At first, stirring was considered indispensable, but it was found that by taking 'an excese of a mixture of magnesia hydrate, with a proper substratum serving as a filtering medium through which the water could pass continuously, the desired effect was obtained without any trouble. When proportionate quantities of finely powdered magnesia oxide and sawdust are mixed with water it will be found that, under the action of heat, magnesia hydrate is formed throughout the whole mass. After cooling, the magnesia hydrate will be discovered so firmly united with the sawdust, so to speak crystallised into it, that it cannot be removed by mechanical means.
This preparation thus possesses the quality of filtering water in a high degree. By tightly filling cylinders of metal with this mixture, and forcing dirty water through, the water, it is said, leaves the first cylinder not only deprived of all lime, but quite clear, the lime carbonate crystallising directly upon the sawdust. The action is so rapid that even water saturated to the fullest extent with lime or gypsum leaves the apparatus with these substances perfectly removed, after 10 minutes' action.
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. 159) of unglazed porcelain is enclosed in another b of metal, and the water to be filtered is admitted to the apace 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 Fro. lis. 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 peuetrated 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.
One familiar form of cellulose as a filtering agent is that of the ordinary laboratory filter-paper. The same may be used on a larger scale by mashing it into a palp with the liquid to be filtered, and allowing it to deposit on a filter cloth automatically, as in the filters made by Maignen for industrial purposes.
This or a similar material is employed by Piefke at the Berlin Waterworks.
The filter shown in Fig. 160 consists of a wrought-iron casing « containing a number of perforated brags b, which form the bottom of flat bell-shaped cast-iron vessels c, the whole grouped one above the other inside the casing. The perforated brags b, rises till it overflows at the outlet. The filtering medium is chemically prepared cellulose or vegetable fibre, and is variously treated according to the purpose for which the filtered water is to be used, or, in other words, according to the degree of purity required in the filtered water. Its price varies accordingly; the beat quality is charged at 75s.. per cwt., and a filter capable of purifying 1000 gal. of water per hour requires, for its first charge, about 3 lb. of filtering material. To charge the apparatus the fibre is mixed with water to a thin paste and admitted through the funnel, when it deposits in an even layer over the perforated surfaces b, and the filter is then quite ready for action. After about 1200 gal. of water have been purified per square foot of filtering area, the latter requires cleaning or washing out; this is performed in a very simple manner by charging the filter with water in the usual manner, and at the same time slowly rotating the vertical spindle /, which carries the scrapers g, and by means of which the filtering material is suspended in the water, the latter washing out the impurities.
As soon as the water runs clear again, the rotary action is stopped and the tap h, on the bottom of the casing, is opened to allow the water to run oil; and the filtering material to settle, when the filter is again ready for use. The quantity of water which may be filtered before it becomes necessary to clean the fibre depends, of course, largely on its state of impurity, and it is advisable to use as a guide the pressure required to force the water through the filter. This should not exceed 3-4 ft. of water pressure, and it is therefore best to place the funnel about that height above the overflow. At each cleaning a small quantity of filtering material is naturally washed away with the impurities; this amounts to about 10 per cent., which quantity should be replaced by admitting it with the water. For the purpose of washing out the filter it is not necessary to use filtered water, nor is water of any particular pressure required; it may be simply charged through the delivery pipe. If at any time it becomes desirable to entirely empty the filter of the filtering material, water is charged through the delivery pipe or into the open vessel, and the tap i, at the bottom of the supply pipe d, is opened, when the fibre will run out with the water.
The apparatus can be recharged as described above, and for the complete operation of cleaning one filter, one man only is required for about 10 minutes. The filter is recommended by the manufacturers for purifying water for all purposes; a small size measuring only 9 in. in diameter and 15 in. high inside, and carrying only about 1 oz. of filtering material, is specially manufactured as a portable filter for military purposes, capable of filtering over 80 gal. of water per hour.
The problem of constructing a filter for steam users and manufacturers that should be able to deal with large quantities of muddy river and canal water, and should at the same time be capable of being easily and efficiently cleaned, has been solved by the adoption of an elastic filtering material, which when compressed forms a compact bed through which the water percolates, but when released immediately expands, freeing itself from the accumulated dirt, and offering little resistance to the flushing current that is then sent through it in the opposite direction. The material employed is sponge contained in a cylinder, and normally compressed between the cylinder end and a piston. While the cleansing operation is being conducted, the piston is alternately raised and lowered, the action on the filtering medium being similar to that ordinarily adopted in washing a soapy sponge; it is first allowed to absorb water until the pores are filled, and then the water is squeezed out, carrying a part of the mud with it, the process being continued until the effluent water is clear.
A successful installation of these filters is in operation at the works of Garton, Hill & Co., saccharum manufacturers, Battersea, London, where it is supplying feed water for 8 boilers, each 30 ft. by 7 ft. When the river is particularly muddy, the effluent water from the filter is bright and clear, and as far as appearance goes, is similar to the company's water, which is also drawn from the Thames, though of course at a higher part of the river, and has been submitted t6 an elaborate process of settling, and filtration by sand beds. The immense quantity of mud eliminated by the filter is made manifest as soon as the cleaning process is commenced, when it pours out in a thick stream, gradually becoming clearer and clearer until the turbidity ceases. It is not contended that sponge has any power to extract the soluble impurities contained in water, or to counteract the ill effects of contamination by sewage. All that is claimed for the filter is that it will rapidly cleanse large quantities of muddy water sufficiently for every manufacturing purpose, and for feeding boilers both on shore and in river boats, and that with a very small amount of care it will remain in good working order for years.
The filters are made in 5 sizes, the smallest of which will pass 100-150 gal. per hour, and the largest 2000-3800 gal.