If electrical power can be obtained at a very low cost, or if the cost is merely nominal, as it is when there is an appreciable difference between the normal consumption and the peak load upon which the rate is based, the electrolytic hypochlorite method offers some advantages but in the great majority of plants it cannot economically compete with bleach. The instability of the liquor and cell troubles have also prevented the process being generally utilised. Baltimore and Cincinnati experimented with this method but did not adopt it. Winslow[6] has reported that, owing to the difficulty in obtaining bleach since the outbreak of war, Petrograd has used electrolytic hypochlorite made from salt.
_Diaphragm Process._ The various types of diaphragm cells that have been commercially operated are of two varieties: (1) cells with submerged diaphragms and (2) cells in which the electrolyte comes in contact with one face only of an unsubmerged diaphragm.
The Le Sueur, Gibbs, Crocker, Billiter-Siemens, Nelson, and Hargreaves-Bird cells are of the submerged diaphragm variety. The Nelson cell has been operated for some time at the filtration plant at Little Falls, N. J. The cells are fed with brine solution previously purified by the addition of soda ash and have given fairly successful results although the cost of maintenance is comparatively high. Tolman[7] has reported that several towns in West Virginia use a bleach solution prepared by absorbing chlorine, manufactured by the Hargreaves-Bird process, in lime water; the solution contains about 1.95 per cent of available chlorine.
The diaphragms in both the submerged and unsubmerged types are usually constructed either with asbestos paper or cloth, placed in such a manner as to divide the cells into two separate compartments: the anodic, into which the brine is fed and where the chlorine is produced; and the cathodic, where caustic soda is formed.
By maintaining the liquor in the anodic compartment at a higher elevation than in the cathodic one, the direction of flow is towards the latter, but owing to osmosis and diffusion the separation is not complete and a portion of the caustic soda passes the diaphragm and produces hypochlorite with a consequent loss of efficiency and rapid deterioration of the anodes. With the exception of the Billiter-Siemens cell, the submerged diaphragm cells operate at not more than 85 per cent efficiency and the cost of maintenance is usually high.