Percolating Filters
In approaching the subject of the design of filters for the purpose of oxidising the organic matters—in solution and in suspension—contained in the liquid which leaves the preliminary process in tanks, the first consideration is the question of site. Where the slope of the ground permits of the construction of the filters on or above the ground level, much expense for excavation may be avoided, so long as the base of the filter can be laid on solid ground. In cases where the site of the works is comparatively flat, it is impossible to avoid excavation, and other means must be adopted to keep the actual cost as low as possible consistent with efficiency.
General Design.—Taking the latter case first, it should be observed that some engineers consider it desirable to construct retaining walls under all circumstances, but the author does not agree with this idea. In the first place the walls do not, of themselves, have any influence on the efficiency of the filters in producing a satisfactory effluent, and if a filter can be constructed without them there is no reason why they should not be omitted. This applies especially where the filters have to be constructed entirely below the surface of the ground. The chief point to be considered is that the effluent shall have a free outlet, with facilities for inspection. In the case of all filters, the best method of securing a free outlet for the effluent is to provide the floor with a suitable slope from the centre to all sides. When the floor is at some depth below ground this requirement necessitates an effluent channel on all sides of the filter. Two methods of carrying this into effect are illustrated, Figs. 59 and 60. Fig. 59 shows one retaining wall for the filter and an additional retaining wall for the surrounding earth, carried up to the surface with the effluent channel between the two walls. In Fig. 60 the arrangement is similar, but the outer wall is a dwarf wall to form the effluent-channel, and the surrounding earth is cut back to a slope of natural repose, the earth bank usually being sown with grass or covered with turf. It is essential that the outer wall shall be carried up above the toe of the earth bank, in order to prevent soil being washed down into the effluent-channel, but a surface-water drain should be laid to take any water that may accumulate at the back of this wall.
These methods of construction would, however, only be adopted by those who consider it essential to provide means for lateral aeration to the filter by constructing the retaining wall for the filter itself of pigeon-hole brickwork. In the author’s opinion, however, lateral aeration on these lines is altogether ineffective. It can only affect the filter material for a distance of a foot or two from the wall. It is true that in some cases horizontal perforated ventilating pipes have been provided, radiating from the centre of the filter to the outer wall and terminating with open ends on the outside. The effect of these is, however, dependent almost entirely upon the temperature of the atmosphere and the direction of the wind, and even if they do induce air currents into the body of the filter the air will pass along the line of least resistance, and therefore find its way through those interstices which are open and not through those which are choked in any way and thus most in need of aeration. The author believes that the aeration of a filter is most effectively secured by the action of the sewage itself, as it falls from the surface to the floor drawing in the air from the top, and that if this is not effective, no amount of lateral openings will produce the desired result. If this contention is correct, there is no need to incur the additional expense involved in the construction of retaining walls, and the filters may be designed on the lines indicated in Fig. 61, where the filtering material fills the whole of the natural excavation, and no walls are required. It is true that this method involves an increase in the amount of filtering material beyond what is actually in use, and the cost of this must be set against the cost of the wall in the alternative method shown in Fig. 62, as the cost of the excavation in either case is about the same. Much will depend upon the cost of the filtering material in different localities.
In both these methods it will be noticed that there is no outer effluent channel, and that the floor slopes from the circumference to the centre, where an effluent receiving chamber is constructed, from which an effluent discharge pipe leads to the next stage in the process. This is not quite so satisfactory as the arrangement shown in Figs. 59 and 60, but it is the most convenient under the circumstances.
When it is possible to place the filter floor on or within 2 feet of the surface of the ground, the method illustrated in Fig. 60 is the design most commonly adopted. Sometimes the arrangement of the floor shown in Figs. 61 and 62 is preferred. There is, however, still another type of floor which is applicable to this case. This is illustrated in Fig. 63, from which it will be seen that the whole of the floor slopes in one direction, and the effluent is thus discharged over one-half of the circumference of the filter, with the result that the effluent channel is only one-half the length of that required in the case of Fig. 60.
Floors of Filters.—In all cases the floors for percolating filters should be of a substantial character. They are usually constructed of cement concrete. The thickness of the concrete will depend upon the nature of the subsoil, but in any case it should be increased at the centre in order to provide a safe foundation for the revolving sprinkler. The surface of the floor should be smooth, so as to facilitate the flow of the effluent, and any suspended solids it may contain, to the outlet. From the preceding illustrations it will be noted that the slope of the floor may be in three directions: (a) from the centre to the circumference, (b) from the circumference to the centre, or (c) from one side of the filter to the other, a uniform slope in one direction along the diameter of the filter. It has been suggested by some that the floor should consist of a series of alternate V-shaped ridges and furrows, with slopes at right angles to the general slope of the floor, but this has the disadvantage that it causes difficulties in arranging the floor tiles and placing the filtering material in position, and further it increases the cost of construction without providing any real compensating advantages.
Sub-drainage.—It has been recommended that the floors should be laid with a gradient of about 1 in 100, but the author believes that it would tend to assist the free discharge of the suspended solids in the effluent if this gradient were increased to 1 in 75, or even 1 in 50, and that if this were done, there would be no need to make any special provision for access to the under drains for cleaning purposes. Another important factor in securing a free discharge of the suspended solids, is the use of proper floor tiles or sub-drains. The old idea of laying rows of agricultural pipes with open joints, or even of perforated pipes, on the floor has been proved to be useless. In some cases it is considered sufficient to have rows of floor tiles, laid at any distance apart up to 10 feet at the circumference. There is, however, very little doubt that the only correct method is to support the whole of the filtering material on a complete false floor, so that the suspended solids which are carried down with the effluent, and thus reach the bottom of the filter at all points of its area, may fall freely into an open space, from which they will be carried away with the least possible obstruction by the flow of the liquid. It is certain that these suspended solids have a great tendency to adhere to any object with which they may come in contact, and that this can only be avoided by providing a free space immediately above the floor and over its whole area. The nearest approach to this acme of perfection is a complete false floor, with openings too small to allow the filtering material to pass through, but large enough to give a free passage to the suspended solids. One of the simplest methods is to form the false floor of bricks, laid flat on rows of bricks on edge. There are, however, several floor tiles on the market specially designed for this work. A number of these are illustrated in the accompanying figures. The Ames-Crosta tile, Fig. 64, has a simple flat top with corrugated edges supported on four short legs. When placed close together, the corrugations form apertures through which the liquid passes, and the flat tops form a table which provides every facility for placing the filtering material in position. A somewhat similar floor tile is the “Newham” shown in Fig. 65. In this case, however, each tile has only two legs, and thus there is less obstruction to the flow of the effluent. The “Stiff” floor tile, Fig. 66, is of much the same type as the two last described. Another material which has been adapted for this purpose is a single layer of the “Dibdin” type of slate slabs and blocks, as shown previously in connection with the preliminary treatment of sewage in tanks, Fig. 49. This method also provides a complete flat false floor, and has the additional advantage of occupying the least possible space. The slabs themselves may be split to any suitable thickness, and the supporting blocks may be cut to any size, as little as 1 inch in thickness if desired. The slabs vary in shape and size, but may be secured of larger area than tiles, so that the necessary supporting blocks are less in number than the feet of the tiles, and thus offer less obstruction to the effluent. The apertures for the passage of the liquid are provided by the holes formed by the irregular edges of the slabs, where they abut on one another.
There are, however, floor tiles of other shapes, such as the Candy tile, Fig. 67, for which it is claimed that the effective depth of the filter may be calculated from the floor, and that the height of the V-shaped opening under the tile provides greater drainage and ventilation openings than other tiles. Other drainage tiles are semicircular in section. The Naylor tile, Fig. 68, and the Albion floor tile, Fig. 69, designed and used by Mr. E. E. Ryder, Surveyor to the Bushey Urban District Council, both have openings at the floor level. The “Mansfield” floor tile, Fig. 70, is the type adopted for the extensive filters at the Birmingham Tame and Rea district sewage farm by Mr. J. D. Watson, Engineer to the Drainage Board.
In some cases, instead of providing a free space above the floor, drainage channels have been formed in the floor, with rebates in the sides to receive flat perforated tiles, as Fig. 71. This method of drainage certainly leaves the floor clear to receive the filtering material, but it has the disadvantage that it does not provide a free space under the whole of the material; and as the suspended solids in the effluent must of necessity travel some distance before they reach a channel, they are, to a great extent, arrested by the material which comes immediately on the floor, however large the separate pieces may be, and thus tend to choke the interstices.
Whatever method of sub-drainage may be adopted, it is advisable to continue the tiles, or channels, right through to the effluent channel or chamber in straight lines, so that the effluent may have a clear way throughout its course under the filter, until it reaches the open effluent channel or pipe.
