Sewage Disposal Works: Their Design and Construction

William Charles Easdale

Sewage Disposal Works: Their Design and Construction

EAN 8596547417019

DigiCat, 2022

Contact: DigiCat@okpublishing.info

Table of Contents

PREFACE

INTRODUCTION.

SCREENS.

STORM-WATER OVERFLOW WEIRS.

DETRITUS TANKS OR GRIT CHAMBERS.

TANKS.

SLUDGE DISPOSAL.

PERCOLATING FILTERS.

EFFLUENT SETTLING TANKS OR HUMUS PITS.

SAND FILTERS.

CONTACT BEDS.

CAPACITY OF PERCOLATING FILTERS AND CONTACT BEDS.

STORM-WATER TREATMENT.

MEASURING APPARATUS.

STERILISATION OF SEWAGE EFFLUENTS.

INDEX

MANUFACTURERS OF APPLIANCES

ADVERTISMENTS.

THE SEPTIC TANK COMPANY, LTD.

S. H. JOHNSON & Co., Ltd.

THE STODDART CONTINUOUS SEWAGE FILTER

Telephone: 1843 AVENUE (3 lines) . Telegrams: KEESH, LONDON. FILTERING MATERIAL FOR SEWAGE BEDS FRANK KEEP 9 and 10 St. Mary-at-Hill, E.C.

NAYLORS’ PATENT AERATING TILES

BURN BROTHERS’ PATENT Automatic Sewage Disposal Apparatus

Geo. Kent Ltd.

MECHANICAL SEWAGE SCREENING APPARATUS (Automatic)

THE MANSFIELD PATENT STONEWARE AERATING TILE FOR FLOORS OF BACTERIA BEDS AND FILTERS

PREFACE

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In the course of the preparation of a series of articles for Surveying and the Civil Engineer, dealing with the numerous and varied types of appliances used in connection with Sewage Disposal Works, it occurred to the Author that it might be useful to many Engineers, and especially to Students, to have the whole series published in a permanent form for reference. At the same time, it appeared to afford an excellent opportunity to include full details of all the various methods of design and construction in general use, and thus provide a complete work dealing with the whole subject. The result is the present volume, which, it is hoped, will prove of value to those engaged in this branch of engineering. In any future editions that may be required, it will be the endeavour of the Author to omit any details which may have become obsolete, and to include particulars of any new methods of construction, systems or appliances, which may be brought into use from time to time, and he will therefore be glad to receive particulars of new appliances and systems as they are introduced.

W. C. EASDALE.

28 Victoria Street,

Westminster, S.W.

1910.

INTRODUCTION.

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In approaching a subject upon which so much has already been written, it may be desirable to point out that the improvements which have taken place in recent years in connection with sewage disposal are so extensive and varied, and have developed at such a comparatively rapid rate, that most treatises now in existence are in many respects more or less out of date. It is true that further developments may be anticipated in the future, but these will probably be concerned more with additions and improvements in matters of detail than of principles, which are now to a great extent agreed upon. The time would thus appear to have arrived when it is desirable to describe in detail the various methods of construction now generally adopted in the design, of present-day sewage disposal works.

In order to avoid a repetition of the usual preliminary details to be found in all the existing literature devoted to this subject, it is assumed that readers are acquainted with the nature of the problem to be solved in the design of sewage disposal works, with the varying characteristics of the different liquids included under the term sewage, and with the engineering formulæ and requirements involved in the design of tanks, filters, and similar constructional works. The present volume will thus consist exclusively of descriptions, illustrated with drawings and photographs, of the various tanks, chambers, filters, beds, and other details of sewage works, including the numerous types of appliances required in connection therewith.

In other works dealing with this subject it has been customary to use as illustrations, drawings of works actually carried out by their authors or other engineers. While these are interesting and valuable to a certain extent, their application under other conditions is limited, and their usefulness is thus much reduced. In the present volume the illustrations of the various details of construction do not, as a rule, represent actual working drawings adapted to any particular set of conditions, but are shown in diagrammatic form for the purpose of serving as suggestions to engineers in search of ideas which they can adapt to meet the requirements of any particular scheme upon which they may be engaged. It will follow that the engineer must in all cases rely upon his own practical experience and judgment in deciding to adopt any of the various methods of design and construction illustrated and described in the following pages; and it may be found that a combination of several types, or even a combination of several details of different types, combined with practical experience and mature judgment, will frequently produce the most suitable and efficient scheme.

SCREENS.

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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.

Fig. 1.—Simple Screen.

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.

Fig. 2.—Rotary Screen.

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.

Fig. 3.—Double Lifting Screen.

Fig. 4.—Mechanical Screen.

Fig. 5.—Mechanical Screen.

Fig. 6.—Mechanical Screen.

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.

Fig. 6a.—Rake Cleaning Gear for Fig. 6.

Fig. 7.—Mechanical Screen.

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.

Fig. 8.—Mechanical Screen with Rake Cleaning Gear.

STORM-WATER OVERFLOW WEIRS.

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The proper design of weirs for diverting the excess volume of sewage in times of storm has not in the past always received sufficient consideration. Too frequently it has been dealt with by rule of thumb. In the first place the position for the weir has not always been well chosen; but, as a result of the recommendations of the Royal Commission on Sewage Disposal, it will be necessary in the future to construct these weirs, in all cases which require the approval of the Local Government Board, after the screen. This is a wise precaution, as it prevents the possibility of a storm-water overflow coming into action as a result of want of attention to the screen. In this position the factor which has the greatest influence upon the proper working of such weirs is the rate of flow into