The main and original features of dry filtration are reviewed. Forced aeration increases the aeration capacity of the process. In addition, the dual layer of media results in a better control of the elimination of both dissolved pollution and that connected with suspended solids. The treatment of various waste water in which the ratio between soluble and insoluble pollution was varied, made it possible to differentiate the way in which each is removed. This study lasted for one and a half years with 10 month's actual testing on two separate pilot plants. The contact time was related to the water and air flow rates as well as the degree of cloqqinq. The effect of the type of support media and the rates of flow of the various fluids were studied and the result led to recommendations being set for application of the process on an industrial scale.