This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
"This method is specially adapted to hospitals and other large public buildings. The salt-water could be decomposed in moderate-sized tanks plated OX) each floor, and allowed to run into the drain-system in small quantities either automatically or by hand regulation. Thus this cleanly and practically inexpensive liquid would fill the traps of the various sinks and basins, and so disinfect all the material while it was yet within the building and before it reached the city sewers."1
Electrolysis consists in breaking up the organic compounds of sewage into their constituent parts, by passing an eleetric current through iron electrodes placed in the flowing sewage, which results in the formation of iron oxides and chlorine. The first produces oxygen, and the- second produces chloric acid, which destroys organic matter. A non-oxidizahle carbon plate is employed for the positive pole, and iron is used as the negative pole, so that, by means of a porous diaphragm between, the component part- of the mineral salts are collected. At the non-oxidizalde plate a solution of chlorine and oxide of chlorine is produced, and at the negative plate ammonia, soda, and potash are formed, which precipitate the maguesium salts and lime in the Liquid A large portion of the solid and dissolved impurities in sewage are thus deposited in the form of sludge.
The foregoing short descriptions of some chemical processes for the clarification, purification, or precipitation of sewage, may be taken as examples of what has been tried in these directions, and it will be noticed that in no case must it be assumed that the effluent water from any sewage that has been treated chemically has been rendered so pure as to be fit for domestic purposes, unless it has been passed. over or through land in order that it may lose all propensity to again decompose.
Professor Henry Robinson, in a paper on "Sewage Disposal" which he read before a congress of the Sanitary Institute held in Dublin in the year 1885, said: "Experience shows that it is impossible at all times and seasons to be sure of a constant and uniformly high standard of purity, and that chemical works should be supplemented by a filtration area however small. The addition of this, however, enables a lower standard of effluent from the precipitation tanks to be admissible, and this can be attained with very simple and inexpensive chemicals."
Speaking of the cost of such works, he stated that it varied from 0. 91 to .166 of a £ per head of the population, and that the average cost of the works he had himself carried out was 123 of a £ per head. The cost of treatment he had found was from 036 to 110 of a £ per head per annum, and an average over several places gave .06 of a £ per head per annum. These figures only applied to places where the very highest standard of purification was sought to be attained; hut when' filtration was added, the cost of works would he about .075 of a £ per head, and the cost of treatment 04 of a £ per head per annum.
1 From a pamphlet by the British Electrozone Company.
The difficulties and objections to be met with in dealing with sewage entirely by chemical processes, may be summed up as follows: -
(1) The varying character of the sewage to be dealt with, not only in different towns but also almost hourly during each day, this being intensified where, as in most cases, the sewage contains various trade and manufacturers refuse or wastcs. which are often very refractory under the influence of the chemicals used for the treatment of the wage.
(2) The tendency of all chemically-treated effluents to revert to secondary decomposition, this, having only been temporarily arrested by the treatment
(3) The first cost of the necessary works and plant, and the subsequent expense of treatment.
(4) The disposal of the sludge which is precipitated to the bottom of the tanks.