The mechanisms involved according to the invention in the filter for water treatment are complex and include chemical, physical and biological processes the coarse gravel layers between 20 and 60 mm according to the invention located over a length at the entrance and at the outlet of the filter (a) and (d) will allow good distribution of the water in the filter and therefore good drainage of the water towards the outlet of the system;the layers of fine gravel (b) placed alternately with bricks based on the local soil (c) are permeable to water, with a fine and uniform diameter as possible of 1-6 mm, improve the distribution of water and reduce the risk of clogging and allow physical treatment by adsorption. The bricks based on the local soil (d) which are layers of soil mixture, constituted by: - soil: influences the chemical and physical adsorption of part of the organic matter and constitutes a support for microorganisms in the filter - charcoal: improves biological decomposition,and the adsorption capacity of the system - sawdust: additional carbon source for microorganisms, and effective adsorbent of polluting substances contained in water thanks to its high content of cellulose - iron: its oxidation to ferric iron or to ferrous iron within the filter considerably increases the adsorption capacity of phosphore in the system the water collection tank at the outlet (4) is a rectangular basin for storing treated water the aeration of the filter is done thanks to the permeable layers composed of gravel which is a natural aeration system
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