On arrival at the disposal works, the first stage of the process through which the sewage passes is generally that of screening for the purpose of arresting the grosser solids in suspension. In a number of cases where the sewage is delivered by gravitation, there are no screens of any kind in use, reliance being placed upon the detritus chambers to perform the duty of arresting the floating solids, as well as the grit and other mineral matters, of such a specific gravity that they are readily deposited by simply reducing the velocity of the flow. Where the levels involve the use of pumping plant, screens are a necessity, and, as the Royal Commission on Sewage Disposal have expressed the opinion that all sewage should be screened, it will apparently be necessary to provide screens in all future schemes.

Simple Screens.—The simplest type of screen is in the form of a grating, consisting of vertical iron bars in a stout iron frame, arranged to fit into grooves cut in the side walls of the screen-chamber, or in channel-iron guides attached to the sides of the chamber. As a general rule the vertical bars are round in section, but some engineers prefer to use flat bars with their longer side parallel to the line of flow, while others even go so far as to use wedge-shaped bars with the thick end facing the flow of sewage. In the latter case, the idea is to facilitate the passage through the screen of those matters which are too small to be arrested on the front of the bars, but large enough to be caught between the bars, and thus possibly choke the intervening spaces. As all simple fixed screens must of necessity be cleaned by hand, they are usually arranged at an angle of about 60 degrees to the floor of the chamber, in order that the matters arrested may be more easily drawn up by a hand rake to the top of the screen. Fig. 1 shows a screen of this type in plan and section, with a large scale detail of the round, flat and wedge-shaped bars previously described. It will be noticed that a narrow platform of boards is shown across the chamber, at the top of the screen, to receive the screenings, which are then thrown into a barrow for removal to their final destination. One important point to be remembered in the design of the chamber for screens of this type is, that the bottom of the screen should be placed in a sump some 12 inches or so below the invert of the incoming sewer, so as to provide space for the accumulation of a certain amount of sludge and screenings without choking the screen. This sump should be provided with a washout valve. It is advisable to have all screen-chambers in duplicate, so that one of them may be in use while the other is being cleaned. The spaces between the bars vary in width with the character of the sewage, but the distance most generally adopted is half an inch. The important point to be considered is, that while the screen should arrest all the larger suspended matters it is intended to intercept, it should allow a free passage to all others without becoming rapidly choked. Another important factor in the efficiency of a fixed screen is its width. The greater the width, the less will be the liability to choke, and consequently it will not require raking so frequently to keep it in proper order.

Rotary Screens.—Where the flow of sewage is sufficient for the purpose, and it is desired to reduce the necessary attention to the minimum, the self-cleansing rotary screen, manufactured by Messrs. John Smith and Co., may be adopted. This is illustrated in Fig. 2, from which it will be seen that it consists of a revolving wire screen, extended between two rollers, one below and the other above the sewage level. The upper roller is rotated by means of a water wheel driven by the sewage. A rotary brush is fitted to the shaft and driven in the opposite direction to the screen roller, so that it brushes off the screenings into a trough, from which they are removed by hand.

Screens for Deep Sewers.—In cases where the depth of the sewer makes it inconvenient to adopt a fixed screen, the double lifting screen, manufactured by Messrs. Adams Hydraulics Ltd., may be used, as shown in Fig. 3. This consists of duplicate screens, arranged to slide up and down in cast-iron guides attached to the walls of the chamber. These screens are raised and lowered by a chain, which passes over a drum revolved by hand. The main screen is in the form of a basket, with a hinged front, which falls to the floor of the chamber when this screen is lowered into position. When it is desired to clean out this basket screen, the other plain guard screen is lowered into position in front of the basket-screen, and the latter is then raised. As the chain by which the basket-screen is raised is attached to the top of the hinged front, the action of raising this screen first draws up the hinged front and this prevents the screenings falling out. After this screen has been emptied, it is again lowered into position, and the guard-screen raised to permit the sewage to flow direct into the basket-screen.

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Mechanical Screens.—In larger schemes, where power is available for the purpose, mechanically operated screens are frequently adopted, as they are not only self-cleansing but the screenings are delivered automatically at or above the ground level, and thus very little labour is involved in removing these matters. Figures 4, 5, 6, 7 illustrate four examples of this type of screen, manufactured respectively by Messrs. Ham, Baker and Co., Ltd., Messrs. J. Blakeborough and Son, Ltd., Messrs. S. S. Stott and Co., and Messrs. J. Wolstenholme and Co. The general features of these screens are an inclined screen or strainer, fixed in the channel or catchpit through which the sewage flows to the tanks or to the pumps, and a raking apparatus with special shaped prongs, which travel in the spaces between the bars forming the screen and remove the refuse. The Stott screen includes a rake cleaning gear, consisting of a revolving steel comb, by means of which the screenings are removed from the prongs of the rake while they are in motion.

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In the case of the screen, manufactured by Messrs. Whitehead and Poole, illustrated in Fig. 8, the bars are of tapered steel, and are so arranged that they can be removed and replaced if necessary. The special friction drive with which this machine is fitted, prevents the breaking of the chain should the rake prongs become caught in the screen. The rake-cleaning gear consists of two swing levers, which carry a cleaning comb and a balance weight to hold it in position over the dirt tray. As the rakes bring up the screenings and reach the delivery position, they pass through the cleaning comb, which is, at the same time, forced down by a catch on the chain engaging with flanged rollers on the end of the swing levers. In this way the rakes are effectively cleaned, and it is impossible for the rake prongs and the comb to foul each other.

In addition to the screens already described, mention may be made of the special drum-shaped screen invented by Mr. Baldwin Latham and the numerous types of mechanically-operated screens in use in Germany, all more or less elaborate in character. Further details of these are probably unnecessary, as the aim of the engineer engaged in the design of sewage disposal works should be to adopt those appliances which are of the simplest possible form consistent with the requirements of the case with which he is called upon to deal. Some engineers prefer to use screens specially designed by themselves to meet the requirements of each particular scheme, and while this method provides scope for the exercise of a considerable amount of ingenuity, it is liable to involve greater expense than would be incurred by the adoption and possible adaptation of one of the various types already on the market.

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