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Purification of Water

Purification of Water for Chemical Purposes

For accurate scientific work water must be freed from most of the impurities detailed above, and the usual procedure of Purification of water is to distil from the ordinary tap supply. The liquid is heated preferably in a copper vessel and the steam condensed by passage through a cooled tube of some material, e.g. tin, silver, or even platinum, which will resist the action of steam. Such "distilled water" is sufficiently pure for most ordinary purposes, but for special purposes a higher degree of purity is necessary; in such cases the water is redistilled after the addition of potassium permanganate and potassium hydroxide; if traces of ammonia are to be avoided yet another distillation with the addition of a little potassium hydrogen sulphate is necessary. In all these distillations the first portion of the distillate should be rejected and also a considerable residue allowed to remain undistilled.

Bourdillon's distillation apparatus
Bourdillon's distillation apparatus.
The need for water of a high degree of purity is especially felt in experimental work on the electrical conductivity of aqueous solutions, and many modifications of the distillation apparatus and process have been proposed for preparing such water. A relatively simple distillation apparatus (Bourdillon) is outlined in Fig.

The boiler A, of approximately 13 litres capacity, is connected by a rubber bung with a copper tube B and cylinder C, which serve to remove particles of spray. In order to prevent excessive condensation B and C are jacketed with non-conducting material. The vertical tin tube D acts as condenser and is water-cooled at the two glass jacket cooling tubes; during the distillation a current of purified air slowly ascends from the inlet E. Tap water is placed in the boiler and a little potassium hydrogen sulphate is added. After a short time the steam is free from carbon dioxide. Passing up the tin tube the steam is condensed at the upper glass jacket, so that the condensed water during its descent is submitted to a " scrubbing " action by the ascending steam and pure air. The water collected is of a high degree of purity, but of course contains dissolved nitrogen and oxygen. Its electrical conductivity is roughly 1/107 reciprocal ohm at 18° C.

Purification of Water for Domestic Purposes

This is a problem of enormous economic importance. Owing to the large number of factors involved, numerous systems of water purification are in use in various parts of the world.

Storage. - Experiment shows that such natural waters as are not very pure to begin with, are greatly improved for potable purposes by storage. Suspended impurities gradually subside, carrying with them a portion of the bacterial content of the water, thus rendering the supernatant liquid considerably purer.

In addition to this, other actions take place involving a diminution in the number of bacteria,1 although sometimes there is an initial rise, followed by decline. The bactericidal effects are induced by a variety of causes, the more important of which are:
  1. Sunlight. - Although diffused daylight has but little action, direct sunlight is a powerful bactericide.
  2. Many common forms of life, such as protozoa, rapidly consume or exterminate pathogenic germs.
  3. Starvation. - The bacteria may consume the whole of their food supply.
  4. Toxins. - The excretory products of the bacteria may accumulate to such an extent as to poison them. Such toxins may even prove dangerous to higher forms of life, so that complete absence of bacteria does not necessarily prove that water is wholesome.
If, however, the water is very good to begin with, as, for example, deep well water, storage cannot improve it; indeed it may be detrimental to store the water, inasmuch as the entry of any pollution would give the accompanying bacteria a free field for rapid multiplication.

In addition to a reduction of the bacterial content, most natural waters undergo during storage several other changes which improve them for potable purposes. Thus the organic matter tends to disappear, either through settling or through oxidation to water, carbon dioxide, etc. The hardness is reduced either by abstraction of soluble calcium salts by plants and animals, or through evolution of some of the dissolved carbon dioxide into the atmosphere whereby the calcium bicarbonate becomes transformed into the normal carbonate and separates out as an insoluble deposit. The nitrogen compounds, such as ammonia, nitrates, and nitrites, tend similarly to disappear.

Purification of water through sedimentation may be greatly assisted by the introduction of powdered substances and particularly of colloids, although these latter take longer to settle. Frankland, in a series of experiments in which some 20 grams of powdered chalk, coke, charcoal, or spongy iron were added per litre of water, was able to effect the removal of from 90 to 100 per cent, of the organisms present in polluted waters. Savage races have long used mucilaginous substances, such as quince seeds, the acid juices of plants, and astringent and tanninoid precipitants, such as Peruvian bark, for purifying water for drinking purposes in the above manner; the sweetening of the waters of Marah by Moses by casting a tree into them is probably an example of this primitive method of treating waters.

It appears that the ancient Egyptians purified their water by allowing it to percolate through earthenware vessels containing alum. Colloidal aluminium hydroxide would thus collect in the pores of the earthenware and assist the process of filtration.

In modern times alum or aluminium sulphate has been added to waters to assist in their purification. It reacts with any dissolved calcium (or magnesium) carbonate converting it into calcium (or magnesium) sulphate, aluminium hydroxide being simultaneously precipitated. Thus:

CaCO3 + Al2(SO4)3 + 3H2O = 2Al(OH)3 + 3CaSO4 + 3CO2.

The precipitate gradually settles, taking down with it organisms and other suspended impurities. Thus, for example, Leeds found that an addition of 0.5 grain of alum per gallon of a certain sample of water reduced its bacterial content from 8000 to 80 per c.c., that is, by 99 per cent. The precipitate also acts as a decoloriser and has been applied in this capacity for clearing the water at Antwerp.

