Anammox

Enrichment culture of the anammox bacterium Kuenenia stuttgartiensis, Radboud University Nijmegen

Anammox, an abbreviation for ANaerobic AMMonium OXidation, is a globally important microbial process of the nitrogen cycle ^[1]. The bacteria mediating this process were identified only 20 years ago and at the time were a great surprise for the scientific community ^[2]. It takes place in many natural environments and anammox is also the trademarked name for an ammonium removal technology that was developed^[3] by the Delft University of Technology.

Process background

In this biological process, nitrite and ammonium are converted directly into dinitrogen gas. This process contributes up to 50% of the dinitrogen gas produced in the oceans. It is thus a major sink for fixed nitrogen and so limits oceanic primary productivity. The overall catabolic reaction is:

NH₄⁺ + NO₂⁻ → N₂ + 2H₂O.

The bacteria that perform the anammox process belong to the bacterial phylum Planctomycetes, of which Planctomyces and Pirellula are the best known genera. Currently four genera of anammox bacteria have been (provisionally) defined: Brocadia, Kuenenia, Anammoxoglobus, Jettenia (all fresh water species), and Scalindua (marine species). The anammox bacteria are characterized by several striking properties: they all possess one anammoxosome, a membrane bound compartment inside the cytoplasm which is the locus of anammox catabolism. Further, the membranes of these bacteria mainly consist of ladderane lipids so far unique in Biology.^{[citation needed]} Of special interest is the conversion to hydrazine (normally used as a high-energy rocket fuel, and poisonous to most living organisms) as an intermediate.^[4] A final striking feature of the organism is the extremely slow growth rate. The doubling time is nearly two weeks. The anammox process was originally found to occur only from 20 °C to 43 °C^[5] but more recently, anammox has been observed at temperatures from 36 °C to 52 °C in hot springs ^[6] and 60 °C to 85 °C at hydrothermal vents located along the Mid-Atlantic Ridge^[7].

History

For a long time the general consensus was that ammonium could only be oxidised under aerobic conditions. The Austrian theoretical chemist Engelbert Broda was the first to recognise the possibility of anaerobic ammonium oxidation in 1977^[8]. The simultaneous removal of ammonium and production of nitrogen gas was observed in an industrial wastewater treatment in The Netherlands in 1986.

Application

The application of the anammox process lies in the removal of ammonium in wastewater treatment and consists of two separate processes. The first step is partial nitrification (nitritation) of half of the ammonium to nitrite by ammonia oxidizing bacteria:

4NH₄⁺ + 3O₂ → 2NH₄⁺ + 2NO₂^- + 4H⁺ + 2H₂O

The resulting ammonium and nitrite are converted in the anammox process to dinitrogen gas and circa 15% nitrate (not shown) by anammox bacteria

NH₄⁺ + NO₂^- → N₂ + 2 H₂O.

Both processes can take place in 1 reactor where two guilds of bacteria form compact granules ^[9][10].

For the enrichment of the anammox organisms a granular biomass or biofilm system seems to be especially suited in which the necessary sludge age of more than 20 days can be ensured. Possible reactors are sequencing batch reactors (SBR), moving bed reactors or gas-lift-loop reactors. The cost reduction compared to conventional nitrogen removal is considerable; the technique is still young but proven in several fullscale installations. The first full scale reactor intended for the application of anammox bacteria was built in the Netherlands in 2002. Other wastewater treatment plants, such as the one in in Germany (Hattingen), where anammox activity is coincidentally observed were not purpose built. As of 2006 there are three full scale processes in The Netherlands. One on a municipal wastewater treatment plant (in Rotterdam), and two on industrial effluent. One is a tannery, the other a potato processing plant.

References

• citeUlike for an up to date overview of relevant scientific literature.

1. ^ Arrigo, R. A. Marine microorganisms and global nutrient cycles. Nature 437, 349–355 (2005)
2. ^ Strous, M. et al. Missing lithotroph identified as new planctomycete. Nature 400, 446–449 (1999)
3. ^ Jetten Michael Silvester Maria, Van Loosdrecht Marinus Corneli; Technische Universiteit Delft, patent WO9807664
4. ^ "Pee power: Urine-loving bug churns out space fuel". Agence France Press. 2011-10-02. http://www.google.com/hostednews/afp/article/ALeqM5g7KNt-gKX2QahA5aTYpXKiGMFwRQ?docId=CNG.7ad23683e364936213965f6a6f6a1c2c.4a1. Retrieved 2011-10-03. 
5. ^ Strous, M., Kuenen, J.G., Jetten, M.S. 1999. Key Physiology of Anaerobic Ammonium Oxidation. App. Environ. Microb. (3248-3250)
6. ^ Jaeschke et al. 2009. 16s rRNA gene and lipid biomarker evidence for anaerobic ammonium-oxidizing bacteria (anammox) in California and Nevada hot springs. FEMS Microbiol. Ecol. 343-350
7. ^ Byrne, N., Strous, M., Crepeau, V, et al. 2008. Presence and activity of anaerobic ammonium-oxidizing bacteria at deep-sea hydrothermal vents. The ISME Journal.
8. ^ Brochier, C., Philippe, H. (2002), A non-hyperthermophilic ancestor for bacteria. Nature, 417, 244-
9. ^ B. Kartal, G.J. Kuenen and M.C.M van Loosdrecht Sewage Treatment with Anammox, Science, 2010, vol 328 p 702-3
10. ^ Knight, Helen (May 7, 2010). "Bugs will give us free power while cleaning our sewage". New Scientist. http://www.newscientist.com/article/dn18872-bugs-will-give-us-free-power-while-cleaning-our-sewage.html. Retrieved May, 2010.