still others to biological oxidation. The last are uneconomical contrivances for effecting the purposes of the first; and, theoretically at least, the greatest efficiency in the plant as a whole is obtained by performing the rough work by some rapid clarification process, and the finishing purification (when necessary) by the more sensitive aerobic filter. But the available area and materials, character of sewage, fall available, etc., may outweigh these purely theoretical conditions. Moreover, the combining of the two general functions in one appliance, although one of them may be effected uneconomically, may produce an economy of combined action greater than would be possible by two appliances or operations, because of the complication of operations thus introduced.

The general structures and appliances for treating sewage consist of strainers for removing coarse suspended matters; tanks for sedimentation; septic tanks for developing septic action in sediment; the treatment of sewage with coagulants to hasten and increase precipitation; "hydrolytic " and Imhoff tanks for utilizing surface adhesion in clarifying sewage and for securing more advantageous septic action; filters of coarse material— contact filters for removing suspended matter by surface adhesion and incidentally by straining, together with bacterial action upon the organic matter so removed; other coarse-grain filters in which suspended organic matter removed by surface adhesion is so modified as to be less putrescible-sprinkling or trickling filters; fine-grain filters in which straining and surface adhesion remove a large part of the suspended organic matter and also bacteria, and effect a large amount of purification by oxidation; the irrigation of cultivated land with sewage; treatment with disinfectants to kill off most of the bacteria; together with some other contrivances and processes advocated for various purposes but not in common use.

ART. 72. STRAINING

All processes of purification involve the removal from sewage of suspended matter. Screens, coarse and fine, are used for

removing all grades of such matter, from sticks, rags, etc., to that passing a % of an inch opening. Coke and other coarsegrain filters are sometimes used for the same purpose. Sedimentation in tanks may remove a large part of all but colloidal matters, and more or less of these, the use of coagulants increasing this amount. By introducing suitable surfaces, part of the colloidal matters is removed by "surface adhesion " to these, the matter so collecting falling off from time to time in large flakes. Tanks have been built containing large numbers of horizontal surfaces, placed a few inches apart vertically, on which suspended matters collect. Other tanks are filled with stones or sand which remove suspended matter partly by straining and partly by surface adhesion. In each kind of tank other processes are undergone by the sewage, which will be discussed under the head of " tank treatment."

When the sewage is to be treated on any kind of filter, the previous removal of coarse suspended matter is more important than is generally appreciated. "The volume of sewage which may be successfully purified upon a given filter area is inversely proportional to the amount of suspended matters in the sewage applied. In other words, if the whole or a part of the suspended matters are removed from the sewage by some treatment preliminary to filtration, the filters can be operated at much greater rates and a smaller area will be required for treatment of a given volume of sewage." (Report of Mass. State Board of Health, 1908.)

The simplest strainers are of rods or wire screens of galvanized iron, copper, etc. These are generally placed vertically between the sewer outlet and the tank or filter, or the pump suction. The iron rods used in this country are generally to inch in diameter and spaced with - to 2-inch clear opening, although the spacing has been as great as 6 inches. Rod strainers are placed either vertical or inclined as much as 45 degrees from the vertical. Material can be removed from them with rakes, and they are cleaned more easily than are screens, but are not so effective.

In some installations, the rods are bent and joined to form a

cage or basket.

At Glasgow, Scotland, a screen of rod links, passing over two wheels like a link belt and inclined 45 degrees, its lower loop being in the sewage, removes the larger matters and raises them to an elevated platform. According to the late Emil Kuichling, coarse screens remove from 3 to 10 cubic feet of material per million gallons of sewage, equivalent to about 104 pounds of dried matter, or 12 parts per million of suspended solids.

