CHAPTER XXIX.

COPY OF THE ENGINEER'S REPORT TO THE LOCAL GOVERNMENT BOARD ON THE SALFORD SEWAGE WORKS.

THE following chapter consists of a verbatim Report by Mr J. Corbett, made to the Local Government Board, on the subject of the Sewage Works of the County Borough of Salford.

The Salford Sewage Works, in their completed state, including a few items not yet carried out, but intended to be finally completed within a few months, may be described as follows. The process is threefold.

1. Chemical precipitation by continuous flow in tanks.

2. Straining through gravel roughing filters.

3. Bacterial purification by aerated filters or beds, supplied through spray-jets.

About one-fourth the total flow of sewage comes in by gravitation, and three-fourths is pumped from the main intercepting sewer, which is laid near the river in its winding course through the borough, crossing below the river at three places, without requiring syphons, and serving to drain the land water. The dry-weather or minimum daily flow of sewage from the borough population of nearly 230,000 is about 8,000,000 gallons, of which about 5,500,000 gallons flow between 9 a.m. and 9 p.m., and 2,500,000 gallons between 9 p.m. and 9 a.m.

The average daily flow is 12 to 13,000,000 gallons, and the greatest flow provided for is 32,000,000 gallons, or fourfold the dry-weather flow.

Of the 8,000,000 dry-weather flow, about 4,600,000 gallons is domestic sewage (the estimated quantity of the water supply), about 2,000,000 is liquid trade refuse, and the remaining 1,400,000 gallons is from subsoil water, this item increasing considerably in wet weather.

The ultimate flow provided for, 32,000,000 gallons, is 139 gallons per head of the present population.

The main intercepting sewer has a sump sunk 2 feet below its invert, from which a chain bucket dredger daily lifts any deposit of silt, by steam power, into tram trucks, which convey it to the sludge tanks, and so out to sea.

There are three screening chambers, controlled by valves, each with an iron grating 6 ft. wide and 13 ft. high, with bars in. apart. The usual raking apparatus is provided, worked by steam power.

The sewage then passes through a 45-in. brick culvert to the pumping engines.

Each of the three pumping engines has a 30-in. suction and delivery pipes, separate throughout.

The two original pumping engines are alike, each being a vertical compound engine, with 24-in. H.P. and 40-in. L.P. cylinders, conuected to cranks at 90°, and with a double-acting pump at some distance below each steam cylinder, 31 in. diameter, all of 6 ft. stroke, and up to 17 revolutions per minute.

(Originally made by Messrs J. Watts & Co., Soho Works, and greatly improved by local firms.)

In 1906 a centrifugal pump and vertical compound tandem engine was added, made by Messrs Tangyes, Ltd., with cylinders 17 in. and 25 in., by 12-in. stroke, and up to 194 revolutions per minute; the fan 60 in. diameter. Inlet and outlet 23 in. diameter.

Each of these three pumps, when in good condition, can raise 13,000,000 gallons of sewage per 24 hours, the gross lift being from 20 to 30 ft. So two engines can deal with four times the dry-weather flow, leaving one engine in

reserve.

There are four steam boilers, each 28 ft. by 7 ft., of the Lancashire type, with Galloway tubes, worked at 64 lbs. pressure. