Filtration means the concentration of sewage on an area of especially chosen porous ground, or on an especially prepared bed as small as will absorb and purify it. The intermittency of application is a necessary requirement even in suitably constituted soils wherever complete success is arrived at. The reason for this is that free oxygen is indispensable to nitrification and the nitrifying organism. This method has been in operation only during the last thirty years, and the correct theory of its action has been found out within the last ten years. Dr. Frankland, in the first report of Rivers Pollution Commission of England, shows that not only a chemical action but also a biolytic action takes place with the assistance of minerals and the oxygen of the air. This biolytic action is known as nitrification, and consists in the oxidation of the nitrogen of ammonia and its ultimate conversion into nitric acid. This change takes place in two stages, each characterized by a distinct organism. The office of one of these organisms is to convert ammonia into nitrite, while the other converts nitrites into nitrate. The bacteria are present or quickly develop in the sewage which may be considered a nutrient medium for them by reason of containing a large amount of their natural nitrogenous food. These organisms, according to Warrington, may be separated by successive cultivations in favorable media; from a potassium nitrate solution with no ammonia we may obtain nitric organism alone; from an ammonium carbonate solution we may obtain nitrous organisms in a pure state. It is a noticeable fact that frost checks nitrification, but several satisfactory methods have been used so that no serious objection can be raised to intermittent filtration on this ground. “The purification of sewage by this method depends upon the oxygen and the time of action” (Hazen). To obtain the highest efficiency the sewage must be applied intermittently and there must be a large percentage of voids in the sand. Experiments made on especially prepared beds at Lawrence, Mass. show that by treating 85000 gallons per acre per day, 94 percent of the organic matter and 98 percent of the bacteria are removed, and by treating 60,000 gallons per acre per day, 97 to 99 percent of the organic matter may be removed. The above data shows that 100,000 may be successfully treated with a five foot bed of sand. The writers inspected the intermittent downward filtration plant at Mendota Illinois and the following data were obtained. The city has a population of about 6000 with 8 miles of sewers and 210 house connections. The average daily flow is about 350,000 gallons although during the rainy season there is a considerable amount of ground water present.