Chapter V: Part 5

_Methods of Distribution._—Much diversity of opinion exists with reference to the question of distribution in filling contact beds. The many methods which have been tried in various places, may be arranged under four headings, viz. (_a_) above the surface of the material; (_b_) at the surface level; (_c_) just below the surface; (_d_) at the bottom of the bed. Some engineers hold the opinion that distribution over a large area of the material has no value, and that it matters little how the bed is filled so long as the liquid finds its way into the interstices of the material with as little disturbance as possible. The simplest method which fulfils these requirements is to allow the sewage to flow over the surface at the inlet end of the bed, but this soon causes the surface at this point to become clogged, and unless it is cleaned at frequent intervals the solids very quickly become washed down into the bed, and in the end this portion of the material will have to be removed and washed before it can be used again. The efforts to avoid this trouble have resulted in the numerous methods of distribution referred, to above.

Taking them in the order given, the idea of discharging the sewage above the surface level (_a_) by means of elevated troughs or pipes, has been to imitate to some extent the method found necessary in the case of percolating filters, and thus aerate the sewage before it enters the bed. The difficulties which arise in this case are that some extra fall is needed, and the provision of the troughs or pipes with suitable supports is costly. It is also extremely difficult to arrange the distribution by these means so that it shall be uniform over the whole area, and unless this is done it cannot be of much advantage.

Distribution at the surface level (_b_) may be provided by means of shallow grips in the material itself, and these have the great advantage that if they become clogged to such an extent as to prevent the sewage from freely passing into the bed, it is a small matter for the manager to cut a fresh grip in another direction and leave the first one to dry up when the sludge in it can be easily removed by hand. Another method is to use rows of stoneware channels, or wooden or iron troughs, with their edges set level with the surface of the material, so that the sewage may flow over the edges or through holes or notches in the sides. This is usually satisfactory, but it is not an easy matter to maintain all the channels at the same level, and after they have been in operation for a time it will be found that the material immediately under the troughs or channels is badly clogged, and can only be cleaned or renewed by removing the channels.

Sub-surface distribution (_c_) is arranged by means of perforated or open-jointed pipes, laid below the surface of the material and thus out of sight. The reasons for adopting this method are: that it avoids the unsightliness caused by surface distribution; that the surface is kept free from obstructions, and thus allows free aeration when the bed is emptied; and last, but not least, it prevents any nuisance arising from the evolution of obnoxious gases in the tank effluent whenever it is over-septicised, a not infrequent occurrence in the case of old-fashioned schemes, or in new works where the volume of sewage for which the tanks were designed has not yet reached its maximum. This method has the disadvantage that when the openings in and between the pipes become choked, more labour is involved in cleaning them than in the case of open channels or troughs on the surface.

Filling from the bottom (_d_) is assumed to possess all the advantages and none of the disadvantages caused by the other methods. The distribution is certainly uniform, as the liquid first fills the sub-drains and then rises at the same level throughout the whole of the material, forcing out any carbonic acid gas that may have accumulated in the lower part of the bed. As the sewage does not appear on the surface at all, there is no unsightliness and no trouble from bad odours. On the other hand, it is evident that the solids in suspension in the sewage or tank effluent are retained at the bottom of the bed, especially in the under-drains, and thus they will appear in large quantities in the effluent. Unless some special provision is made, by means of an effluent settling tank or sand-filter, to arrest these solids in suspension in the final effluent, they will be liable to cause trouble in the stream, and will, in any case, seriously affect the results of any analyses that may be made. The usual manner of arranging this method of filling, is to cause the sewage to flow into an open or covered chamber at the inlet to the bed, the walls of the chamber being provided at the floor level with openings connected to the sub-drains laid on the floor of the bed.

Whatever method of distribution is adopted, it is desirable that the surface of the filter material shall be not less than 3 inches above the highest level to which the sewage will rise, so that the liquid may not be visible at the surface.

