The effect of this filtration is chiefly mechanical; it separates any suspended matter, whether organic or inorganic. A certain amount of biological action possibly also takes place. Piefke found that a perfectly cleaned and sterilised filter when first used, increases the microbes in water, instead of decreasing them. Gradually a gelatinous layer of slimy matter is formed on the top of the sand; the water now filters through much more slowly, but it gradually becomes freer from microbes, these being intercepted by the slimy layer. It is important that this layer should not be disturbed by too rapid or forced filtration, and that when the surface layer requires to be removed, because the filter has become impervious, time should be allowed for another thin film to form before the filtered water is again utilised. Koch concluded that the rapidity of filtration should never be allowed to exceed 100 millimetres (about 4 inches) per hour; and that the number of microbes per c.c. in the filtered water should never exceed 100. Some oxidation of organic matter, as well as detention of microbes, may take place during the filtration of water, nitrates being formed by the vital activity of certain “nitrifying” microbes in the filter. (On nitrification, see pages 195 and 274.) P. Frankland’s observations show that the number of microbes in Thames water is reduced by filtration through sand and gravel beds, as practised by the London Water Companies, so that only 3·4 per cent. of those originally present remained. He also concludes that the majority of the microbes present in filtered water are derived from post-filtration sources. Thus the number is greater in tap-water than in water derived from near the reservoirs.
Other materials besides sand have been used for filtration on a large scale, but none with proved success.
Domestic Filtration ought, as already explained, not to be needed, but circumstances often arise in which the public supply is open to suspicion, and a second domestic line of defence against infection through the water supply is desirable. When this is so, the form of filter which will best protect the household is one attached to the house-tap, so that all drinking-water is perforce filtered. When filtering involves the transfer of water from the tap to the interior of the filter, opportunity is left for carelessness or forgetfulness. The one essential point of a domestic filter is that it will prevent the passage through it of microbes. Every filter must be tested from this standpoint.
On this point the experiments of Woodhead and Cartwright Wood are conclusive. They first of all experimented on various filters with fine artificial ultramarine containing particles 16 µ to 0·6 µ or even less in diameter in suspension; and milk containing granules and globules of fat 0·5 µ to 30 µ or more in diameter, freely diluted with water.
| TIME IN MINUTES REQUIRED FOR FILTRATION OF 1 PINT OF WATER. | PRESENCE OR ABSENCE OF ULTRAMARINE IN FILTRATE. | PRESENCE OR ABSENCE OF MILK IN FILTRATE | |
|---|---|---|---|
| Silicated carbon filter | 68 | ++ | +++ |
| Carbon filter | 18 | + | +++ |
| Maignen’s Filtre Rapide | 4 | 0 | ++ |
| Spongy iron filter | 14 | 0 | +++ |
| Pasteur-Chamberland filter | 420 | 0 | 0 |
| Berkefeld filter | 140 | 0 | 0 |
The number + indicates the relative amount of the experimental substances that made their way through the filtering medium.
Experiments were then made with the actual microbes of certain infectious diseases, and it was found that certain filters allow these to pass. Thus a silicated carbon filter allowed 1,000 out of 15,000 typhoid bacilli suspended in water to pass through its substance; a manganous carbon filter allowed 600 to 800 out of 10,000 cholera vibrios to pass through; Maignen’s filter on the second day of experiment allowed 150 out of 5,000 cholera vibrios to pass through; Lipscombe’s charcoal filter experimentally only reduced typhoid bacilli from 20,000 to 5,000; the magnetic carbide filter only reduced them from 20,000 to 10,000; the spongy iron filter from 20,000 to 3,000; while, on the contrary, the Pasteur-Chamberland and the Berkefeld filter completely stopped all microbes and produced a sterile water. (As to these two, see page [98].)
Of the materials enumerated animal charcoal was formerly regarded as an excellent filtering medium. It is capable of oxidising organic matter dissolved in water, but so far from sterilizing water, it favours the growth of microbes in it. Water filtered through charcoal, after the first few days of use of the charcoal, deteriorates, as the charcoal yields up impurities to it.
Manganous Carbon consists of animal charcoal and black oxide of manganese mixed with oil, and heated strongly together out of contact with the air. The oxidising power of the carbon is said to be thus greatly increased. It shares the objections to carbon.
Silicated Carbon consists of 75 per cent. of charcoal and 22 per cent. of silica, with a little oxide of iron and alumina. It is not an efficient filtering medium.