PROGRESS OF FILTRATION
A copious and pure supply of water became a vital necessity as soon as men gathered together in communities, and ancient history teems with more or less vague allusions to methods of disease prevention and toward securing a satisfactory water supply. Moses insisted upon burying the camp refuse and excrement beyond the confines, and Elizah, in King II, selected the site for the Israelitic town and purified the river water then by coagulation. The Roman practice of providing reservoir which they called “Breathing places,” where the water in passing from the surface to the consumer rested, really subsided, was somewhat aerated and acquired a degree of purification, anticipated our present system of settling basins, and the Hindoo and Egyptian practice of using aluminiferous clays and even alum to precipitate the impurities of the said water in vessels foreshadowed our present coagulating methods—so essentially a part of mechanical filtration. This coagulation we express by chemical formula and prove its harmlessness through chemistry of which they knew very little, as for example: (1) Where alum is used:
Where sulphate of alumina is used:
When alum or sulphate of alumina is introduce into waters containing alkaline carbonates, a reaction occurs, changing the introduced soluble sulphae into an insoluble hydrate and this hydrate and its faculty of aggregating together the suspended impurities with which it comes into contact into masses of greater specific gravity than the water itself causing subsidence or precipitation is the basic principle upon which rests Mechanical Filtration particularly and slow filtration as a prop to remedy its failure as an independent process of purification.
As the communal habit grew and men grouped themselves into steadings, villages, towns and cities it became imperative to protect the water supply against disease propagation, and as these communities grew in wealth and refinement the demand for clean, clear water grew in proportion, all manner of schemes for protection by prevention have proven abortive and it is generally recognized by intelligent races that it is necessary to cleanse and purify the supply through filtration and sterilization to ensure safety. In ten more years, London, England, can celebrate the centennial of the first municipal water filtration plant ever built so far as the record shows. It was of the slow sand bed type sometimes called the English system of filtration and did not differ in principle from the slow sand plant now in use at Providence, R. I.; Pittsburgh, Pa.; Philadelphia, Pa., and Washington, D. C. Those responsible for their construction did not expect more than the removal of the mud and suspended matter. It was not then known that bacteria were themselves in suspension and would be removed in more or less exact ratio to the removal of the other suspended matter. Typhoid fever w-as discovered 20 years later, 1829, and it was suspected that water carried and disseminated this disease. In 1893 the cholera outbreak at Hamburg, Germany, where the waters of the Elbe, unpurified, constituted the supply and the remarkable fact that Altoona nearby, using the same source of supply—filtered—passed through the same period immune, drew the attention of the scientific world and established the value of Filtration as a preventative against water-born epidemics. In 1875, the first slow sand filter was built in America, at Poughkeepsie—and in 1885, the first municipal filter plant of the Mechanical type was installed at Somerville, N. J. This was a pressure plant substantially as shown by Figure J.
Although the germ theory so far as water was concerned was partially believed in, it was qot fully accepted by other than such scientists as Koch and his school, until the discovery of cholera spirillium in the water of the Ganges in India, by Koch, in 1883, and what, perhaps, is more to the point culture methods, whereby the living growing organisms taken from water on plates were developed so as to be observed, counted and differentiated. It is now firmly established that impure water docs carry and distribute pathogenic germs and that certain epidemical diseases, e. g., typhoid—are largely due to use of such contaminated supplies. Mechanical filtration became a factor in the purification of public water supplies following the issuance of the Isiah Hyatt patent in 1884. This covered the use of a coagulating salt as applied to a running supply of water passing to a filter, and introduced prior to filtration. This is known as the alum process patent, though it was not confined to any particular salt capable of coagulating water. The two methods of filtration, slow sand and the mechanical or rapid system of filtration, have been in competition since that date. Before separating them and going into the greater merit of the mechanical system, I will show you the two, side by side, or rather together on the same supply at York, England. Figures C, E and H.
