The Sewerage of Sea Coast Towns Part 5

The extra cost of the sewers to carry the additional quant.i.ty of storm water might also be taken into account by working out and preparing estimates for the alternative schemes.

The actual cost of the fuel may be taken at approximately 1/4 d. per 1,000 gallons. The annual works and capital charges, exclusive of fuel, should be divided by the normal quant.i.ty of sewage pumped per annum, rather than by the maximum quant.i.ty which the pumps would lift if they were able to run continuously during the whole time. For a town of about 10,000 inhabitants these charges may be taken at 1-1/4 d. per 1,000 gallons, which makes the total cost of pumping, inclusive of capital charges, 1-1/2 d. per 1,000 gallons. Even if the extra cost of enlarging the sewers is added to this sum it will still be considerably below the sum of 6 d., which represents the cost of providing a separate system for the surface water.

Unless it is permissible for the sewage to have a free outlet to the sea at all states of the tide, the provision of effective storm overflows is a matter of supreme importance.

Not only is it necessary for them to be constructed in well- considered positions, but they must be effective in action. A weir constructed along one side of a manhole and parallel to the sewer is rarely efficient, as in times of storm the liquid in the sewer travels at a considerable velocity, and the greater portion of it, which should be diverted, rushes past the weir and continues to flow in the sewer; and if, as is frequently the case, it is desirable that the overflowing liquid should be screened, and vertical bars are fixed on the weir for the purpose, they block the outlet and render the overflow practically useless.

Leap weir overflows are theoretically most suitable for separating the excess flow during times of storm, but in practice they rarely prove satisfactory. This is not the fault of the system, but is, in the majority of the cases, if not all, due to defective designing. The general arrangement of a leap weir overflow is shown in Fig. 17. In normal circ.u.mstances the sewage flowing along the pipe A falls down the ramp, and thence along the sewer B; when the flow is increased during storms the sewage from A shoots out from the end of the pipe into the trough C, and thence along the storm-water sewer D. In order that it should be effective the first step is to ascertain accurately the gradient of the sewer above the proposed overflow, then, the size being known, it is easy to calculate the velocity of flow for the varying depths of sewage corresponding with minimum flow, average dry weather flow, maximum dry weather flow, and six times the dry weather flow.

The natural curve which the sewage would follow in its downward path as it flowed out from the end of the sewer can then be drawn out for the various depths, taking into account the fact that the velocity at the invert and sides of the sewer is less than the average velocity of flow. The ramp should be built in accordance with the calculated curves so as to avoid splashing as far as possible, and the level of the trough C fixed so that when it is placed sufficiently far from A to allow the dry weather flow to pa.s.s down the ramp it will at the same time catch the storm water when the required dilution has taken place. Due regard must be had to the altered circ.u.mstances which will arise when the growth of population occurs, for which provision is made in the scheme, so that the overflow will remain efficient. The trough C is movable, so that the width of the leap weir may be adjusted from time to time as required. The overflow should be frequently inspected, and the acc.u.mulated rubbish removed from the trough, because sticks and similar matters brought down by the sewer will probably leap the weir instead of flowing down the ramp with the sewage. It is undesirable to fix a screen in conjunction with this overflow, but if screening is essential the operation should be carried out in a special manhole built lower down the course of the storm-water sewer. Considerable wear takes place on the ramp, which should, therefore, be constructed of blue Staffordshire or other hard bricks. The ramp should terminate in a stone block to resist the impact of the falling water, and the stones which may be brought with it, which would crack stoneware pipes if such were used.

In cases where it is not convenient to arrange a sudden drop in the invert of the sewer as is required for a leap weir overflow, the excess flow of storm-water may be diverted by an arrangement similar to that shown in Fig. 18. [Footnote: PLATE IV] In this case calculations must be made to ascertain the depth at which the sewage will flow in the pipes at the time it is diluted to the required extent; this gives the level of the lip of the diverting plate. The ordinary sewage flow will pa.s.s steadily along the invert of the sewer under the plate until it rises up to that height, when the opening becomes a submerged orifice, and its discharging capacity becomes less than when the sewage was flowing freely. This restricts the flow of the sewage, and causes it to head up on the upper side of the overflow in an endeavour to force through the orifice the same quant.i.ty as is flowing in the sewer, but as it rises the velocity carries the upper layer of the water forward up the diverting plate and thence into the storm overflow drain A deep channel is desirable, so as to govern the direction of flow at the time the overflow is in action. The diverting trough is movable, and its height above the invert can be increased easily, as may be necessary from time to time. With this arrangement the storm-water can easily be screened before it is allowed to pa.s.s out by fixing an inclined screen in the position shown in Fig. 18. [Footnote: PLATE IV] It is loose, as is the trough, and both can be lifted out when it is desired to have access to the invert of the sewer. The screen is self- cleansing, as any floating matter which may be washed against it does not stop on it and reduce its discharging capacity, but is gradually drawn down by the flow of the sewage towards the diverting plate under which it will be carried. The heavier matter in the sewage which flows along the invert will pa.s.s under the plate and be carried through to the outfall works, instead of escaping by the overflow, and perhaps creating a nuisance at that point.

