material at the bottom. Thirteen of the earlier beds are 5 ft. deep, two are 8 ft., and the newer ones are about 7 ft. deep.

Owing to these beds having been worked for several years before the roughing filters were completed and provided with cleansing apparatus, there is much more clogging of the beds than was expected; but after they have been once thoroughly cleansed, it is expected that they will require but little renewal or cleansing in future years. Their surface is raked by hand labour

each few days.

The spray-jets, when working under the full available head of about 8 ft., deliver at the rate of 50 gallons per sq. yd. per hour, or 1200 gallons per 24 hours if run continuously. There are 39,000 sq. yds. of beds, so each hour's flow of all the spray-jets will discharge 1,950,000, say 2,000,000 gallons. So, with the minimum or dry-weather flow of 8,000,000 gallons per day, each bay must be worked about 4 hours and rested 20 hours; and with the maximum flow provided for, fourfold the minimum flow, or 32,000,000 gallons per day, each bay must be worked 16 hours and rested 8 hours; but the working hours can be extended if desired by checking the flow by the control valves.

It has been found best to run each bed for two hours and then rest it for ten hours or less, according to the quantity to be dealt with.

Thus the minimum daily flow per sq. yd. of bed is about 205 gallons, the average about 330 gallons, and the maximum 820 gallons; but this large flow will only occur when the sewage is greatly diluted with rain water.

As the beds average 2 yds. in depth, these flows are only, per cub. yd. of medium, 103 gallons per day minimum flow, 165 gallons average; and 410 gallons maximum flow when diluted with rain water.

The filtrate from these beds flows into the old culvert which formerly conveyed the intercepted sewage of the borough into the river Irwell before the sewage works were constructed; but the Irwell is now transformed into the Manchester Ship Canal.

Turning now to the disposal of the sewage sludge from the precipitation tanks, the floor of each tank slopes to an outlet pipe controlled by a valve in a subway below the central channel, and discharging into a channel along the said subway, and thence through an 18-in. pipe and a further length of open channel to the sludge tanks.

Men with push-boards sweep the sludge across the tank floor to the outlet, wading in it with waterproof thigh-boots.

The outlet pipe in each tank has above it two 2-ft. lengths of cast-iron pipe, with valve seatings, so that the outlet is first used to discharge the supernatant water of the tank down to 4 ft. above the outlet level.

Then one length of pipe can be raised and the water run down 2 ft. above the outlet level; and the flow along the subway channel is all this time turned into the main sewer. But when the second pipe is raised, and the sewage sludge deposited on the tank floor begins to flow, the channel is connected to the sludge tanks.

There are two sludge tanks, circular and saucer-shaped, 100 ft. diameter and 9 ft. deep in the middle, the two holding 3000 tons of sludge. There is a supply channel around each sludge tank, with holes through the wall to distribute the sludge. There is also a sludge supply trough extending to near the centre of each tank. From the centre of the tank bottom an 18-in. suction pipe leads to the sludge pumps; and also at the centre is a 6-in. skimming pipe leading to the sewer, and fitted with a "telescope-pipe" and

supporting float to skim water from near the surface, whether the tank be full or not. Thus the densified sludge sinks to the bottom, and the separated liquid is passed to the sewer, and so again through the tanks, etc.

Between the two sludge tanks is the sludge pump-house, with two sludge pumps of "Tangye's Special Pump" type, each with steam cylinder 16-in. diameter, double-acting piston pump 12-in. diameter, both 24-in. stroke, running up to about 35 double strokes per minute, each pump delivering about 200 tons of sludge per hour. These pumps work very satisfactorily, even when dealing with sludge containing 20 or 30 per cent. of solid matter. From these pumps an 18-in. pipe, 200 yds. long, conveys the sludge to the sludge steamer Salford, at a wharf on the Ship Canal.

This steamer is 170 ft. long, 32 ft. beam, about 11 ft. draught loaded, and carries 600 tons of sludge in four tanks formed above the light-load line so as to discharge through eight 18-in. valves and pipes through the bottom of the ship.

