Col. Ducat calls his filter the "Aerated Bacterial Self-Acting Filter", and the following is taken from a circular letter issued by him on the subject: -

"By this method of disposal, the sewage, taken direct from the sewer without any preliminary treatment, is run on to a specially-pre pared filter of an inexpensive construction, where, by the life-action of micro-organisms, the sludge or solid matter in the sewage is broken down and liquefied, in which condition much of the carbon, combining with oxygen, forms carbonic acid gas, which is dissipated inoffensively; and much of the nitrogen in the sewage combines with the hydrogen, forming ammonia. In this Aerated Filter, which is specially designed for the purpose, the process of purification by oxidation is fostered and furthered; the nitrogen of the ammonia and of the organic matter, in combination with the oxygen of the air, which this Aerated Filter alone can supply automatically in sufficient quantity, forma nitric acid, which combines with the lime, soda, potash, or other suitable base in the sewage, forming nitrates or nitrites, which are entirely harmless, in the effluent.

"This elaborate process of the laboratory of nature is practically utilized and applied in the most scientific but at the same time the very simplest and cheapest manner possible. The sewage, without precipitation of sludge or treatment of any kind whatever, merely runs direct from the sewer on to the top of the filter and issues automatically from the base of the same, - an effluent bright, purer than that exacted by any Rivers Board, absolutely inodorous, and fit to go into any stream without causing offence or injury of any kind

"As this method of sewage-treatment dispenses with the use of chemicals (which might be injurious to fish life), it will be found especially well adapted for use at the numerous seaside places where valuable oyster-beds <»r fishing-banks are endangered by the discharge of untreated sewage in their vicinity, and where the problem of the substitution of a harmless effluent is very difficult of solution.

"Sewage purification of a like high order cannot, by any other method, be effected so inexpensively as by this process: no chemicals are necessary, there is no sludge to deal with, the filters require a minimum of attention as there are no valves to open or adjust, the filtering material never wants washing or changing, and one man can superintend the purification of one million gallons of sewage a day, the only expense beyond the man's wages being a mere trifle for coke in winter to warm the air supplied to the microbes, and such small contingent charges as are incidental to all works of an engineering character.

"The facility with which a high degree of purification can be obtained, at a very trifling cost for fuel, during a long severe frost, when any ordinary filtration would be impossible, will, in many cases, make this filter especially valuable.

"The bacterial analysis of the effluent from the filter at Hendon (near London), where 250 gallons of raw sewage were being treated on each square yard of filter per day, equivalent to about one million gallons on an acre, has made it perfectly clear and certain that any required degree of purification can be obtained by this method of treatment, and rank sewage can, if necessary, be rendered as chemically pure as a high-class drinking-water. It is merely a question of adapting the height of the filter to the quality of the sewage to be treated and the purity of effluent desired; but that a very foul sewage can be rendered pure enough for all practical purposes by treatment in a filter eight feet high, is shown by the chemical analyses which have been made." l

1 The reader must remember that these are Col. Ducat's remarks about his own invention, and must take them with the proverbial grain of salt The statement that "the bacterial analysis of the effluent" at Hendon proves that

Tables XXXVI. And XXXVII. Prepared By Dr. A. C. Houston. .M.B., D.Sc., Give The Analyses Of The Crude Sewage, And Of The Effluent From The Tanks At

Hemlon. takes last year

Plate XVI. explains the construction of a "Ducat" filter. It will be seen that the sides of the filter are made with open drain-pipes, so placed that the outer end is abort the inner, and thus the external air can blow in, but the sewage does not come oat The upper layers of the filter, where liquefaction takes place, are composed of coarser material, while the finer material is below. Various materials are suitable for this description of filter, as polarite, burnt ballast. gravel, eoke. coke-breeze, and latterly coal, which is said to have considerable merit as a filtering medium.

The apparatus for warming the air supplied to the filter is placed in a chamber under the filter, as shown in the Plate.

Mr. W. J. Dibdins process of filtration is much the same, but has not been patented, and consequently can be used by anyone without payment of royalty. Mr. Dibdin, at the Sanitary Congress in Leeds (1897), asked his hearers "not to call any particular system ' Dibdin's', because he had nothing to do with any system except in the way of scientific investigation, carried on in the interests of sanitation";1 for want of another name, however, we must use the proscribed one, but with due apologie

In order to complete the oxidation of nitrogenous animal matter, the ammonia - to use the words of Messrs. Dibdin and Thudichum - "has to be converted into nitric acid. This change, as is well known, is effected by the action of a micro-organism, or of a series of such; and it is thus established that a typical excretory substance can be entirely destroyed by the aid of fermentathe or allied action of minute organisms without any adventitious assistance. That non -nitrogenized substances, such as starch, can, by the life-action of microorganisms, be gradually oxidised and finally converted into carbonic acid and water, is, of course, a matter of common knowledge. The problem, therefore, resolves itself into the question of how this natural method of oxidation may be best controlled and expedited."