Aerobic biological wastewater treatment in a continuous flow reactor

A method for the purification of substrate-containing wastewater in a continuous flow-through aerobic biologically activated sludge reactor, wherein substrate-containing wastewater and biological sludge are provided to a selector, to form a sludge/water mixture,

• wherein at least part of the biological sludge is conditioned in the selector under anaerobic or anoxic conditions with at least part of the substrate-containing wastewater
• such that at least 20 wt % of the biological sludge in the selector-has a residence time in the selector which is at least 20% greater than the hydraulic residence time of the sludge/water mixture in the selector, to obtain a conditioned sludge/water mixture, by subjecting the sludge/water mixture in the selector under anaerobic or anoxic conditions to discrete changes in flow velocity achieved by varying a feed flow rate of the wastewater and/or by varying a feed rate of the sludge;
• after which the conditioned sludge/water mixture, optionally after subjecting it to an additional anaerobic or anoxic contact step, is fed to the aerobic biologically activated sludge reactor where it is subjected to aerobic treatment, to obtain sludge-containing aerobically treated wastewater.