Filter

“ 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. ”

“ In the best filters an automatic arrangement for the measurement of the supply to each separate filter, and for the regulation of the quantity within certain limits, is adopted, and the resistance at outflow is so arranged that not more than a certain head of pressure, about 21⁄2 ft., can under any circumstances come upon the surface film, while a depth of several feet of water is maintained over the sand. It is essential that during the working of the filter the water should be so supplied that it will not disturb the surface of the sand. ”

“ 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. ”

“ Now, from his experience of water filtration, Koch arrived at several important conclusions. In the first place, he maintained that the portion of the filter-bed which really removed micro-organisms effectively was the slimy organic layer upon the surface. This layer is produced by a deposit from the still unpurified water lying immediately above it. The most vital part of the filter-bed is this organic layer, which, after formation, should not be disturbed until it requires removal owing to its impermeability. ”

“ The error of too narrow a filter is a common one. It causes a delayed filtration, which means undesirably long contact of water and coffee and also the cooling of the liquid which in a correct, undelayed filtration is smoking hot at completion. The bag should also not be too long or be allowed to hang or soak in the liquid. A filter bag set tightly into a pot against its sides, thus surrounded with impenetrable walls, is greatly reduced in filtering surface, and the filtration is thereby slackened. ”

“ If the filter be a muslin bag, free on all sides, the filtering surface is considerable and permits the necessary quick passage of water through the grounds, provided the bag is of a wide enough diameter as to prevent too great a depth of grounds through which the water cannot quickly penetrate. ”

“ I noted that he was very particular in his examination of it, apparently being very careful to separate whatever it was he was looking for on the inside and the outside surfaces.
"That," he explained to me at length as he worked, "is what is known as a Berkefeld filter, a little porous cup, made of porcelain. The minute meshes of this filter catch and hold bacteria as if in the meshes of a microscopic sieve, just like an ordinary water filter. It is so fine that it holds back even the tiny bacillus fluorescens liquefaciens which are used to test it. ”

“ A cool tool for saving filter settings; a “Watchlisted pages” filter; a “Flagged Revisions” filter (for some wikis only) ; a “Last revision” filter. Tell us your ideas—but don’t wait: The team is focused right now in making changes to the New Filters beta. But in a few months, we’ll be moving on to other projects. So now is the time to give the new tools a try and then tell us what you think. What works well? What could make things better for you? ”

“ 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. ”

“ A small excess over that demanded by theory should be sufficient. [Pg 31]
Filtration.—Solutions are best filtered hot whenever the assay allows of this being so done. The precipitate should be allowed to settle, and the clear liquid decanted on the filter with the aid of a glass rod if necessary. The filter-paper must not be too large, but at the same time it must not be overloaded with the precipitate. There should be ample room for washing. For general use three sizes of filter-paper are sufficient. Common quick filtering-paper (English) is best for most work in assaying. ”

“ 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. ”

“ Much has been said as to the effect of variation in the rate of filtration upon the efficiency of filters. Experiments have been made at Lawrence and elsewhere which have shown that, as long as the maximum rate does not exceed a proper one, and under reasonable regulations, and with the filter in all respects in good [Pg 313] order, no marked decrease in efficiency results from moderate fluctuations in rate. There is probably a greater decrease in efficiency by stopping the filter altogether, especially if it is done suddenly, than by simply reducing the rate. ”

“ When a filter has been emptied, and is being re-charged, the water should be introduced from a neighbouring filter, and should pass upwards in the filter to be charged, until the surface of the sand has been covered. ”

“ THE WITNESS: Yes. Whatever type of signal gets through those filters, it would have to have -- the amount that got through the 90 filter and the amount that got through the 150 filter would have to be fairly constant, so that the difference between the two is constant, and the needle would deflect to a consistent value.
MR. PHILLIPS: In looking at your example of filter response versus frequency on the bottom of the chart, it would appear that approximately halfway in between the 90 and 150 hertz frequencies, at about 120 hertz, the filters would be the least selective, is that true? ”

“ 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. ”