Walls of Filters.—The great diversity of methods adopted to retain in position the material of filters constructed above the level of the ground, must be extremely perplexing to anyone who investigates the subject. They vary from walls of such thickness and substantial material that they would be quite suitable for resisting the pressure of a head of water equal to the depth of the filter, down to no walls at all. In many cases walls 9 inches thick throughout and 6 feet in height are found to be quite satisfactory, while in a large number of cases the material itself, when it consists of clinker, is found standing in an almost vertical position with perfect success. The question naturally arises, if it is possible to construct filters satisfactorily without any retaining walls, why incur unnecessary expense in providing walls? The answer is that clinker is not universally adopted as a filtering material, and that the strength of circular walls is not sufficiently taken into consideration. The thickness of the wall will naturally depend upon its height, but assuming this to be 6 feet, and that the wall is in all cases carried down to a solid foundation, the extreme thickness that would be needed is probably 18 inches at the bottom and 9 inches at the top, as Fig. 72. As a general rule, however, it will probably be found quite sufficient to construct the wall of brickwork in cement, with the lower half 14 inches and the upper half 9 inches in thickness. In a large number of the most recently constructed filters, the walls have been omitted altogether when clinker has been used for the filter. Large pieces of this material have been selected, and carefully packed in the form of a dry rubble wall with perfect success, even when laid with a batter on the outside of as little as 1 in 6 or even 1 in 8. It is obvious that this method of construction needs special care and supervision, especially in providing a rough kind of bond between the different layers, if it is to be successful. A rough idea of this method is given in Fig. 73. As a kind of happy medium between this system and a wall to the full height of the filter, a dwarf wall has been adopted in a number of cases, and notably by Messrs. Willcox and Raikes, Civil Engineers, who have also designed a special form of coping, made in fire-clay or terra-cotta, as shown in Fig. 74. Again, in several places where suitable local stone, or stones and bricks resulting from the demolition of old houses or walls, was available at a cheap rate, these materials have been utilised to form dry rubble walls, similar in construction to the clinker walls illustrated in Fig. 73.
Occasionally the walls of filters are carried up several feet above the surface of the filter with the idea of forming a screen to protect the revolving sprinklers from the influence of wind. It is, however, obvious that these measures can only have an effect upon the outer ends of the revolving arms, and, as the action of the revolving sprinklers is only affected by wind when the head of sewage by which they are driven is at its minimum and the force of the wind is simultaneously at its maximum, the necessity for windscreens seldom arises. In actual practice, with an average head of sewage, no real difficulty is caused by wind.
So far, it has been assumed that the walls and floors of the filters can be constructed upon solid ground. Unfortunately this is not always possible. If the subsoil is of an unsubstantial nature it is advisable to lay the floor of a suitable thickness and diameter, so that the wall may be built upon the floor itself, and thus distribute the load over as large an area as possible. In some cases the levels necessitate the raising of the filter floor above the surface of the surrounding ground. Under these circumstances the footings of the walls should be carried down to solid ground, and no reliance whatever should be placed upon made-up ground even to carry the floor alone. The outer edge of the floor should be supported by the footings of the wall, and the remainder carried upon piers or cross-walls of brickwork, concrete or masonry, extended down to solid ground. The piers or cross-walls should be sufficient in number to support the floor with safety. A smaller number may be used if steel joists are provided between them to take the weight of the floor and filter, or the whole floor may be of a properly designed reinforced concrete construction, supported at the circumference on the footings of the outer wall and at the centre on a substantial pier of concrete carried down to solid ground, with intermediate piers if the diameter of the filter is excessive.
Planning of Filters.—The preceding notes with regard to floors, sub-drainage and walls, apply generally to all filters, whether they are for revolving or travelling distributors or for sprays or fixed troughs. The most suitable plan for travelling distributors is naturally rectangular, but where more than one such filter is required it will be found economical to arrange them in pairs, with a central supply channel feeding two distributors, one on each side. This applies equally to other types of fixed distributors, but care should be taken to divide the total area into reasonable units. In all except the smallest schemes (which should consist of not less than two units) it will be found that three, or multiples of three, units form a very convenient method of arrangement. This suggestion with regard to the subdivision of the total area applies to filters for revolving distributors, so far as their number is concerned, but there are several methods of arranging the area of the filters themselves. When the filters are separated from one another they may be placed in regular order of one kind or another if the site is uniformly flat, or irregularly to suit the contour of the ground if the site is uneven. It has been considered an objection to the use of circular filters that even if they are placed close together a considerable amount of space is rendered useless. There may be some justification for this objection in cases where the area of the site is limited. On the other hand, the objection has been overcome by arranging the total filter area on one common floor, fixing the revolving distributors so that the circumferences of the areas they cover meet where possible, and leaving the spaces not covered by the distributors free of material to provide some lateral aeration for whatever it is worth, or as an alternative to provide a convenient position for chambers to receive the effluent from the contiguous filters. This arrangement is illustrated in Fig. 75. If other arrangements are made for the discharge of the effluent, then the spaces in question may be filled with material and utilised as filters by fixing smaller suitable distributors, as Fig. 77. This has been done at Kingston-on-Thames in converting existing rectangular contact beds into percolating filters, while at Darwen, Lancs, the intermediate spaces not covered by the revolving distributors are utilised as filters and the distribution effected by means of fixed sprays.
In other cases the filters for revolving distributors have been constructed octagonal in plan instead of circular, as shown in Fig. 76. By constructing suitable retaining walls the whole scheme presents a very good appearance, and the intervening spaces can be utilised for effluent chambers or as filters with smaller distributors, as previously described, and practically the whole area is thus utilised.
Filtering Material.—On this subject there exists a great diversity of opinion. Some engineers are satisfied to use any kind of material which will not disintegrate rapidly, while others pin their faith to one particular kind. Again, the grading of the material is a matter upon which it is seldom possible to find two engineers in complete agreement. The opinion is frequently expressed that true economy consists in utilising local material as far as possible, sometimes even to the extent of adopting a local product, even though it is admittedly not so good as some other material which may cost slightly more for carriage from a distance.
Undoubtedly the first consideration is to secure a material which will not disintegrate, but this is not the only essential qualification. The author has had many opportunities of observing the results obtained from various materials, and, for dealing with an average sewage, he has never seen a better material than hard-burnt vitrified furnace clinker. This material, of the proper kind, is practically equal to stone or gravel in its ability to withstand the various influences which tend to cause disintegration, but it possesses the advantage over stone and gravel of having numerous cavities, which apparently form the most suitable means of assisting the development of the bacterial gelatinous growth, which appears to be the essential factor in causing the deposition of the organic matters in suspension and in solution in the tank effluent. It would seem as if the smooth surfaces of gravel or broken stone cannot retain this growth, and that it is washed away as soon as it begins to form. It is true that excellent effluents are obtained from filters of gravel or stone, but, so far as the author is aware, only by providing a larger cubic capacity of filter than would be required if proper clinker were used. It is thus questionable whether the lower cost of local stone or gravel does, in fact, result in ultimate economy, if a smaller quantity of the right kind of clinker, at a slightly higher price per cubic yard, will secure the same result.
It will be noticed that stress is laid upon the necessity of using the right kind of clinker. This is intentional, as the word “clinker” appears to cover a large variety of material. House refuse, cinders, and over-burnt bricks, as well as the products of refuse destructors, are all considered as “clinker,” especially in cases where a contractor finds he has taken a very low price for filter material in making up his tender. In the author’s opinion, the only kind of clinker, indeed the only kind of material, which should be used for percolating filters is the extremely hard clinker from boiler furnaces, more or less vitrified throughout, and not only of irregular shape, with a rough surface, but possessing numerous cavities on all sides. Clinker of this type is occasionally to be obtained from destructor furnaces, but it depends upon the character of the original refuse, and probably to some extent upon the method of stoking. In any case it necessitates the exercise of experienced judgment and discrimination in selection, and in some cases destructor clinker is so soft, and so evidently certain to undergo rapid disintegration, that it should be rejected at all costs.
Among other materials which are used for percolating filters in various parts of the country are coal, broken saggars, stone of various kinds, including granite, gravel, broken bricks, coke, cinders, coke-breeze, and slag from ironworks, but in the author’s opinion none of these are so satisfactory as the right kind of clinker described above.
Grading of Filtering Material.—There is probably as much, if not more, diversity of opinion on this point as in the matter of the kind of material most suitable for filters. The sizes in actual use vary from ⅛ inch to 3 inches and even 6 inches. Some engineers stipulate for a uniform grade throughout the filter, others prefer to have different grades at different depths, while still others are satisfied to allow small and large pieces to be mixed together indiscriminately. In the author’s opinion the last mentioned method is the least satisfactory of all. One of the essential factors in obtaining the maximum efficiency from percolating filters is a free passage for air to enter into all parts of the filter. This can only be secured if the interstices between the pieces of material are kept clear at all times, but when small and large pieces are mixed together, the small have a natural tendency to fall into the spaces between the large pieces and thus choke them. Even if this were not of importance, the usual methods of filling a filter do not permit of the uniform distribution of the finer particles among the larger, so that the usual result is that some portions of the filter consist almost entirely of fine and others of coarse material, and the results must of necessity be unequal.
On the face of it, the division of a filter into layers, each consisting of a different grade of material ranging from coarse at the bottom to fine at the top, would appear to be an excellent idea, but it is only those who have attempted to put this idea into practice who appreciate the extreme difficulty of carrying it out. With the exception of very small filters, where constant supervision and an unlimited amount of labour is available, it will be found impracticable under ordinary circumstances.
