Part 5

_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.