A process (10) for treating sulphate-containing water (28) includes feeding the water into a biological sulphate reduction stage (12) containing a complex carbon source (18). The water is subjected to biological sulphate reduction in the presence of an electron donor source, thereby to reduce sulphates in the water to sulphides and to produce alkalinity. The complex carbon source is contacted with water containing higher sulphide and alkalinity concentrations or levels than the sulphate-containing feed water, thereby converting complex carbon molecules to electron donors. The, or another, electron donor source is regularly added to the biological sulphate reduction stage (12).