Nature, however, is not content with coagulating and filtering water, but at the first op portunity sends it tumbling over some precipice, to fall against rocks and lie dashed into spray until it reaches the bottom a mass of foam. In doing this Nature effects in a simple way something that has greatly perplexed engineers to iinitate—i. e., to aerate water in a practical way. This ;titration fills the water with myriads of minute hulibles of air. The surface of contact between the water and air is immense, owing to the enormous number of air-bubbles. In this way the water is subjected to the powerful influence of the oxygen of the air, which destroys the dissolved organic impurities, and not only kills many of the lower forms of life, but makes the life of many others hazardous by removing the organic matter on which they feed. The artificial aeration of water has been effected in the following way: A large verti cal pipe many feet in length is turned back on itself so as to form a great U. Into one end of this the water is injected and falls tangling up the air with it and emerging from the other end as foam. Water so rii;rated takes hours to lose its air, so minute are the bubbles. The effect of this Oration is to oxidize the dissolved organic matter and greatly purify the water. To return now to the filter. After it certain duration of filtration the filter-beds become so clogged with the separated coagmluni and filth that filtration becomes difficult, and if allowed to go on would soon yield a foul writer from the growth in them of iniero-organisms. rind instead of purifying would render the water organically less pure. Long Itny danger of such a catastrophe the cleaning of the filter-beds takes place. To accomplish lids the cur rent of water is reversed, and. instead of th»ving down through the filter-bed, is sent with great force tip through it from the bottom. The entire bed of sand is thus lifted and floats, as it were, on the ascending stream of water, yielding up all if s impurities, which escape with the water through a wasti.i-pipe. The washing of the tiller is continued until the wash-water runs clear. when, by turning a few valves, the flow is reversed again and filtration is resumed. So simple are the operations of filtration and washing the beds, that one man can handle a plant millions of gallons per day.
The effect of this method of filtration on the purity of water is most remarkable. Thus the analyses of the water of the city of Atlanta, Oa., before and after filtration furnish incon testable proof of t lie success of the process there employed: This city has a battery of 12 filters with a capacity of nearly 4,000,000 gals. per day. Be fore the introduction of the filtering plant the water could not be used except for sanitary purposes. Now the filtered water is the best there is in the city.
The remarkable action of mechanical filtration in the removal of organic life in water is also marked and is of the greatest importance. It is now a well-recognized fact that many diseases are conveyed by water, and reach us in the forms of microbes, or disease-seeds. From the standpoint of the hydraulic engineer, however, so long as the microbe is a particle of in soluble matter it can be removed as easily as any other particle of solid matter—clay, for in stance. The microbe and the particle of clay become alike entangled in the gelatinous coag uhun, and are removed by the filter-bed. Dr. Charles V. Chapin, Superintendent of Health of Providence, P. I., has made some most interesting investigations in the water filtered by the filter plant at Long Branch, which is one of the finest yet built. In the unfiltered water he found in 1 c. c. 208 organisms. In the filtered water only two. Nature, herself, can not
do better than this. (See Pure Water for our Cities, by Dr. Peter Austen, Engineering _Maga zine, No. 1, p. 95.) The Hyatt Filtering System, invented by Isaiah Smith Hyatt, of Morristown, N. J., coag ulates the impurities in the water and then filters it. The filter proper is simply a body of ordinary sea sand supported in a perforated false bottom, the whole being inclosed in a wrought-iron cylindrical vessel.
The filter is connected with the supply-pipe in such a manner that a by-pass is formed around the filter; or, in other words, it is so arranged that the filter may be disconnected without disturbing the flow of water through the main pipe. A small portion of the muddy water to be treated, not more than a fraction of 1 per cent of the total volume, goes through an attachment to the main filter, containing lumps of alum. A minute amount of alum is thus dissolved and passes into the filter, where it is mixed with the main body of water, the quantity of alum used being less than 1 grain per gal. of water. The suspended clay and other earthy matter which is of a basic nature, has the effect of precipitating the alumina of the alum, causing it to separate all through the water in the form of gelatinous flocks. These minute particles bring together, or coagulate, the finely suspended matter, converting it into such a form that the filter will easily and completely remove it. The supply of water to this coagulator is governed by a valve regulated by a scale, each division of which corresponds to a given quantity of alum dissolved. In consequence of this reaction, the minute amount of alum employed is entirely destroyed, as such, and is removed from the solution, the fine silt which could not otherwise be removed by filtration is converted into such a form as to be easily removable, and the resulting filtered water is perfectly bright and clear, no matter how dirty and muddy it may have been previously. For the purpose of cleaning the filter-bed, provision is made by which the current of water can be reversed, and the accumulation of dirt, etc., is removed through a special discharge-pipe.
Fig. 1 represents one of the largest filters of the Hyatt system. It is constructed of wrought iron and steel, with a capacity of 250 gals. per min., or 325,000 gals. per 24 hours. It is 10 ft. in diameter, 13 ft. high, and requires 392 bush, of filtering material. It is specially adapted for the requirements of large factories and industries where a great volume of water is used daily. The operation is as follows : The water enters the filter through the main inlet pipe below the partition and above the filtering material, passing downward and out through a system of cone-valves at the bottom, which are so constructed as to prevent the filtering material from escaping, and at the same time allowing the water to flow freely to the outlet pipe. When washing, the water passes from the inlet-pipe to the outlet-pipe, entering the filter at the bottom through the cone-valve outlet system and up through the filtering mate rial, agitating and loosening the same and producing pressure which causes the material to be discharged into the upper tank, which is always filled with water, through the 7 discharge pipes. The material, being heavy, settles immediately to the bottom, displacing the water which flows out through the waste-pipe, carrying with it all the arrested silt and impurities. After the material has all been discharged into the upper compartment it is allowed to settle hack into the lower chamber or filter proper, displacing the water in this compartment, which flows out through the lower waste-pipe.