Part 6

Silicated Carbon.--This was one of the earliest modifications of the simple carbon block. Figs. 13, 14 show respectively the forms adopted for downward and upward filtration. In the former, the stoneware receptacle is divided into two parts by a diaphragm upon which there is fixed, by a porcelain stay, a silicated carbon block, which entirely closes the apertures in the diaphragm. The upper surface and corners of the filtering block are non-porous, consequently the water has to enter at the edges and follow the course indicated by the arrows, before it can reach the clear water compartment below. In cleaning the filter, it is only necessary to unscrew the nut, when the block can be lifted out and soaked in boiling water, after which the surface can be scrubbed.

The ‘Army Medical Report’ says of filters employing carbon in porous blocks that “These are powerful filters at first, but they are apt to clog, and require frequent scraping, especially with impure waters. Water filtered through them and stored, shows signs of the formation of low forms of life, but in a less degree than with the loose charcoal. After a time, the purifying power becomes diminished in a marked degree, and water left in contact with the filtering medium is apt to take up impurity again, though perhaps in a less degree than is the case with the loose charcoal.” The advantages of combining silica with the carbon are not at first sight apparent.

Maignen combines charcoal with lime to produce a compound which he calls “carbo-calcis.” At the same time he employs an asbestos filtering cloth. The arrangement of his filter is shown in Fig. 15. The hollow, conical, perforated frame _a_ is covered with asbestos cloth _b_; _c_ is a layer of finely powdered carbo-calcis, deposited automatically by being mixed with the first water poured into the filter; _d_ is granular carbo-calcis filling up the space between _c_ and the sides of the containing vessel; _e_, unfiltered water; _f_, filtered water; _g_, tube for admitting air to aërate the water and correct the usually vapid flavour of filtered water. This filter has remarkable power; wine passed through it will come out colourless and tasteless. Moreover the cleansing and renewal of the filtering media are simple in the extreme.

Prof. Bernays, of St. Thomas’s Hospital, has taken out a patent for a new filtering material, consisting of charcoal combined with a reduced manganese oxide. The well-known purifying action of charcoal (animal and vegetable), which in its ordinary state is liable to certain difficulties and objections, is in this invention supplemented and improved by heating it in covered crucibles with 5 to 15 per cent. or more of powdered manganese black oxide (the mineral pyrolusite), together with a very small quantity of some fixed oil, resin, or fat. Having ascertained that the simple admixture of the manganese dioxide with the charcoal without previous heating had no utility as a filtering medium, and was even injurious by reason of the diminution of the porosity of the charcoal, Prof. Bernays devised the above method with the object of oxidising the hydrogen and other oxidisable impurities of the charcoal, and hence approximating it to pure carbon in a state similar in efficacy to platinum black rather than in its ordinary less powerful analogy to spongy platinum. The heating is of course out of contact with air, and the temperature sufficiently high to cause the reduction of the manganese dioxide at least to manganous-manganic oxide, which afterwards acts as a carrier of oxygen, and thereby much prolongs the purifying action of the medium. Another method of obtaining charcoal in combination with manganous-manganic oxide is to saturate charcoal with manganous chloride (or even manganese residues) and afterwards subject it to a strong heat in closed crucibles. The charcoal prepared in the above manner may be employed in the filtration of water in layers with sand and other filtering material in the usual manner.

A filtering material which has all the properties of animal charcoal, and is said to give higher results, is magnetic carbide, discovered by Spencer, many years ago, and consists of iron protoxide in chemical combination with carbon. It is considered that the purifying effect is produced by its power of attracting oxygen to its surface without the latter being acted on, the oxygen thus attracted being changed to ozone, by which the organic matter in the water is consumed.

There can be no doubt of the value of this filtering material. Its manufacture is very simple, as it is obtained by roasting hematite iron ore with granulated charcoal for 12 to 16 hours at a dull red heat, and used in a granular form. Another form for making this material is to heat the hematite (iron red oxide) with sawdust in a close vessel. The product is magnetic, and never loses its activity until the pores are choked up. The Southport Water Company formed their filtering beds of this material, and after years of use it is still giving satisfaction.

