Iron(II) should be oxidised to iron(III) before the neutralisation of acid water with limestone, otherwise the oxidation will occur downstream of the neutralisation plant with the formation of acid (reactions 1 and 2). This study aimed at investigating the effect of surface area of the medium that supports bacterial growth on the rate of biological iron(II) oxidation. The study showed that the biological iron(II) oxidation rate is directly proportional to the square root of the medium specific surface area.