There still remains the first-mentioned alternative, viz. to have the material as nearly as possible of a uniform grade throughout the filter. Whether the grade should be fine, medium or coarse, will depend upon the strength and character of the liquid to be treated, but in either case the best practice from all points of view is, in the opinion of the author, to provide all of a uniform grade. In making this statement, it is assumed that there will be a layer about 6 inches deep of coarser material all over the floor, as this is necessary to prevent the finer grade above it being washed through in the effluent or choking the apertures in the sub-drains. In some cases also a modification is adopted in providing the top layer, about 6 inches to 12 inches in depth, of finer material than the bulk of the filter, in order to arrest any suspended solids that may be present in the tank liquor, upon the surface of the filter, from which they can be more readily removed than if they were allowed to enter the filter. On the other hand, some prefer to allow these solids to enter the filter, but to provide ample clear interstices and sub-drainage, so that these solids after treatment in the filter may be washed out as humus or “converted products” in the effluent, from which they are easily removed by settlement in suitable tanks to be described later.
Methods of Distribution.—The question of distribution is the most important factor in the successful operation of percolating filters. At first it was considered sufficient simply to spread the liquid as evenly as possible over the surface of the filter by any convenient method. Gradually improvements in methods were introduced with varying results, and at the present time a large number of different appliances are to be found in actual use, some producing jets, others a fine spray, and others again what is termed a “thin film.” All of these have their advocates, and all undoubtedly can be made to give satisfactory results. One of the essential requirements of any method is that it shall give even distribution per unit of superficial area of the filter. Whether the jet or the spray or the thin film is the most efficient in this respect is a matter of opinion.
On this question of distribution, one of the most important points to be taken into consideration in forming an opinion as to the best method to adopt is, what happens after the liquid is discharged on to the surface of the filter? What happens beneath the surface, in the body of the filter? The generally-accepted theory is that the liquid trickles slowly over the surface of the separate pieces of the material, dropping from one to the other and ultimately falling to the floor of the filter, and thence flowing to the effluent drain. No definite investigations into this question appear to have been made until the year 1909, when a series of extremely interesting experiments were carried out by Mr. W. Gavin Taylor, M. Am. Soc. C.E., Resident Engineer, Sewage Disposal Works, Waterbury, Conn., U.S.A. The results of these tests were published in the “Engineering Record” of June 5, 1909, and illustrated by excellent graphic tables, which show that, “Percolating liquids were more equally diffused in the fine material than in the coarse; that the plotted curves of diffusion approximate parabolas having their apexes at the surface of the filter material; and that, in general, about one-half of the total lateral movement taking place within a 6 foot depth was effected within the uppermost foot of material.” These observations apply to the effect produced by single drops applied at the surface. Further tests made to ascertain the effect of distribution on several adjacent points at the surface of the filter demonstrated that, “The liquid from each point of application spread out through the material in the normal cone of diffusion, until its cone was intersected by that from an adjacent point of application; that there the two or many liquid films united, as tributary sources, into minute streams which interrupted, to a considerable degree, the continuance of diffusion and tended to descend through the remaining material at a higher velocity and along lines of least resistance. The streaming tendency increased rapidly as the rates of application were made greater.” Among the final conclusions drawn from these tests the most important is that, “The desideratum in the application of sewage to percolating filters is to attain perfection in aerial distribution, and that a high efficiency in sub-surface distribution is fostered by a slow continuous rate of application rather than by an intermittent application at a higher rate.” This last sentence fully confirms the author’s own opinion with regard to the use of dosing tanks which are dealt with later under the heading “Methods of Feeding Percolating Filters.”
In the following pages a number of the many appliances which have been introduced for distributing tank effluents upon percolating filters are illustrated and described in detail, the various types being grouped under separate headings.
Automatic Revolving Distributors.—One of the first, if not the first, of the revolving distributors for percolating filters was the “Candy-Whittaker” sprinkler in its original form. This is supplied by the Patent Automatic Sewage Distributors, Ltd. It is now made in three different forms: the “Open” type, the “Enclosed” type, and the “Buoyant” distributor, as shown in Figs. 78, 79, and 80. The most prominent feature of these sprinklers is the mercury seal joint, for which it is claimed that it gives an absolutely watertight and frictionless joint, that it cannot freeze, and that no renewal of the mercury is required. It is stated that the use of the patent “check-ring” in combination with the mercury seal prevents any loss of the mercury, no matter what the head may be. Another feature is that the ball-bearings are moisture-proof, due to the special methods of construction. In addition to the above, the makers point to the special value of their compensating arms system, by means of which their distributors will work continuously with a very small volume of sewage and still be capable of dealing with any larger volume of sewage that may be required. The names of the “Open” and “Enclosed” types are self-explanatory, but the “Buoyant” type is specially designed to reduce the friction on the bearings. The revolving portion is supported by a float or buoy in the form of a cylindrical tank, which floats in a small chamber at the centre of the filter. The removal of all weight friction on this distributor reduces the power required to rotate it to the minimum.
Another make of revolving distributor is well-known as the “Cresset.” This is manufactured by Messrs. Adams Hydraulics, Ltd., and is illustrated in Fig. 81. In this case the special feature is the joint between the revolving and fixed portions of the apparatus. It consists of a simple air-lock, formed by a cushion of air between two water columns. This is clearly seen in the illustration. It is obvious that this is an absolutely frictionless joint, so that there is no loss of head. It also has the effect of removing the strain to which the overhead ball-bearings are generally subject, so that the revolving body swings freely in the true vertical line from the cross head above. Another point is that no expense is involved, and very little trouble is incurred, in renewing this joint whenever it may be found necessary, but it has been maintained in perfect condition without requiring renewal for very long periods. Special means are provided for removing and replacing the ball-bearings in the cross-head without dismantling the distributor.
Another type of distributor manufactured by Messrs. Adams Hydraulics, Ltd., is their “Sypho-Jet,” as shown in Fig. 82. As its name implies, it is siphonic in action, and combines the functions of a sprinkler and intermitting valve in one apparatus. It may either be used in connection with a dosing tank or connected direct to the settling tank, from which it draws off a certain depth of water every time it fills to a certain height.
Two types of revolving sprinklers are manufactured by Messrs. Mather and Platt, Ltd. These are illustrated in Figs. 83 and 84. The special feature claimed for the “Pipe Arm” distributor is the overflow centre, as this eliminates the use of joints or seals. Another point of interest in connection with this apparatus is that there are two sets of ball-bearings at the head, one to take the weight of the revolving portion, the other to take lateral movement. In addition there is a set of roller-bearings at the centre, which are arranged to prevent lateral movement, and they can be adjusted to take up any wear that may occur. The “Open-trough” type, Fig. 84, is provided with a special turbine arrangement at the centre, which it is claimed gives an immediate motion when the sewage is turned on. The distributing troughs being open assist in aerating the liquid, and as it is possible to provide holes in the bottom of the troughs these are able to drain completely dry, and can also easily be cleaned. This apparatus is provided with ball and roller bearings, as described in connection with the “Pipe Arm” type mentioned above.
Messrs. George Jennings, Ltd., manufacture a revolving distributor, the special feature of which is that the sewage is delivered from the central fixed column to the revolving tank, to which the arms are attached, by means of a syphon formed in the central column itself, but this syphon is disconnected from the supply pipe by a fixed cylinder and port-holes, which make the syphon independent of the pressure at the inlet to the supply pipe outside the filter bed. The pressure acting upon the syphon is therefore the atmospheric pressure applied direct to the surface of the liquid in the fixed cylinder, thus operating without any loss of head. In addition to the ordinary overhead ball-bearings, a gun-metal bearing is provided to check any excessive lateral movement. The syphon described above when once started remains sealed, as the outlets are trapped by the liquid at the bottom of the revolving cylinder, and as the top lip of the revolving cylinder is arranged to be 6 inches above the top water level in the tanks, any flooding at the centre of the bed by the liquid overflowing at this point is impossible (see Fig. 85). When required to deal with fluctuating flows this sprinkler can be fitted with compensating arms.
Messrs. Whitehead and Poole manufacture a revolving sprinkler, as shown in Fig. 86, in which a float is used to carry the weight of the rotating parts, so that wear and tear and friction are reduced to the minimum. The chamber containing the float is completely closed, and this is below the surface of the filtering material, so that they are not affected by frost. In this sprinkler the joint between the fixed and rotating parts is made by utilising a little of the buoyancy of the float to form an upward bearing at the neck of the float chamber. It is claimed that this joint is perfectly watertight, with the minimum of friction.
