The invention relates to a water purification device and method, in which air (L) that is fed to a waste water activation station (1) is compressed with the aid of a compressor (3). The air feeding apparatus that connects the compressor (3) to the activation station (1) is coupled to a sludge dewatering unit (4) in such a way that heat or energy from the compressed air (L) is delivered to the sludge drying process taking place in the sludge dewatering unit. The invention makes it possible to improve water purification and particularly save energy.