_Sub-Drainage._—Reference has already been made to the fact that lack of ample under-drains has often been the cause of the failure of contact beds in the past. The general practice for a long time consisted in placing a layer of coarse material on the floor of the bed, and providing a few rows of ordinary agricultural drain-pipes laid with open joints. In some few cases special perforated pipes were used, in others the pipes were partly embedded in the concrete floor. In the opinion of the author, however, no drains in the form of pipes are satisfactory, as they do not leave a space _at_ the floor level as a free exit for the solids in the effluent. Where pipes are used it will generally be found after a few months’ operation that these solids, in the form of black sludge, have accumulated along the sides of the pipes and among the material at the floor level, and when this once commences the accumulation continues to take place, rising gradually in the bed until the interstices are choked to such an extent that the liquid capacity of the bed is reduced to a fraction only of its original volume. The trouble was intensified by the comparatively small number of the pipe drains usually found in the beds. It was evidently assumed that the matters would travel laterally through the layer of coarse material at the floor level. Unfortunately an additional impediment to the free flow of these matters was caused in many cases by the want of sufficient fall on the floor itself. Very little consideration will show that a large area of floor requires a considerable slope in order to produce the velocity of discharge necessary to remove matters in suspension, yet it was seldom that a gradient of more than 1 in 200 was provided, and in a few cases the surface of the floor was absolutely flat. Under these conditions it is difficult to see how any other result could be expected. It may be argued that it was not properly understood in those days that the solids in suspension (converted organic matters, the products of oxidation) must be removed if the filtering material is to retain its working capacity, but this fact has long been recognised in connection with percolating filters, which have in most cases been constructed upon complete false floors, provided with perforations, and with a suitable slope on the surface of the actual floor.

There is very little doubt that the question of providing ample means of sub-drainage deserves special consideration; and, in the author’s opinion, the floors of all contact beds should be laid with much greater fall to the outlet than in the past, and they should, in addition, be covered entirely with a false floor of special floor-tiles of the kind described in connection with percolating filters (pages 91 to 94). If the usual bottom layer of coarse material is then laid upon the false floor, it will be found that the beds will maintain their normal working capacity for a much greater length of time than in beds constructed on the old style. Instead of arranging the slope on the floor from the inlet end to the outlet end of the bed, it is preferable to construct an effluent channel with a suitable fall down the centre of the bed to the outlet, and arrange the floor with a cross-fall from the sides to the centre channel, which may be covered by slabs of concrete or stone laid upon the top of the floor tiles where they abut upon the edges of the channel as suggested, Fig. 139_a_.

_Material for Filling Contact Beds._—The remarks made under the heading “Filtering Material” for percolating filters (pages 101 to 103), apply with equal force to contact beds. In none of the many cases which have come under the observation of the author has it been possible to obtain such satisfactory results from other material as from clinker, when used under the same conditions as to the strength and volume of the sewage treated. It is true that excellent effluents have been produced by beds filled with burnt clay, broken bricks or stones, but it will usually be found that in such cases the material is more or less clogged, and that the volume successfully treated per day is considerably less than that which would be dealt with by clinker. The reasons are the same as those already mentioned in pages 101 to 103, and there is no need to say more here than to recommend as strongly as possible the use of hard-burnt vitrified furnace clinker as already described for percolating filters.

The foregoing remarks apply more particularly to the material for coarse and medium-grain contact beds. Clinker of the kind described is equally suitable for fine-grain contact beds, but it is difficult to break it to the required grade except at great cost, and a considerable loss in bulk due to the production of fine dust. For fine-grain beds, requiring material specified to pass a ¼-inch square mesh, and to be retained on a ⅛-inch square mesh, clean coke-breeze from gas-works (not ashes) will be found to be the most efficient. The important point to be observed in preparing material for fine-grain contact beds is that, while none of the particles should exceed ¼ inch in diameter, as many as possible of the finer particles, ⅛ inch and 3/16 inch in diameter, should be retained, but all dust should be removed even if it necessitates washing the material for that purpose. Even the finer particles down to 1/16 inch diameter might be used, but it will be found extremely difficult to arrange the sifting process to arrest these without retaining the dust as well, especially if the material is at all damp, as the fine meshes required for the purpose quickly become clogged with the dust, and the sieve or screen is rendered useless.

