masses of organic residues may provide significant assistance in ameliorating growing climate chaos.

 

A note to Readers: I do apologize for the length of this discourse. But “WHY” has usually been of prime importance to me – and often personally energizing. I will now attempt to address “HOW” and “WHAT.” But I also suggest investigation of the plethora of information available through searching anaerobic digestion and biogas on the web. The EPA’s AgStar website has masses of information, mostly for larger-scale systems. There are a great many videos of varying quality on YouTube.I also recommend most highly the information and most of the perspectives provided in David House’s Biogas Handbook.