Filter (water treatment)

“ The reason for this is simply that a material open enough to allow water to pass through it rapidly is not fine enough to stop such small bodies as the bacteria. The filters which are so often sold as “germ-proof,” consisting of sand, animal charcoal, wire-cloth, filter-paper, etc., do not afford protection against any unhealthy qualities which there may be in the raw water. Animal charcoal removes color without retaining the far more objectionable bacteria. ”

“ If the rate of filtration is not moderately fast, the consumer will not buy. The maker realizes this, and slights efficiency in order to gain a more rapid flow. The filtering material is made coarser or more porous, and, with the resistance decreased, the water passes through more readily, and so do the bacteria.
“No filter will continuously yield a perfect effluent without being often cleaned. Whatever a filter takes out of the water remains in the filter until removed by cleansing; and if that cleansing be delayed the filter will dispose of the accumulated filth by way of the passing water. ”

“ 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. ”

“ The supply for drinking purposes is often purified by means of domestic filtration. This is especially desirable with cistern water. Domestic filters should act not only as strainers by removing suspended impurities, but they ought also to act chemically by oxidizing a part or all of the dissolved organic matter. Various materials are used for domestic filters, amongst them being sand, sponge, flannel, cotton, animal charcoal and spongy iron. Nothing is more erroneous than the supposition that a filter, once started, will continue to act, without further attention, forever. ”

“ 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. ”

“ In addition, the water filtered will in a great majority of cases have enough air dissolved in itself to produce whatever oxidation there is time for in the few hours required for it to pass the filter, and it is only at very low rates of filtration that intermittent filters produce effluents of greater chemical purity than by the ordinary process. The yellow-brown coloring matter present in so many waters appears to be quite incapable of rapid nitrification ”

“ In the country, or in towns that have no water mains, it will be easy to devise an arrangement for giving the necessary pressure. An increase in the porosity of the filtering tube is not to be thought of, as this would allow very small germs to pass. This filter being a perfect one, we must expect to see it soil quickly. Filters that do not get foul are just the ones that do not filter. ”

“ The efficiency of filtration under various conditions has been studied by a most elaborate series of experiments at Lawrence with small filters to which water has been applied containing a bacterium (B. prodigiosus) which does not occur naturally in this country and is not capable of growing in the filter, so that the results should represent only the bacteria coming through the filter and not include any additions from the underdrains. ”

“ Buck says, "There is no material known which can be introduced into the small space of a tap-filter and accomplish any real purification of the water which passes through at the ordinary rate of flow."
The various complicated closed filters, filled with any material which can not be removed for cleansing, condemn themselves. No amount of pumping water through them at different angles, which is at all likely to be used, can cleanse them of the impurities that adhere to the mass and in the pores of the filtering material used. ”

“ With a leaky bottom there is either a loss of water when the water in the filters is higher than the ground-water, or under reverse conditions, the ground-water comes in and mixes with the filtered water, and the latter is rarely improved and may be seriously damaged by the admixture. And with very bad conditions water may pass from one filter to another, with the differences in pressure always existing in neighboring filters, with most unsatisfactory results. ”

“ 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 filter serves to retain any solid impurities which may still remain suspended in the water; but it will be found that the escaping water is nearly pure.
The dark brown fatty acid is mixed with a large amount of impurity, such as short wool fibers, burrs, sand, and dye stuffs washed from the wool. To remove water more completely, the semi-fluid mass is pumped from the tank, and delivered into hair-cloth filters; the liquid which drains from these bags finds its ways to the sand filters joining the drainage which formerly passed out from the tank through the sluice. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ The use of preliminary filters, which involves a large investment, is not always to be commended, particularly because at times of reasonably good water the removal of some of the organic matter is really injurious and lessens the effect of the final filters. ”

“ Under the most favorable conditions it is unwise to discharge the effluent of a sewage filter in the near vicinity of a source of water supply. Under farm conditions filters are usually neglected or the sewage is improperly applied, resulting in the clogging and befouling of sand filters and the discharge from stone filters of an effluent which is practically as dangerous and even more offensive than raw sewage. ”

