Part 11

Messrs. Glenfield and Kennedy, Ltd., also manufacture apparatus for operating contact beds, as shown in Fig. 144. This arrangement of the apparatus delivers the sewage to six beds. There are two valve boxes, A, internally divided into three compartments. Three sets of tube valves, B, on each valve box, control the inlets to the several compartments of the valve box and also to the beds, which are connected up to the valve boxes with suitable pipes. As the sewage collects in the measuring chambers, it raises the float C—the float chamber being in communication—which, through the rack-and-pinion shown, turns the shaft D. To a sleeve, E, over the shaft D, a hammer, F, is keyed, while a stopper catch, G, is mounted freely. Keyed to the shaft D are lifting levers H and K. The lever K lifts the hammer F to the vertical, and, being free to rotate with the sleeve E, it falls and strikes on one of the copper buffers, L, in the turning plate M, causing it to turn. In like manner the stopper catch G is thrown over by the lifter H—a little in advance of the hammer—and, resting on the stopper plate N, drops into one of the notches, O, thus stopping the gear at the proper place. The turning of plate N causes the roller lever P, keyed to the vertical shaft Q, to rotate—through the agency of the mitre gearing and horizontal shaft—thus actuating the lever R, and raising the tube valve B. Two valves are operated simultaneously, one on each valve box. The valves are held open by the levers P, until, the water being run off, the weight of the float descending puts the gear in motion again—by returning the hammer to its original side—and moves the roller levers P off the end of the levers R, thus allowing the tube valves B to close and the water to collect once again. The force of the blow of the hammer as it strikes the buffer L can be regulated within certain limits, for, on the outer end of the sleeve E carrying the hammer, a lever, S is keyed, which, as it works in unison with the hammer, and is attached to the piston of the swivel cataract adjustable oil cylinder T, has the effect of cushioning the fall of the hammer.

Another type of syphonic apparatus for contact beds is manufactured by Messrs. Burn Bros., as shown in Fig. 145. In this case the primary filters are usually supplied with sewage from a collecting or dosing tank in which two or more discharge syphons are fixed, or they may be filled from a supply channel under certain circumstances. In the former case a syphon discharges immediately the collecting tank is full. A “Sequela” relief apparatus is attached to each syphon, and causes these to discharge alternately or in rotation. The relief apparatus is divided into three compartments, and depends for its working on the transference of oil, of a special nature, from one compartment, A, to another compartment, C, via compartment B, in stages corresponding with the number of syphons under control, each relief apparatus at the commencement being set a stage in advance of the one next to it. After a syphon has discharged, the oil which has been transferred to the compartment C in the relief apparatus is automatically returned to the compartment A, and the apparatus is then ready for another series of operations. Thus the oil, which is non-evaporative and non-freezing, is used over again and again, and as it does not come in contact with the sewage, it remains quite pure and serviceable for years. A discharge syphon is fixed to each filter, and, in order to ensure a proper period of contact of the sewage with the filtering material, each syphon is provided with a “Horometer” relief apparatus. This apparatus can be set to give a period of contact varying from twenty minutes to twenty-four hours. The “Horometer,” like the “Sequela,” depends upon the transference of oil from one compartment to another, but in this case only two compartments are necessary, A and B. As the filter fills, the oil is forced, by air pressure, to rise in a vertical pipe from compartment A above the level of a regulating tap, which is set to pass the oil into compartment B in the time determined upon for the contact of the sewage in the filter, and as soon as the necessary quantity of oil has been transferred through the tap, the syphon discharges. After the syphon has discharged, the oil is automatically returned from compartment B to compartment A, and the apparatus is again ready for use. No watertight brick chambers are required in the filter in connection with the apparatus, thus effecting considerable economy in structural work. It is only necessary to construct a screen in dry brickwork or perforated iron round the syphons to hold up the filtering material.

The syphons manufactured by Messrs. George Jennings, Ltd., actuated by air-valves as described under the heading of “Dosing Apparatus,” can also be adapted for filling and emptying contact beds.

The Enock apparatus for contact beds, Fig. 146, manufactured by Messrs. A. G. Enock and Co., Ltd., is a simple device working on the principle of the ball valve. A float, which takes the place of the ball, is raised by liquid entering a pit, which pit is outside the bed or tank which has to be emptied by the valve. The valve is attached to a vertical rod in connection with a horizontal weighted lever, at the other end of which the float is fixed. When a tank is full, it flows into the float chamber, and the rise of the liquid in this pit lifts the float and opens the valve, thereby allowing the contents of the tank to escape. The float pit then slowly empties itself by means of a small outlet pipe, and the valve closes so that the tank is ready to receive more liquid. This apparatus can be arranged so as to fill a number of beds in rotation, the inlet valve to each pit being either opened or closed as required by the overflow of liquid from each contact bed in turn. The outlet valves to the contact beds are similar to those already described and if the first beds are filled in rotation, no further connection between the apparatus in the lower beds will be required, each valve working absolutely independently of the others.