Among other materials which may be adopted for fine-grain contact beds, broken saggars in the pottery districts, or slag in the neighbourhood of ironworks, are probably the next best, but only if they are properly graded in the manner described above. Indeed the chief difficulty in securing this fine-grade material is the preparation and grading, particularly where large quantities are required and the situation of the beds involves much handling of the material after it has been sifted. It is, however, of such extreme importance to have it as fine as possible, without including any dust, that the stipulations in the specification with regard to this material should be made very clear and definite, so that the contractor may make sufficient provision in his prices to enable him to comply with the specification in its strictest sense.

In addition to the bottom layer of floor-tiles and coarse material prescribed in the section relating to sub-drainage, it will be necessary to provide an additional layer above this, about 3 inches in depth, of a medium grade, to support the very fine material and prevent it being washed through the interstices in the coarse bottom layer, and special attention must be devoted to the method of distribution at the surface in order to avoid disturbance of the fine material.

_Automatic Apparatus for Contact Beds._—Although contact beds can be operated by hand, this involves continuous and regular supervision by a man, and, unless he is under strict control, the proper cycle of operation may not be adhered to, and the result will, in that case, not be satisfactory. In the early days of contact beds, several types of apparatus were designed for the purpose of operating these beds automatically, so that the cost of manual labour would be reduced to a great extent, and in addition, the possibility of mismanagement avoided. Among these appliances, one of the most widely known is that manufactured by Messrs. Adams Hydraulics, Limited; and shown in Fig. 140. In small installations, the low draught syphons described above in connection with dosing apparatus are used to give alternate fillings to pairs of contact beds. Where more than two beds are in use the automatic air-lock feed is used. This consists of a cast-iron inverted U-pipe, both legs of which are trapped, either in self-contained castings or in separate chambers. The sewage flows through one, the others being charged with air. When the sewage in the bed has reached the proper height it overflows into a small chamber in which a dome is fixed. This dome is connected by means of an air-pipe to the cast-iron inlet feed, and as the sewage rises in the chamber round the dome, the air contained therein is forced up into the feed which it fills, thus forming an air-lock, which prevents the sewage from passing through and stops the supply to the bed. At the same time that the feed is stopped by the transfer of air in the manner described, the compression of the air in another small dome in the same pit has forced a water seal on the air-pipe leading from this dome to the feed in the next bed, and thus liberates the air-lock in the feed of that bed and allows the sewage to flow to this in rotation. The whole operation is then repeated with each bed in rotation, the last bed in the series, when full, starting the feed in the first bed again. These same feeds may be arranged to hold up sewage in a collecting or dosing tank, so as to ensure the accumulation of sufficient sewage to fill each bed at one dose within a reasonable time.

In order to ensure that the sewage is held up in each bed for a suitable period for contact, the outlet is provided with an automatic syphon, which is arranged in such a way that the filling of the bed alone can start it. When the bed is full, the sewage flows through a pipe with an adjustable orifice into the timing pit. When this pit is full, the compression of the air in a small dome placed in the pit, and connected to the syphon by means of an air-pipe, releases the air in the syphon and allows it to start at the end of the desired period of contact. A special feature of these syphons is that they are so arranged that after they have emptied the bed they continue in action as syphons, taking the drainings from the bed, however small in volume they may be, and stopping off only when the bed commences to fill again, thus ensuring a thorough draining of the material. Another special feature of this apparatus is that there are no moving parts, the whole operation depending upon the transfer of air by means of the head of liquid. It is important to note that by this apparatus the period of contact in any bed can be arranged for any particular length of time, quite irrespective of the rate of flow of the sewage to the works, and independent of the filling and emptying of the other beds in the series.