The revolving distributor manufactured by Messrs. Ham, Baker and Co., Ltd., shown in Fig. 87, consists of a revolving trough to which the distributing pipes are connected. The trough is supported upon ball bearings carried upon a pillar, and the special feature of the apparatus is that the incoming sewage is delivered into the central trough in such a way that it strikes upon blades fixed in this trough, and it is claimed that in this way full advantage of the initial head of sewage is obtained. By this means also no special joint is required between the fixed and revolving parts. A special method of gaining access to the ball-bearings for inspection and cleaning is provided. In connection with this distributor the makers supply an automatic cleaning gear for the spray holes, Fig. 88. This consists of a rocking bar attached to each arm of the distributor and provided with a number of small fingers or pickers, one opposite to each hole. An arm or a lever attached to the rocking bar, and suspended from same to within a short distance of the surface of the filter, comes into contact with a block of wood or other material fixed in the filter itself at each revolution, and thus causes a movement of the rocking bar which actuates the finger points and causes them to pass in and out of the spray holes in the arms. By this means any obstruction is removed and forced out by the pressure of water within the arm, and the holes are kept free.
The “Facile” rotary distributor shown in Fig. 89 is manufactured by Messrs. W. E. Farrer, Ltd. The special advantage claimed for this distributor is that it has no joint, so that there is no friction to overcome beyond that in the ball-race, upon which the whole weight of the revolving portion is supported. It is provided with a syphonic attachment to the central column, and thus acts as a self-dosing distributor, and, if desired, can be connected direct to the septic tank. A patent gauging tube is fitted, so that the volume of the dose may be gauged whether it is fed from the dosing chamber in the ordinary way or direct from the septic tank. Access is provided to all parts, and the ball-race can be removed without dismantling the apparatus.
Among the later types of revolving sprinklers is the “Carlton Rotor” distributor, Fig. 90, manufactured by the Carlton Engineering Company. This is specially designed to obviate the use of a dosing tank, to work satisfactorily with any variation of flow, and to prevent all possibility of choking. It is claimed that these advantages are secured by the use of a special water-wheel which rotates round the central column on an inclined plane. The sewage is delivered to this water-wheel by means of a bend connected to the central column. As each section of the water-wheel is filled, it rotates and delivers its contents into an annular trough, to which the distributing arms are attached. These are, however, not perforated with spray holes, but are provided with small bends on the top of the arms, which deliver the sewage into distributing troughs fitted with baffle plates. These baffle plates are so arranged that the sewage is delivered on to the surface of the filter in a thin film and at a uniform rate over the whole surface.
A novel apparatus for the purpose of providing an automatic method of cleaning the orifices of the arms of revolving or travelling distributors is shown in Fig. 91. This is supplied by the Carlton Engineering Company, in connection with their ordinary type of revolving distributor, also shown in Fig. 92. In the distributing arm B apertures are made in the form of elongated slots A, each slot having a brass cleaning blade E inserted. These blades are carried on brackets fixed by a set-screw D, on to a reciprocating rod or tube C, mounted on runners F. Two bevel wheels G are strapped on to the arm, and are connected by an eccentric rod to the reciprocating rod C. At every revolution which the distributing arm makes, the bevel gearing G comes in contact with the stop H, causing a partial revolution of the gearing. This in turn causes the backward and forward motion of the cleaning blades in the apertures. The apertures are thus kept entirely free from grit or any other solid matter, and the flow is broken up into a fine spray.
The “Simplex” revolving distributor is manufactured by the Ames Crosta Sanitary Engineering Company, Ltd., and illustrated in Fig. 93. This sprinkler revolves on a ball-bearing which runs in a bath of solid grease, and is protected from moisture and dust by means of special washers. The whole of the weight is carried from the top of the pillar, no bearing being submerged in the sewage. The seal is arranged by means of two gun-metal rings with special annular grooves turned in them; the top ring is carried from the revolving bucket by means of a rubber or copper washer, thus the ring carries no part of the weight of the apparatus, and is free to compensate for any oscillation. Rollers running on ball-bearings are placed under the bucket, and these take up side movement due to wind and other causes. The revolving arms are provided with a special cap at the ends, which can be removed by a quarter turn to enable the arms to be cleaned. There is no head lost, as the level of water in the bucket is equal to the head in the supply chamber or channel. The apparatus is not affected by frost, and is remarkably sensitive, as 2 inch head of water above the holes in the arms will start the apparatus in motion.
Messrs. Burn Bros. manufacture a revolving sprinkler, “Burn and Brown’s Patent,” shown in Fig. 94, which has one or two special features. These consist of a self-regulating apparatus for dealing with the variation of the flow of sewage and at the same time cleaning the orifices. A shaft carried on each of the arms, and mounted on suitable bearings, is provided with tapered fingers fixed in a position corresponding to the holes in the arms. By means of a float in the circular revolving tank connected to a simple lever the shafts on the arms are given a slight movement, which causes the fingers to enter the orifices in the arms as the flow decreases, and to be withdrawn as the flow increases. The result is that the orifices are enlarged or reduced as the flow varies, and the movement of the tapered fingers tends to make the orifices self-cleansing. It is claimed for this distributor, that it can be made to deal with any variation in flow of sewage up to 6 volumes, with a maximum head of less than 18 inches above the surface of the material of the filter.
A new type of automatic revolving sprinkler is the “Hodgson” overflow type, manufactured by Messrs. George Kent, Ltd. As will be seen from the illustration, Fig. 95, the sewage passes first into a feed chamber attached to the central column, from which it flows into the distributing chamber D, to which the arms are attached. It is claimed that by use of the weirs W in the distributing chamber, a uniform speed of rotation and consequently a uniform distribution, is secured. The ball-bearing upon which the rotating portion is carried is stated to take both the side and end friction.
The revolving distributors previously described are all constructed on the Barker’s Mill principle, to some extent if not entirely. The Fiddian type of distributor, supplied by Messrs. Birch Killon and Co., and illustrated Fig. 96, is based upon an entirely different principle. It consists of an elongated water-wheel, which revolves around a circular filter. It is pivoted at the centre of the filter upon the supply pipe, and the outer end of the distributor is carried upon a rail track. In the case of large filters, from 70 feet to 110 feet diameter, two rail tracks are used. The liquid passes through the supply pipe and is delivered into the buckets of the water-wheel, from which it falls in the form of a film, so that the water-wheel not only drives itself over the filter by means of the weight of the liquid, but it also distributes the liquid which drives it. A very small quantity is sufficient to start the distributor and keep it in motion, the head required, measured from the surface of the filter, being about 18 inches. The buckets are provided with graduated weirs, the width of which is proportionate to its distance from the centre of the filter. By this means uniformity of distribution is secured. It is claimed that as there are no small holes to be choked, the apparatus can be employed for all kinds of liquid with the minimum of attention, and that it does not require any dosing apparatus.
Power-driven Revolving Distributors.—One of the first revolving distributors in which it was decided to adopt power to drive it independent of the sewage, was the “Scott-Moncrieff and Alliott” distributor, manufactured by Messrs. Manlove, Alliott and Co., Ltd., and erected at the Birmingham Sewage Farm for Mr. J. D. Watson. In this case a horizontal arm is attached to and pivoted upon the vertical stand-pipe in the centre of the filter which is connected to the supply pipe. The outer end of the horizontal arm is carried by a rail, and is fitted with an oil engine which travels along on the outer rail on the end of the arm. The rotating arm consists of a large main trough or carrier into which the sewage from the central stand-pipe is delivered, and on one side of the main trough is fitted a row of small troughs, each of which is supplied with sewage from the main carrier through a porthole specially proportioned to the volume of sewage to be discharged from the trough and to its distance from the centre of the bed. By this means uniform distribution is secured over the whole area of the filter. From each of the small troughs the sewage flows over the edges of the trough, and thence, trickling down the outside of the trough, is discharged in a row of fine rain-like columns upon the filter. It is claimed for this distributor that absolute uniformity of distribution is secured, that it will work equally well with any volume, that there are no holes to become choked, that it requires the minimum of supervision, and that it will deal with any variation up to 3 times D.W.F. without attention (Fig. 97).
In the “Hartley” circular power-driven distributor, the apparatus (manufactured by Messrs. Hartley, Causton and Richmond, and shown in Fig. 98), is arranged in a form similar to that used in connection with the Barker’s mill type of distributor, with this difference, that it is driven by an electric motor attached to the end of one of the arms, which is used as the distributing arm; and the arm on the opposite side is the balancing arm to counteract the dead weight of the distributing arm and act as a balance against wind pressure. The current is brought to the distributor by an overhead cable, connected to a revolving contact at the top of the central standard. The method of distribution from the distributing arm is different from the usual type, as it takes place through a number of sectional distributing pipes C (Fig. 99), which are connected to the main tube H. These are arranged in echelon, so that by opening the covers P P a cap or cleaning rod may be passed through each tube from end to end. Each sectional distributing tube, C, is suspended from the main tube by an attachment D. This attachment carries a screw, G, by which is regulated a valve shown open at E and closed at E2. The flow may be increased or decreased to any extent, or stopped altogether, by this valve. Each space between the dotted lines on the plan is covered by one sectional tube, and the sewage flow can be increased or decreased, or stopped upon each section. The spray nozzles are of a special type, and by use of the deflection or spray plates M, a smaller number of spray holes or jets are required, so that they can be made larger, and they are thus not so easily choked. Among the advantages claimed for this distributor are the following. That it can be regulated to work with any variation in the volume, from the smallest quantity to 2 million gallons per acre per day, without increasing the consumption of power. Separate sections of the filter may be removed or rested entirely by adjusting the valves G. The distributor can work with any head of sewage from 6 inches to 6 feet, or more if desired. It is not affected in any way by wind pressure; and as the balancing arm empties at the same time as the distributing arm, it is always in balance. The horse-power required to drive it is very little, less than half horse-power per acre. The rate of distribution on any of the sections of the filter can be varied from 50 to 400 gallons per yard per day.