The chief advantage claimed for this type of apparatus is, that it can be adjusted so as to suit any required level of liquid in any particular bed.

_CAPACITY OF PERCOLATING FILTERS AND CONTACT BEDS._

Although this separate section is devoted to the question of calculating the capacities of percolating filters and contact beds, it is mainly for the purpose of stating that it is impossible to formulate any rules which admit of general application. It may reasonably be pointed out that this last statement is a truism, and affords no assistance to those in search of information on the subject. There is, however, so great a tendency in some quarters, to rely upon results obtained in one place under certain conditions as a guide in designing a scheme in another place under possibly totally different conditions, that it is impossible to repeat the statement in question too often.

In the first place, long and practical experience is necessary to enable an engineer to come to a decision as to what are the conditions under which any particular scheme is to be carried out, and which of them will have a bearing upon the methods to be adopted in the design of the works. A careful study of the fifth report of the Royal Commission on Sewage Disposal, will show that they assume over 70 different sets of conditions under which percolating filters and contact beds may be adopted. The capacity of the filters required to produce the desired results will depend upon the strength of the sewage to be treated, the type of tank adopted for the preliminary process of sedimentation, the grade of material to be used, the amount of fall available, the final destination of the effluent, and other factors, all of which again may be affected by other circumstances, which must of necessity be taken into consideration. As the basis for calculating the capacity of filters may vary between 15 and 200 gallons of the daily dry-weather flow per cubic yard of material, it is evident that there is a wide margin for error, and the only safe course to adopt is to allow for the worst possible conditions and thus provide a large margin of safety. Although the suggestions made in the fifth report of the Royal Commission with regard to the provision to be made under various sets of conditions may be taken as a guide, to some extent, it should be borne in mind that the figures given represent the minimum which should be allowed in each case, and the only really safe guide in these matters is long practical experience of a large number of works under the greatest possible variety of conditions.

In order, however, to provide a rough guide for the purpose of making preliminary estimates, it may be stated here that, under ordinary conditions, with sewage of average strength, a properly designed preliminary process, suitable material of medium grade, and not less than 4 feet deep for percolating filters, it should be possible to produce an effluent which will not create a nuisance by providing—

(_a_) Percolating filters, at the rate of one cubic yard for every 84 gallons of the daily dry-weather volume, or

(_b_) Contact beds, at the rate of one cubic yard in each series for every 56 gallons of the daily dry-weather volume.

In other words, the ratio of the cubic capacity of filter material to the daily dry-weather volume of the sewage for all ordinary purposes may be taken as—

(_a_) 2 to 1 for percolating filters. (_b_) 3 to 1 for contact beds.

TABLE, GIVING THE RATIO OF THE TOTAL CUBIC CAPACITY OF PERCOLATING FILTERS AND CONTACT BEDS TO THE DAILY DRY-WEATHER VOLUME OF THE SEWAGE UNDER VARYING CONDITIONS.

──────────────────────────┬────────────────────────────────────── │ Percolating Filters ├────────────────────────────────────── │ Strength of Sewage ├────────────┬────────────┬──────────── │ Strong │ Average │ Weak ├────────────┴────────────┴──────────── Preliminary Process. │ Grade of Material (See pages 23, 29). ├──────┬─────┬──────┬─────┬──────┬───── │Coarse│ │Coarse│ │Coarse│ │ or │ Fine│ or │ Fine│ or │ Fine │medium│ │medium│ │medium│ ──────────────────────────┼──────┼─────┼──────┼─────┼──────┼───── Detritus tanks │11·20 │ — │ 6·72 │ — │ 4·20 │ — │ │ │ │ │ │ Septic tanks │ 3·73 │ — │ 2·40 │ — │ 1·68 │ 1·68 │ │ │ │ │ │ Continuous flow settlement│ 3·73 │ — │ 2·40 │ — │ 1·68 │ 1·68 │ │ │ │ │ │ Quiescent settlement │ 3·36 │ 6·72│ 1·68 │ 2·40│ 1·29 │ 1·29 │ │ │ │ │ │ Continuous flow chemical }│ │ │ │ │ │ precipitation }│ 2·58 │ 3·36│ 1·68 │ 2·10│ 1·12 │ 0·96 │ │ │ │ │ │ Quiescent chemical }│ │ │ │ │ │ precipitation }│ 1·68 │ 2·58│ 1·29 │ 1·29│ 0·98 │ 0·84 ──────────────────────────┴──────┴─────┴──────┴─────┴──────┴─────

