Abstract: In a reactor for water treatment of the type that contains particle-like carriers on which a biofilm grows that generates a desired decomposition of the contamination, a forced current is created such that the water in another current flows downwardly. In a special embodiment the upwardly flowing current is created in a tube. The reactor proves to be exceptionally effective and can be utilized together with other water purification and water treatment equipment. It is a distinctive feature that the downflowing current tears air down thus causing an exceptionally high oxygen concentration in the water. The reactor can furthermore be used for denitrification. Among other things the invention provides a minibiological purification plant servicing a single or very few households. As micro film carriers are preferably used, particles having an outwardly open cell structure for increasing the total effective surface area.