Sterilisation of Water

By Sterilisation of Water is understood the destruction of all organisms in the water, whether pathogenic or not. In the absence of suitable mechanical filters or in cases of doubt as to the efficiency of the filters in removing pathogenic germs, sterilisation should be resorted to, for this is the only sure method of preventing the spread of water-borne diseases.

The Forbes steriliser

One of the oldest and best known methods consists in boiling the water whereby very thorough sterilisation results. The boiled water is apt to be insipid and flat to the taste in consequence of the expulsion of its dissolved gases, but this is a matter of minor importance in the preparation of beverages such as tea, coffee, and cocoa. As ordinarily carried out, boiling would be too expensive a process to carry out on a very extensive scale, but the cost may be greatly reduced by utilising the heat of the sterilised water to warm up fresh crude water, so that the purified water shall leave the steriliser at a temperature very little above that at which it entered. This is effected in the Forbes Patent Water Steriliser - to mention one among many. It has been adopted by the American Army authorities and operates in the manner indicated in fig.

The feed water passes upwards through a series of narrow, vertical chambers B, C, D, E, separated by thin, metallic walls from the hot, sterilised water, and enters the boiler F at a high temperature. Here it is raised to boiling and escapes into G, whence it passes down the tower, giving up its excess of heat to the cold, raw water within the metallic partitions, and escapes at H at a temperature only a few degrees above that at which it entered the apparatus. As it passes down the tower it also draws with it from G the dissolved gases which have been expelled during boiling, and reabsorbs them. The sterilised water, as it leaves H, is devoid of the insipidity so characteristic of water that has been boiled in the ordinary way.

The Puech Chabal ultra-violet light steriliser

Ultra-violet light, such as that emitted by a mercury-vapour lamp, exerts a powerful germicidal action on water and ice. In a series of experiments carried out at Marseilles it was observed that a lamp working with 3 amperes at 220 volts destroyed pathogenic organisms in water within a radius of 2½ inches in two seconds. In order to ensure complete sterilisation in a stream of water, the latter is made to flow, by means of a series of baffle plates, close to the lamp on three successive occasions, as shown in fig.

The efficiency of the process appears to be independent of the amount of dissolved oxygen, the germicidal action being a purely physical action not involving the production either of hydrogen peroxide or of ozone.

Very efficient sterilisation of suspected waters may be effected by chemical methods, and the cost may be materially reduced by first purifying the water as far as may be by mechanical filtration. The final stage of purification is then effected with a minimum quantity of the chemical reagent.

One of the most valuable substances for this purpose is ozone, for it acts rapidly, imparts no taste to the water, and leaves no solid residue. Numerous ozone installations have been erected for this purpose both in Europe and in America. In 1913 France possessed some thirty different plants, the largest being at the Parisian waterworks of St. Maur. Here the water is first sand filtered and then ozonised, over 24 million gallons being treated daily. The ozone is prepared by commercial methods such as are described in Chapter V., and conducted into sterilisers, of which various forms have been designed.

The Howard-Bridge steriliser

The Howard-Bridge Steriliser is shown in diagrammatic section in fig.

Crude water enters at A, and draws in from pipe B any unused ozonised air which has collected at C. As the water travels round the first bend, all the ozone is usually extracted from the air, which now escapes from the system at D. The thereby partially sterilised water now receives a charge of freshly ozonised air through the pipe E, and passes round the second bend and between the baffle plates until it reaches C. At this point it discharges its excess of air and escapes at F in a highly purified condition. The excess of air at C is continuously drawn away to B to partially sterilise the crude entrant water as explained above.

A Vosmaer steriliser

This system possesses the decided advantage that no external power has to be applied to force the ozone into the water. The gas is drawn in by suction as the water descends the first limb of the apparatus.

Diagrammatic representation of Siemens-Halske steriliser

Several forms of ozone sterilisers have been described by Vosmaer, a recent (1916) development of which is shown diagrammatically in Fig. The feed water enters through the top left-hand tube, and as it passes down the cylinder meets the ascending stream of ozone which effects its sterilisation, and escapes at A. In the Siemens-Halske steriliser (fig.) the feed water passes down a tower containing lumps of some such material as flint, coke, or gravel, to increase its surface, and meets an ascending current of ozone which effects its sterilisation. The escaping air at the top of the vessel is still rich in ozone, and is dried and passed through the ozoniser instead of ordinary air, and thus raised to its previous ozone concentration. About 1.3 grams of ozone will suffice to sterilise 1 cubic meter (1000 litres) of average water.

An ingenious apparatus has been designed, in which the ultra-violet light, produced simultaneously in the silent discharge employed for preparing ozone, is utilised to assist in the sterilisation of water. The last named is first acted on by the light, the partial sterilisation thus induced being completed immediately after by contact with the ozone.

Hydrogen peroxide theoretically constitutes an ideal steriliser, for, like ozone, it destroys bacteria without adding any foreign chemical to the water. One part in 10,000 suffices to destroy ciliate infusoria, although a minimum of 1 part per 1000 is necessary for the destruction of bacteria, and even with this concentration the action is rather slow. Owing to its instability and high cost, however, hydrogen peroxide can only be used for sterilising water on a small scale. More convenient are certain metallic peroxides, for these can be obtained in a solid state, of greater stability and sterilising efficiency. Sodium peroxide, Na₂O₂, for example, possesses a marked germicidal action, and at the same time serves as a softener for hard waters, sodium carbonate being generated by the action of dissolved carbon dioxide, thereby effecting the precipitation of calcium (or magnesium) carbonate, in the case of temporary hardness, by removing the solvent, namely carbonic acid, and in the case of permanent hardness, by double decomposition with the calcium (or magnesium) sulphate. Thus:

Na₂O₂ + CaH₂(CO₃)₂ = Na₂CO₃ + H₂O₂ + CaCO₃;
or
Na₂O₂ + CO₂ + H₂O = Na₂CO₃ + H₂O₂;
Na₂CO₃ + CaSO₄ = Na₂SO₄ + CaCO₃.

Calcium peroxide, CaO₂, and magnesium peroxide, MgO₂, have also been recommended, the former being sold under the name of " bicalzit," whilst an impure form of the latter is used in the sterilisation of bottled mineral waters.

Chlorine is a powerful germicide, and is becoming increasingly popular for this purpose. It may be used in the form of liquid chlorine or as chlorine water. It is sometimes more convenient, however, to use bleaching powder, 30 lb. of which are, in general, sufficient to sterilise 1 million gallons of water. Sodium hypochlorite is also employed for the same purpose.

Sodium hydrogen sulphate, NaHSO₄, in tablet form has been recommended for travellers, and was used in the Boer War, about 15 grains per pint or 1.75 grams per litre being required.

Citric acid, 1 part per 1000, is germicidal, but imparts a strong acid flavour to the water. Carbon dioxide under pressure is also effective; and bottled mineral waters, such as soda water, are usually very safe to drink.

Alkalies are also powerful germicides. The addition of lime to water in accordance with Clark's process for removing temporary hardness effects the removal of the bulk of the contained bacteria. This is largely due to the mechanical effect of sedimentation which causes the bacteria to be carried down with the lime, only a portion of them being killed. After a time they rise again into the water. By addition of excess of lime, the germicidal action is increased, but even with 0.2 per cent, of lime (calculated as CaO) or 20 parts per 10,000, the sterilisation is not usually complete, whilst at this concentration the water is too strongly alkaline for most purposes.