──────────────────────────┬─────────────────────────────────────────── │ Contact Beds ├─────────────────────────────────────────── │ Strength of Sewage ├──────────────┬──────────────┬───────────── │ Strong │ Average │ Weak ├──────────────┴──────────────┴───────────── Preliminary Process. │ Number of Series (See pages 23, 29). ├────┬────┬────┬────┬────┬────┬────┬────┬─── │ │ │ │ │ │ │ │ │ │ ×1 │ ×2 │ ×3 │ ×1 │ ×2 │ ×3 │ ×1 │ ×2 │ ×3 │ │ │ │ │ │ │ │ │ ──────────────────────────┼────┼────┼────┼────┼────┼────┼────┼────┼─── Detritus tanks │ — │ — │6·72│ — │6·72│ — │ — │4·42│ — │ │ │ │ │ │ │ │ │ Septic tanks │ — │ — │5·09│ — │4·42│ — │2·24│2·54│ — │ │ │ │ │ │ │ │ │ Continuous flow settlement│ — │ — │5·09│ — │4·42│ — │2·24│2·54│ — │ │ │ │ │ │ │ │ │ Quiescent settlement │ — │ — │3·81│ — │3·36│ — │1·68│ — │ — │ │ │ │ │ │ │ │ │ Continuous flow chemical }│ │ │ │ │ │ │ │ │ precipitation }│ — │5·09│ — │ — │3·36│ — │1·26│ — │ — │ │ │ │ │ │ │ │ │ Quiescent chemical }│ │ │ │ │ │ │ │ │ precipitation }│ — │3·90│ — │ — │2·54│ — │1·26│ — │ — ──────────────────────────┴────┴────┴────┴────┴────┴────┴────┴────┴───

KEY: ×1 = Single ×2 = Double ×3 = Triple

In the case of contact beds these figures give the cubic capacity of each series, and they must be doubled or trebled respectively for double and triple contact. Both percolating filters and contact beds, constructed on this basis, would be capable of treating up to three times the dry-weather flow in times of storm.

Having given the above method of calculation, in a form not usually adopted in connection with sewage disposal, and bearing in mind the misunderstandings which frequently arise in comparing the various methods in use at the present time in different countries, it may be useful to set out in this form the figures which, it is understood, have been adopted by the Local Government Board, on the basis of the fifth report of the Royal Commission on Sewage Disposal, as the minimum which they consider suitable under varying conditions. In the Table opposite, the figures given represent the ratio which the cubic capacity of the filters bears to the daily dry-weather volume of the sewage, whether it be in gallons, cubic feet, cubic metres, vedros, or any other term of measurement.

_Examples._—1. A daily dry-weather volume of 10,000 gallons of sewage, of average strength, is to be treated upon percolating filters of medium sized material, after preliminary treatment in septic tanks.

10,000 gallons × 2·40 = 24,000 gallons = 3,840 cubic feet = total cubic capacity of filters

2. A daily dry-weather volume of 3,000 cubic metres of weak sewage is to be treated upon single contact beds, after preliminary treatment in continuous flow settlement tanks.

3,000 cubic metres × 2·24 = 6,720 cubic metres = total cubic capacity of beds

For the purpose of the above Table, the strength of the sewage is estimated according to the amount of oxygen absorbed from permanganate of potash in four hours, as indicated in the fifth report of the Royal Commission as follows:—

Parts per 100,000 “Strong” sewage = oxygen absorbed 17 to 25 “Average” ” = ” ” 10 to 12 “Weak” ” = ” ” 7 to 8

A quick method of converting gallons into cubic feet is to multiply the gallons by the reciprocal 0·16. This can be done rapidly (frequently by mental calculation) by multiplying the gallons by 4, and the product again by 4, and inserting the decimal point between the second and third figures from the right-hand side, thus:—

24,000 × 4 = 96,000 × 4 = 384,000 = 3840·00 = cubic feet

This is useful in calculating the capacity of tanks, and for all similar purposes.

