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SEWAGE DISPOSAL WORKS

_THEIR_ _DESIGN AND CONSTRUCTION_

BY W. C. EASDALE, M.S.E. M.R.SAN.I.

AUTHOR OF “THE PRACTICAL MANAGEMENT OF SEWAGE DISPOSAL WORKS”

155 ILLUSTRATIONS

London: E. & F. N. SPON, LIMITED, 57 HAYMARKET

New York: SPON & CHAMBERLAIN, 123 LIBERTY STREET

1910

PREFACE

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.

CONTENTS

PAGE INTRODUCTION 1

SCREENS: Simple—Rotary—For deep sewers—Mechanical—Other types 3

STORM-WATER OVERFLOWS: Diverting plate—Fixed weirs—Movable weirs 16

DETRITUS TANKS: Capacity—Dortmund type—Apparatus for sludge removal 23

TANKS: Various types—Capacities—General construction details—Flow through tanks—Sludge well—Roofs—Inlets and outlets—Precipitation tanks—Dortmund types—Hydrolytic tank—Imhof tank—Skegness tank—Candy-Whittaker tank—Non-septic cylinder—“Kessel”—Separator—Fieldhouse tank—Dibdin slate beds 29

SLUDGE DISPOSAL: Sludge removal—Apparatus—Chemical mixers—Sewage mixers—Sludge presses—Hydro-extractor for sludge—Sludge draining beds 71

PERCOLATING FILTERS: General design—Various types of construction—Floors of filters— Sub-drainage—Floor-tiles—Walls of filters—Planning of filters—Filtering material—Grading of material —Methods of distribution—Appliances for distribution —Automatic revolving distributors—Power-driven revolving distributors—Automatic travelling distributors—Power-driven travelling distributors—Fixed distributors—Troughs—Spray-nozzles—Methods of feeding percolating filters—Continuous versus intermittent supply—Supply channels and pipes-Dosing apparatus 85

EFFLUENT SETTLING TANKS OR HUMUS PITS 182

SAND FILTERS 185

CONTACT BEDS: Causes of failure—General principles of design—General construction—Methods of distribution —Sub-drainage—Material for filling contact beds —Automatic apparatus 189

CAPACITY OF PERCOLATING FILTERS AND CONTACT BEDS: Relative capacities—Table of requirements 222

STORM-WATER TREATMENT: Stand-by tanks—Methods of construction and operation 227

MEASURING APPARATUS 232

STERILISATION OF SEWAGE EFFLUENTS: Purpose and practicability—Apparatus for manufacture of hypochlorite solutions—Appliance for injection of chemicals 240

INDEX 251

SEWAGE DISPOSAL WORKS

_THEIR DESIGN AND CONSTRUCTION_

INTRODUCTION.

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._

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.

_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.

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.

_STORM-WATER OVERFLOW WEIRS._

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 the detritus tanks, i.e. the area of the inlets to these tanks. It is true that these may be regulated by the use of valves, but unless these valves, when once adjusted to the correct height, can be permanently locked in that position, it leaves them at the mercy of an unscrupulous workman, who may, if he wishes, close them entirely, and thus cause the entire flow of sewage to pass over the storm overflow weir in order to save himself the trouble of attending to the tanks and filters. It is probably with the intention of preventing the possibility of such mismanagement that the Local Government Board object to valves on the inlets to the detritus tanks. One method of preventing trouble is to use simple hand-stops, and provide the frames in both inlets but only _one_ door, so that it is impossible for the man to close both inlets at the same time. The Local Government Board are also usually averse to the use of any type of movable weir, and prefer the simple fixed weir.

_Diverting Plate._—Many ingenious devices have been adopted in the past for the purpose of ensuring the diversion of all the excess volume above a certain fixed quantity. One of these is shown in Fig. 9, where it is assumed that all in excess of the volume which is taken by the sewer flowing four-fifths full is to be discharged over the overflow. In order to facilitate this result, an iron plate is fixed at the level of the weir (say four-fifths of the diameter of the sewer), over the whole of the outlet end of the chamber or man-hole, with a sharp edge on the side facing the flow, so that when the sewage in the chamber rises above this level, the excess volume above that flowing at a depth of four-fifths of the diameter of the sewer, is automatically diverted by the plate and caused to pass away over the weir. The invert of the chamber must naturally correspond with the diameter of the sewer.