Another apparatus which has been used largely in the past is that supplied by the Septic Tank Company, Ltd. This apparatus is made in two forms, illustrated in Figs. 141 and 142. The chief difference between the two types is that in the one case, Fig. 141, the period of contact is controlled by the filling of the next bed, and is thus dependent upon the rate of flow of the sewage, while in the other case, Fig. 142, the period of contact is “timed” for from one to two hours, and is independent of the rate of flow of the sewage. In both cases each set of gear is built up on its own bed-plate, and comprises the inlet and outlet valves and the connecting pipes to and from the same. The valves are of the simple spindle-type, and are connected by rods to rocking levers and actuated by buckets or floats, working in chambers or pits which are in communication with the different beds. By means of overflows from the beds to the buckets, and other devices, the various portions of the gear are actuated in such a manner that they automatically fill the several beds in each set in regular rotation, hold them full for contact, and eventually discharge the treated liquid to the effluent drain. Full details of the method of operation can be obtained from the manufacturers, who claim that the gear will work satisfactorily without attention other than the oiling of the bearings and joints every few weeks.

Another type of automatic apparatus for contact beds is that manufactured by Messrs. J. Blakeborough and Sons, as used in the Triple Tank System of sewage treatment, Fig. 143. In this case the beds are arranged in sets of three, and the filling of each is effected by the overflow from the last bed filled, the discharge being effected by the rising of the liquid in the last bed filled. The apparatus consists broadly of a slide-valve controlling the outlet-pipe, and connected by levers to floating cylinders located in separate chambers. One chamber controls the opening and the other the closing of the valve. The outlet is provided with a rising and falling arm, which is connected by a lever to a balance float fixed in a chamber, and coupled by means of a pipe to the float-chamber of another bed. The method of operation is us follows:—The tank effluent flows by gravitation to bed A, the filtered effluent thus rising in outlet chamber A, and also in the overflow chamber which is connected by pipes to outlet chamber A. As soon as bed A becomes full, the filtered effluent overflows into closing chamber A, which is coupled by a pipe to opening chamber B, and the floating cylinders in each case are raised, with the result that the valve of bed A closes and the valve of bed B opens, the liquid thus commencing to run on to bed B. The same action as above is repeated when bed B is full, and bed C is to be filled, whilst bed C in turn is coupled to bed B, so that the triple action is repeated over and over again so long as sewage continues to flow to the beds. As bed A fills, the liquid rises in outlet chamber A, and, this being coupled to float chamber C, the liquid rises to a corresponding height in float chamber C. Some time elapses before the liquid rises to such a height in the float chamber as to sink the mouth of the outlet pipe below the surface of the liquid in the outlet chamber, this space of time (which can be regulated as desired) representing the length of time that the bed is allowed to stand full before commencing to empty. When the liquid has risen to a given height in float chamber C, the balance float is raised, this action tipping the lever and lowering the rising and falling outlet pipe in outlet chamber C, thus drawing off the effluent from the top slowly, and without disturbing the whole contents of the bed. (_Note_: Bed C is assumed to be full). After the liquid in the bed has been drawn off, the rising and falling outlet pipe remains stationary at the bottom of the chamber until the next action takes place, which is as follows:—When bed A becomes full, it is allowed to stand full until the liquid in bed B (now filling) has risen to a given height, when it raises the balance float in the float chamber A in a similar manner as described above, and thus empties the bed A, at the same time emptying the float chamber C, in which is fixed the balance float connected to the rising and falling outlet pipe of the bed C (now standing empty), thus raising the outlet pipe and rendering the bed again ready for use.

