In a recent number of Engineering of London appeared a most interesting account of the Hyatt system of water filtration as carried on at Long Branch, N. J., and Atlanta, Ga. Illustrations and descriptions of both of these plants have been already given in our columns, but the following abstract of the article in Engineering gives some details of the process and apparatus, which, we think, will make good reading for those interested in the question of obtaining pure water:

The factors of the problem of purification by this system are as follows :

1. CliemlCO-mechamcal coagulation of the soluble and the fine suspended impurities of the water.

2. Rapid filtration through fine sand and coke compacted by downward pressure of the water.

3. A thorough scouring and rinsing of the entire filtering material at least once in every twemy-four hours.

4. Copious automatic aeration under pressure, to oxidize any organic residuum and neutralize organic germs that may escape the process of coagulation and hltralion. These processes, though distinct in principle, are practically blended, concurrent and constant in operation.

The most vital part of the work is to eliminate the organic matters and products in solution, which no mechanical agency can touch in that condition ; and with them the infinitude of micro-organisms that have translated, and are translating, the organic impurities into their own propagation, in what is known superficially as fermentation and decay. There are also various suspended impurities so fine that no large filtration arrests them, and that scarcely settle by months of undisturbed repose. Mineral salts are the agents by which these intangible pollutions are brought into new and tangible combinations, agglomerated and so arrested in the meshes of the filter. The salts of iron, calcium, potassium and aluminium are the most common and the best known for their coagulant action on soluble impurities, having been more or less used for a long time to facilitate the settling of turbid water. In the Hyatt system the influent water receives into its current, as it passes a certain point, an injection of coagulant solution, exprnmcntally adjusted in volume to its requirements from time to time by proper tests and admitted through graduated valves which control the volume to a minute fraction ; the object, ol course, being to supply all that the coagulable matters will take up, without any excess to pass through the filter in solution. This adjustment can be effected with a reasonable degree of accuracy, and the coagulant, being Itself transmuted and piecipitated in common with the impurities with which it combines, does not become an ingredient of the water, but is left behind in the filter.

The installation at Long Branch consists of eight cisterns, each ten feet in diameter, and all connected with a common inlet and outlet pipe. The water having first been aerated and coagulated, Hows from the main supply pipe to and into tlie filters above the surface of the filter beds, and in passing downwards is relieved of all objectionable constituents, issuing through a series of wire-bound outlet screens into a common delivery pipe, and being carried by the continuous pressure to the various consumers. At stated periods (ordinarily once each day) the arrested impurities are thrown oil’from the beds of filtering material into a waste pipe leading to the sea, each filter being renovated independently while the others are performing their work of purification. During this operation the intake pipe to the filter undergoing the operation is cut off from the main inlet, and water passes through a central vertical pipe connecting with a horizontal radial pipe at the bottom of the bed. The water issuing through this horizontal pipe saturate s the bed immediately around and above it, the arrested impurities being detached and carried off by the current. While this current is flowing through the horizontal washing pipe, the latter is gradually moved by means of a lever outside of the filter, and by the time it has passed ail around the interior, agitating and scouring the mass in succession until it has arrived back to its original position, the entire filter bed will have become cleansed, and the process of filtering is then resumed. The automatic aeration is accomplished before the water reaches the pumps. After leaving these it flows through the main inlet to the filters and thence to the consumers.

The plant at Atlanta, Ga., differs in construction from that at Long Branch, in the fact of having two stories or chambers, one above the other, the upper comprising the washing chamber, separated from the lower compartment, or filter proper, by means of a partition or diaphragm, this partition being indented with funnel-shaped depressions to facilitate the return flow by gravitation of the filtering material to the lower chamber. The unpurified water enters at a point just below the diaphragm, flows downward through the filter bed, issues at the bottom through a series of valves all connected in one system, an 1 is delivered into a clear water basin, from which it is pumped by the Holly system of pumps directly to consumers. The principles of coagulation and filtering, as exemplified in this plant, are precisely the same as at Long Branch, the difference in construction consisting in the method of renovating or washing the beds. In this case the beds are washed by means of vertical pipes, through which the entire contents of the lower chamber are forced up by ordinary water pressure and deposited in the upper or washing chamber. The combined effect of attrition in passing through these pipes and violent contact with the water contained in the upper chamber causes a complete separation of the filtering material from the impurities, which flow with the current out through the pipe leading from the upper chamber, thence to aseweror other outlet. This operation having been accomplished, the filtering material is permitted to return to the lower chamber by gravity through the conical apertures in the dividing partition. When thus restored to its original position all openings are closed, excepting the inlet and outlet, and the process of filtration is immediately resumed.

The simple apparatus for the purpose of automatic aeration is little more than a U-shaped well with a syphon from the bottom, and an air injector connected with the intake, on the principle of the Sprengle air pump. The well at Long Branch is 100 feet deep, and the pressure at the bottom of this column of water is, therefore, equivalent to three atmospheres. The rising syphon branch discharges into the bottom of the pumping well, and the copious volumes of air injected at the intake with the descending water are made to blend at every point where screens and deflectors can be placed to agitate and break up the water, and under the powerful pressure at the bottom part the oxygen is absorbed in solution, the remainder of the air converting the entire mass to white foam, in which condition it issues by the syphon into the bottom of the pumping well. Here its brief passage to the pumps and to confinement again under pressure suffices to let the nitrogen escape, while the oxygenated water enters into the mains and filters. Experiments have been published by Professor P. C. Austen of Rutgers College, N. J„ proving that the oxygen absorbed continues to act in reducing impurities for at least a month. The following is a comprehensive analysis made at a time when the water, as will be observed, was comparatively pure :

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