CHAPTER IX.

WIND AND WINDMILLS.

In small sewerage schemes where pumping is necessary the amount expended in the wages of an attendant who must give his whole attention to the pumping station is so much in excess of the cost of power and the sum required for the repayment of the loan for the plant and buildings that it is desirable for the economical working of the scheme to curtail the wages bill as far as possible. If oil or gas engines are employed the man cannot be absent for many minutes together while the machinery is running, and when it is not running, as for instance during the night, he must be prepared to start the pumps at very short notice, should a heavy rain storm increase the flow in the sewers to such an extent that the pump well or storage tank becomes filled up. It is a simple matter to arrange floats whereby the pump may be connected to or disconnected from a running engine by means of a friction clutch, so that when the level of the sewage in the pump well reaches the highest point desired the pump may be started, and when it is lowered to a predetermined low water level the pump will stop; but it is impracticable to control the engine in the same way, so that although the floats are a useful accessory to the plant during the temporary absence of the man in charge they will not obviate his more or less constant attendance. An electric motor may be controlled by a float, but in many cases trouble is experienced with the switch gear, probably caused by its exposure to the damp air. In all cases an alarm float should be fixed, which would rise as the depth of the sewage in the pump well increased, until the top water level was reached, when the float would make an electrical contact and start a continuous ringing warning bell, which could be placed either at the pumping station or at the man's residence. On hearing the bell the man would know the pump well was full, and that he must immediately repair to the pumping-station and start the pumps, otherwise the building would be flooded. If compressed air is available a hooter could be fixed, which would be heard for a considerable distance from the station.

[Ill.u.s.tration: PLATE IV.

"DIVERTING PLATE" OVERFLOW.

To face page 66.]

It is apparent, therefore, that a pumping machine is wanted which will work continuously without attention, and will not waste money when there is nothing to pump. There are two sources of power in nature which might be harnessed to give this result--water and wind. The use of water on such a small scale is rarely economically practicable, as even if the water is available in the vicinity of the pumping-station, considerable work has generally to be executed at the point of supply, not only to store the water in sufficient bulk at such a level that it can be usefully employed, but also to lead it to the power-house, and then to provide for its escape after it has done its work. The power-house, with its turbines and other machinery, involves a comparatively large outlay, but if the pump can be directly driven from the turbines, so that the cost of attendance is reduced to a minimum, the system should certainly receive consideration.

Although the wind is always available in every district, it is more frequent and powerful on the coast than inland. The velocity of the wind is ever varying within wide limits, and although the records usually give the average hourly velocity, it is not constant even for one minute. Windmills of the modern type, consisting of a wheel composed of a number of short sails fixed to a steel framework upon a braced steel tower, have been used for many years for driving machinery on farms, and less frequently for pumping water for domestic use. In a very few cases it has been utilised for pumping sewage, but there is no reason why, under proper conditions, it should not be employed to a greater extent. The reliability of the wind for pumping purposes may be gauged from the figures in the following table, No. 11, which were observed in Birmingham, and comprise a period of ten years; they are arranged in order corresponding with the magnitude of the annual rainfall:--

TABLE No. 11.

MEAN HOURLY VELOCITY OF WIND

Reference | Rainfall |Number of days in year during which the mean | Number | for |hourly velocity of the wind was below | | year | 6 m.p.h. | 10 m.p.h. | 15 m.p.h. | 20 m.p.h. | ----------+----------+----------+-----------+-----------+-----------+ 1... 3386 16 88 220 314 2... 2912 15 120 260 334 3... 2886 39 133 263 336 4... 2656 36 126 247 323 5... 2651 34 149 258 330 6... 2602 34 132 262 333 7... 2516 33 151 276 332 8... 2267 46 155 272 329 9... 2230 26 130 253 337 10... 2194 37 133 276 330 ----------+----------+----------+-----------+-----------+-----------+ Average 314 1317 2507 3308

It may be of interest to examine the monthly figures for the two years included in the foregoing table, which had the least and the most wind respectively, such figures being set out in the following table:

TABLE No. 12

MONTHLY a.n.a.lYSIS OF WIND

Number of days in each month during which the mean velocity of the wind was respectively below the value mentioned hereunder.

