Filtration

“ In the filtration of sewage only the coarsest particles of suspended matter are removed by mechanical straining. The success of the filtration is dependent on biologic action. The desirable form of life in a filter is the so-called nitrifying bacteria which live in the interstices of the filter bed and feed upon the organic matter in the sewage. Anything which injures the growth of these bacteria injures the action of the filter. In a properly constructed and operated filter, all matter which enters in the influent, leaves with the effluent, but in a different molecular form. ”

“ Ordinary filtration will remove from one fourth to one third of the yellow-brown color of peaty water. A larger proportion can be removed by the addition of alum, which by decomposing forms an insoluble compound of alumina with the coloring matter, while the acid of the alum goes into the effluent either as free acid, or in combination with the lime or other base in the water, according to their respective quantities. ”

“ In those cases where there are no previous records showing the possibilities of the works and the influence of the local conditions, especially the character of the raw water, and until such information is obtained, it is to be taken as the rule that a satisfactory filtration will never yield an effluent with more than about 100 bacteria per cubic centimeter. ”

“ Different raw waters vary so much, and the requirements of filtration are so different, that it is not possible to outline any general procedure or combination of procedures, but each problem must be taken up by itself. ”

“ For domestic use the filter is attached by its open end to the water tap and the pressure from the mains forces the water through it. In laboratory uses, denser filters of smaller diameters are used, and the filter is surrounded by the fluid to be tested. The open end of the filter passes into a vessel from which the air is exhausted and filtration takes place from without inward. The test of the effectiveness of the filter is made by adding to the filtering fluid some very minute and easily recognizable bacteria and testing the filtrate for their presence. ”

“ Where the pollution is less, so that the dissolved oxygen contained in the raw water is sufficient for the oxidation of the organic matters, continuous filtration will give substantially as good results as intermittent filtration, and in other respects it has important advantages. ”

“ When the loss of head exceeds 4 feet the rate of filtration cannot be maintained at 3,000,000 gallons per acre daily with the outlet devices provided, and, in order to maintain the rate, the filter must be cleaned. ”

“ Perhaps the most important single condition is the rate, the numbers of bacteria passing increase rapidly with it. Next, fine sand and in moderately deep layers tends to give high efficiency. The influence of the loss of head, often mentioned, is not shown to be important by the Lawrence results, nor can I find [Pg 89] satisfactory European results in support of it. Uniformity in the rate of filtration on all parts of the filtering area and a constant rate throughout the twenty-four hours are regarded as essential conditions for the best results. ”

“ One of the first requirements of successful filtration is that the rate of filtration shall be practically the same in all parts of the filter. This is most difficult to secure when the filter has just been cleaned and the friction of the sand layer is at a minimum. If the friction of the water in entering and passing through the underdrains is considerable, the more remote parts of the filters will work under less pressure, and will thus do less than their share of the work, while the parts near the outlet will be overtaxed, and filtering at too high rates will yield poor effluents. ”

“ The difference in temperature and increased hardness of the spring water, determine the class of water.
FILTRATION is the artificial method of clarification by mechanical and chemical means. By the mechanical system heavier impurities are held in suspense by percolation of water through carefully prepared beds of sand, gravel, coke, shells, and like substances.
The total area of the London filter beds in 1874 was 68 acres, and rate of filtration per hour in inches and depth of water, or head on beds, were:
The efficiency of filtration is inversely to rate of flow. ”

“ I think every one who has carefully studied the recent history of water filtration in its relation to disease has been convinced that filtration carefully executed under suitable and normal conditions, even if not an absolute, is at least a very substantial protection against water-carried diseases, and the few apparent failures to remove objectionable qualities have been without exception due to abnormal conditions which are now understood and in future can be prevented. ”

“ The rapidity of filtration depends upon—the porosity of the filtering medium—the extent of the filtering surface—the relative viscidity or mobility of the filtering liquid—the pressure or force by which the liquid is impelled through the pores of the filter, and—the porosity and fineness of the substances it holds in suspension. The most efficient filter is produced when the first two or the first three are so graduated to the others that liquid filters rapidly, and is at the same time rendered perfectly transparent. ”

“ Each filter was to be controlled by itself, be independent of the others, and deliver its water into a pure-water reservoir lower than itself, so that it could never be affected by back-water, and so large that there would never be a demand for sudden changes in the rate of filtration. ”

“ The conditions necessary for the removal of bacteria and turbidity are very well understood, and it can be stated with the utmost confidence that no system of filtration through sand at rates many times as high as are used in ordinary sand filtration, and without the use of coagulants, will be satisfactory where either bacterial efficiency or clarification is required. ”

“ By filtration, water may be deprived of living beings and of all suspended impurities; but substances held in solution, cannot thus be separated. Ebullition destroys the vitality of both animals and vegetables; expels air, or carbonic acid, and causes the precipitation of carbonate of lime, but the water should be afterwards subjected to the process of filtration. Distillation, when properly conducted is the most effectual method of purifying water. ”

“ Numerous filters have been suggested, and a few have been constructed for the use of much higher rates of filtration than are usually employed with sand filters, but without the use of coagulants. The results obtained depend upon the requirements and upon the character of the raw water. If a reservoir water contains an algæ growth, it can often be removed by a coarse and rapid filter. The organisms in this case are many times larger than the bacteria, and many times larger than the clay particles which constitute turbidity. ”