_STORM-WATER TREATMENT._

In connection with Sewage Disposal Works, the term “storm-water” is generally understood to mean the extra volume which reaches the works in times of rainfall, in excess of three times, up to and including six times, the average dry-weather flow; so that the volume of storm-water for which provision should be made is equal to three times (volumes) the daily dry-weather flow. Prior to the publication of the fifth report of the Royal Commission, it was usual to provide a rough straining filter for the storm-water, or to reserve a portion of the land for the purpose of dealing with it by broad irrigation. In either case the area of filter surface or land required was 1 superficial yard for every 500 gallons of storm-water (D.W.F. × 3/500 = super-yards). As the result of their investigations, the Royal Commission came to the conclusion that “storm-water” filters, as generally constructed under these conditions, were useless for the purpose for which they were required, and this confirmed the views of most engineers of experience. Where suitable land can be secured for the purpose, and arranged in such a manner that it is reserved solely and entirely for treating the storm-water, this method may still be adopted. If this is not possible, stand-by tanks may be constructed for the purpose of receiving the storm-water. These tanks are to be not less than two in number, and should have a total capacity of not less than one-quarter of the average daily dry-weather flow. The only overflow at the outfall works from which storm-water may be discharged direct to the stream, or other final effluent outlet, must be from these stand-by tanks, and it should not come into operation until these tanks are full. Having regard to these recommendations, it is necessary in every scheme to provide at least two special storm-water stand-by tanks, with a total capacity of ¼ D.W.F.; and the drawing, Fig. 147, illustrates a simple method of constructing these. In this case the inlets are in the form of weirs, running the full width of the tank, so that if the channel leading to these tanks is in communication with, and at the same level as, the inlets to the detritus tanks, and the latter are provided with slotted doors (see Fig. 10, page 19), the weir at the inlets to the stand-by tanks will act as the actual storm-overflow, and, being of considerable length, the maximum height to which the water will rise in passing over this weir will be very small, and will thus have very little effect upon the rate of flow to the sedimentation tanks.

As the only overflow discharging direct to the stream must be from these tanks, and must only come into operation when they are full, the outlet is also constructed in the form of a weir discharging into a channel from which a pipe would be laid to the stream. A further requirement in connection with these tanks is that they should always be kept empty, ready to receive the excess of storm-water at any time. From this it is evident, that these tanks must be emptied after each heavy shower or storm which increases the rate of flow of sewage to the works beyond three times the dry-weather flow. Unfortunately, no directions are given as to the manner in which this is to be accomplished. In the absence of any definite statement to the contrary, it might be inferred that, after the overflow from these tanks has ceased, their contents may also be discharged direct to the stream. As, however, this would necessitate outlets at or near the bottom of the tanks, there would appear to be a possibility of the suspended matters deposited in the tanks being discharged to the stream—the very thing the tanks are designed to prevent. With such an arrangement, also, there will be a risk of the man in charge of the works, either wilfully or by an oversight, leaving the outlets at the bottom of the tanks open, and thus permitting the storm-water to pass direct to the stream without the settlement which it is anticipated by the Royal Commission (Fifth Report, page 233, par. 352) will be provided for all storm-water arriving at the works.

It is obvious that these tanks must be emptied after every heavy shower or storm, and that facilities must be provided both for drawing off the supernatant water and for removing the deposit which will accumulate at the bottom. In the author’s opinion, the only safe method is to provide floating arm outlets for the supernatant water, and to discharge this to land or to a special filter for further treatment, or, better still, to pump it up to the detritus tanks to be treated again with the ordinary sewage. In schemes where the whole of the sewage is pumped at the works, the contents of these storm-water stand-by tanks should certainly be discharged into the pump-well, as this would not involve the provision of special pumping plant. With regard to the sludge from these tanks, this should be drawn off by means of special outlets, and dealt with on sludge draining beds in the manner previously described (page 83).

The difficulties which frequently arise in designing suitable and convenient methods of dealing with storm-water, render it desirable that very careful consideration should be given to the question as to whether it would not be more satisfactory, from the point of view of both economy and efficiency, to omit the stand-by tanks, and increase the capacity of the filters required to deal with the _ordinary_ sewage, to such an extent that they will be capable of dealing with the whole volume of sewage and storm-water combined up to six times the dry-weather flow, and thus obviate the necessity for any storm-overflow at all at the outfall works. If this idea were universally adopted, it would necessitate greater care in the construction of any storm-overflow required on the line of the outfall sewer itself before it reaches the works, but there are (or should be if the sewers were properly constructed) so few cases where the excess of flow, even during the heaviest rainfall, ever reaches six times the dry-weather flow, that the extra cost involved cannot be considered excessive if the greater certainty of securing satisfactory results at all times is taken into consideration.

_MEASURING APPARATUS._