This type of distributor was first erected at the Hanley Sewage Works in 1902, in order to comply with Messrs. Willcox and Raikes’ specification fora power-driven distributor suitable for a large circular filter, on which a Scott-Moncrieff distributor was previously employed.
A somewhat similar type of distributor is the “Adams-Cutler” Fig. 100, manufactured by Messrs. Adams Hydraulics, Ltd. In this case the distribution is from spray holes in the pipe arms in the same way as the ordinary Barker’s Mill type of distributor, but instead of relying upon the motive power due to the head of sewage, the distributor is operated by a rope drive; and where there are a number of distributors in one installation, each of them is operated by a cross-drive taken from a main drive rope, so that any one or more can be cut out of the circuit or controlled, or the speed varied as may be required. It is claimed that a very small amount of power is required to operate a revolving distributor, and the ease with which it is carried upon a central column, are arguments in favour of the revolving over the rectangular types where absolute control is essential and a natural working head is not available.
Messrs. Mather and Platt, Ltd., also manufacture an electrically-driven revolving distributor of the Open Trough type, as shown in Fig. 101. This machine is of special design, and particularly suited for large diameter beds. The troughs are similar to those of the ordinary type of distributor previously mentioned, but are supported on the Warren Girder principle. There are four pipe arms, the end of each being fitted with a special carriage running on rails fixed outside the bed at the floor level. On one of the carriages an electric motor is fixed and connected to the rail wheels by means of chain drives. The current is transferred to the motor from the main at the top of the central, pole of the distributor by means of slip rings.
Automatic Travelling Distributors.—There are several types of these on the market at present. The first is manufactured by Messrs. Birch Killon and Co., Fig. 102, and is constructed on the Fiddian water-wheel principle. The design is similar to that described in connection with the automatic revolving distributor, but it involves two rail tracks supported on walls or piers on each side of the filter. The head required for this type is about 2 feet from the surface of the filter to the top water level in the carrier, and the distributor is fitted with a syphon which feeds it from the carrier or trough. In large schemes the trough is placed along the centre of the filter, and supplies a separate distributor on each side.
Another type is manufactured by Messrs. Ames Crosta Sanitary Engineering Company, as shown in Fig. 103. This sprinkler can be adapted to existing contact beds. It is carried from the walls of the central channel in cantilever form, no other bearing or rails being required. On each side of the machine a hopper is carried in which water-wheels revolve. The water in its passage from the feed channel to the distributing arms is conveyed on to the water-wheels, and the power generated there is transmitted by means of a chain drive to the wheels of the carriage, and thus the whole apparatus is moved along the beds. When the machine nears the end of the bed a reversing lever comes in contact with a buffer, and by this means the direction of motion is changed, and the machine travels backwards along the bed. Two distributing arms are carried on each side of the apparatus, one feeding half the bed in one direction and the other feeding the remaining portion of the bed in the opposite direction, thus giving a regular intermittent feed to the bed. A great advantage of this class of machine is that the whole of the machine is supported from the central channel, and as the wheels are close together, the long lengths of shafts with their risk of twisting, retarding and wearing are dispensed with.
Messrs. Ham, Baker and Co., Ltd., have for some time supplied an automatic travelling distributor. This is illustrated in Fig. 104, from which it will be seen that it consists of a water-wheel mounted on rollers and divided into sections composed of a number of buckets. The sewage is delivered through a syphon and patent reversing valve, in such a way that alternate sections only of the filter bed receive sewage in each direction of travel, thereby insuring equal inter-delivery periods.
Power Driven Travelling Distributors.—The well-known Hanley Distributor was specially designed by Messrs. Wilcox and Raikes, Civil Engineers, for use on their schemes at Hanley, Fenton, Ilkeston and elsewhere, as illustrated, Fig. 105. The object of this distributor is to ensure intermittent discharge at regular intervals, and at a predetermined rate, in the form of a thin film over the surface of fine grade material, and in order to secure these results it was decided to drive the distributor by electric power rather than depend upon the variable and limited amount of power obtainable from the head of the sewage itself. The distributor consists of a pipe extending across the whole width of the bed and supported on wheels, so that it may be drawn backwards and forwards by means of the wire rope attached to the centre. The distributing pipe tapers towards the end farthest from the supply trough, and the sewage is discharged by means of nozzle pipes attached to the flat side of the distributor about 3 inches apart, and connected with it near the top so that the main pipe always remains full. The discharge of the sewage commences and stops along the whole length of the pipe almost simultaneously, and any sediment lies in the bottom of the main pipe where it cannot obstruct the outlets of the nozzles. The nozzles are about ⅝ inches in diameter, and provision is made for cleaning them by removing a plug in the top of each. In order to secure a uniform interval of time between each dose of sewage discharged on to any part of the filter, the distributors are arranged to discharge when travelling in one direction only, the valves on the outlets of the feed syphons being automatically turned off and on by means of a lever, which is actuated by stops fixed at each end of the iron trough or at any intermediate point desired. In this way it is possible to arrange for any portion of the filter to be thrown out of action when necessary for cleaning or repairs. The speed at which the distributor travels may be varied by adjusting the gear wheels attached to the electric motor. At Hanley it has been found that the best results are obtained when the distributors occupy about 7 minutes in making one complete journey, the filters being 200 feet long, so that when the filters are working at the normal rate of 1,000,000 gallons per acre per day, each dose represents one gallon per square yard. In other words, the film one-fifth of an inch in thickness is applied 200 times per day, and thus a total depth of 40 inches of sewage is distributed over every portion of the filters in operation in 24 hours. The power required to drive the distributors is about 1½ B.H.P. per acre of filter. Electric motors are usually adopted, but small oil or gas engines may be substituted when electric current is not available. The winding drums for the wire ropes are driven by belt gear of the usual type with fast and loose pulleys. The reversal of the drum when the distributor reaches either end of the filter is accomplished by an automatic belt striking gear, and a float in the supply trough is used to cut out a distributor, or continue in action at a reduced rate, when the supply of sewage becomes insufficient to maintain the normal rate of distribution. These distributors are manufactured by Messrs. Blakeborough and Sons, and have now been in regular use since 1902 on the Hanley Sewage Disposal Works, where the remarkably satisfactory results obtained are largely due to the highly efficient system of distribution on the filters.
Fixed Distributors. Trays and Troughs.—One of the best known of the fixed distributors is the “Stoddart,” as illustrated in Figs. 106 and 107. It will be seen that it consists of trays of galvanized sheet iron, formed into a number of V-shaped troughs. The ridges between the troughs are provided with diamond-shaped perforations at frequent intervals, and in the bottom of the troughs drip points are inserted through holes to project on the under side. The distributing trays are bolted to specially designed supply channels, connected to a common channel, and the whole of the troughs are mounted on adjustable chairs (F, Fig. 106) carried on tees, which are in turn supported on concrete or brick piers. As the liquid passes into the V-shaped troughs, it fills them and overflows through the diamond-shaped slots in the ridges and thence trickles down on the under side of each trough and drops off from the drip points on the filter below in the form of rain. It is claimed for this method of distribution that the whole of the available fall is utilised; that it adapts itself without adjustment to all rates of flow, however varied, that the sewage and storm-water may all be treated on the same area, and that there is a total absence of moving parts. This distributor is supplied by the patentee, Mr. F. W. Stoddart, of the Western Counties Laboratories, Bristol.
Another type of fixed distributor is fixed only so far as actual distributing channels are concerned. This is manufactured by Messrs. W. E. Farrer, Ltd., and illustrated in Fig. 108. The fixed troughs or channels are laid directly over the filter-bed, and fitted at the inlet end with concentrating chutes which receive the discharges from a double-acting tipping trough, which is supported at its extreme ends by heavy gun-metal bearings. If it is over 10 feet in length, an additional support is provided at the centre in the form of a specially designed rocker. The chutes and the channels are provided at the bottom with ¼-inch holes at 3-inch centres, through which the sewage is spread on to the surface of the filter, which is below the underside of the trough. The channels are arranged at 1-foot centres, so as to distribute the sewage over the whole area of the bed. By means of the double-acting tipper each half of the filter is dosed alternately, and suitable periods of aeration are thus provided. This apparatus is suitable for a single small house with 4 or 5 inmates, and for larger filters of 600 square feet in area and upwards.
The Ducat filter is provided with what may be termed a fixed distributor. It consists of a series of small tipping troughs fixed across the filter, each of which when full tips its contents on to the filter below. The liquid is thus discharged at the same point each time, but intermittently in small doses. As, however, the distributor does not move it is included under this heading.
