The method and an arrangement are for dewatering and washing lime slurry before the dewatered lime mud is passed to a lime mud kiln. The lime slurry is dewatered in a pressurized lime mud filter of disc or drum type. The pressurized lime mud filter is pressurized through a compressor drawing on its suction side (s) gas phase from the filtrate tank and pressurizing on the pressurized side (p) of the compressor the pressure vessel on the lime mud side of the filter. A certain pre-determined amount of gas phase in the gas circulation system is vented and an equivalent pre-determined amount of fresh air is added to the recycled gas phase with the aim of maintaining the partial pressure of oxygen gas above a certain minimum level.