Membrane bioreactors for the removal of anionic micropollutants from drinking water Joa ˜ o G Crespo  , Svetlozar Velizarov and Maria A Reis Biological treatment processes allow for the effective elimination of anionic micropollutants from drinking water. However, special technologies have to be implemented to eliminate the target pollutants without changing water quality, either by adding new pollutants or removing essential water components. Some innovative technologies that combine the use of membranes with the biological degradation of ionic micropollutants in order to minimize the secondary contamination of treated water include pressure-driven membrane bioreactors, gas-transfer membrane bioreactors and ion exchange membrane bioreactors. Addresses CQFB/REQUIMTE, Department of Chemistry, FCT, Universidade Nova de Lisboa, P-2829-516 Caparica, Portugal  e-mail: jgc@dq.fct.unl.pt Current Opinion in Biotechnology 2004, 15:463–468 This review comes from a themed issue on Biochemical engineering Edited by Manuel Carrondo and John G Aunins Available online 28th July 2004 0958-1669/$ – see front matter # 2004 Elsevier Ltd. All rights reserved. DOI 10.1016/j.copbio.2004.07.001 Abbreviation IEMB ion exchange membrane bioreactor Introduction The contamination of drinking water sources with inor- ganic compounds is a matter of concern, because of their harmful effect on human health. Some of these com- pounds are highly soluble in water and dissociate com- pletely, resulting in ions that are chemically stable under normal water conditions. Examples of polluting anions include nitrate, nitrite, perchlorate, bromate, arsenate and ionic mercury, for which the proposed guideline values for drinking water quality are quite low (in the range of mg/L to a few mg/L) owing to their carcinogenic effects or other risk factors to public health [1  ,2–5]. Some of them may be present simultaneously in contaminated water. Technologies available for the treatment of water con- taminated with inorganic anionic compounds include physical, chemical and biological processes [6–10]. In the first type of process, ions are concentrated rather than destroyed. A brine stream containing a high concentration of ions is generated, which must then undergo additional treatment and disposal. Biological conversion is a promising technology for the effective and economical removal of anions from water. Several bacteria capable of degrading anions to harmless products have been identified. These bacteria carry out anaerobic respiration using anions as electron acceptors and organic or inorganic compounds as electron donors (e.g. ethanol, acetate, hydrogen gas). Electron donors must be added to the contaminated water; however, the dosage must be carefully controlled in response to fluctuations in ion concentration, and this constitutes one of the serious drawbacks to the biological process. In the presence of chemical oxidants, some organic elec- tron donors, added in excess, can serve as precursors for the creation of novel contaminants (e.g. disinfection by- products), which have been recognized as potential car- cinogens [1  ,4]. Furthermore, any overdosing of organic electron donors, which are readily biodegradable, can promote microbial growth in water distribution systems, thus requiring post-treatment to produce safe and biolo- gically stable water. Instead of an organic compound, hydrogen has been used to prevent the chemical contamination of water, but it requires special hydrogen supply devices due to safety reasons [11]. Most traditional techniques for the biological removal of anions from contaminated water use high cellular con- centration bioreactors, in which the hydraulic and cell retention times are decoupled. This type of reactor is suited for the treatment of high flow rates of contami- nated water with a concentration of anions in the range of mg/L to a few mg/L, for which slow degradation kinetics are usually obtained. The most common configurations include immobilized cell reactors (e.g. packed bed, flui- dized bed or membrane-supported biofilms) [5,9,12,13], in which cells come into direct contact with the water stream containing the pollutant. The concept of an active layer (biofilm) is important for understanding the functioning of membrane bioreactors. In the case of the anionic compounds mentioned above, the biofilm corresponds to a reaction zone where a redox process takes place involving the oxidation of the added electron donor and reduction of the anionic pollutant www.sciencedirect.com Current Opinion in Biotechnology 2004, 15:463–468