nitrification, which is mediated by aerobic ammonia- and nitrite-oxidizing bacteria and denitrification carried out by denitrifiers, which reduce nitrate to with the input of suitable electron donors. Aeration and input of organic substrates (typically methanol) show that these two processes are: Highly energy consuming. Associated with the production of excess sludge. Produce significant amounts of green-house gases such as and and ozone-depleting NO. Because anammox bacteria convert ammonium and nitrite directly to anaerobically, this process does not require aeration and other electron donors. Nevertheless, oxygen is still required for the production of nitrite by ammonia-oxidizing bacteria. However, in partial nitritation/anammox systems, oxygen demand is greatly reduced because only half of the ammonium needs to be oxidized to nitrite instead of full conversion to nitrate. The autotrophic nature of anammox bacteria and ammonia-oxidizing bacteria guarantee a low yield and thus less sludge production. Additionally, anammox bacteria easily form stable self-aggregated biofilm (granules) allowing reliable operation of compact systems characterized by high biomass concentration and conversion rate up to 5–10 kg N m−3. Overall, it has been shown that efficient application of the anammox process in wastewater treatment results in a cost reduction of up to 60% as well as lower emissions. Disadvantages The doubling time is slow, between 10 days to 2 weeks. This makes it difficult to grow enough sludge for a wastewater treatment reactor. Also the recovery time after the loss of sludge by accident is longer than in conventional nitrogen removal systems. On the other hand, this slow growing rate is an advantage due to the reduction of surplus sludge that needs to be removed and treated. Depending on the exact species, the optimum pH level is 8. Therefore, it can be necessary to adjust the pH of the wastewater by adding caustic. References Nitrogen cycle Environmental microbiology