Treatment of waste water from a biomass-to-liquid process, said process comprising producing synthesis gas from biomass and conversion of said synthesis gas into liquid hydrocarbons by a Fisher-Tropsch process. The waste waters obtained are purified in common with waste waters from another industrial process to which said biomass-to-liquid process is integrated, such as in forestry, power and/or heat generation, waste incineration or a process in a metal-, petrochemical and/or oil refining industry. The biomass-to-liquid. process and said another industrial process may have a common feed water process unit, a common cooling water process unit and a common waste water treatment unit. The waste water treatment process may comprise a biological purification process and said Fischer-Tropsch process may utilize a cobalt catalyst. An integrated factory comprising a biomass-to-liquid plant and another industrial facility in which both plants are connected to a common waste water treatment facility is also described.