Biodegradation, or the breakdown of chemical substances by living organisms, is one of the major processes that determine the fate of organic chemicals in the environment. Microorganisms, particularly bacteria and fungi, play a major role in biodegradation because of their abundance, species diversity, catabolic versatility, and ability to adapt to a wide variety of environmental conditions. Biodegradation can occur under both aerobic and anaerobic conditions. In the natural environment, the former occurs in soil or water where oxygen is present, whereas anaerobic degradation occurs in sediments or ground water where oxygen is generally absent. Biodegradation is important in wastewater treatment plants where both aerobic and anaerobic processes may be involved.
In contrast to other environmental fate processes, such as hydrolysis or photochemical reactions, biodegradation is unique in that the end result is often the complete conversion of the organic substance to inorganic products (e.g., carbon dioxide and water). Complete break down is often referred to as "ultimate degradation" or "mineralization." Alternatively, a single alteration of the parent compound is sufficient for "primary degradation." Because biodegradation in natural environments is due to the concerted actions of multiple microbial populations with different metabolic capabilities, complete degradation is often the final result.
The US EPA and OECD have developed a series of laboratory screening tests that can be used to determine the "ready" and "inherent" biodegradability of organic compounds. For these screening tests to be applicable to a broad range of organic substances, degradation in such tests is often monitored by measuring loss of dissolved organic carbon, oxygen consumption (biological oxygen demand) or formation of carbon dioxide.
Ready biodegradability is determined under the most stringent of test conditions, using a very small amount of microbial inoculum, and where the test chemical is present as the sole carbon source at low concentrations. Consequently, chemicals that are shown to pass a ready biodegradability test also rapidly degrade in wastewater treatment plants and in the natural environment. Inherent biodegradability tests are intended to provide more favorable conditions for biodegradation to occur. Such tests are often conducted using higher microbial inoculum concentrations, higher test chemical concentrations, and are conducted under conditions which allow for acclimation of the microorganisms. Chemicals that pass an inherent biodegradability test are considered non-persistent, although the breakdown of the chemical in the environment may be slow.
In summary, biodegradation is an important natural process, which converts chemicals released into the environment into other chemicals and, eventually, into carbon dioxide. Most hydrocarbon and oxygenated solvents have been shown to be biodegradable and do not persist in the environment.