A reactor for the biological treatment of wastewater, includes a chamber capable of containing a mixture of wastewater and sludge comprising various levels, each level being defined by a sludge concentration and/or density; means for determining a minimum level and a maximum level of sludge extraction in the chamber, comprising: measurement means capable of measuring the sludge concentration and/or density at various levels of a mixture of wastewater and sludge; selection means capable of selecting a maximum sludge concentration and/or density value and a minimum sludge concentration and/or density value; deduction means capable of deducing a minimum extraction level corresponding to the maximum concentration value selected and a maximum extraction level corresponding to the minimum concentration value selected; extraction means capable of extracting sludge at variable levels between the minimum extraction level and the maximum extraction level.