“ In all precipitations the reagent should be added slowly, with constant stirring, and should be hot when circumstances permit. The slow addition is less likely to occasion contamination of the precipitate by the inclosure of other substances which may be in the solution, or of the reagent itself.
FUNNELS AND FILTERS Filtration in analytical processes is most commonly effected through paper filters. In special cases these may be advantageously replaced by an asbestos filter in a perforated porcelain or platinum crucible, commonly known, from its originator, as a "Gooch filter." ”

“ Bag in/bag out housings are specifically designed to allow for removal of a dirty air filter while minimizing worker exposure [Filtration Group Inc. 2012] . In these systems, a plastic liner is attached to a service port on the filter unit, as shown on the following page in Figure 19. When the filter is ready for replacement, the facility maintenance worker, wearing appropriate PPE, removes the filter into a liner. This process contains the filter with its contaminants so the worker is not exposed and the particulates are not resuspended in the workplace environment. ”

“ Filter.—A bee glass or a common flower-pot may soon be made into a filter. Thrust a piece of sponge into the hole in the bottom, and upon it lay a stratum of washed sand and powdered charcoal. Pass the water through it, and it will be purified, and saturated with oxygen at one and the same time. With the river-tank, the simplest way of reviving exhausted stock is wholly or partially to change the water; with marine stock, such a change is not easy, and the filter comes more legitimately into use. ”

“ Their presence invariably gives rise sooner or later to sub-surface clogging at the point of junction with the coarser sand, as has been found by repeated tests at Lawrence as well as in some of the Dutch filters where such layers were tried; and as there is no object in putting a coarser sand under a finer, the filter sand is best all of the same size and quality from top to bottom.
UNDERDRAINING.
The underdrains of a filter are simply useful for collecting the filtered water; they play no part in the purification. ”

“ The loss of head of water passing a freshly cleaned filter is comparatively slight, and the water of the second filter is allowed to fall a few inches below the high-water mark, at which level it will take the effluent from the other filter. The connections between the filters are made by siphons of large pipe, the summits of which are considerably above the high-water line. These siphons are filled by exhausting the [Pg 199] air, and when opened to the air there is no possibility of a flow of water through them. ”

“ A well-constructed filter, especially one in which sand is the medium, will practically remove all the bacteria. Generally speaking, a filter should be cleansed once a month in summer, and every two months in winter. ”

“ Whichever method is adopted, care should be taken to provide a valve or penstock on the connection to each separate filter, in order to be in a position not only to throw any filter out of work when desired, but also to regulate the rate of supply to each filter independently of the others. ”

“ 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. ”

“ The principal advantage of a trickling filter over other methods of treatment is its high rate which is from two to four times faster than a contact bed, and about seventy times faster than an intermittent sand filter. The greatest disadvantage is the head of 12 to 15 feet or more necessary for its operation. Sedimentation of the effluent is usually necessary to remove the settleable solids. ”

“ An artist who views color subjectively and is rarely familiar with the spectral basis may match a green leaf perfectly with a mixture of pigments. A photographic plate, a visual filter, or a spectroscope will reveal a difference which the unaided eye does not.
Some time before the Great War began, it occurred to the writer that colored filters could be utilized in aiding vision by increasing the contrast of the object to be viewed against its surroundings. [9] Studies were made of various filters, made with the object of the experiment in mind, in viewing the uniforms of various armies. ”

“ The cost of handling sand has been very greatly increased, because the filter is built in one bed, and all work upon it has to be done during the comparatively short intervals when the filter is not in use, an arrangement which is not at all economical in the use of labor. ”

“ 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. ”

“ Remove the filter holder plugs and attach the filter holder to the personal sampling pump with a piece of flexible tubing. Clip the filter holder to the worker's lapel. Air being sampled should not be passed through any hose or tubing before entering the filter holder.
(4) Prepare the field blanks at about the same time as sampling is begun. These field blanks should consist of unused filters and filter holders from the same lot used for sample collection. Handle and ship the field blanks exactly as the samples, but do not draw air through the field blanks. ”

“ To make a perfect filter, which should have the double action of arresting the finest suspended matter and removing the matters held in solution, and the whole to cost but little and capable of being made by any housewife, has long been an object of much attention, and, after many experiments and testing various substances in many combinations, the following plan is suggested as giving very perfect results, and costing only about 8s. ”

“ 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. ”