_Fixed Weirs._—Even this ingenious method of diversion is, however, not accurate, as no provision is made to counteract the effect of the increased head on the outlet from the chamber, due to the backing up of the sewage in passing over the weir. Where a fixed weir is alone permissible, the only really satisfactory method of securing the desired result, is to increase the width of the overflow weir to such an extent that the maximum depth of storm-water, which may possibly flow over the weir, is reduced to the minimum, say one inch, and thus the effect of this head on the normal outlet from this chamber (i.e. on the inlet to the detritus or sedimentation tanks) is also reduced to the minimum. This will necessitate careful consideration, and a special set of calculations in each case. Where it is found that the execution of the above suggestion involves the construction of a weir of abnormal and unpractical width, it will be found convenient to arrange the normal dry-weather outlet from this chamber in the form of a narrow vertical slot, which can be most easily provided in a simple door or stop in a grooved frame, fixed in the outlet from this chamber. Fig. 10 shows an example of this slotted door, and when the correct width of the slot has been ascertained by actual experiment, the door should be bolted to the frame, so that it cannot be removed or altered by any unauthorised person. From the drawing it will be seen that it is not difficult to calculate the dimensions of the slot orifice, so that with the head due to the height of the storm overflow weir it shall discharge the desired volume (say three times the dry-weather flow), and if the width of the overflow weir is then calculated to take the excess volume with a depth of one inch of water over the weir, this extra one inch of head will have very little effect on the discharge through the slot outlet.

_Movable Weirs._—If, however, it is desired to provide for an absolutely correct diversion of the storm-water, this can only be done by the use of a movable weir. There are two types of this form of weir on the market at present, both manufactured by Messrs. Adams Hydraulics, Ltd. Fig. 11 shows a floating weir, circular in form, arranged by means of floats to rise and fall freely with the level of the sewage in the chamber. The joint between the fixed and moving portions of the apparatus consists of an air-lock, and is thus frictionless. The floats are adjusted to bring the lip of the weir at such a depth below the top water level, that the volume which can pass over the weir without raising it is the maximum volume which it is desired to pass to the tanks and filters. As soon as the flow of sewage exceeds this volume, it naturally causes the floats to raise the lip of the weir, and in this way the volume passing to the tanks and filters can never exceed the predetermined fixed volume, and all in excess must pass over the overflow weir. Fig. 12 shows a swinging syphon, which has the same effect as the floating weir. In this case the syphon has both legs trapped, so that it acts as a continuous syphon, and it is pivoted on the top of the division wall to swing freely. To the inlet leg, on the sewer side of the division wall, is attached an adjustable float, of sufficient buoyancy to raise this leg of the syphon (and with it the outlet leg as well) as the sewage rises in the chamber. It will be seen that the difference in level between the lip of the inlet leg and the buoyancy point of the float, represents the head which controls the maximum rate of flow through the syphon, and that immediately this is exceeded the float rises, and with it the syphon leg, so that all the excess volume of sewage, above the fixed maximum rate of flow through the syphon, must of necessity pass over the storm-water overflow weir.

_DETRITUS TANKS OR GRIT CHAMBERS._

The function of these tanks is to arrest all mineral matter, such as stones, sand, road-grit, and similar substances which cannot be decomposed in the subsequent stages of treatment, and would thus choke the tanks and filters. The essential factor in their operation is a reduction of the rate of flow of the sewage, so that all matters of a greater specific gravity than the water and the organic matters in suspension may be deposited by subsidence. At the same time the velocity should not be reduced to such an extent as to allow the organic matters in suspension to settle out, as these can be more suitably dealt with in the subsequent tanks provided for that purpose. From this it will be seen that considerable care is needed in designing these tanks if they are to have the desired effect. Further, it is very essential that every facility shall be provided for removing the matters which are deposited with as little trouble as possible.