Iron.--From experiments made by allowing water to filter through spongy iron on to meat, it has been found that after 6 weeks the meat remained fresh. Another test was made by preparing a hay infusion, which was kept till it showed abundance of organic life. The infusion was filtered through spongy iron with layers of pyrolusite, sand, and gravel, and then was kept in contact with meat for many weeks. The meat showed no signs of putrescence. In some of the experiments filtered air was supplied, which proves conclusively that bacteria or their germs are not revived when supplied with oxygen after the filtration; this is a result of importance, as it demonstrates that by filtration through spongy iron, putrefaction of organic matter is not only suspended for a time, but that it ceases entirely until reinstated by some putrefactive agent foreign to the water. The peculiar action of spongy iron is believed to be thus explained. If a rod be inserted into a body of spongy iron which has been in contact with water for some time, gas bubbles are seen to escape. These are found to contain carbon and hydrogen, and experiments lead to the conclusion that the carbon is due to the decomposition of organic matter.

The material was introduced for filtration purposes some years ago by Prof. Bischof. His ordinary portable domestic filter consists of an inner, or spongy iron, vessel, resting in an outer case. The latter holds the “prepared sand,” the regulator arrangement, and the receptacle for filtered water. The unfiltered water is, in this form of filter, mostly supplied from a bottle, which is inverted into the upper part of the inner vessel. After passing through the body of spongy iron, the water ascends through an overflow pipe. The object of this is to keep the spongy iron, when once wet, constantly under water, as otherwise, if alternately exposed to air and water, it is too rapidly oxidised.

On leaving the inner vessel, the water contains a minute trace of iron in solution, as carbonate or ferrous hydrate, which is separated by the prepared sand underneath. This consists generally of 3 layers, namely, commencing from the top, of pyrolusite (manganese black oxide), sand, and gravel. The former oxidises the protocompounds of iron, rendering them insoluble, when they are mechanically retained by the sand underneath. Pyrolusite also has an oxidising action upon ammonia, converting it more or less into nitric acid.

The regulator arrangement is underneath the perforated bottom, on which the prepared sand rests. It consists of a tin tube, open at the inner, and closed by screw caps at its outer end. The tube is cemented water-tight into the outer case, and a solid partition under the perforated bottom referred to. It is provided with a perforation in its side, which forms the only communication between the upper part of the filter and the receptacle for filtered water. The flow of water is thus controlled by the size of such perforation. Should the perforation become choked, a wire brush may be introduced, after removing the screw cap, and the tube cleaned. Thus, although the user has no access to the perforation allowing of his tampering with it, he has free access for cleaning. Another advantage of the regulator arrangement is that, when first starting a filter, the materials may be rapidly washed without soiling the receptacle for filtered water. This is done by unscrewing the screw cap, when the water passes out through the outer opening of the tube, and not through the lateral perforation.

Various modifications had, of course, to be introduced into the construction of spongy iron filters, to suit a variety of requirements. Thus, when filters are supplied by a ball-cock from a constant supply, or from a cistern of sufficient capacity, the inner vessel is dispensed with, as the ball-cock secures the spongy iron remaining covered with water. This renders filters simpler and cheaper.

As the action of spongy iron is dependent upon its remaining covered with water, whilst the materials which are employed in perhaps all other filters lose their purifying action very soon, unless they are run dry from time to time, so as to expose them to the air, the former is peculiarly suited for cistern filters.

Cistern filters are frequently constructed with a top screwed on to the filter case, by means of a flange and bolts, a U-shaped pipe passing down from this top to near the bottom of the cistern. This tube sometimes supplies the unfiltered water, or in some filters carries off the filtered water, when upward filtration is employed. This plan is defective, because it practically gives no access to the materials; and unless the top is jointed perfectly tight, the unfiltered water, with upward filtration, may be sucked in through the joint, without passing at all through the materials. This is remedied by loosely surrounding the filter case with a cylindrical mantle of zinc, which is closed at its top and open at the bottom. Supposing the filter case to be covered with water, and the mantle placed over the case, an air valve is then opened in the top of the mantle, when the air escapes, being replaced by water. After screwing the valve on again, the filter is supplied with water by the siphon action taking place between the mantle and filter case and the column of filtered water, which passes down from the bottom of the filter to the lower parts of the building. These filters are supplied with a regulator arrangement on the same principle as ordinary domestic filters. The washing of materials, on starting a filter, is easily accomplished by reversing 2 stop-cocks, one leading to the regulator, the other to a waste pipe.

The use of spongy iron has now been applied on a large scale to the water obtained from the river Nette, for the supply of the city of Antwerp. Dr. Frankland has visited the Antwerp Waterworks at Waelheim, about 15 miles above that city, and reported on the result of his inquiry. He attaches especial value to the fact that spongy iron filtration “is absolutely fatal to _Bacteria_ and their germs,” and he considers it would be “an invaluable boon to the Metropolis if all water supplied from the Thames and Lea were submitted to this treatment in default of a new supply from unimpeachable sources.”

