Anammox

Anammox, a process of Anaerobic Ammonium Oxidation, is an innovative technological advancement in the removal of ammonia nitrogen in wastewater. This quite new process combines ammonia and nitrite directly into dinitrogen gas (SHIVARAMAN and SHIVARAMAN 2003) instead of passing through a two-stage process of aerobic nitrification and anaerobic denitrification. It was discovered about 15 years ago and has resulted in new opportunities for research and development of sustainable nitrogen removal systems. Compared to conventional nitrification/denitrification in activated sludge systems, Anammox eliminates the necessity of an organic carbon source for nitrification, reduces energy demand for aeration and has a smaller production of excess sludge and lower CO₂ emissions (BERTINO 2010). The Anammox reaction can be represented as NH₄- + NO₂- = N₂ + 2H₂O. This reaction is carried out by Anammox bacteria (Candidatus Brocadia anammoxidans) belonging to the group of planctomycete (SHIVARAMAN and SHIVARAMAN 2003).

Two Reactor Nitritation-Anammox Process

Different setups have been developed to apply Anammox in wastewater treatment. In the “two reactor nitritation-Anammox process“, ammonium is first partially oxidised to nitrite in an aerated reactor (partial nitritation). In a second stage the nitrite is reduced to elemental nitrogen using the remaining ammonium in the Anammox reactor. So far, it has only been used for ammonium rich effluents (FUX 2003; BERTINO 2010). There are alternative names for these two-stage processes, e.g. SHARON-Anammox process (XING and CLARK n.y.).

One Reactor Nitritation-Anammox Process

The one-step reactor process also has several names (see XING and CLARK n.y.). A common name is the CANON Process for ‘Completely Autotrophic Nitrogen removal Over Nitrite’ where aerobic ammonium oxidising bacteria (nitrifiers) and Anammox bacteria simultaneously perform a two-step reaction under oxygen-limited conditions in one reactor. However, this process is limited to the laboratory and pilot-plant scale due to the complex controlling conditions and the slow growth of Anammox bacteria (HUMBERT 2011).

One Reactor Denitrification-Anammox Process

This process is called DEAMOX and stands for Denitrifying Ammonium Oxidation. It is the coupling of denitrification and Anammox processes. Research is still ongoing. It can be applied to the treatment of wastewater with high nitrogen concentrations and high organic carbon levels, e.g. landfill leachate and wastewater from digested animal waste (e.g. large anaerobic digestion reactor).

The Anammox process requires much less oxygen than conventional nitrification/denitrification processes resulting in important energy savings. Because autotrophic bacteria carry out Anammox, there is no need for organic carbon sources, which saves on chemical dosage costs. The biomass yield of Anammox is very low, which saves on sludge treatment costs. However, the long start-up time and the high sensitivity of the bacteria to oxygen concentration and nitrite accumulation limit the application of the Anammox process (LI et al. 2008).
 

As it is still a new technology, operation of an Anammox reactor requires a lot of knowledge. It is a high-tech system, which needs to be monitored (temperature, load, chemical compounds of wastewater, etc.). The problems that can occur are manifold and experts must be available for maintenance. During the start-up of the first full-scale Anammox reactor in Rotterdam, the team gained a lot of experience. Read more about it in VAN DER STAR et al. (2007) .

Adapted from VAN DER STAR 2007

The full-scale Anammox reactor is a 70m³ reactor designed by Paques BV (www.paques.nl). The reactor combines a high loading rate with efficient biomass retention, characteristics the Anammox process has in common with anaerobic wastewater treatment. In the lower compartment (approx. 40m³) influent is mixed with downward gas flow and gas recycled from the top of the reactor. On top of the lower compartment, gas is collected for the riser of the gas lift. The liquid moves from the lower compartment to the less mixed and thus stratified upper compartment, serving mainly for biomass retention and effluent polishing. The feed is introduced from the bottom of the reactor and is mixed with an additional recirculation flow from the effluent of the reactor when loads are lower than ca. 8m³/h or 150 kg-N/d. This allows adequate up flow velocity and shear stress to favour granule formation. The design load is 500 kg-N/d (7.1 kg-N/m³/d) but the practical maximum load is determined by the amount of nitrogen in the sludge digestate (700 kg-N/d on average).