A different type of fixed distributor is that manufactured by Messrs. J. Blakeborough and Sons. This apparatus, patented by Messrs. Haller and Machell, Civil Engineers, is used in the triple tank system of sewage treatment. Fig. 109 illustrates the “Aerat” type, which consists of cast-iron channels supported on wrought-iron joists carried upon piers. Each length has six orifices on each side, through which the sewage passes on to small trays provided with radiating grooves on the top and dripping points on the under side at the edge. As the distributors may be fixed within a few inches of the surface of the filter, very little head is required to operate them.
A very ingenious form of fixed distributor, Fig. 110, is supplied by the Septic Tank Company, Ltd. From its name, “Capillary Trough Distributor,” it will be observed that during normal and minimum rates of flow, capillary attraction is utilised to deliver the sewage on the filter in the form of drops. The tank effluent is delivered to each trough distributor over brass V-notches, set in the side of the main channel, into which the tanks discharge. These V-notches are adjustable, and are set so as to prevent their being tampered with subsequently. The trough distributors are of artificial stone, true from end to end, and set level. Over both their edges copper wires are fixed, at intervals of a few inches, each wire extending from within a short distance of the bottom of the trough channel over the edge and down to the underside of the trough. On a trough being filled the liquid overflows right along the edge, and is attracted by and runs down the wires, so that if the flow is at all excessive a little stream will run down each; on the flow subsiding to the ordinary, a drop-by-drop delivery commences; should the flow of sewage cease altogether for any length of time, the wires will go on dripping until they have drawn off the contents of the trough, almost to the end of the wires. On a fresh supply of sewage arriving at the installation, and the troughs refilling to a higher level, the rate of delivery from the wires will increase until it keeps pace with the flow, thus ensuring an even distribution on the filter throughout the day. There is no loss of head in connection with this apparatus beyond the depth of the distributing troughs themselves.
Spray Jets.—This type of distribution is not used in many installations in this country, but it has been adopted in large schemes in the U.S.A. There are a number of different types, including those in which the sewage is forced upwards through a nozzle and falls back on the surface of the filter, and others in which the sewage flows through inverted nozzles, and each jet impinges upon a fixed cup or disc of metal, or other material, and is thus spread out on to the filter. Among the former, one of the simplest types is that manufactured by Messrs. Adams Hydraulics, Ltd., Fig. 111, in which, by means of a square baffle plate, provided with suitable grooves, the spray is forced out in such a manner that it covers a square area of the filter. It is claimed for this that it covers the whole surface and no portion of the filter is wasted, as in the case of round jets which cover circular areas. The same firm also supply several other types of fixed sprays, as shown in the illustration.
Another type is manufactured by Messrs. Ham, Baker and Co. This is shown in Fig. 112, from which it will be seen that it can be adapted to any existing installation, and to standard spigot pipes. Each jet being in the form of a stop-valve, any individual spray may be put out of action when necessary.
Messrs. W. E. Farrer, Ltd., manufacture a type shown in Fig. 113. The special feature of this is a loose disc inside the nozzle, which oscillates as the sewage passes through, and so tends to clear away any flocculent matter which may accumulate in the orifice of the nozzle. The sewage itself is delivered from the nozzle in the form of a very fine film, and it is claimed that by this means any nuisance from smell is reduced to the minimum.
Messrs. Burn Brothers also manufacture a fixed spray, Fig. 114, called the Burn and Brown’s Patent “Whirl” Spray. The special feature of this is that it consists of an inverted conical spreader, provided with vanes revolving on a spindle. The sewage, as it leaves the nozzle, impinges upon the face of the cone, causing it to revolve rapidly and spread the sewage in the form of fine drops. Owing to the special construction of the serrated edges of the cone, the sewage is well distributed over the whole area covered by the spray.
The “Acme” type of fixed spray, manufactured by the Ames Crosta Sanitary Engineering Company, is shown in Fig. 115. The special feature of this spray is that it has a loose metallic cone, which is provided with a helical groove, inside the nozzle. This cone rotates as the sewage passes through the nozzle, and thus gives a maximum spray with a small head. These sprays have a free waterway, and the rotation of the cone tends to prevent the choking of the orifice.
A simple form of fixed spray consists of perforated pipes arranged at suitable intervals over the surface of a filter, and provided with dosing apparatus to give intermittent supply under pressure, and thus deliver the sewage in the form of jets from the perforations in the pipes. An improved form of this type of distribution is supplied by the Septic Tank Company. This is illustrated in Fig. 116, from which it will be seen that orifices in the top of the distributing pipes are fitted with nozzles, set at an angle of about 45 degrees to the vertical. The discharge from these nozzles, it is claimed, gives the maximum efficiency of distribution.
A self-cleansing spray jet has been brought out by the Carlton Engineering Co. From the illustration, Fig. 117, it will be seen that a plug B is fitted into the orifice and supported upon a spindle, which is attached at its lower end to a lever connected to an automatic tipper on the outside of the nozzle. The tipper is fed by the spray from the nozzle, and as it falls alternately on one side and on the other it raises and lowers the plug B in the orifice, which tends to prevent any clogging, and also varies the area of the orifice and the rate of discharge, as well as the area of the filter-surface covered by the spray.
A simple type of fixed spray is supplied by Messrs. George Kent, Ltd., and is shown in Fig. 118. In this case the liquid issuing from the nozzle impinges upon a curved surface or cone, by which it is deflected and spread out, leaving the surface at an angle of 45 degrees to the vertical. One of the advantages claimed for this type is that it has no small holes to become clogged.
A novel form of this type of apparatus is manufactured by Messrs. Daniel Adamson and Co. Above the orifice in the nozzle is suspended a revolving deflecting plate (Fig. 119). The vanes of this plate are shaped in such a way that, as it revolves, the jet impinges against a different angle every instant. None of the energy is wasted in churning or throttling, as the plate merely directs the water in such a way that no two drops ever fall twice on exactly the same spot. It is stated that the diameter of the wetted area of the filter surface is more than double the head.
There are a considerable number of other types of nozzles used for fixed sprays. Several of these are illustrated in Fig. 120, where A is the type in use at Salford; B, the type used at Birmingham; C, a nozzle with a double spreader in use at Waterbury, Conn., U.S.A.; D shows another type in use at Waterbury, with a single spreader, but this is so arranged that it can be adjusted to give a smaller or larger area of orifice. E shows the type adopted at Columbus, Ohio, U.S.A.
In connection with some elaborate studies of methods of distribution for percolating filters, which were carried out for the Sanitary Research Laboratory and Sewage Experiment Station of the Massachusetts Institute of Technology, by Messrs. Winslow, Phelps, Storey and McRae, a special series of investigations were made into the value of what is referred to as the “gravity method of spraying.” This consists in supplying the sewage from troughs or pipes supported above the filter, and discharging it through orifices on the under side of the pipes or troughs in the form of solid jets, which are made to impinge upon concave discs fixed near to the surface of the filter. The effect of this impingement is to cause the liquid to splash upwards and outwards in the form of a fine spray, and, as a result of a long series of tests with various forms of discs and various heads, it was found that the best average results were obtained with a concave metal disc 3 inches in diameter having a curvature corresponding to a radius of 2 inches. With a total head of 4 feet and a head of 3 feet from disc to the pipe above, it gave the best results obtained with that total head. With a total head of 6 feet and a head of 4 feet from disc to supply pipe, it gave the best results obtained by any of the forms of this type of distribution that were tested.
A method of distribution similar to this type of gravity disc spray is manufactured by Messrs. Glenfield and Kennedy, Ltd. The details of this disc, with the methods of supporting same and the method of supplying the sewage from an overhead pipe, are shown in Fig. 121.
In connection with sewage disposal works where chemical precipitation has been adopted for preliminary treatment, the tank effluent has been distributed over fine-grained percolating filters, by means of dosing tanks discharging comparatively large volumes intermittently over the surface of the filter. In such cases the aim has been to deliver the liquid at such a rate that it would flood the entire surface of the filter in a very short space of time, and then percolate through and leave the surface free to aerate for as long a period as possible before the next discharge took place. This process involved the use of small filters, in order that the area to be flooded should not be too large for the liquid to spread over it rapidly, and the use of very fine material to prevent the liquid from passing through too quickly. Among the difficulties encountered by this method were the displacement of the surface layer of the material, due to the high rate of delivery; and the labour involved in cleaning the surface of the filter. A further trouble which arises when effluents from septic tanks are treated by this process is the evolution of evil-smelling gases, which are certain to cause a nuisance if there are any houses in the vicinity. Efforts have been made to obviate this difficulty by distributing the sewage through open-jointed or perforated pipes laid a few inches beneath the surface of the filter, as described later in connection with contact beds (page 200). While this is satisfactory to a certain extent, especially where the volume to be treated is small, and the filter capacity is comparatively large, neither this method, nor that of flooding the surface of the filter, can be said to comply with “the desideratum in the application of sewage to percolating filters,” quoted previously (page 106), nor do they avoid the difficulties encountered in securing uniform distribution dealt with in the following pages. The defects of these methods of distribution may also have been aggravated by the very optimistic anticipations on the part of the designers of the schemes as to the volume of sewage which could be satisfactorily purified under such conditions. It is, therefore, desirable to point out that, in the event of either of these methods of distribution being adopted, care should be taken to secure a very high degree of clarification (reduction of matters in suspension) in the preliminary tank treatment, and to provide filters of ample cubic capacity.