Many preparations of iron have long been known to possess a purifying influence on water containing organic impurities. Thus Scherer, years ago, recommended a solution of iron sulphate where the impurities were present in large quantity. Later still, iron chloride was proposed as suitable, the salt being precipitated in the presence of organic matter as ferric oxide, the oxide thus formed acting also mechanically on the suspended impurities in course of precipitation, very much as white of egg acts in clarifying liquids, when it coagulates and carries impurities with it to the bottom. Other iron preparations have a similar action, notably dialysed iron, while several oxidising agents, such as potash permanganate, are also well known to possess a powerful effect on organic impurities. It will at once be seen, however, that all such substances are inadmissible as filtering media, or purifying agents for potable waters, for the reason, that in the case of some at least of the agents mentioned, decompositions take place, which in themselves might prove dangerous, while in the case of all an excess (and it would be almost impossible to avoid an excess) of the purifying agent would be equally bad, and would render the water quite unfit for domestic purposes. It has been found, however, that various kinds of native rock containing iron protoxide effect the filtration of water very completely, and Spencer, acting on this idea, after experimenting, found that when the iron protoxide was isolated as magnetic oxide, it both freed the water from turbidity and effected decoloration very quickly. Thus bog-water, as dark as porter, when filtered through it speedily lost its colour and became clear and sweet, the carbonic acid given off during the process of decomposition rather tending to improve the water. The purifying power of the magnetic oxide does not deteriorate with use. The oxide gets coated with a slimy deposit, owing to the deposition of decomposed organic matter, but this being removed, it is as powerful as ever in its purifying action. Unfortunately this iron rock is not found native to any extent, but the fact of its action being determined, Spencer continued his experiments with the result that it can now be produced artificially, and forms one of the most efficient and useful filters for domestic purposes.

Metallic iron is employed by Jennings & Hinde. The filtering material consists of fine iron or steel shavings, filings, turnings, or borings obtained from the swarf or skin of cast iron, wrought iron, or steel; this material may either be used by itself, or it may be used with other materials, either mixed with them or in separate layers. The iron or steel shavings, &c., are obtained from iron or steel that has been brought to a state of fusion either by melting or the processes necessary for making cast iron, wrought iron, or steel, and being separated from many of the impurities contained in the ore from which it was obtained, will have but a comparatively small portion of earthy impurities mixed with it, and will be for this reason superior to iron which is obtained from native ores or oxides without fusion.

By filtering water through small divided swarf or skin of cast iron, wrought iron, or steel, free oxygen will be withdrawn from the water, and consequently any insects or animalculæ contained in the water will be deprived of life, and any germs contained in the water will be deprived of the oxygen necessary for their development and life, and the water will be consequently purified and rendered wholesome. A convenient way of forming a filter is to use a layer of the turnings, shavings, &c., together with layers of other filtering material resting upon a perforated partition placed across a closed vessel. The materials are cleaned by boiling them in hot water with a small quantity of ordinary washing soda, to remove any oil or grease that might accidentally be associated with the materials above mentioned. Afterwards the iron borings should be well washed before being put into the filter. The filter vessel may be of any ordinary construction and shape. If sand is used in conjunction with the above-mentioned materials, it is preferable to place some of the sand at the bottom of the filtering vessel, and the iron or steel materials, or both, over the sand, and then more sand over them. These materials are disposed so that they may be partially separated from each other by perforated plates of earthenware, glass, or other suitable material. But this partial separation, though convenient, is not essential, as the perforated plates may be dispensed with and the material placed over and under each other in layers without plates to separate them.

Porous Pottery.--Chamberland has found that the liquid in which microbes have been cultivated becomes absolutely pure if passed through unglazed porcelain. Its purity can be demonstrated by mixing it with liquids sensitive to the action of microbes, such as veal broth, milk, and blood, in which it produces no alteration.

A tube _a_ (Fig. 16) of unglazed porcelain is enclosed in another _b_ of metal, and the water to be filtered is admitted to the space between the two by turning a stop-cock. Thence it slowly filters through to the inside of the porcelain tube, and flows out at the bottom. Under a pressure of 2 atmospheres, or 30 lb. to the sq. in., a tube 8 in. in length, with a diameter of 1 in., will yield about 5 gal. of water daily. For a larger supply, it is only necessary to increase the size or the number of the tubes.

In cleansing the filter, the porcelain tube is removed, and the microbes and other matter that have accumulated on the outer face of it are brushed off. The tube may also be plunged in boiling water in order to destroy any germs that may be supposed to have penetrated beneath its surface; or it may be heated in a gas jet or in a furnace. In fact, it can be more readily and more thoroughly cleaned than most of the domestic filters in ordinary use.

