Potential and Limits of Biodegradation Processes for the Removal of Organic Xenobiotics from Wastewaters Davide Dionisi [1] * www.ChemBioEngRev.de ª 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ChemBioEng Rev 2014, 1, No. 2, 67–82 67 Abstract In the removal of organic xenobiotics from waste- waters, 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 biodegrad- able as sole carbon and energy source under aerobic conditions, while under anaerobic conditions many chlorinated hydrocarbons can be biodegraded by re- ductive dechlorination processes. The main limits of biodegradation processes are the limited time for adaptation to the xenobiotics, the very low xenobio- tics concentration in the influent, the competition with abiotic removal mechanisms, the limited avail- ability 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. Keywords: Biodegradation, Biological processes, Wastewater treatment, Xenobiotics Received: December 04, 2013; revised: January 20, 2014; accepted: February 14, 2014 DOI: 10.1002/cben.201300008 1 Introduction Xenobiotics are usually defined as man-made synthetic chemi- cals which are foreign to the biosphere [1]. Important classes of xenobiotics are polycyclic aromatic hydrocarbons (PAHs), ha- logenated aromatic and aliphatic compounds, pharmaceuticals, azo compounds, polychlorinated byphenyls (PCBs), etc. In a broader sense, which is adopted in this paper, the term xeno- biotics can also include substances which are naturally present in the biosphere, such as petroleum hydrocarbons and natural hormones, but which due to human presence or industrial ac- tivity are present in some water bodies at concentrations higher than their natural levels. In industrial sites xenobiotics may be present in wastewaters due to releases from manufacturing processes, while in munici- pal, i.e., urban, wastewaters the presence of xenobiotics is mainly due to household releases [2, 3]. Commonly used prod- ucts such as personal care products, cosmetics, and detergents contain a wide range of xenobiotics such as surfactants, bio- cides, oils, and fragrances. Pharmaceuticals and steroid hor- mones are other important classes of xenobiotics released by households [4]. Many xenobiotics are toxic for the ecosystem even at very low concentrations [5, 6], therefore, when they are present in wastewaters they need to be removed as much as possible. Many chemical and physical technologies are used to remove xenobiotics from wastewaters: (i) adsorption on activated car- bon or other adsorbents, (ii) chemical oxidation, including cat- alytic oxidation, photocatalysis, Fenton reaction, and ozonation amongst others, (iii) membrane filtration, and (iv) incineration [7–13]. These techniques are usually very effective and allow very high removal of most xenobiotics from wastewaters. How- ever, they also suffer from several disadvantages. Some of them do not alter the chemical structure of the xenobiotic but simply transfer the substance to a different phase, e.g., a solid phase for activated carbon adsorption. While in the short term this is a benefit since the contaminant is removed from the phase where it can be the most harmful to the environment, in the long term the problem of xenobiotics presence has not been solved since the substance is still present in the environment and may still pose a toxicity risk. Other technologies such as chemical oxidation and incineration remove the substance from the environment. However, these technologies are often very expensive and in some cases may generate toxic products, e.g., PAHs can be released to the atmosphere during incinera- tion of industrial wastewaters [14]. It is interesting to observe that the UK has launched a program [15] aimed at evaluating technologies for the removal of endocrine disrupting com- ————— [1] Dr. Davide Dionisi Materials and Chemical Engineering Group, School of Engineer- ing, University of Aberdeen, Aberdeen, AB24 3UE, UK. E-Mail: davidedionisi@abdn.ac.uk