Manganese can be present in natural water in concentrations exceeding 10mg/l. If not treated well, Mn(II) ions can easily escape through the water treatment systems. Mn(II) in the distribution systems could form manganese dioxide, which is insoluble in water, causing several problems such as water discoloration, turbidity, metallic taste, odour, corrosion and biofouling. Oxidation is a common method for Mn(II) removal. Oxidation process can become more efficient with catalytic effects of several substances. High Mn(IV) concentrations can be maintained only by sludge recycle which eventually leads to sludge ages as high as 10days. The aim of this study was to determine whether ageing of MnO2 sludge affects its catalytic effect on the oxidation of manganese by aeration. In order to reach this aim, five different groups of experiments were carried out by MnO2 sludge aged for a period of 0-10days. It has been demonstrated that the catalytic effect of MnO2 sludge on the oxidation of manganese, contrary to what is commonly expected, aeration increases with increasing sludge age. It has been concluded that, catalytic oxidation rate constant, k(cat) obtained from the batch systems can safely be used in design of continuous flow Mn(II) oxidation reactors with sludge recycle. As a result, the catalytic effect of MnO2 sludge on the Mn(II) oxidation by aeration increases with increasing sludge age up to approximately fourth day.