It is interesting to remark that some of the earliest filtering vessels of which we have any knowledge are simply made of porous earthenware. After all our modern researches after antiseptic filtering media, we are reverting to the ways of our remotest forefathers.

Filtering Cisterns.--The following is a description of a filter which purifies foul water from organic impurities held in solution as well as from suspended solids. Take any suitable vessel with a perforated false bottom, and cover it with a layer of animal charcoal, on the top of that spread a layer of iron filings, borings, or turnings, the finer the better, mixed with charcoal dust; on the top of the filings place a layer of fine clean siliceous sand, and you will have a perfect filter. Allow the foul water to filter slowly through the above filter, and you will produce a remarkably pure drinking-water. Before placing the iron filings in the filter, they must be well washed in a hot solution of soda or potash, to remove oil and other impurities, then rinse them with clean water; the filings should be mixed with an equal measure of fine charcoal. If the water is very foul, it must be allowed to filter very slowly. The deeper the bed of iron filings is the quicker they will act.

In Bailey-Denton’s cistern filter, the principal novelty is that it runs intermittently, and thus allows the aëration of the filtering material, and the oxidation of the impurities detached from the water. The oxidation is effected by the perfect aëration of the filtrating material, which may be of any approved kind, through which every drop of water used in the kitchen, bedrooms, and elsewhere must pass as it descends from the service cistern for use. As water is withdrawn from this filter, fresh water comes in automatically by the action of a ball-tap; and this fresh water immediately passes through the aërated material into a lower chamber, forming the supply cistern of filtered water for the whole house. The advantages claimed for the filter are that it secures pure water for the whole house. It is attached by pipe to, but is distinct from, the service cistern; it can be placed in any part of the house, and it cannot get out of order. Any approved filtering material may be used, and being aërated between each passage of water through it, oxidation is made certain.

A slate or iron cistern and filter combined may be made by dividing the cistern with a vertical partition perforated at the bottom, and placing in the half of the cistern which receives the water, a bed of filtering material, say 6 in. of gravel at the bottom, 6 in. animal charcoal in granular form in the middle, and 6 in. clean sharp sand at the top, covering all by a perforated distributing slab.

Fig. 17 illustrates a method of preparing an ordinary house cistern for filtering. The pipe and fittings should be of galvanised iron; black or plain iron is better as long as it lasts, as it rusts fast; in either case it is better to waste the water first drawn, for the water absorbs both the zinc and the iron when standing overnight. The zinc is not healthy, and the taste of the iron is unpleasant.

The perforations should equal 3 or 4 times the area of the suction pipe, which in ordinary cisterns may be 1¼ in. pipe, while the branches may be ¾ in. pipe. The holes, if ⅛ in., should number at least 200, distributed along the lower half of the pipes. Smaller holes are preferable; of 1/16 in. holes, 800 will be required.

For the filtering material we recommend a layer of fine gravel or pebbles for the bottom, 3 or 4 in. in depth, or heaped up over the perforated pipes; upon this a layer of sharp, clean sand, 9 in. in depth; upon this a stratum of pulverised charcoal, not dust, but granulated to size of peas or beans, or any of the material above mentioned, 4 in. deep; and upon this a stratum of fine, clean sand 6 to 12 in. in depth.

Such a filter should be cleansed at least twice in a year by pumping out all the water, taking out the mud or settlings, and one-half the depth of the top layer, and replacing with fresh sand.

The double filter cistern, Fig. 18, has much to recommend it, having a large receiving basin which in itself is a filter placed in a position for easy cleaning. The recess at the bottom may be covered with a perforated plate of galvanised sheet iron, upon which may be laid a filter bed of gravel, sand, charcoal, spongy iron, and sand in the proportions as stated above. This enables the frequent cleaning by removing the top layer of the filter bed without disturbing the water supply. The cover should fit tight enough to keep out insects and vermin.

A double-bottomed basin perforated and filled with clear, sharp sand and charcoal should be attached to the bottom of the pump pipe, as shown.

This enables the small filter to be drawn up and cleaned, without the necessity of emptying the cistern or interrupting the water supply.

The half barrel or keg filter, as illustrated in Fig. 19, is a convenient form of cistern filter where filtered water is required from cisterns already filled.

This is also a convenient form for readily cleaning or changing the filter without the necessity of discharging the water from the cistern.

This filter can be made from an oak keg or half barrel, such as is used for liquors or beer. Take out one of the heads and cut away the edge, so that it will just drive into the end of the keg, fasten 2 battens of oak across the head with oak pins left long enough to serve for legs for the filter to rest upon.