The foregoing, however, are not purely mechanical effects. Organic matters, particularly coloured constituents of upland and peaty waters, are usually colloid in character and exhibit electrophoresis, migrating to the anode or cathode according as they are negatively or positively charged. In most cases their charge is negative, so that these are precipitable by positive ions and positive colloids. Inasmuch as the precipitating power of an ion is a function of its charge or valency, aluminium with a valency of three has a very much greater precipitating effect than sodium, of valency equal to unity. This serves to explain the particular effectiveness of salts of aluminium and ferric iron which has long been observed. It must be remembered, however, that whilst a positive ion tends to precipitate a negative colloid, a negative ion tends to stabilise the same. Hence the total effect of, for example, an aluminium sulphate solution is the difference between the opposing actions of the aluminium and sulphate ions. By reducing the number of the latter, therefore, the precipitating effect is enhanced. This explains the greater efficiency of basic alums, which are now used in the Brooklyn (U.S.A.) filters, and in which the average deficiency of SO3 is some 8 per cent, of that required to form a neutral salt. Electrical methods for the removal of colour, based on electrophoresis, have been recommended and are of considerable scientific interest to the colloid chemist.

One disadvantage of the use of alum is the conversion of the carbonates of calcium and magnesium into sulphates, whereby temporary hardness is converted into permanent. This is objectionable for steam-raising purposes. This disadvantage is not shared by the metallic iron treatment which has been applied successfully to certain American waters. According to this, iron turnings are introduced into the water whereby a portion of the metal dissolves yielding soluble ferrous hydrogen carbonate, FeH2(CO3)2. Thus

Fe + 2H2O + 2CO2 = FeH2(CO3)2 + 2H.

The nascent hydrogen reduces any nitrates or nitrites to ammonia. Atmospheric oxygen converts the ferrous salt into colloidal ferric hydroxide, a reaction that is facilitated by cascade aeration. As it coagulates and settles, the ferric hydroxide purifies the water in an analogous manner to that described for aluminium hydroxide above. The process is rather expensive, however, and for this reason some American towns have added ferrous sulphate direct to their waters. This is less satisfactory as the salt is not oxidised so readily as ferrous bicarbonate; in addition to this, sulphuric acid is introduced into the water.

Removal of Iron. - Ferruginous waters are bitter to the taste, even 1 part of iron per million of water being perceptible to the average individual. Such waters, which are widely distributed in Germany and the Netherlands, in America, and elsewhere, are particularly favourable to the growth of minute organisms, such, for example, as the Crenothrix, the vitality of which appears to be connected with the secretion of iron within the tissue of its cell walls. This organism flourishes in waters containing 0.3 parts of iron per million of water, and may lead to serious choking of water-mains. Aeration and treatment with lime or with colloidal substances have been adopted with more or less success. Thus, for example, at Amsterdam, intense aeration followed by filtration has reduced the iron content of its water supply from 0.8 parts per million to nil. Iron may also be completely removed from water by passage through a manganese permutit filter.

Algae may be removed by treating the water with copper sulphate, 1 part in 10 million of water proving algicidal.

Filtration. - Although storage may improve water, further purification by filtration is necessary if the water is to be regarded as safe for domestic purposes. On the large scale water is usually filtered through beds of sand laid in layers of increasing coarseness from the top downwards. The process was originally introduced about the year 1810 in Scotland and Lancashire, and a few years later adopted at Chelsea, with the object of clarifying the water, bacteriology being then an unknown science. It was not until 1885 that Frankland systematically applied bacteriological water tests to the London water supply, and found that more than 99 per cent, of the micro-organisms present in Thames water at Hampton were removed by passage through the sand filters. This has since been confirmed times without number, and the Report of the London Metropolitan Water Board for 1913 is of interest in this connection:

No. of Micro-organisms per c.c. of Water taken from
ThamesLeeNew River
Raw water525092632172
Filtered water163114

A very striking instance of the value of sand filtration was afforded by the outbreak of cholera in Hamburg in 1892. The city drew its water from the Elbe and used it in its raw condition for domestic purposes. No fewer than 1250 per 100,000 of the population perished through cholera. The contiguous town of Altona lost but 221 per 100,000, despite the fact that it drew its domestic water from the Elbe below Hamburg, after it had received the sewage pollution from the latter city. This relative immunity was due to the fact that the Altona authorities purified their water by passage through sand filters.

Sand filtration is largely employed in this country, sand being laid to varying depths upon gravel which increases in coarseness with the depth, which ranges from 6 to 8 feet in toto. The water is drained away through pipes at the base.

Although it was formerly believed that a filter bed was most efficient when freshly laid, it is now known that such is not the case. The best results are obtained when the filter has been in operation sufficient time to allow a film or " dirt cover " to form on the surface of the bed, which is the chief level at which purification proceeds. The water passing through the filter should not be used for domestic purposes until after the lapse of the necessary 44 filming time." The film obtained from natural waters is essentially organic and of a semi-colloidal nature. By adding an inorganic colloid, such as alumina, a film may be artificially formed on the sand surface in a short space of time, thus saving delay in the use of the filter. When once such a film, whether natural or artificial, has been satisfactorily formed, micro-organisms will only pass through in small numbers. As time progresses the film becomes increasingly thicker, until ultimately the rate of filtration becomes too slow to be economically efficient, and the bed must be cleaned.

In Great Britain the sand filters generally deal with from 2 to 3 million gallons of water per acre per day, that is, with a downward travel of about 10 to 12 cm. (4 to 5 inches) per hour. The head of water ranges from 60 to 100 cm. (2 to 3¼ feet).

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