Abstract: A process for treating waste water such as sewage wherein waste water to be purified is brought successively into contact with a number of net units which are arranged in a treating tank with certain intervals therebetween. Organic substances in the waste water are removed due to decomposition caused by microorganisms which adhere to the net units. In the net units, water being in contact with the individual net units is always replaced, a disturbance is given to the water resulting in easy approach of the organic substances to the microorganisms, contact area between the organic substances and the microorganisms is large, and various kinds of microorganisms propagate. Thus, removal of the organic substances proceeds effectively and promptly. An apparatus which is suitable for carrying out the process is also proposed.