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._

In recent years the provision of proper means of measuring and recording the flow of sewage at disposal works is becoming more general, but there are still a very large number of works at which it is impossible to obtain any trustworthy information as to the volume of sewage treated. As long as all works smoothly, and there is no trouble with the effluent, it is considered superfluous to trouble about the quantity of the liquid which passes through. When, however, difficulties arise, and it becomes necessary to investigate the cause of the trouble, it is of the utmost importance to be in a position to ascertain the daily volume of the sewage arriving at the works and the variations in the rate of flow, as well as the quantities dealt with by each separate tank and filter. It is also of great assistance, in making investigations at such times, to have a definite record of the volumes treated day by day during the preceding six months; indeed the possession of a complete record of the daily flow of sewage over the whole period during which a sewage works has been in operation, is a valuable asset not only to those responsible for the works themselves but also to the authorities who control the streams and watercourses, and to investigators in search of information to be used for the public benefit. The initial cost of suitable measuring and recording appliances is not excessive, but, when once the works are completed and in good working order, the local surveyor or manager has great difficulty, and usually finds it impossible, to persuade his committee that the outlay is justified. It is, therefore, desirable that engineers should in all cases make provision for such apparatus in the contract for the construction of the works, and thus make sure that it will be available when required.

Various types of measuring appliances are in use. Among these the simplest is the gauge weir penstock Fig. 148, manufactured by Messrs. Adams Hydraulics, Ltd., from which the depth of water flowing over the weir can be observed at any time. This, however, does not provide a record of the volume flowing at all times. For this purpose the same firm supply several types of recorders, among which the simplest is the type shown in Fig. 149. This is supplied with 24-hour or 7-day movement, and charts which register the volume in gallons per minute, per hour, or per 24 hours.

Another measuring apparatus is that supplied by Messrs. George Jennings, Ltd., Fig. 150. This consists briefly of a hollow copper float attached to a brass rod, which is carried up through a hollow column into the indicator box provided with a glass hinged door. The brass rod is provided with pointers, which move up and down with the rods and indicate the water level.

Messrs. George Kent, Ltd., supply a large number of various types of recorders and measuring instruments, including the well-known “Venturi” meter and others actuated by floats to indicate the discharge over a weir. The Venturi meter type of measuring apparatus adapted for sewage, is shown in Fig. 151. In this case the Venturi tube may form part of the ordinary supply pipe, or it may be fitted in a chamber built in the supply channel, the tube connected through the chamber with its ends terminating in the channel at either side. This measuring apparatus may be fitted with the following types of recorders. A diagram only, counter only, or combined counter and diagram in one instrument. The recorder may be placed in any convenient position within a thousand feet of the meter tube.

Several types of measuring apparatus are manufactured by Messrs. Glenfield and Kennedy. One of these is illustrated in Fig. 152. This consists of an apparatus for recording the volume of water or sewage flowing over a weir, a chart being revolved by clockwork, and the volume indicated by a pen actuated by a float and cord working over pulleys. This instrument can be fitted with cam and pen carriage to show the rate of discharge on the chart (24 hours or 7 days), in gallons of cubic feet per minute over a V-notch, rectangular weir, or in an open channel of known dimensions. It is claimed that by simply taking the area of the diagram, the total discharge for any period can be ascertained much more quickly and more correctly than from a diagram simply giving the height flowing over the weir.

A somewhat novel form of apparatus, recently introduced by Messrs. Adams Hydraulics, Ltd., for measuring the flow of sewage in a channel, consists of a water-wheel by means of which the velocity of the flow is registered. In order to maintain the paddles of the wheel at a uniform depth below the surface of the liquid, the wheel is carried upon a shaft supported by two floats provided with vertical guide rods working in brackets attached to the sides of the channel. By this means the whole of the apparatus rises and falls with the liquid in the channel. The wheel-shaft is provided with a bevel-toothed wheel, which engages with another similar wheel attached to a flexible shaft, and this drives a set of geared wheels similar to a flush-tank counter, which thus record the number of revolutions made by the paddle-wheel.