Methods of Feeding Percolating Filters.—Under the heading “Methods of Distribution” (page 106), reference is made to some experiments, from the results of which the conclusion was drawn that “a high efficiency in sub-surface distribution is fostered by a slow continuous rate of application rather than by an intermittent application at a higher rate.” This agrees in every respect with the author’s own experience, and confirms his opinion that intermittent discharges to percolating filters should only be resorted to in cases where it is absolutely necessary in order to assist in securing uniformity of distribution, or to ensure a sufficient volume for the operation of the appliances adopted for distribution. Among the former may be cited fixed spray nozzles and jets from fixed pipes, in which cases an intermittent supply is useful in causing a regular variation of the head upon the orifices, thus varying the distance to which the jets or sprays are thrown, and producing greater uniformity of distribution per unit of area covered. Examples of cases, where intermittent supply is necessary in order to discharge the volume required to operate the appliances adopted for distribution, are found in connection with most types of fixed distributors and nearly all types of automatic revolving distributors. In the case of the latter, it is well known that a certain minimum head is necessary to overcome the friction due to the resistance of the air and to the weight of the apparatus itself on its bearings. However small this friction may be, it needs a volume of sewage slightly in excess of that required to fill the spray-holes, which must be large enough to take the maximum flow of sewage when working under the maximum head. It frequently occurs, especially in small schemes, and in schemes where the percolating filters are preceded by contact beds or slate beds, that the rate of flow of the sewage is at times so low that it is not equal to the minimum volume required to operate the distributor, and, in the absence of any arrangement to overcome the difficulty, the distributor would cease to revolve and the sewage would trickle through the spray-holes without proper distribution. The same difficulty arises in connection with many of the fixed methods of distribution, and it is most readily overcome by the use of a dosing tank fitted with an automatic syphon or valve, by means of which the sewage is held-up in the tank until it reaches a certain predetermined level, and is then discharged at a given rate to the filter. When the tank is empty the discharge ceases, and the sewage is again held-up as before.
In the case of large installations, or schemes where the whole of the sewage is pumped and the rate of flow to the filters is thus under control, it is not necessary to use a dosing tank for the purpose of providing the rate of discharge to the filters necessary to keep the distributor in motion. It is, however, maintained in some quarters that intermittent supply is desirable in any case, in order to secure alternate periods of work and rest for aeration. There is one obvious disadvantage in this method of working. Assuming that during the maximum rate of flow of the sewage the volume which comes down in 5 minutes is stored in a dosing tank, and discharged to the filters in 2½ minutes, it is clear that the rate of distribution is twice as great as it would be if the distribution were continuous over the whole period of 5 minutes. It is claimed that the disadvantage of the higher rate of distribution is counteracted by the 2½ minutes of rest and aeration, but on this point there is room for doubt, especially when the conditions which come into play during the average and minimum rates of flow of the sewage are taken into consideration. Taking the average rate as equal to one-half the maximum rate, it will be seen that the dosing tank will discharge every 10 minutes, but the time in which its contents are delivered to the filter will still be 2½ minutes, so that under these circumstances the rate of distribution on the filter will be four times as much as it would be if the distribution were continuous. During the minimum rate of flow of the sewage the conditions are still worse, and it is difficult to accept the theory that periods of rest for 7½ minutes will compensate for excessive rates of distribution at four times the rate under continuous operation for periods of 2½ min. at a time.
Those who advocate intermittent supply under all circumstances would appear to have lost sight of the fact that very ample periods of rest are already provided by all revolving distributors. When working at the rate of one revolution per minute, the average time taken by each arm to pass over any one point on the surface of the filter may be taken as 0·5 second. Before the succeeding arm reaches the same point a period of 15 seconds will have elapsed, so that under these conditions the ratio of the periods of work to periods of rest is as 1 to 30. In other words, even during continuous distribution, the time allowed for rest and aeration is thirty times as much as that during which the sewage is actually being delivered to the filter, and there would thus appear to be no reason for unnecessarily adding to the periods of rest by means of dosing tanks, especially as such a course involves a greatly increased rate of distribution at all times when the discharge actually takes place.
There is, of course, the possibility of adopting the happy medium, which would consist of a dosing apparatus of such a type that it would provide a continuous supply during the maximum flow of the sewage, and only act as an intermitting appliance during the minimum rate of flow. This is probably the best arrangement to adopt in all cases where a dosing apparatus is absolutely essential.
As previously stated, however, there are many cases where the conditions render the use of a dosing tank absolutely necessary to prevent the stopping of the distributor, and these have brought about the introduction of several types of automatic syphons and valves, all arranged specially for the purpose of giving intermittent discharges to filters, contact beds, areas of lands, and so forth. There are various methods of constructing dosing tanks, all dependent upon the arrangement of the preliminary processes and the filters which follow them. In nearly all cases, however, it is found necessary to reduce the fall taken up by the dosing tank to the minimum, and the different syphons and valves have thus been designed to work with the least possible head. One point in the construction of dosing tanks should not be overlooked, i.e. the provision of a washout valve for use when it becomes necessary to clean out the tank. It may be noticed here that neither the Fiddian type of distributor, on the water-wheel principle, nor the power-driven distributors, require any dosing tank to keep them in motion during the periods of minimum flow of the sewage. In the case of the Fiddian distributor the apparatus remains standing until the buckets are filled, and each time these discharge their contents the distributor is moved forward a short distance. Under these conditions, however, the disadvantage of long intervals of rest between comparatively large discharges of sewage still remains. Undoubtedly the most even rate of distribution under all conditions is secured by the use of power-driven distributors, whether of the rotary or travelling type, but these can only be economically adapted for use in large installations or where power is available at a very low cost.
Apart from the question of continuous versus intermittent methods of feeding percolating filters, there still remains the problem of deciding whether the actual connections between the tanks and the filters shall be in the form of channels (carriers) or pipes. For fixed sprays and revolving distributors, which involve the use of pressure due to a column of liquid, the actual connection to the apparatus must be in the form of a pipe extending at any rate to the outside of the filter. Whether it should be continued beyond this point in pipe form or in a channel depends entirely upon local conditions, such as the slope of the ground, the planning of the filters, and the relative cost of the two methods. There is one thing to be said in favour of pipes, i.e. that there is less chance of a nuisance being created by the evolution of foul gases from the sewage than in the case of open channels. On the other hand, channels may be, and frequently are, covered in. Whichever method is adopted, care should be taken to provide a valve or penstock on the connection to each separate filter, in order to be in a position not only to throw any filter out of work when desired, but also to regulate the rate of supply to each filter independently of the others. Washout valves should also be provided on the channels or pipes, in suitable positions. Where the filters are arranged in groups, as suggested in Figs. 75 and 77, the intervening spaces may be used to accommodate a simple receiving tank, or a dosing tank, to which the sewage or tank effluent may be conveyed by an elevated channel or by a pipe under pressure. The connections to the adjacent filters may then be provided in the form of pipes leading from this central tank direct to the distributor at the centre of each filter. If the receiving tank or dosing tank in such cases is built upon piers, the floor of the tank will come above the level of the surface of the filters, and the space below may be utilised for an effluent receiving chamber or even for a settling tank to arrest the solids in suspension in the effluent. There are many ways in which the supply to filters may be arranged, and the preceding notes are merely intended as suggestions, which may be elaborated as found desirable to suit conditions which vary almost in every case.
Figs. 121a and 121b have been prepared to show conditions under which channels and pipes respectively would be suitable for supplying the tank effluent to the filters. Fig. 121c is the plan of a works where a dosing tank is used to deliver the tank effluent intermittently through separate supply pipes to each of the filters. If the valves on the supply pipes are properly adjusted, each filter will receive an equal proportion of each discharge from the dosing tank. An alternative method is to provide an apparatus by means of which each separate discharge from the dosing tank is delivered in rotation to each of the filters.
In cases where the appearance of the works must be taken into consideration, the method of arranging the tanks and filters suggested in Fig. 121d may be adopted. By this means the space covered by the works is utilised to the utmost, and the tanks are of the form by which uniformity of distribution and the maximum reduction of velocity is secured. On the other hand, it is probable that the slope of the site will only rarely suit this arrangement.
Dosing Apparatus.—Various methods of providing intermittent supply are in use. Among these one of the first was a simple form of balanced valve with float. This, however, has been superseded by other types, among which the syphons are the simplest form. One of the first of this type specially designed for the purpose in question was the low-draught syphon, manufactured by Messrs. Adams Hydraulics, Ltd., and shown in Fig. 122. The special feature of this apparatus is that it can be arranged for a small depth of sewage in the dosing tank. It will work with any depth from 6 inches upwards. These syphons can also be arranged to work singly, in pairs, each one coming into action alternately, or in numbers of three and upwards, when they can be adjusted to work in sequence or simultaneously.
