Part 6

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 E_{2}. 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.