The accepted rate at which slow sand beds are to be operated is taken at 3,000,000 gallons per acre per day, while the rate advocated for the rapid or mechanical filter is 125,000,000 per acre per day, so that the mechanical type operates more than 40 times as fast as the slow sand filter, consequently, occupies very much less land, a matter of great consideration for a plant necessarily located near a growing city.
The slow sand beds were designed to be hand cleaned, the beds were to be hand raked and the impurities conveyed by labor to a place where the dirty sand could be cleaned and replaced or wasted. Some more or less successful attempt at minimizing this crude method by the use of apparatus of the scraper and scoop idea have been made, but the fundamental idea of the slow sand system was, and is, to clean the beds by hand methods. From the start and to-day. Mechanical filters not only derives its name, but depends on mechanical means of cleaning the sand forming the filtering media; several methods are applied for this purpose as you will see from the cuts. Figures D, K and L. Briefly by the use of revolving rakes, assisted by the upward volume of pure water through the underdrains or strainer system—up through the sand and flushing the dislodged accumulation of impurities into the gutters provided and thence to the sewer. By air and water under pressure upward in like manner, or by water alone under high velocity, a vertical rise of 24 inches or more. The East Providence Mechanical Filter Plant installed under the supervision of Edmund B. Weston, C. E., of Providence, was the subject of much debate and was carefully observed by the Rhode Island Board of Health, the President, Dr. Swarts, evincing the greatest interest and freely testifying to the success of the process. The results compiled by E. B. Weston have been distributed through several edition throughout the civilized world and were really the first tabulated results of mechanical filtration scientifically presented and accepted. The East Providence plant now enlarged is of the subsidence tank type with agitators or rakes.
While in this country the rectangular shaped steel reinforced concrete unit of filtration washed with air and water or water alone has largely replaced the circular wooden type it is proper to record the fact that no better efficiency has been reached by any type of filter. As an example of the most modern construction in Rhode Island, of the rectangular concrete type, Bristol, Warren and Newport are noteworthy. Figures B and I.
Nowhere outside of Rhode Island is there so good an example of the several types of Mechanical Filtration removing color from practically the same highly colored supplies and the practical filter man has greatly benefitted by experience in this state—as against theoretical conclusions. A cut is herewith shown giving the results physically obtained in Beakers, Figure G. The same reactions and results occur on a large scale within the open or enclosed coagulating basins, when the alumnia salt is applied to the water of a municipal supply passing to mechanical filters. The next cut discloses a section through a typical filter plant of the concrete rectangular type. Figure A.
It is a fact fully proven and accepted that the reaction between the applied alumina salt and the alkaline carbonates in the water fullv decomposes the applied salt or sulphate, which is soluble, into the insoluble Hydrate of Alumina and no trace of the original sulphate injected into the water can be obtained by the most delicate chemical tests known. Figure F.
So that the effluent or filtrate through the system of mechanical filtration contains no added chemical and in fact has less than is in the original water as supplied to the filter. In general practice, an average of one to two grains of sulphate of alumina is applied to each gallon of water to be purified. In the less turbid waters of the eastern portion of our country this average is greatly reduced. It requires about seven parts of Alkalinity to decompose each grain of sulphate of alumina. The waters of the United States range in native alkal.nity from seven parts of alkalinity to two hundred parts per million.
There are about six hundred municipal mechanical filtration plants in the United States and Canada, and about twelve slow sand filter plants. No new slow sand filter plants have been built for cities or municipalities since 1910; while over one hundred rapid sand or mechanical filters have been installed. Some slow sand plants have been dismantled and converted or rebuilt into mechanical filters, as e. g., Rock Island, Illinois. Every other slow sand plant in the United States except Providence, R. I.; Lawrence, Mass., and New Haven, Conn., has needed and accepted some essential feature of mechanical filtration such as coagulation, preliminary filters of the mechanical type, etc., though the original excuse for the excessive expenditures in land and construction was to avoid the use of these accessories.