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.

The disposal plant was constructed under the supervision of the Iowa Engineering Company of Clinton, Iowa, on a 15 acre tract of land, one and a half miles south of the city. Although original plans called for 18 beds with five feet of gravel, only four beds each 275 feet by 75 feet were finished, and instead of 5 feet of ground only 24 inches was used. The plant was complete in July 1899 and was operated until December, when it was discontinued on account of cold weather, it not being considered necessary to operate it during the winter. From December until April the sewage was discharged directly into a creek which runs along one side of the land.

The beds have no automatic device for distributing the sewage at definite intervals of time, but are connected with the main sewer by a number of gate valves which were regulated by the attendant. The sewage is distributed by means of a wooden sluice way which extended the entire length of the bed and has six inch openings on the side every 16 feet which gave a fairly even distribution.

Although no analysis of the sand and gravel was made, it was apparent that the uniformity coefficient was very large.

The material was shipped a distance of 90 miles and resembles ordinary ballast used for rail roads. The largest pieces were thrown along the sluice way to prevent excessive washing. The finest of the sand was of a size which would be considered too fine for good mortar sand. No attempt was made to screen the material. The gravel would not be considered first class for filtering beds.

Samples of the sewage and effluent were taken at two different times. The first was between 9:00 and 10:30 A. M., April 20, 1900. Until the day before no sewage had been applied to the beds since last fall. The second collection was made two weeks later, when samples of the sewage and effluent were collected at intervals of one hour during the day, and mixed into composite samples. The sewage was taken from a manhole just above the beds, and the effluent from the outlet of the underdrain about 900 feet below the beds.

The analysis is given in Table I. and the results show that the beds are not working as efficiently as might be expected from a first class plant. This may be accounted for from the fact that the filtration is too rapid and that the material which is less than two feet in depth is not suitable for such a purpose.