The primary objective of the thesis was to adapt partial nitritation and anammox technology to the challenges of mainstream municipal wastewater treatment. Issues resulting from low temperature, low nitrogen concentration, slow growth and relatively low anammox activity as well as extreme difficulty in controlling partial nitritation under these conditions had to be resolved. As a result, a pre-treatment technique was developed to reduce the start-up times of biofilm anammox reactors. Anammox activity was four times higher with the bioprimer before inoculation as compared to the control. Bioprimer acted as a scaffold that allowed for a very quick attachment of anammox bacteria. The start-up of anammox moving bed biofilm reactors (MBBR) was also quickly achieved by inoculating with waste sludge from granular systems like DEMON®. This study showed that granular biomass can be easily transitioned to a biofilm. A novel control method for cold partial nitritation was developed. Mainstream partial nitritation was achieved by combining two control strategies. The target level of ammonium oxidation was achieved by controlling dissolved oxygen to total ammonium nitrogen (DO/TAN) ratio and nitritation was obtained by free ammonia (FA) inhibition of nitrite oxidizers at high pH. Long term stability was obtained at DO/TAN ratio of 0.06 and at FA concentration of 1.1±0.2 mg NH3-N L-1. A sidestream partial nitritation-anammox (PNA) MBBR reactor treating anaerobically digested sludge dewatering centrate was operated in order to farm anammox biomass used for bio-augmentation purposes. Very fast start-up of PNA process could be achieved by incorporating the bioprimer technique. Nitrogen removal of 2.5 g-N m-2 d-1 was achieved in as little as 56 days. Moreover, FA was found to be the predominant inhibitor of nitratation rather than selective wash-out of nitrifying biomass. Finally, a mainstream PNA reactor incorporating all previous techniques was developed. The proposed configuration comprised of partial nitritation-MBBR and anammox-IFAS reactor has achieved nitrogen removal rates comparable with conventional mainstream nitrogen removal treatment. Partial nitritation-MBBR achieved partial nitritation at 2.0±0.3 g-N m-2 d-1 and nitrogen removal reached 0.45±0.1 g-N m-2 d–1 in the anammox-IFAS reactor at 19±3 °C with an average effluent nitrogen concentration of 11±4 mg-N L-1.