(158) Chemical Processes for the Treatment of Sewage. The number of processes for the clarification of sewage which have been proposed and patented are very numerous, as may be seen from the following list of patents:

The chief processes which have survived, and are in actual use are those in which lime, salts of iron, and salts of alumina are used either singly or in combination, examples therefore will be considered under these respective headings; but, it will first be useful to state the nature of the liquid to be treated, and next the chemical action by which a precipitate is formed and the method, of its action.

(159) Composition of Sewage.

Se wage is a very complex liquid, varying according to the season, water supply and manufactories in reaction and content of solid matter. It is usually alkaline, save the sewage of towns like Birmingham, in which vast volumes of "pickling" or strongly acid liquors are cast into the sewers. It contains soluble and insoluble compounds of fatty acids with bases, ammonia, sulphide of ammonia, finely divided cellulose derived from paper, fatty matters,

and many other soluble and insoluble substances. According to the Rivers Pollution Commission, the average composition of water closet towns sewage, in 100,000 parts is 44 69 parts of suspended matters, of which 24 18 are mineral, the rest organic, 72.2 parts of solid matters are in solution; that is to say, a sample of carefully filtered sewage on evaporation would leave solid matter in that proportion. A gallon therefore contains 50-5 grains of dissolved matter, a quantity which might about fill a teaspoon; the total nitrogen in the 100,000 parts is about 77 but the extremes vary widely. The dry weather sewage of the metropolis is estimated as containing 60 tons of sewage per annum per head (224 gallons weigh a ton); the lowest money value which can be assigned to this is over £1,000,000.

(160) General Reactions on the addition of Lime, Iron, or

Alumina.

If to this complex liquid either hydrated lime or a solution of lime be added a precipitate is formed, the lime combines with the free carbonic acid or it decomposes carbonates of the alkalis, and slowly sinks to the bottom as lime carbonate; in sinking it carries down a considerable portion of organic matter and thus clears the liquid.

If a salt of iron is added, such as sulphate of iron, the reaction is more complex, the sulphuric acid combines with the lime salts, and the result is that protoxide of iron is first formed. This acts as a carrier of oxygen, and the precipitate as before carries down in subsiding large quantities of organic matter some of which it may oxidise. If the iron salt is used alone the effluent is slightly acid. Subsequently any sulphide of ammonium present is decomposed and iron sulphide formed, and the sewage becomes from this reason more or less black. Sulphate of alumina when added to sewage is decomposed very similarly to sulphate of iron, hydrated alumina oxide being formed and slowly sinking to the bottom, lime or carbonate of lime in the presence of sulphate of alumina is converted into sulphate of lime.

(161) Lime Process.

Save when the sewage is acid, the proper amount of lime to add according to Dr. Stevenson should not exceed the hardness of the

water supply, that is to say, if the water supply possesses 12 degrees of hardness, then add 12 grains per gallon of lime. The lime is usually added in the form of milk of lime, that is to say slacked lime is mixed with water until a thin white sort of emulsion is formed. The Commission reported that the lime process was very simple and the least costly of any, but that it could not be profitable in an agricultural sense, and did not purify the sewage.

The lime process has been tried at Tottenham, at Blackburn and at Leicester.

(162) Alumina Processes.

The phosphate process of Messrs. David Forbes, F.R.S., and A. P. Price is an alumina process; phosphate of alumina is dissolved in hydrochloric or sulphuric acid and the solution added to the

sewage.

Bird's process is the addition of a crude sulphate of alumina to sewage, the sulphate is formed by decomposing clay (that is alumina silicate) by sulphuric acid. The proportions used are 6 cwt. of pulverised clay and 120 lbs. of sulphuric acid to 200,000 gallons of sewage. It is in use at Stroud. The effluent is acid.

(163) Salts of Iron.

The "Sewage Purification Company" treat a powdered magnetic iron ore with sulphuric acid; the result is of course a solution of sulphate of iron, this is added in due proportion to the sewage; the effluent is acid.

(164) Various Mixtures.

In General Scott's process 10 cwt. of lime, and 5 cwt. of clay are added to 400,000 gallons of sewage, the sludge is burnt in a kiln, and ground into cement. General Scott's process was tried at Ealing.

Hille's process was tried at Wimbledon. It consists in adding to the sewage a mixture of 100 parts of lime, 6 of tar and 12 of calcined magnesium chloride.

Holden's process is the use of a precipitant consisting of a mixture of sulphate of iron, lime, and coal dust.

The A, B, C process consists in adding a mixture of ammonia, alum, clay, charcoal, Epsom salts (magnesic sulphate) and blood to sewage. According to the second report of the Rivers Pollution Commission

the practical application of the process was as follows: 1,027 gallons of river water held suspended or dissolved 3 cwt. of ammonia alum, 6 cwt. of moist clay, 15 lbs. of animal and 20 lbs. of vegetable charcoal, 20 lbs. of Epsom salts, and 4 lbs. of fresh blood in a magma of clay; this liquid the Commissioners found discharging into the sewage at the rate of about 210 gallons per hours at Leamington.

Dr. Anderson's process has been tried at Coventry. The sewage was mixed with a saturated solution of sulphate of alumina heated to the boiling point, the crude sulphate being made on the spot by means of sulphuric acid acting on clay or shale; after this treatment the sewage flowed on and was treated with milk of lime. According to the Commissioners the money results were not encouraging.

(165) Sewage Sludge.

From any of the above processes result a sludge and an effluent; the latter is sent into the nearest watercourse, the former requires special treatment. In some cases, as at Coventry, the sludge is reduced in bulk and made fairly dry by compression in Johnson's filter presses. These presses consist of a number of grooved discs arranged in series, each disc having a central perforation and being separated from the disc on each side by means of a filtering cloth. The liquid sludge is filtered through these cloths by air compressed at a pressure from 100 to 120 lbs. per square inch. The liquid escapes fairly rapidly, while the solid matters remain behind in a more or less solid cake: this is found the least expensive way of drying. In some cases the sludge is burnt, as at Acton, where the solid matters are mixed with house refuse and consumed without nuisance in a destructor.

(166) Summary of Precipitation Processes.

Dr. Corfield sums up the matter with regard to the processes of precipitation as follows: "All these precipitation processes to a certain extent purify the sewage and prevent the pollution of rivers, chiefly by removing the suspended matters from the sewage; but they all leave a very large amount of putrescible matter in the effluent water, and at least all the ammonia contained in the sewage (sometimes they add to it); the greater part of the phosphoric acid is