Month | Year of least wind (No. 8) | Year of most wind (No. *8*) | | 5 10 15 20 | 5 10 15 20 | | m.p.h. m.p.h. m.p.h. m.p.h. | m.p.h. m.p.h. m.p.h. m.p.h. | ------+-------+-----+-------+-------+-------+------+------+-------+ Jan. 5 11 23 27 3 6 15 23 Feb. 5 19 23 28 0 2 8 16 Mar. 5 10 20 23 0 1 11 18 April 6 16 23 28 1 7 16 26 May 1 14 24 30 3 11 24 31 June 1 12 22 26 1 10 21 27 July 8 18 29 31 1 12 25 29 Aug. 2 9 23 30 1 9 18 30 Sept. 1 13 25 30 1 12 24 28 Oct. 5 17 21 26 0 4 16 29 Nov. 6 11 20 26 3 7 19 28 Dec. 1 5 19 24 2 7 23 29 ------+-------+-----+-------+-------+-------+------+------+-------+ Total 46 155 272 329 16 88 220 314

During the year of least wind there were only eight separate occasions upon which the average hourly velocity of the wind was less than six miles per hour for two consecutive days, and on two occasions only was it less than six miles per hour on three consecutive days. It must be remembered, however, that this does not by any means imply that during such days the wind did not rise above six miles per hour, and the probability is that a mill which could be actuated by a six-mile wind would have been at work during part of the time. It will further be observed that the greatest differences between these two years occur in the figures relating to the light winds. The number of days upon which the mean hourly velocity of the wind exceeds twenty miles per hour remains fairly constant year after year.

As the greatest difficulty in connection with pumping sewage is the influx of storm water in times of rain, it will be useful to notice the rainfall at those times when the wind is at a minimum. From the following figures (Table No. 13) it will be seen that, generally speaking, when there is very little wind there is very little rain Taking the ten years enumerated in Table No. 11, we find that out of the 314 days on which the wind averaged less than six miles per hour only forty-eight of them were wet, and then the rainfall only averaged .l3 in on those days.

TABLE No. 13.

WIND LESS THAN 6 M.P.H.

-----------+-------------+------------+--------+---------------------------------- Ref. No. | Total No. | Days on | | Rainfall on each from Table | of days in | which no | Rainy | rainy day in No. 11. | each year. | rain fell. | days. | inches.

-----------+-------------+------------+--------+---------------------------------- 1 | 16 | 14 | 2 | .63 and .245 2 | 15 | 13 | 2 | .02 and .02 3 | 39 | 34 | 5 | .025, .01, .26, .02 and .03 4 | 36 | 29 | 7 | / .02, .08, .135, .10, .345, .18 | | | | and .02 5 | 34 | 28 | 6 | .10, .43, .01, .07, .175 and .07 6 | 32 | 27 | 5 | .10, .11, .085, .04 and .135 7 | 33 | 21 | 2 | .415 and .70 8 | 46 | 40 | 6 | .07, .035, .02, .06, .13 and .02 9 | 26 | 20 | 6 | .145, .20, .33, .125, .015 & .075 10 | 37 | 30 | 7 | / .03, .23, .165, .02, .095 | | | | .045 and .02 -----------+-------------+------------+--------+---------------------------------- Total | 314 | 266 | 48 | Average rainfall on each of | | | | the 48 days = .13 in

The greater the height of the tower which carries the mill the greater will be the amount of effective wind obtained to drive the mill, but at the same time there are practical considerations which limit the height. In America many towers are as much as 100 ft high, but ordinary workmen do not voluntarily climb to such a height, with the result that the mill is not properly oiled. About 40 ft is the usual height in this country, and 60 ft should be used as a maximum.

Mr. George Phelps, in a paper read by him in 1906 before the a.s.sociation of Water Engineers, stated that it was safe to a.s.sume that on an average a fifteen miles per hour wind was available for eight hours per day, and from this he gave the following figures as representing the approximate average duty with, a lift of l00 ft, including friction:--

TABLE NO. 14 DUTY OF WINTDMILU

Diameter of Wheel.

10

12

14

16

18

20

25

30

35

40

The following table gives the result of tests carried out by the United States Department of Agriculture at Cheyenne, Wyo., with a l4 ft diameter windmill under differing wind velocities:--

TABLE No. 15.

POWER or l4-rx WINDMILL IN VARYING WINDS.

Velocity of Wind (miles per hour).

0--5 6-10 11-15 16-20 21-25 26-30 31-35