In a process to treat water effluent within a biological water treatment plant, the water subject to treatment is released into a vessel containing microorganisms with the addition of oxygen as required. The novelty is that the temperature of the water is increased directly or indirectly by the introduction of heat energy such that the activity of the microorganisms is increased. The heat energy required is derived from the waste heat from other processes such as a dried sludge incinerator or a power station.
