average air-pressure being 4.5 inches of water. The quantity applied when the most satisfactory results were obtained was at the rate of 263,780 gallons per acre, per day; so that at this rate the area required per million gallons of effluent of the same impurity as that experimented with, would be 3.8 acres. The dry weather flow of the sewage experimented upon is 16 gallons per day per head of the population, so that the quantity treated at the most efficient rate is equal to that from 16,486 persons per acre.

In his later development for treating the Wolverhampton sewage, Mr. Lowcock has substituted coke breeze for gravel in the body of the filter, and improved the arrangement for distributing sewage over the surface of the filter. The following table summarizes Mr. Lowcock's. results in October, 1895:

Analysis of the effluent from the settling tank as applied to filter, and the resulting effluent from filter in purts per 100,000.

The percentages of reduction are calculated on the tank effluent; if calculated on the sewage, the results of the whole treatment, tank and filter, would be a reduction of considerably over 90 per cent.

The Wolverhampton sewage is a most difficult one to deal with, as it contains a large quantity of manufacturers' and acid

waste.

As regards the refuse from dye-works and woollen factories, Mr. Lowcock states that his experiments appear to show that when the refuse is properly treated in tanks first, and the resultant tank effluent is slightly alkaline, there is no prejudicial effect on the filters or on the quality of the effluent; and in the treatment of refuse from dye-works the organic colouring matter is removed by the filter at the same time as the albuminoid ammonia.

Mr. Scott Moncrieff has also proposed a cultivation filter bed, which would seem to have been designed chiefly for Institutions or large residences.

"The filter bed is about 3 feet deep and 2 feet wide, and 10 feet in length. The entire sewage discharge and waste waters from a household of from ten to twelve persons (with

the exception of the grease, which is held back as far as possible by a grease trap) finds its way into one end of this filter bed. The liquid portion rises through a false bottom, and then through successive layers of flint, coke, and gravel, till it reaches the level of the overflow pipe, which is about two inches below the level of the invert of the drain. The depth of the filtering medium is only about fourteen inches. The cubic capacity of the filter bed is thus so small that the natural expectation would be that in a few days the filtering medium would become choked, and a nuisance result. As a matter of fact, however, the reverse of this happens, the effluent up to a certain point actually improving in quality as time goes on, and the whole process continuing to work satisfactorily and uninterruptedly for months together without constituting a nuisance."

The filter beds and channels are in duplicate to allow of periodical aeration.

My friend, Col. George Waring, the well-known sewage engineer in America, has pursued the same line of investigation upon a practical scale with a somewhat different arrangement from Mr. Lowcock's. He obtained permission in 1894 to treat a portion of the sewage of Newport, Rhode Island. The sewage was taken from "the main outlet sewer. The sewer at this point is 5 ft. wide and 5 ft. deep, and with a grade of 1 to 2,000. At the end of the wharf, 104 ft. beyond the point selected, the sewer delivers into a large settling chamber, and from this an iron pipe, laid on the bottom of the inner harbour, leads beyond the breakwater and discharges into the main channel. A storm overflow in the settling chamber. . . allows the direct discharge of sewage into the inner harbour at times when the flow is unusually large. This overflow is provided with low tide gates, but very high tides sweep over these and flood the settling chamber with salt water. The city is sewered according to the combined system. The street inlets deliver into large catch-basins, which in dry weather are little better than cesspools. Many of the sewer connections are merely overflows from the cesspools, receiving only liquid which is stale and putrid.

"The main sewer, which of necessity has very little fall, is a sewer of deposit, in which putrefaction is constantly going on. Because of these conditions, the sewage used in the experiments was often far from 'fresh,' although the analyses showed it to be of normal composition and fair average strength. It contained practically no manufacturing wastes, although at times there was evidence of the presence of gas liquor. The sewage was raised by means of a 10-inch diaphragm pump, which was

placed with a 3-inch galvanized iron suction running to within about 3 inches of the bottom of the sewer.

