The changes occurring in these filters are of the nature of oxidation, with the result that the proportion of the oxidised nitrogen increases (as nitrites and nitrates), the ammonia becomes less, and the total solids and organic nitrogen almost disappear. It will thus be seen that the work of these filters is not merely a straining action. It is true that particulate matter in the effluent from the tank is caught on the surface by the film (resulting from previous effluents), but the real work of the bed is _nitrification_, an oxidation of ammonia into nitrites and nitrates. This change obviously begins when the tank effluent flows over the "weir" on to the filter-beds, and the oxygen thus obtained by the effluent is carried down in solution into the coke-breeze. Upon the surface of the filtrant are oxidising bacteria. When the effluent is on the bed they oxidise its contained products; when the bed is empty and "resting" they oxidise carbon. An advantage arising from the periodical emptying and filling of the filter is that the products of decomposition which would eventually inhibit the action of the aërobic bacteria are washed away, and pass into the nearest stream, where they become absolutely innocuous. The "filter" is more correctly termed a _cultivation bed_, for its purpose is to furnish a very large surface upon which the nitrifying organisms present, as we have seen, in all soils, may flourish, and thus feeding upon the organic matter of the sewage, may perform their function of oxidation. It is not possible to lay down exact limits as to where denitrification ends and oxidation begins. To a certain extent, and in varying degree, they overlap each other. But roughly we may say that in the tank there is a breaking down (denitrification and decomposition) and in the filter-beds a building up (nitrification). The case is precisely parallel to similar changes occurring in soil, and which we have dealt with elsewhere. The advantage indeed of this biological treatment of sewage is that it exactly follows the processes of nature, in contradistinction to the mechanical and chemical methods hitherto adopted. At Sutton and some other places the same principles are applied,--that is to say, bacterial filtration,--but there is no tank. A metal screen in some measure takes its place, and holds back solid matter from being carried on to the beds. The filtrant is burnt clay, and it is forked over occasionally to let in oxygen. The crude sewage is run over the top of the burnt ballast, where it is left for two or three hours. It is then slowly run off on to a finer filter, where it also stays two hours. Thence the effluent is run into the stream. [Illustration: FILTER-BEDS As Used at Sutton]