Potential and limits of biodegradation processes for the removal of organic xenobiotics from wastewaters

In the removal of organic xenobiotics from wastewaters, biodegradation is an interesting alternative to conventional physical and chemical processes. Biodegradation in biological wastewater treatment plants has the potential advantages of removing the xenobiotics from the environment, transforming them into nontoxic products such as carbon dioxide and new biomass, and being relatively low in cost. Many xenobiotics have been found to be biodegradable as sole carbon and energy source under aerobic conditions, while under anaerobic conditions many chlorinated hydrocarbons can be biodegraded by reductive dechlorination processes. The main limits of biodegradation processes are the limited time for adaptation to the xenobiotics, the very low xenobiotics concentration in the influent, the competition with abiotic removal mechanisms, the limited availability of external substrates for cometabolism, and the intrinsic nonbiodegradability. Existing biological wastewater treatment processes can be modified by using higher values of the solids retention time, bioaugmentation, addition of readily biodegradable substrates to support cometabolism, and the use of anaerobic-aerobic processes.