Method and system for purification of waste water

Method for the purification of waste water, in which method a number of different purification operations are performed to remove organic matter contained in the waste water, to remove nutrient compounds and/or to remove other harmful compounds from the waste water, characterised in that

• all purification functions, including nitrogen and phosphor removal, are performed in one and the same reactor, in which several alternating oxidation and reduction zones are created in unclarified waste water flowing through it by supplying each zone with gas mixtures with different oxygen content as needed for the purification function to be performed in the zone, in such manner that oxygen-rich gas containing 30-50% oxygen is supplied for oxidation while gas with a low oxygen content of 0-5% is supplied for reduction;
• a solid support for increasing biomass and the biomass adhering to it are exposed to alternating oxidation and reduction zones so that the waste water to be purified and the biomass are subjected to an abrupt variation of conditions, in which phosphor is alternately bound in biomass growth and dissolved and nitrogen compounds are oxidised into nitrate, to be reduced in the next zone into nitride and further into nitrogen gas;
• the process is controlled and regulated by optimising the oxygen content of the gas used;
• the solids level in the reactor is controlled and excess solid matter is removed directly from the reactor space via flotation;
• and
• the surface sludge obtained from the reactor is removed entirely without returning any portions of it into the process;
• the water leaving the reactor being so clean that it can be passed directly into the water system without a conventional sedimentation plant.