The distance to beyond the North-west Lightship off Liverpool is 64 miles, or 128 miles the round trip; and the steamer usually makes four or five trips a week, going with each alternate tide. Her fastest round trip was made in about 15 hours.

The preparation of the precipitants, lime and iron salts, used for the sewage, is provided for in the lime-house.

A branch from the L. & N.-W. R. Co.'s dock line brings the lime trucks directly from the Hoffman kilns at the Buxton quarries into the lime-house, where the lime is stored for use. A workman fills a wheelbarrow with lime,

passes with it up a lift to a raised floor, tips it into one of the lime-slaking pans, and there it is dissolved. The apparatus is in duplicate to provide for repairs, etc.; each set includes two slaking pans with revolving mixers and mixing pan.

When the pan of lime is slaked more water is added, and it flows through a grid, where stones, etc., are intercepted, and so into the mixing pan, which is like a mortar mill, with two heavy rollers revolving in it, by which any unslaked pellets of lime are crushed, and thence it flows to the mixing chamber already described.

The salts of iron used are a by-product from a local chemical works, very similar to " copperas." This is easily dissolved in cold water in two mixing pans, but steam pipes are also provided for use if required.

The water used is the tank effluent, raised by small centrifugal pumps. A duplicate couple of steam engines work the machinery.

The chemical laboratory adjoins the lime-house; here the resident chemist makes daily analyses of samples taken hourly, and mixed, to show each day's average of the crude sewage, the tank effluent, the roughing filter effluent, and the final filtrates from the different bacteria beds. The sewage sludge is also sampled and analysed.

There are also on the works a smiths' and mechanics' shop, ample stores, offices, men's rooms, weighing machines, etc., also the manager's house, with a large committee-room.

The site of the works is 30 acres (some land having been sold for the Ship Canal), and of this about 9 acres is still unused, and 2 acres is only used for gardens, etc., the actual sewage works thus occupying only 19 acres.

CHAPTER XXX.

SANITATION AND HOUSE DRAINAGE.

The Ventilation and Warming of Buildings.

Like

ONE of the commonest constituents of the atmosphere is carbon dioxide. all other gases, it is subject to physical changes of state as the result of increased pressure and diminishing temperature, and can be made to undergo

Pressure.

FIG. 701.

liquefaction. Fig. 701 represents diagrammatically the changes it undergoes when subjected to increased pressure. At a temperature of 50° C. a fairly uniform curve is produced (No. 1), but No. 3, plotted when the temperature has fallen to 30° C., shows the tendency 2 of the gas to liquefy. This temperature is called the critical temperature for carbon dioxide; it is the highest temperature at which increased pressure produces liquefaction. In other words, carbon dioxide cannot be converted into a liquid by pressure until it has been cooled down to this critical pressure of 30° C. In the diagram the white area shows the gaseous and the shaded area the liquid states. The cross-hatched portion

shows the period of liquefaction (fig. 701).

Atmosphere and its Pollution.-The atmosphere is composed approximately of 78 per cent. of nitrogen

21

oxygen
argon, etc.

by volume.

Argon is a peculiar gas. It is about 1 times heavier than atmospheric air, and will not combine with any other substance.

The atmosphere also contains traces of carbon dioxide, ammonia, organic matter, ozone, and watery vapour. Ozone is a form of oxygen only present in pure atmosphere. It is written as Og.

It is found that the carbon dioxide (CO2) present in air, as a result of expiration, corresponds very closely to the organic matter from the same source, and hence the reason that we take the amount of CO, present in any atmosphere as a standard of purity. Now when CO, is present in as much as 2 parts in 1000 it is apparent to the senses; that is to say, added CO2

in addition to the 3 to 4 parts in 1000 usually present. Therefore if we know the average quantity of CO, which a person gives off per hour, which we generally consider to be equal to 6 cub. ft. per hour in repose, we find that each individual will require 3000 cub. ft. of air-space per hour; and as the air in a room cannot be changed more than three times per hour with comfort, in cold weather, 1000 cub. ft. space should be the minimum allowance for each person. As examples, however, we have barracks-allow 600 cub. ft.