_STERILISATION OF SEWAGE EFFLUENTS._

It has been recognised in many quarters that, although it is possible by modern methods of sewage disposal to secure a high degree of purification from a chemical point of view, it may be necessary in certain cases to take steps to remove the large numbers of bacteria present in all such effluents. Experiments have been made which demonstrate that, when a pure culture of some specific organism is added to a sewage in a sufficiently large quantity, it may pass through the tanks and filters, and appear in the final effluent. This result is not necessarily a conclusive proof that the bacteria in sewage effluents are dangerous, as the experiments do not represent normal practical conditions. On the other hand, it is true that, in ordinary practice, sewage effluents contain large numbers of _B. coli_, which is admittedly of intestinal derivation, and although this bacillus is not a disease organism itself, its survival in an effluent is considered an indication of the presence of sewage matter, and consequently of the possibility of the survival of any pathogenic germs which may be present in the crude sewage. On this basis, scientists argue that sewage effluents are _potentially_ dangerous—that there is a possibility of the pollution of drinking-water or shell-fish by the bacteria present in sewage effluents. This being so, it is evident that an additional process will, in some cases, be required to remove the bacteria, and in a few cases sand filters have been provided for this purpose. These, however, involve a comparatively high initial expense, and a considerable annual outlay for maintenance, and in some quarters it is considered that sterilisation may possibly be a means of securing the desired result at less cost, and with a higher percentage of removal of bacteria, and consequently with a higher degree of safety. It is true that a number of scientists, in replying to a question which was submitted to them on this subject by the Royal Commission on Sewage Disposal, stated that in their opinion sterilisation was impracticable, but this was in the year 1903, and there is good reason for assuming that if the same question were put to the same men to-day, the replies would in many cases be modified, if not entirely different. Practical experiments in the sterilisation of sewage effluents have been few and far between in this country, but in the United States of America a large number of reliable experiments under varying conditions have been made, and the results published. From these it is evident that sterilisation is not only possible, but economically practicable. Unfortunately, both in America and elsewhere, attempts were made to sterilise crude sewage and tank effluents, and the results of these experiments were so unsatisfactory, both in efficiency and cost, that they gave the impression that sterilisation was impracticable.

In the opinion of the author, sterilisation should be restricted to the destruction of living organisms, and should only be used in the case of liquids with a high degree of chemical purity, and a low content of matters in suspension. It is quite possible at the present time to produce sewage effluents which comply with these conditions, by means of properly designed, constructed and managed sewage disposal works, as long as these include suitable effluent settling tanks for the removal of the solids in suspension. Effluents which comply with the provisional standard suggested by the Royal Commission, and drinking-water supplies which are very slightly polluted, would be very suitable.

A number of different methods of sterilisation have been tried, but so far as the existing knowledge of the subject extends, the application of chlorine in one form or another is generally admitted to be the most efficient and economical process. The chlorine may be applied in the form of a solution of chloride of lime, or as a hypochlorite of sodium, or of magnesium. The solution of chloride of lime may be prepared by the sewage works manager, but this necessitates a considerable amount of care and knowledge in order to secure the correct strength at all times, and it is, of course, essential that the chloride of lime itself should always be of a known strength. The hypochlorite of sodium may be produced chemically, and this can be purchased of known strength from chemical manufacturers. For large volumes of sewage effluent, however, it will probably be found most economical to utilise an electrically produced hypochlorite of sodium, as this can be prepared on the works as and when required, of a uniform strength, and at a comparatively low cost.

In the case of the Digby process, briefly stated, this consists in passing an electric current through a solution of sodium chloride, with the result that the sodium chloride is broken up into its component parts, and chlorine is liberated at the positive pole, while sodium is deposited at the negative pole. The sodium and the chlorine are then allowed to recombine in the form of hypochlorite, and this solution is then ready for application to the liquid to be sterilised. Other processes are similar in principle but vary in detail.