“ Though less extensive than the experiments above mentioned, they are of historic interest as giving the first adequate demonstration of the possibilities of that method of purification.
The system of mechanical filtration, or the 'American System,' as it is sometimes called, differs from natural sand filtration by the use of alum or some similar coagulating substance before sedimentation and filtration, by the higher rate of filtration employed and by the use of certain mechanical devices for cleaning the sand beds. ”

“ In continuous filtration, which alone has been previously considered, the air is driven out of the sand as completely as possible before the commencement of filtration, and the sand is kept continuously covered with water until the sand becomes clogged and a draining, with an incidental aeration, is necessary to allow the filter to be scraped and again put in service. ”

“ In the removal of tastes and odors from pond or reservoir waters which are not muddy, but which are subject to the growths of low forms of plants, which either by their growth or decomposition impart to the water disagreeable tastes and odors, intermittent filtration may have a distinct advantage. In such cases there is often an excess of organic matter to be disposed of by oxidation, and the additional aeration secured by intermittent filtration is of substantial assistance in disposing of these matters. ”

“ If it be true that efficient sand filtration is a safeguard against pathogenic germs like typhoid and cholera, then there can be but one criterion of efficiency, viz., their absence in the filtered water, which can only be ascertained by regular examination. But it is not alone for pathogenic germs that filtration is proposed. Filtered water containing more than 100 micro-organisms of any kind per cc. is below the standard in purity, and should on no account be distributed for drinking purposes. ”

“ In connection with this plant is a small laboratory in which are made daily bacteriological examinations of the water before and after filtration. Physical, chemical and microscopical examinations are also made at frequent intervals. The results obtained indicate the amount of purification that is taking place, and they already have shown that the filter is rendering efficient service in protecting the community from water-borne diseases. ”

“ As a result of the first of these epidemics a series of experiments were made at Stralau by Fränkel and Piefke in 1890. Small filters were constructed, and water passed exactly as in the ordinary filters. Bacteria of special kinds not existing in the raw water or effluents were then applied, and the presence of a very small fraction of them in the effluents demonstrated beyond a doubt that they had passed through the filters under the ordinary conditions of filtration. ”

“ When, however, appliances of the character to be described in Chapter IV are used for the regulation of the incoming water, and with a steady rate of filtration, this provision becomes quite superfluous.
With open filters a depth of water in excess of the thickness of any ice likely to be formed is required to prevent disturbance or freezing of the sand in winter. ”

“ Its sole advantage99 over the Pasteur-Chamberland filter is the more rapid rate of filtration; and against this is to be set the greater liability to fracture and the lack of continuance of efficient filtration. Woodhead and Wood in the report already quoted, state: “The Berkefeld filter appears to have the largest pores among the efficient filters, as is evidenced by the fact that the water organisms were not apparently weakened, that more species of organisms appeared in its filtrate, and that lowering the temperature to 11° C. did not prevent their appearance. ”

“ The operation of household filters is necessarily, with rare exceptions, left to the kitchen-girl and luck. Scientific supervision is practically impossible. With a large filter, on the other hand, concentrating all the filters for the city at a single point, a competent man can be employed to run them in the best-known way; and if desired, and as is actually done in very many places, an entirely independent bacteriologist can be employed to determine the efficiency of filtration. ”

“ No more striking example could be found, demonstrating on a large scale the efficiency of filtration as a preventive of water-borne diseases than that of the cholera epidemic of Hamburg in 1892, yet, at the present writing, there are people holding public offices throughout the United States who do not believe in the value of filtration as a public prophylactic, or who are so indifferent as not to [113] advocate its adoption. ”

“ The mechanical treatment of water is commonly known as filtration, and there are several forms of public water-filtration plants now in use by American cities. The simplest is the system of slow sand filters in which the water is treated by allowing it to percolate slowly through a bed of sand and crushed stone, thereby becoming rid of noxious bacteria. ”

“ The third process of securing a good water-supply is by means of filtration of surface waters which would otherwise be unsuitable for domestic purposes. The methods of filtration, which it is the purpose of this volume to explain, are beyond the experimental stage; they are now applied to the purification of the water-supplies of European cities with an aggregate population of at least 20,000,000 people. In the United States the use of filters is much less common, and most of the filters in use are of comparatively recent installation. ”

“ The gas is lighted and the water jacket maintained at a high temperature until filtration is completed. If the steam steriliser of the laboratory is sufficiently large, it is sometimes more convenient to place the flask and filtering funnel bodily inside, close the steriliser and allow filtration to proceed in an atmosphere of live steam, than to use the gas ring and hot-water funnel. ”

“ The third epoch is marked by the experiments at Louisville, Pittsburg and Cincinnati, which have greatly increased our knowledge of the treatment of waters containing enormous quantities of suspended matter, and have reduced to something like order the previously existing confused mass of data regarding coagulation and rapid filtration. ”

“ The nitrifying action in sewage filtration is shown by the analysis of sewage passing through a trickling filter, as given in Tables 86 and 87. It is shown by the reduction of the content of organic nitrogen, free ammonia, oxygen consumed, and the increase in nitrites, nitrates, and dissolved oxygen. The reduction of suspended matter is interrupted periodically when the filter “unloads.” The suspended matter in the effluent is then greater than in the influent. ”

“ The economies which may possibly result by the use of an inferior filtration are comparatively small, and it is believed that in those American cities where filtration is necessary or desirable it will be found best in every case to furnish filters of the best construction, fully able to do what is required of them with ease and certainty. ”