"The mouth of this suction was open full bore, so that a fair sample of the sewage, solids as well as liquids, might be had . . . The capacity of the pump at full stroke was 81 gallons, but its average throw during the experiment was about 28 gallons, and its average speed was five to eight strokes per minute A chemical laboratory was provided, suitably equipped for the determination of free and albuminoid ammonias, consumed oxygen, dissolved oxygen, chlorine, &c., and for the recording of meteorological conditions. The . . . pump delivered the flow . . . at will to either side of a partition which divided into two sections (for alternate use) a shallow bed of coarse broken stone . . . The function of this bed was to catch and retain the coarser solids contained in the sewage, before passing it to the tanks below." When one section was choked the flow was turned into the other. "The impurities in the section thrown out of use disappeared rapidly in its interval of rest."

The function of the straining tanks was mere mechanical sedimentation. Each tank had a diameter of about six feet at bottom; the outer sides slightly tapering inwards, and a depth of about five feet six inches, of which five feet was filled with the filtering material. See Illustrations on pages 10, 12, and 14. "From the straining apron, the sewage, freed from its coarser solids, passed to the straining tanks. Of these there were originally four similar in construction, but filled with different materials. Each tank has a total capacity of about 985 gallons. The top of No. 1 was about four inches below the delivery of the straining apron, and each succeeding tank was six inches lower than the one next before it, so that the tanks could be used in series if desired, the overflow of No. 1 delivering into No. 2, the overflow from No. 2, in turn passing to No. 3, and so on. The internal arrangement of one of these tanks is shown in Fig. 3, page 10. A is a false bottom of plank, perforated with 4-inch holes about four inches apart, and supported a few inches from the bottom on cleats. B is a galvanised iron airpipe, six inches in diameter, branching from a 12-inch air main, and delivering through the false bottom into the open space below. C is a layer of coarse broken stone (1 to 24-inch) six inches thick. . . . D is a cylindrical diaphragm, of hooped staves, resting upon the broken stone C, and dividing the surface of the tank into a circle and a ring of equal area. E is the material with which the main body of the tank, inside and outside of the diaphragm, was filled. In tank No. 2 it was fine broken stone (3 to 2-inch); in No. 3, round pebbles, of diame

ter ranging from to -inch; and No. 4, coarse white gravel, of very uniform size, free from sand, each grain being about inch in diameter. Each of these tanks was fitted with a drainage cock F near its bottom, and a hole bored through the bottom and closed with a wooden plug G, provided means for the rapid and complete emptying of any tank when desired for the purposes of cleansing.

"A smaller cock, H, was placed near the top of each tank, just below the overflow line, and from this samples were taken for analysis and examination. The partially strained sewage from the apron was delivered on the surface of one of these tanks in the circle enclosed by the diaphragm. It passed down through the central cylinder of filtering material and under the diaphragm, and rose again through the annular space outside of the diaphragm overflowing through a spout into a gutter. This gutter led the liquid either to the central circle of the next straining tank, when two or more of these tanks were used in series, or to the aerating tank, for further treatment."

As has been stated, the function of these four "strainers" was mere mechanical sedimentation. The liquid flowed slowly through them and the suspended matters, which were more or less fibrous or gelatinous in their nature, became attached to the particles of the filter, the coarser of them being deposited near the surface of the central cylinder and the finer progressing further and further into the mass. It was found that practically all of the solid matters were deposited in the central core during the downward flow of the water, and that very little work remained to be done as the liquid rose in the outside ring. This was the case when the sewage was applied at the maximum rate attained in the experiment, 8,950,194 gallons per acre, the water moving through the tank at the rate of about three feet per hour.

As a rule, sewage was passed through a strainer until the resistance from the collection of matter was so great that the liquid in the inner compartment overflowed the diaphragm. The rate at which this clogging matter gathered was very variable. It was noticed that an admixture of salt water materially increased it, evidently from precipitation of soap; and that putrefaction, as has been indicated, tended to decrease it by making soluble compounds.

The suspended matter or sludge thus removed from the sewage, mechanically, by the strainers was afterwards destroyed by emptying the strainer through the plug G. The liquid was drawn slowly through the cock, to prevent such disturbance of the sediment deposited upon the particles of the stone as a rapid flow would have caused. Air was then forced through the filter so as to induce bacterial action. By this means the cleansing was easily effected.

The effluent from the strainers was led to a distributing box, from which it escaped over a level weir, the flow being divided by movable knife-edge gates, which regulated the amounts applied to the aerators.