“ When the turbidity of the applied water is not too great it is entirely removed in the course of filtration. With extremely muddy raw waters, however, turbid effluents are often produced with sand filters. The conditions which control the passage of the finest suspended matters through filters have been studied by Mr. Fuller at Cincinnati at considerable length. They are similar in a general way to the conditions which control the removal of bacteria. That is to say, the removal is more complete with fine filter sand than with coarse sand; with a deep sand layer than with a shallow sand layer ”

“ It is a recognized fact that many diseases are contracted through drinking impure water, yet many are so careless as not to take the simple means of removing this danger. It only requires boiling the water to destroy the germs. This, however, does not remove the foreign matter, such as decayed vegetable growth and other substances, therefore it is well to filter as well as to boil water. Many good filters are made which are cheap and easy to clean. The Gate City Stone Filter is perhaps the simplest one, being an earthen crock with a porous stone bottom. ”

“ The rates of filtration used in the experiments have ranged from less than 100 to about 130 million gallons per acre daily. To employ a rate much higher than this involves the use of a much coarser sand, or an increase in the height of water upon the filter to an impracticable extent. There would seem to be no material advantage in the use of lower rates within certain limits, while the cost of filters would be greatly increased. ”

“ 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. ”

“ The operation of a filter is to be regarded as satisfactory when the filtrate contains the smallest possible number of bacteria, not exceeding the number which practical experience has shown to be attainable with good filtration at the works in question. ”

“ 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. ”

“ Filtration.—The object of filtration is to remove the impurities of water. The most dangerous impurities are suspended in it, especially the microbes causing infectious diseases. Hence the most perfect filter is the one which most completely prevents the passage through it of microbes. If the water supply is pure, domestic filtration is not only useless, but likely to do more harm than good. ”

“ Intermittent filtration may be considered in those cases where it is proposed to use a water polluted entirely beyond the ordinary limits, and for waters containing large quantities of decomposable organic matters and microscopical organisms ”

“ Most people imagine that by once going to the expense of a filter they have secured for themselves a safeguard which will endure throughout all time without further trouble. No mistake could be greater, for without preserving constant watchfulness, and bestowing great care upon domestic filtration, it is probable that the process will not only entirely fail to purify the water, but will actually render it more impure than before. ”

“ A housemaid's pail with its three or four gallons will come once in ten or fifteen minutes, so that the filter is always being emptied, and as the water drains off the fresh air follows it. The water of a fixed bath is practically clean, and gives the filtration gutter a vigorous stir, which does nothing but good. The domestic intermissions are invaluable. When sewage is collected in a tank and is then allowed to flow without intermission for six hours at a stretch, it is doubtful if the greatest purifying power is obtained from the filter. ”

“ Household filters may be used to remove mud or iron rust from the tap water, or to remove the bacteria in case the latter is sewage-polluted, or to do both at once. Perhaps oftener they are used simply because it is believed to be the proper thing, and without any clear conception either of the desired result or the way in which it can be accomplished. I shall consider them only in their relations to the removal of bacteria, as I credit the people who employ them with being sufficiently good judges of their efficiency in removing visible sediment. ”

“ 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. ”

“ When, however, a water is to be used for a domestic water supply and transparency is an object, the still finer particles which would not show themselves in paper, but which are still able, in bulk, to render a water turbid, should be as far as possible removed, thus necessitating a finer filter; and, when there is reason to think that the water contains the germs of disease, the filter must be fine enough to remove with certainty those organisms so extraordinarily small that millions of them may exist in a glass of water without imparting a visible turbidity. ”

“ The sand was washed by the use of a reverse current, the sand being stirred by a revolving rake at the same time. The amount of coagulant employed was about 0.7 of a grain per gallon. The raw water was practically free from turbidity, and the filter was operated to remove color and bacteria.
The removal of color, as stated in Mr. Weston’s report, amounted to from 70 to 90 per cent. The experiments extended over a period of ten months. The rate of filtration employed was about 128 million gallons per acre daily. ”