More than 50% of the electrical power needed to treat pulp and paper industry effluents is used for aeration in biological treatment stages. A large share of the oxygen that passes through the wastewater is not consumed and will be found in the off-gas. Energy can be saved by aerating under conditions where the oxygen transfer is most efficient, for example at low concentrations of dissolved oxygen Consider the sludge as an energy source; electricity can be saved by avoiding sludge reduction through prolonged aeration. High oxygen transfer efficiency can be retained by using the oxygen consumption of biosolids. Quantified savings in the form of needed volumes of air while still achieving sufficient COD reduction are presented. The tests have been made in a bubble column with pulp mill process water and sludge from a biological treatment plant. These were supplemented with case studies at three pulp and paper mills.