The laws relating to common lodging-houses only require 300 ft.; dairies should provide for 800 cub. ft. per cow; and in estimating these quantities 13 ft. may be taken as the maximum useful height of a room. The chief gaseous impurities in air are—

nitrous acid, suspended matter, dust, seeds of plants, spores of fungi and bacteria.

Now the artificial light in rooms will tend to foul the atmosphere to a certain extent, except, of course, electric light. For instance, when 1 cubic foot of coal gas is burnt we evolve 2 cub. ft. of CO2, while 1 pound of oil burnt will give off 23 cub. ft. of CO2. It can be proved that in 4% of CO2 (it is, of course, a question of the amount of O present; CO2 is merely a diluent, and not a poison) a candle will cease to burn, and that the air is dangerous to human life. Wholesome air should also contain about 40% of moisture. It is very often necessary to obtain the amount of moisture which is present in any atmosphere, and the process is carried out by an instrument called a hygrometer. There are three kinds in practical use

The absorption hygrometer.

The condensation

The evaporation

Fig. 702 shows Saussure's hair hygrometer, acting on the absorption principle, while fig. 703 shows Daniell's condensation hygrometer.

The hair hygrometer shows the presence of moisture, because when it absorbs moisture the hair will contract and move the needle over the graduated scale. In fig. 703 the process is different entirely. Here we have

the black bulb filled with ether and the other covered with muslin over which ether is poured. This will very quickly evaporate and cause the ether in the

other bulb to cool. Dew is formed on the outside thereof; at the same instant the thermometer inside will denote the temperature at which the dew was formed, and by means of a table the amount of moisture can be found which is present in the air.

=

=

Again, if we know the elastic force at the temperature of the wet-bulb thermometer obtained by a table = f, and the difference of level of the two thermometers d, the height of the barometer h, Felastic force at dewpoint temperature, we have the following formulæ *If the temperature is above 32°,

Ventilation of Buildings.-The ordinary chimney fireplace will carry off about 29,000 cub. ft. of air per hour. Again, we can find out this quantity with some degree of accuracy by the following equation, if we know

N being a constant '02-'06, according as we have clean or dirty flues. Regarding the inlet and outlet ventilation of a room, it is usual to allow 24 sq. in. per head, or 1 sq. in. for every 60 cub. ft. of space; or, again, if they are of the form that warm air is admitted round the fireplace, 1 sq. in. for every 120 cub. ft. The inlets should always be placed at or about breathing level, and the outlets actually in the ceiling. The rate at which these two gases will intermix will be inversely as the square root of their densities. One B.T.U. will raise 52 cub. ft. of air 1 degree Fahr. And an ordinary man will give off 284 units of heat per hour, of which 93 are absorbed by vapour, leaving 191 units to heat up the air. A cubic foot of coal gas would produce about 650 B.T.U.'s when burnt.

There are three principal factors in ventilation, viz. diffusion, wind, and unequal weights of air. The first two are of minor importance. In any system of ventilation it is advisable to have more inlets than outlets. But the total sectional area of the two should be equal. To reduce the effects of friction, circular tubes are preferable to square ones; sharp angles must be avoided, and the tubes must be as short and smooth as possible. Outlet shafts are often placed close to flues, to take advantage of the warmth and so get a greater discharging capacity. All joints in the tubes must be carefully made and quite air-tight. Ceiling outlets must be continued right out through the roof into the outer atmosphere. It is the practice of unscrupulous builders to take them into the roof space only, and provide outlets in the gables, even if they take the latter precaution at all. A circular disc should always be provided, as shown by fig. 704, to prevent any down-draughts. Outlet shafts may be taken direct into the chimney, but when so constructed Suitable tables will be found in Hurst's Pocket Book, pp. 207-208.

*