Strainers of coarse particles such as coke or buckwheat coal have been used but little in this country. Coke and coal are used partly because of the possibility of burning the organic matter removed by the strainer, by using the strainer material as fuel when it is removed. Tests by the Massachusetts State Board showed that coke breeze (including pulverized coke) in a bed 12 inches thick (gradually reduced to 3 inches by removing clogged material) removed 57 per cent of the bacteria and 74 per cent of the suspended albuminoid ammonia. Screened coke removed 72 per cent of the bacteria and 59 per cent of the suspended albuminoid ammonia. Fine bituminous coal removed 70 per cent of bacteria and 65 per cent of suspended albuminoid ammonia. Buckwheat anthracite (between -inch and 1-inch mesh) removed 56 per cent of bacteria and 56 per cent of albuminoid ammonia. All were operated at a general rate of 1,000,cco gallons per acre daily. From the breeze bed were removed 8 cubic yards of coke per 1,000,000 gallons of sewage strained; from the screened coke 0.4 cubic yard; from the bituminous coal 0.8 cubic yard; and from the anthracite o.8 cubic yard.

At Columbus screened 1-inch coke was used to strain sewage at the rate of 1,500,000 to 3,000,000 gallons per acre per day. The results were fully as good as those just described, but there was considerable putrefaction with objectionable odor; and 5.5 cubic yards of coke were removed per 1,000,000 gallons strained. They were cleaned at from two- to eight-week intervals. Drying the coke for burning required from two to four weeks when it was spread upon land, and an objectionable odor was given off. The results obtainable do not seem to warrant the general use of this method, with its objectionable features; although improve

ments in structure or operation may perhaps be devised to meet the objections.

Fine screens of modern design are comparable to sedimentation tanks in the amounts and fineness of suspended matter removed. There is no accepted limit between fine and coarse screens. Kenneth Allen would make it of an inch, but others would place it as low as 1 of an inch. The finer the screen the greater the percentage of removal of

suspended matters, of

course.

FIG. 65.-DIAGRAMS OF VARIOUS TYPES OF SCREENS.

The following is greatly condensed from a paper by Mr. Allen before the Am. Soc. of Civil Engineers and a discussion thereof, this being the most exhaustive compilation on the subject up to the year 1918. The use of fine screens is just beginning to be seriously considered in this country, but will probably increase in the near future.

Fine screens may be divided into five general types:

1. The band screen, consisting of an endless flexible band, of either wire mesh or links, which passes over upper and lower rollers like a belt.

2. The wing screen, formed of vanes, as in a paddle wheel, which are composed of radial bars at uniform distances apart.

3. The shovel-vane screen, similar to the wing screen, but with semicircular wings and a different method of removing the screenings.

4. The drum screen, consisting of a cylinder or truncated cone of perforated plates or wire mesh, which rotates on a horizontal axis.

5. The Riensch-Wurl screen, which consists of a perforated disk surmounted by a truncated cone, also perforated, both mounted on an inclined shaft.

The first type is represented in this country by the Jennings screen, which has been in service at the Chicago stock yards since July, 1913. It consists of separate sections of removable screens of Monel metal, 40 meshes per inch. The screen is cleaned by blowing off the matter adhering to it by use of air under 1 pounds pressure. Screen and blower together are operated by a 10 H.P. motor at a cost of 12 to 15 cents per hour, 1 to 1 million gallons a day being screened. A band screen of parallel links in Hamburg, Germany, is cleaned by a comb scraper. One of wire mesh in Göttingen is cleaned by a brush and water jets.

Wing screens are not used in this country, but are found at three German installations and one English one. Each wing or vane is 5 to 10 feet long and is formed of parallel bars set to

inch apart. The vanes revolve against the current. Each vane is cleaned by a hard rubber scraper which, as the vane rises, travels from its inner to its outer end, and is followed by a brush. One screening, 21 to 26 million gallons a day, requires 2.3 H.P. to operate and cost II cents per year per capita of population served.

The shovel-vane screen is a modification of the wing. The vanes are semicircular in section and the stationary axle is hollow and quite large, with its top third open. As each vane rises it scoops up the suspended matter, which slides slowly toward and finally drops into the hollow axle. In this axle is a belt conveyor, which receives and removes the screenings. An arm carrying a brush at its end swings about a pin at the center of the arc of each vane and automatically brushes off adhering matter as the vanes revolve. The screen is about 12 feet diameter.