Chapter XI: Other Methods of Filtration

WORMS TILE SYSTEM.

This system, invented and patented by Director Fischer of the Worms water-works, consists of the filtration of water through artificial hollow sandstone tiles, made by heating a mixture of broken glass and sand, sifted to determined sizes, to a point just below the melting-point of the glass, in suitable moulds or forms. The glass softens and adheres to the sand, forming a strong porous substance through which water can be passed. These tiles are made hollow and are immersed in the water to be treated, the effluent being removed from the centre of each tile. They are connected together in groups corresponding in size to the units of a sand-filtration plant. They are washed by a reverse current of filtered water. These tiles have been used for some years at Worms, Germany, and at a number of smaller places, and were investigated experimentally at Pittsburg. Some difficulty has been experienced in getting tiles with pores small enough to yield an effluent of the desired purity, and at the same time large enough to allow a reasonable quantity of water to pass. In fact, with other than quite clear waters, it has not been found feasible to accomplish both objects at the same time, and it has been necessary to treat the water with coagulants and preliminary sedimentation or filtration before applying it to the tiles. The problem of making the joints between the tiles and the collection-pipes water-tight when surrounded by the raw water also is a matter of some difficulty.

THE USE OF ASBESTOS.

It has been suggested by Mr. P. A. Maignen that the surface of sand filters should be covered with a thin layer of asbestos, applied in the form of a pulp, with the first water put onto the filter after scraping. The asbestos forms a sort of a paper on the sand which intercepts the sediment of the passing water. The advantage of the process is in the cleaning. When dried to the right consistency this asbestos can be rolled up like a carpet, and taken from the filter without removing any of the sand.

This procedure is almost identical with that which has occurred naturally in iron-removal plants, where algæ grow in the water upon the filters, and form a fibrous substance with the ferric oxide removed from the water, which can be rolled up and removed in the same way as the asbestos. The advantages of the process, from an economical standpoint, are less clear.

FILTERS USING HIGH RATES OF FILTRATION WITHOUT COAGULANTS.

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. The requirements in this case are rather in the nature of straining than 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. The application of such systems of filtration would therefore seem to be somewhat limited.

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HOUSEHOLD FILTERS.

The subject of household filters is a somewhat broad one, as the variety in these filters is even greater than in the larger filters, and the range in the results to be expected from them is at least as great. I shall only attempt to indicate here some of the leading points in regard to them.

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.

In the first place, as a general rule, which has very few if any exceptions, we may say that all small filters which allow a good stream of water to pass do not remove the bacteria. 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.

The other household filters have filtering materials of much finer grain, unglazed porcelain and natural sandstone being the most prominent materials, while infusorial earth is also used. The smaller sizes of these filters allow water to pass only drop by drop, and when a fair stream passes them the filters have considerable filtering area (as a series of filter-tubes connected together). On account of their slow action, filters of this class are, as a rule, provided with storage reservoirs so that filtered water to the capacity of the reservoir can be drawn rapidly (provided the calls do not come too often). Some of these filters are nearly germ-proof, and are comparable in their efficiency to large sand-filters. There is no sharp line between the filters which stop and which do not stop the bacteria; but in general the rule that a filter which works rapidly in proportion to its size does not do so, and _vice versa_, will be found correct.

In thinking of the efficiency of household filters we must distinguish between the filter carefully prepared for an award at an exhibition and the filter of the same kind doing its average daily work in the kitchen. If we could be sure in the latter case that an unbroken layer of fine sandstone or porcelain was always between ourselves and the raw tap-water we could feel comparatively safe. The manufacturers of the filters claim that leaky joints, cracked tubes, etc., are impossible; but I would urge upon the people using water filtered in this way that they personally assure themselves that this is actually the case with their own filters, for in case any such accident should happen the consequences might be most unpleasant. The increased yield of a filter due to a leaky joint is sure not to decrease it in favor with the cook, who is probably quite out of patience with it because it works so slowly, that is, in case it is good for anything.

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. With the methods of examination now available, and a little care in selecting the times and places of collecting the samples, it is quite impossible for a filter-superintendent to deliver a poor effluent very often or for any considerable length of time without being caught. The safety of properly-conducted central filtration is thus infinitely greater than that from even the best household filters. Further, it may be doubted whether an infected water can be sent into every house in the city to be used for washing and all the purposes to which water is put except drinking, without causing disease, although less than it would if it were also used for drinking.

The use of household filters must be regarded as a somewhat desperate method of avoiding some of the bad consequences of a polluted water-supply, and they are adopted for the most part by citizens who in some measure realize the dangers from bad water, but who cannot persuade their fellow-citizens to a more thorough and adequate solution of the problem. Such citizens, by the use of the best filters, and by carefully watching their action, or by having their drinking-water boiled, can avoid the principal dangers from bad water, but their vigilance does not protect their more careless neighbors.