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Application of Anammox for Nitrogen Removal from Wastewater

Figure 1

Right from the start of our research into the exciting Anammox process, support was provided by the Netherlands Foundation for Applied Science (STW) and the Foundation of Applied Water Research (STOWA) together with industry, because the possible application of the anammox process in wastewater treatment had obvious advantages and might lead to very considerable savings in terms of energy and operational costs for nitrogen removal.

Conventional procedures for removal of ammonium from waste water make use of the well known nitrification/denitrification process according to the following aerobic/anaerobic reactions (1) – (3):

Aerobic nitrification

NH₄⁺ + 1.5 O_2à NO₂^- + H₂O + 2H⁺ (1)

Aerobic nitrification

NO₂^- + 0.5 O_2à NO₃^- (2)

Heterotrophic denitrification

6 NO₃^- + 5 CH₃OH + H⁺ à 3 N₂+ 5 HCO₃^-+ 8 H₂O (3)

Anoxic ammonium oxidation (Anammox)

NH₄⁺ + NO₂^- à N₂ (4)

Combined nitritification and anammox

2 NH₄⁺ + 1.5 O₂ à N₂ + 3 H₂O + 2 H⁺ (5)

The nitrification proceeds in two steps by nitrifying bacteria such as the ammonium-oxidizing Nitrosomonas or Nitrosospira species (1) and the nitrite-oxizing Nitrobacter or Nitrospira species (2), whilst the denitrification takes place under anoxic conditions by a large variety of bacteria using a variety of organic compounds as the electron donor. In reaction (3) this is exemplified by methanol used in the daily practice of wastewater treatment when no other cheap carbon source is available. The disadvantage of this process (1)-(3) is the relatively high (energy-) cost for aeration (total of 2 O₂ required per ammonium) and methanol. Introduction of the Anammox reaction (4) in combination with partial oxidation to nitrite (1) will allow reaction (5) that can save oxygen because only half of the ammonium needs to be oxidized to nitrite. Most importantly, the Anammox reaction will totally replace methanol use. In the earlier stage of the development of the Anammox process for practical application it was feared that the slow growth of the responsible bacteria such as Candidatus Brocadia anammoxidans would become an unsurmountable problem. However the research in Delft in collaboration with Mark van Loosdrecht and Udo van Dongen now has shown that extremely high biomass (20-40 kg of dry weight per m³) densities can be grown in systems with very efficient biomass retention. These systems possess very high volumetric loading rate (2-8 kg of N converted per m³ per day) competitive to aerobic nitrification. The Anammox process is particularly suited for wastewaters that contain high ammonium and little organic compounds (COD), such as sludge digestors effluent and a variety of industrial wastewaters. It is estimated that this may lead to reduction in operational cost by a factor of 10. Feasibility studies on a 20-liter scale with sludge digestor effluent have shown that the composition of the influent did not negatively affect the ammonium removal efficiency. The nitrogen load of the Anammox reactors could be increased from 0.2 kg N m^-3 day ^-1 to 2.6 kg N m^-3 day^-1 over 1 year. Over the years, different types of processes have been patented. In one of these processes the aerobic reaction (1) is applied under the name SHARON (single reactor system for high ammonium removal over nitrite, now in operation on 1,800-m³ scale) to convert about 50% of the ammonium to nitrite without the need of any pH control or biomass retention. The effluent is then fed into a second reactor with Anammox to obtain anaerobic reaction (4), as shown in the figure. Cost estimates are $ 0.70 kg^-1 N, which is low compared to the $ 2-5 kg^-1 N calculated for other processes tested on a pilot scale for the same purpose.

The discovery that the Anammox process, though anoxic, can very well be run under oxygen limitation, also allowed the use of a single reactor to convert ammonium to dinitrogen gas in the ‘CANON’ reaction [completely autotrophic N-removal over nitrite, (1) + (4) à (5)]. Difference of this reaction with other processes such as ‘Oland ‘ and ‘aerobic deammonification’ is that the latter two make use of the denitrification activity of conventional aerobic nitrifiers instead of Anammox used in the CANON system.

The first full-scale Anammox reactor in the Netherlands for the treatment of the ammonium-rich water from sludge digestor is planned early 2001.