Another type of syphonic dosing apparatus is that supplied by the Patent Automatic Sewage Distributors, Ltd., and shown in Fig. 123. In this case a syphon is combined with a mechanical valve operated by floats, hence the name “auto-mechanical syphon.”
A further type of syphonic dosing apparatus is manufactured by Messrs. George Jennings, Ltd., as shown in Fig. 124. These syphons are arranged to work in regular sequence, and are operated by means of air valves, which are shown mounted on a frame and connected by means of air pipes to the syphons. The sequence is obtained by means of cams, fixed in different positions on a shaft, which is rotated by means of a float actuated by the rise and fall of the liquid in the dosing tank.
A different type of dosing apparatus is the “Coleman” valve, manufactured by The Ames Crosta Sanitary Engineering Company, Fig. 125. This consists of a balanced valve, suspended from a hollow lever which contains mercury, and has a branch provided with a float actuated by the rise and fall of the liquid in the tank. As the sewage rises in the tank, the float overcomes the resistance of the balance weight on the lever, which is lifted, and the mercury falls to the lower end of the lever. The combined action of the float and the mercury opens the valve, and the sewage is discharged. When the tank is emptied, the apparatus resumes its former position ready for the next dose.
Another type of dosing apparatus is manufactured by Messrs. Mather and Platt, Ltd. This consists primarily of a flap valve, counterbalanced weights, and a balance vessel or drum, which is filled gradually with liquid, the whole being pivoted about a horizontal axle. When the measuring chamber and balance drum are both empty, the counterbalance weights raise the latter and allow the flap valve to close, the balance drum then being in its highest position. As the measuring chamber fills, a portion of the sewage is allowed to pass from it into the balance drum, which is thus gradually filled, and at a given level its weight will suffice to overcome the counterbalance weights and the pressure of the sewage on the flap valve, and will open the latter, thus allowing a measured quantity of sewage to be discharged into the troughs or pipes for spreading over the filter. The balance drum discharges its contents meanwhile, and thus, when the measuring chamber is empty, the flap valve closes again by the action of the counterweights, and the chamber is ready to receive another quantity of sewage (Fig. 126).
Messrs. George Kent, Ltd., have introduced several different types of dosing apparatus, which can be arranged to draw off less than 6 inches head of water and any greater depth. In one case the apparatus, Fig. 127, consists essentially of a seating S, through which the liquid discharges, a valve-cap C, attached to arms which rotate about the pin P and nearly balanced by the weights W, also by floats F F attached to a frame, also rotating about P and carrying two adjusting weights A A. The apparatus is actuated by the rise of the liquid, raising the floats and lifting the cap C, which returns to its position after the discharge has taken place. For greater depths a different type is used, as shown in Fig. 128. In this case the apparatus consists of a valve seating S, through which the liquid discharges, two floats A and B, a controlled device for giving a sudden release and for regulating the levels at which the valve is opened or closed, and an alternating gear G, which is fitted with two or more valves placed in the chamber and operated in rotation. The action of the apparatus is as follows: When the liquid in the dosing chamber reaches the top level, the float A has acquired sufficient buoyancy to cause the shoulders of the plate K to suddenly push aside the rollers at the ends of the weighted levers L L of the control device. The lugs T T then come in contact with the collar R, and the valve-cap C and the float B are lifted. The tank discharges until the floats A and B have lost sufficient buoyancy to cause the bottom shoulders of the plate to push aside the rollers and allow the valve to close.
This apparatus is applied in various ways for different purposes in connection with sewage works, and the same firm also manufacture special valves for giving a measured discharge under various heads.
The Ames Crosta Sanitary Engineering Company, Ltd., also supply a very ingenious method of providing intermittent supply to filters, as shown in Fig. 129, which is equally suitable for percolating filters, for contact beds, or for intermittent filtration on land. This machine is arranged to give any desired timed discharge to the beds in rotation, or to discharge measured volumes to the beds in rotation. By means of valves or weirs, the machine is arranged so that additional beds in the series are automatically brought into operation as the increase in flow demands, and the additional beds brought into action go out of action as the flow decreases. The illustration shows a machine for delivering timed volumes on a series of six beds. The square chamber is divided into six divisions by means of iron plates; each division is connected to one of the six beds by means of a pipe. A circular dish or vessel is suspended over these divisions, and revolves on ball-bearings carried on the bridge. The circular dish is provided with a weir at its circumference, and the sewage or tank effluent which enters the dish from the pipe in the centre is diverted by means of the weir or outlet to one of the divisions supplying the beds. The upright shaft from which the revolving dish is suspended is fitted with a six-toothed ratchet wheel, and, by the rise and fall of a float in the liquid of the actuating tank, a pawl is moved along until it engages with the next tooth of the ratchet wheel. When the pawl has caught the tooth of the ratchet wheel the outlet valve in the actuating tank is opened, and as the liquid flows out of the tank the float descends, and by means of levers the pawl is moved one-sixth of a revolution; the pawl being engaged with the ratchet wheel, the dish is revolved one-sixth of a revolution, and the weir is thus brought over the next division, and consequently the next bed receives the flow. Extra weirs can be arranged at various levels, so that if more sewage is coming than that required for one bed, a portion of the liquid would flow over one or more of the weirs, and so on, to the respective beds. The flow to the actuating tank is taken from the feed channel, and can be set to fill the actuating tank in any desired period of time. The discharge from the actuating tank can be conveyed either to a special plot of land, or into the revolving dish. An ingenious device is arranged so that any bed in the series can be instantly shut out of action should it become overworked. In an apparatus for feeding revolving sprinklers, arrangements are made for bringing the extra beds into operation by means of valves controlled by the flow of sewage.
Messrs. Glenfield and Kennedy also manufacture a balance-valve type of dosing apparatus operated by means of floats and buckets. This is shown in Fig. 130.
The “Uniform” automatic distributor, manufactured by Messrs. Whitehead and Poole, is designed for the automatic distribution of measured volumes of liquid to filters or other areas in regular rotation. This is shown in Fig. 131, and the principles on which it is constructed involve rotating arms carried on a float operated in a closed central chamber. The arms are rotated by the reaction of the discharge of the liquid, but are locked so as to remain stationary in one position, continuing to feed one filter until the head of liquid actuating a regulating cock and tipper releases the locking device and permits the rotation of the arms to the next bed, where the process is repeated, and so on to all the filters in rotation.
The “Ponding or Intermitting Valve,” supplied by the Septic Tank Co., is shown in Fig. 132. The essential features of this apparatus are two cylindrical vessels of cast-iron or other suitable material, approximately of the same shape, weight and displacement. One vessel is used as a bucket and the other as a bell. They are suspended from two equal arms of a pivoted lever at approximately the same distance from the standard carrying the lever. On the same end of the lever to which the bell is hung, an ordinary lift-up valve is attached, the seat of which is at or below the low-water level in the ponding chamber. This valve is connected with the lever by means of a linked rod, chain, or other suitable arrangement. The cubical content of the bell and bucket is sufficient to ensure the displacement of the liquid, in excess of that required to lift the valve from its seat, by rocking the lever to which it is attached. In the bottom of the bucket a draw-off valve is provided for the purpose of emptying same. This valve is constructed so as to prevent the liquid passing through it into the bucket as it rises in the chamber, and to open and allow the contents of the bucket to escape when the water in the ponding chamber has almost been discharged. The operation of the apparatus is as follows. The ponding valve being closed, and the supply of liquid being turned into the ponding chamber, the liquid will rise. On reaching the small ball in the bottom of the valve of the bucket, the ball will float and close this bucket valve, so as to prevent the entrance of the liquid through and into the bucket. The liquid continuing to rise, will gradually and simultaneously submerge the bell and the bucket. The upper edge of the bucket is fixed at the predetermined height to which the liquid is to rise in the ponding chamber. The bell and bucket being of the same size and weight, and being hung so that the one is level with the other, the displacement of both will be equal up to the time the liquid reaches the upper edge of the bucket, and consequently no movement of the lever will take place. On the liquid rising above the top edge of and filling the bucket, it will cause the displacement of the bucket to be reduced to much less than the displacement of the bell, and this difference is sufficient to lift the valve from its seat, and discharge the contents of the ponding chamber. As the liquid in the chamber recedes, the bucket and the bell will both be gradually left suspended in the air. Until the liquid in the chamber drops below the bottom of the bucket, the latter will remain full; on the water continuing to fall, the ball in the small valve at the bottom of the bucket will drop, allowing the contents of the bucket to escape. On the bucket being emptied of its contents, the weight of the bell, together with the valve and chain or rod, will overcome that of the bucket, the valve will close, and the liquid commence to pond in the chamber again.
The special syphons manufactured by Messrs. Burn Bros., described under contact bed apparatus, are also suitable for use as dosing syphons, and work singly or in sequence, and give intermittent discharge to filters or to land (see Fig. 145, page 218).
An auto-mechanical type of syphon for dosing tanks is supplied by the Carlton Engineering Co., and is illustrated in Fig. 133.