Page:The Journal of Tropical Medicine, volume 6.djvu/225

This page needs to be proofread.

June 15, 1903.)

THE JOURNAL OF TROPICAL MEDICINE.


a a! ee a 2 small seale for households and individuals. The ordinary filters for public water supplies are usually in water- tight basins of 10 or more feet in depth, with sides built of masonry and bottoms cemented, or of brick eemented. In the floors are drains or channel ways for _ collection of the filtered water. The filter itself is usually 5 or 4 feet in depth, and made up from the bottom of broken stone or pebbles, covered by a layer of coarse gravel, on which is placed a layer of coarse sand, and finally a layer of fine sand. The obj2ct of this arrangement is to prevent the fine sand from being carried through into the drains, and to so regulate the flow that there shall not be any currents or streams. The water rests several feet deep in the filter, and should not be allowed to flow through the filter at a greater rate than 4 inches an hour.

The filtration process is far from being a simple one, and has only recently been understood. At one time its action was thought to be mechanical, the sand separating out the suspended matter. Later, in addi- tion to the screening properties, it was held that the oxygen in the filter acted on the organic matter, oxidising it. Recently it has been discovered that the oxygen of the air has by itself little action on organic matter in water without the aid of microbes, and that the purifying effect of a filter is mainly due to a mem- _ branous layer of microbes on or near the surface of the filter. On the surface of a sand filter there is a slimy deposit, composed of finely-divided clay with strong absorbent properties, and a gelatinous mass of inter- cepted bacilli and streptococci, micrococci, alge and | other bodies, and immediately below this film is a - Jayer of nitrifying organisms.

It is this slimy layer on the sand which is the impor- _ tant factor in removing micro-organisms and the organic | matter from the water. By the film the pathogenic micro-organisms are intercepted and destroyed, the organic matter is broken up into carbonic acid and | ammonia, while by the nitrifying organisms the ammonia is resolved into nitrous and nitric acid.

The important points to be borne in mind with Teference to sand filtration are that (1) there should be a thin layer of slime composed as above on the surface ; (2) that this slime should not be disturbed the process of filtration; and (3) that when

the slime becomes too thick for the water to pass “through it should be removed by slicing off. If a fi er be employed for too long a period without pleansing there is a gradual growth of the surface yacteria through the filter.

whe filter bed requires cleansing at certain periods, eae vise the oe layer would completely choke up

» filter and render it impermeable. The period that el pses between the cleansing depends on the quality

f the water and the finencss of the sand.

” The method of cleansing is to remove a thin layer of tand half an inch in thickness, and disturb the upper art of the remainder of the sand by a fork, so as to mXpose it to the air, after which it is smoothed’ over.

this process is repeated until the upper fine sand is feduced to a foot in thickness, after which the whole

f the filter is taken up and cleansed. The formation of the slimy layer on the filter may be du ing a matter of only a few hours, or it may take twenty- four hours or even longer. The water at this time that passes through the filter 1s not free from impurities, and arrangements should be made to allow it to run to waste. The filter bed should be filled from the top, and the water should be allowed to stand in it at a depth of 3 feet for at least twenty-four hours.

For efficient filtration the following conditions have been laid down by Koch: (1) The sand should not get below a foot in thickness; (2) the rate of fiow through the filter should not be greater than 100 mm., or 3-95 inches, per hour, or not more than 200,000 gallons per acre per hour; for the more rapid the rate of flow through the filter the less is the efficiency of the filtra- tion ; (3) the quantity of microbes in the filtered water should not exceed 100 per cc. The mere provision of filters is not enough to secure good water; they must be well constructed, supervised, and frequently ex- amined, especially in hot weather. in winter and in times of epidemics.

Care has to be taken that the fine film of bacteria, slimy alg, and suspended matter is not broken. This may happen from the water being allowed to pass too rapidly through the filter, or from intense heat or cold on an exposed filter. Under these circumstances the filter is defective. In cold climates it is better for the filters to be covered over to prevent them from freezing ; while in hot countries it is necessary in order that the water shall not be exposed too much to the sun, which renders the water warm, and at the same time favours the growth of alge. The covering of the filters also excludes dust, which in some tropical countries is im- portant. The storage tanks after the water is filtered should under every circumstance be covered over.

The very best sand filters cannot keep back ali the micro-organisms in a water, but they possess the power of keeping back 98 to 99 per cent. of the bacteria. They have an enormous controlling power, as is seen by a comparison of the number of organisms in the water as discharged on the filters and the few that appear in the filtered water.

In Calcutta river water the germs were In settling tank a In settling tank with alge After filtration :— In clean settling tank re ft Worn 3 In settling tank with alge 250s ‘tris

It will be seen that storage before filtration has also a very important effect on the reduction of micro- organisms.

There are two systems of sand filtration, the slow and the rapid, and in either system the filtration may be applied continuously or intermittently.

The slow sand filtration is the process which has been described, and is dependent on the gravitation of the water through the filter. There are conditions, however, in which the filtration through sand requires to be rapid, and for this purpose, the sand filters being usually limited in size, mechanical contrivances and pressure are brought into requisition. Mechanical filters are in greater use in America than elsewhere, and appear to be well suited for the muddy water derived from the

250,000 per ce. 20,000 ,, 100,000

»

This article is issued from Wikisource. The text is available under Creative Commons Attribution-Share Alike 4.0 unless otherwise noted. Additional terms may apply for the media files.