existing in the crude water will be arrested. The sand, which is nominally the filter, has interstices about thirty times as wide as the largest dimensions of the larger microbes; and the reason why these, and, still more, why organisms which were individually invisible under any magnifying power, and could only be detected as colonies, were arrested, was not understood. In process of time it became clear, however, that the worse the condition of a filter bed, in the then general acceptation of the term, the better it was as a microbe filter; that is to say, it was not until a fine film of mud and microbes had formed upon the surface of the sand that the best results were obtained.
Even yet medical science has not determined the effect upon the human system of water highly charged with bacteria which are not known to be individually pathogenic. In the case of the bacilli of typhoid and cholera, we know the direct effect; but apart altogether from the presence of such specific poisons, polluted water is undoubtedly injurious. Where, therefore, there is animal pollution of any kind, more especially where there is human pollution, generally indicated by the presence of bacillus coli communis, purification is of supreme importance, and no process has yet been devised which, except at extravagant cost, supersedes for public supplies that of properly-conducted sand filtration. Yet it cannot be too constantly urged that such filtration depends for its comparative perfection upon the surface film; that this surface film is not present when the filter is new, or when its materials have been recently washed; that it may be, and very often is, punctured by the actual working of the filters, or for the purpose of increasing their discharge; and that at the best it must be regarded as an exceedingly thin line of defence, not to be depended upon as a safeguard against highly polluted waters, if a purer source of supply can possibly be found. Such filters are not, and in the nature of things cannot be, worked with the precision and continuity of a laboratory experiment.
In fig. 21 a section is shown of an efficient sand-filter bed. The thickness
of sand is 3 ft. 6 in. In the older filters it was usual to support
Fig. 21.—Section of Sand-Filter Bed.
this sand upon small gravel
resting upon larger gravel,
and so on until the material
was sufficiently open to pass
the water laterally to under-drains.
But a much shallower
and certainly not less
efficient filter can be
constructed by making the
under-drains cover the whole
bottom. In fig. 21 the sand
rests on small gravel of
such degree of coarseness
that the whole of the grains
would be retained on a sieve
of ⅛-in. mesh and rejected
by a sieve of ½-in. mesh in
the clear, supported upon a
3-in. thickness of bricks laid
close together, and constituting
the roof of the under-drains,
which are formed by
other bricks laid on thin
asphalt, upon a concrete
floor. In this arrangement
the whole of the materials
may be readily removed for cleansing. 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
2½ 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. 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 unfiltered water may then be allowed to flow
quietly and to fill the space above the sand to a depth of 2 or 3 ft.
It would appear to be impossible with any water that requires filtration
to secure that the first filtrate shall be satisfactory if filtration
begins immediately after a filter is charged; and if the highest
results are to be obtained, either the unfiltered water must be permitted
to pass extremely slowly over the surface of the sand without
passing through it, or to stand upon the sand until the surface film
has formed. With waters giving little or no sediment, which are
often the most dangerous, some change, as by the first method, is
necessary. It has been proposed, on the other hand, to allow the
filter to act slowly until the surface film is formed, and to discard the
first effluent. This course can scarcely fail to introduce into the sand
many bacteria, which may be washed through when the full working
of the filters is begun; and it should not, therefore, be adopted when
the source of the supply is known to be subject to human pollution.
The time for the formation of an efficient surface films varies, according
to the quality of the raw water, from a few hours to a few days. Judging
from the best observations that have been made on a large scale,
the highest rate of efficient filtration when the surface film is in good
condition is about 4 in. downwards per hour of the water contained
above the sand, equivalent to about 50 gallons per day from each
square foot of sand. When the surface film has once been formed,
and the filter has begun its work, it should continue without interruption
until the resistance of that film becomes too great to permit of the
necessary quantity of water being passed. That period will vary,
according to the condition of the water, from eight or ten days to four
weeks. The surface film, together with half an inch to an inch of
sand, is then carefully scraped off and stored for subsequent washing
and use. This process may be repeated many times until the thickness
of the fine sand is reduced to about 18 in., when the filter bed
should be restored to its full thickness.
A lately discovered effect of sand filtration is a matter of great importance in connexion with the subject of aqueducts. A brown slimy sediment, having the appearance of coffee grounds when placed in clear water, has been long observed in pipes conveying surface waters from mountain moorlands. The deposit grows on the sides of the pipes and accumulates at the bottom, and causes most serious obstruction to the flow of water. The chemists and bacteriologists do not appear to have finally determined the true nature and origin of this growth, but it is found in the impounded waters, and passes into the pipes, where it rapidly increases. It is checked even by fine copper wire-gauze strainers, and where the water passes through sand-filter beds in the course of an aqueduct, the growth, though very great between the reservoir and the filter beds, is almost absent between the filter beds and the town. Even the growth of the well-known nodular incrustations in iron pipes is much reduced by sand filtration. From these facts it is clear that, other things being the same, the best position for the strainers and filter beds is as close as possible to the reservoir.
Some surface waters dissolve lead when bright, but cease to do so when the lead becomes tarnished. More rarely the action is continuous, and the water after being passed through lead cisterns and pipes produces lead poisoning—so called “plumbism.” The liability to this appears to be entirely removed by efficient sand filtration.
Sand filtration, even when working in the best possible manner, falls short of the perfection necessary to prevent the passage of bacteria which may multiply after the filter is passed. Small, however, as the micro-organisms are, they are larger than the capillary passages in some materials through which water under pressure may be caused to percolate. It is therefore natural that attempts should have been made to construct filters which, while permitting the slow percolation of water, should preclude the passage of bacteria or their spores. In the laboratory of Pasteur probably the first filter which successfully accomplished this object was produced. In this apparatus, known as the Pasteur-Chamberland filter, the filtering medium is biscuit porcelain. It was followed by the Berkefield filter, constructed of baked infusorial earth. Both these filters arrest the organisms by purely mechanical action, and if the joints are water-tight and they receive proper attention and frequent sterilization, they both give satisfactory results on a small scale for domestic purposes. The cost, however—to say nothing of the uncertainty—where large volumes of water are concerned, much exceeds the cost of obtaining initially safe water. Moreover, if a natural water is so liable to pathogenic pollution as to demand filtration of this kind, it ought at once to be discarded for an initially pure supply; not necessarily pure in an apparent or even in a chemical sense, for water may be visibly coloured, or may contain considerable proportions both of organic and inorganic impurity, and yet be tasteless and free from pathogenic pollution.
There are several materials now in use possessing remarkable power to decolourize clarify, chemically purify and oxidize water; but they are too costly for use in connexion with public water supplies unless a rate of filtration is adopted quite inconsistent with the formation of a surface film capable of arresting micro-organisms. This fact does not render them less useful when applied to the arts in which they are successfully employed.
Attempts have been made, by adding certain coagulants to the water to be filtered, to increase the power of sand and other granular materials to arrest bacteria when passing through them at much higher velocities than are possible for successful filtration by means of the surface film upon sand. The effect is to produce between the sand or other grains a glutinous substance which does the work performed by the mud and microbes upon the surface of the sand filter. Elsewhere centrifugal force, acting somewhat after its manner in the cream separator, has been called in aid.
