A process for producing a biomass for use in the treatment of Fischer-Tropsch (FT) reaction water includes introducing a nutrient component comprising Carbon (C), Nitrogen (N) and Phosphorus (P), and water into an aerobic reaction zone containing a sewage sludge, and maintaining, in the aerobic reaction zone and under aerobic conditions, a F/M ratio of 0.25-2 kg COD/kg MLSS, where F/M=Food to Microorganism Ratio; COD=Chemical Oxygen Demand, expressed as mg oxygen/B of liquid in the aerobic reaction zone; and MLSS=Mixed Liquor Suspended Solids, expressed as mg solids in the aerobic reaction zone/B of liquid in the aerobic reaction zone. The F/M ratio is maintained for a period of time, to produce a biomass suitable for use in the treatment of FT reaction water.