Completely autotrophic nitrogen removal by anammox bacteria: past, present and future

Abstract

The recent discovery of the metabolism of anammox bacteria (ANaerobic AMMonium OXidation) has revolutionized the understanding of the nitrogen cycle. Unlike the conventional process of nitrification and denitrification, by which the nitrogen removal from wastewater is usually performed, the anammox bacteria catalyze the oxidation of ammonium to molecular nitrogen through the reduction of nitrite. The autotrophic nitrogen removal process, as often named the process that exploits the anammox metabolism, consists of two stages: a first stage of oxidation of about half of the ammonium to nitrite (Partial Nitritation, PN) by aerobic ammonium oxidizing bacteria (AOB), followed by a second phase where anammox bacteria remove nitrogen through the production of dinitrogen gas released into the atmosphere. The lower aeration demand, the autotrophic metabolism of these bacteria and the lower production of excess sludge, make the PN/anammox process much cheaper than conventional methods, with overall savings higher than 40% in terms of operational costs. Furthermore, the PN/anammox process uncouples the nitrogen removal from the oxidation of the organic matter, thus allowing the recovery of the associated chemical energy through the production of methane-rich biogas in the anaerobic digestion process. In this review, after a few hints about the anammox metabolism and the benefits arising from its implementation in wastewater treatments, the fullscale technological applications employing the PN/anammox process will be described focusing on their differences, both in terms of reactor configuration and process management. The discussion ends with a review of the new fields of application and the topics on which international research is currently focusing.