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.
At Wimbledon the alumino-ferric is applied in the form of blocks about 30 inches x 20 inches x 3½ inches, piled on edge in a large wooden vessel through which a stream of water is made to pass, and the saturated solution thus formed is drawn off at the bottom of the vessel and mixed with the sewage. The proportion of alumino ferric used in this case is about one-third of a ton per million gallons of sewage treated, and one gallon of the solution is added to 9000 gallon-. of sewage. Half a ton of pure lime is also added to each million gallons of sewage, or 7.84 grains per gallon, but this amount of course varies with the different conditions of the sewage, and in very dry hot weather a small quantity (about 2 grains per gallon of sewage) of manganate of soda is added, which is stated to haw a very beneficial effect in the prevention of secondary decomposition and of offensive odours. After the necessary treatment, the sewage passes into the settling tanks, and the remainder of the process is similar to nearly all precipitation-works. The effluent, however, does not pass direct into a river or stream, but is led in carriers over land, where it is further purified before entering the adjoining river.
1 Sewage Disposal Works, by W. Santo Crimp. M . Inst C.E , page 219.
The "Natural" Purification or Cosham's System has for its principle feature -and one which is of considerable importance - the fact, claimed to have been discovered by Mr. Cosham. that in treating sewage with any chemical, and notably alumino-ferric, the consumption of chemicals can be sensibly reduced, and their good effect extended, by prolonging the period during which contact is maintained between the chemical and the sewage, and the more effectually this is done the better the result. This is effected by means of speeial tanks, which Mr. Cosham has designed, and which are shown in Fig. 444. page 61.
After special treatment with the requisite amount of chemicals. the sewage is conveyed by a pipe discharging into the central portion of the continuous automatic precipitating tank, where it has a downward flow which greatly assists the precipitant in its work; it then passes under the divisional wall, and rises up the portion of the tank on the other side. The central-tank effluent then passes up the flocculent-Hue (which traps back the bulk of the floeculent matter, and so prevents it from passing into the divisional chambers and so into the first of the series of divisional chambers; it then passes through each chamber in succession, having a combined flow in each, viz., a downward, followed by an upward, which is obtained by the floeculcnt-flues, and finally is taken for filtration to the "natural" filter beds, or prepared area of ground.
In the first portion of the downward flow in the central portion of the tank, the sewage gets thoroughly and evenly impregnated with the precipitant, the precipitation actually commencing to take place about two-thirds down; and as the sewage rises on the other side of the divisional wall, the precipitation continues, and the albuminoids in solution, on rising, are rapidly coagulated by chemical combination, and so caused to descend,. leaving a clarified effluent for further treatment in the specially-constructed trapped floeculent-arresting chambers.
The bulk of the sludge precipitates in the central portion of the tank, and accumulates in the sludge-sump formed at the bottom of it. The flocculent matter precipitates in the series of divisional chambers, and accumulates at the bottom of each, more precipitation taking place in the first two or three cham-liers than in the later ones. The sludge from the central compartment is first extracted, and the sewage lowered in the central compartment; after which. by lifting the valves placed at the bottom of each divisional chamber, the head of effluent presses out the precipitate from the bottom of each of the divisional chambers into the central sludge-sump. It is claimed that the removal of the sludge is thus greatly facilitated over the usual methods.
The effluent is dealt with either over land, or in one of the "natural" filters, composed entirely of sand, gravel, and pebbles, arranged in layers of varying thickness and size.
This process will be again referred to when dealing with the subject of the disposal of of sewage from isolated houses. It is sufficient here to say that the tanks are most ingenious, and have much to recommend them.
The Persulphate of Iron Process is a chemical process of precipitation in combination with lime, invented by Mr. Thomas Wardle, the author of a book on Sewage Treatment and Disposal, and who claims for it that organic matter in solution is precipitated together with all suspended matter, and that persulphate of iron is an unfailing germicide. A weak solution of pel sulphate of iron, to the extent of about 1 ton to each million gallons of sewage, is used, to which are sometimes added lime, alumina salts, and othei chemicals,"according to the nature of the sewage to be treated". The sewage after being dosed is passed through settling tanks in the usual way. The author is not aware whether this process has been tried anywhere, or with what success.
The Hermite Process practically consists of a process to electrolyse sea-water. by electricity applied through platinum and zinc electrodes of considerable superficial area. Current at a pressure of 6 volts is passed through these plates, and certain salts rich in oxygen are the result, together with chlorine. It is stated that this chlorine can be obtained at a cost of 4d. in fuel for every 1000 grammes, and also that 1 gramme of chlorine every 24 hours is sufficient to deodorize and partially disinfect the sewage of one person of the population. The electrolysed solution is added to the sewage in the sewers, and it is claimed that under thin process the sewage reaches the outfall in a perfectly inoffensive condition. It is stated that about 6½ lbs. of chlorine can be made every hour from one electrical horse-power, and that this quantity will purify about 14,000 gallons of sewage. A hundred horse power would thus purify about 1 ½ million gallons of sewage in about 10 hours.