Abstract: The present invention relates to a process for treating wastewater containing suspended solids comprising adding to the wastewater a treating substance in an amount sufficient to enhance at least one of (a) the settling rate of the solids, (b) the bulk density of the solids and (c) the filterability of the solids, said treating substance being selected from the group consisting of (i) bauxite refinery residue known as red mud, and (ii) red mud that has been at least partially reacted with calcium and/or magnesium ions so as to have a reaction pH, when mixed with five times its weight of water, of less than 10.5. In addition, the invention relates to processes for reducing the concentration of dissolved phosphorus-containing species in wastewater, for decreasing the odour of a material having an odour due to the presence of one or more sulphur-containing substances and for decreasing the propensity to develop odours and a composting process.