A number of different methods of sterilisation have been tried, but so far as the existing knowledge of the subject extends, the application of chlorine in one form or another is generally admitted to be the most efficient and economical process. The chlorine may be applied in the form of a solution of chloride of lime, or as a hypochlorite of sodium, or of magnesium. The solution of chloride of lime may be prepared by the sewage works manager, but this necessitates a considerable amount of care and knowledge in order to secure the correct strength at all times, and it is, of course, essential that the chloride of lime itself should always be of a known strength. The hypochlorite of sodium may be produced chemically, and this can be purchased of known strength from chemical manufacturers. For large volumes of sewage effluent, however, it will probably be found most economical to utilise an electrically produced hypochlorite of sodium, as this can be prepared on the works as and when required, of a uniform strength, and at a comparatively low cost.
In the case of the Digby process, briefly stated, this consists in passing an electric current through a solution of sodium chloride, with the result that the sodium chloride is broken up into its component parts, and chlorine is liberated at the positive pole, while sodium is deposited at the negative pole. The sodium and the chlorine are then allowed to recombine in the form of hypochlorite, and this solution is then ready for application to the liquid to be sterilised. Other processes are similar in principle but vary in detail.
Fig. 153.—The Digby Meridioniser.
This process naturally involves the use of an electric current and an apparatus for producing the hypochlorite. Of the latter there are several on the market, and an illustration of the Digby Meridioniser, which is manufactured and supplied by Messrs. Adams Hydraulics, Ltd., is given in [Fig. 153]. The particular feature of this apparatus consists in the manner in which the re-combination of the anode and cathode products are secured. Instead of the re-combination taking place in the main body of the electrolyte, it can only take place in the special porous compartment enclosing the electrodes. The re-combination may take place in either the anode or the cathode compartment, the products of the one compartment being conveyed to the other compartment. Thus the caustic hydrate from the cathode cells flows by gravity into the anode compartment, the two compartments being connected by a glass pipe. Fresh water or water containing an excess of alkali is run into the cathode cell. This process gives hypochlorite solution of low saline content, the only salt present in the resultant liquor being that due to diffusion, depending upon the porosity of the closely covering compartment walls, or upon such a reaction as that covered by the Blount hypothesis. A cross-section of this apparatus is shown at [Fig. 153], in which A is the positive lead; BB negative leads, C outflow, D inflow, EE cathodes, F anode.
Another machine is that supplied by Messrs. Oxychlorides (1907), Limited. This machine consists essentially of a graphite anode of circular cross-section (except as to about 4 in. at the top, which is left open for the free escape of gases evolved during the electrolysis), and within it a metallic cathode of smaller circular cross-section. The annular space between the anode and cathode is filled with a solution of common salt, or with sea-water, through which a current is passed from a low-potential dynamo. In either case the ultimate result is to obtain some of the chlorine of the salt in an active or available form, the only difference being that in the case of using strong salt solution, a concentrated form of available chlorine may be obtained, while with sea-water a weaker solution results. Where sea-water is readily obtainable it is naturally more economical to make use of it, and to employ the larger volume of less concentration; while where sea-water is unobtainable and salt is expensive, or where chlorides in the effluent are objected to, it is more advantageous to prepare concentrated solutions, and to dilute them when required for use. This machine is made in various sizes to suit varying conditions. It was used by the Royal Commission on Sewage Disposal in connection with their experiments at Guildford, referred to in their Report of 1908, pages 198 to 201.
Electrolytic hypochlorite of magnesium is being produced daily, by the Borough of Poplar, for use as a disinfectant. The highly effective qualities of this solution, as well as the low cost of production, makes it a very valuable disinfectant, and for the results obtained every credit is due to Dr. F. W. Alexander, the Medical Officer of Health, who initiated the scheme, and to whose unbounded energy and enthusiasm the success of the work is due. This solution is, of course, equally suitable for the sterilisation of sewage effluents. The cost of production of this solution, of an average strength of from 4·5 to 5 grammes of available chlorine per litre (·45 to ·5 per cent. solution), is estimated at under one penny per gallon. The apparatus in use at Poplar has been supplied by the Farringdon Engineering Co., and an illustration of the plant is shown in [Fig. 154].
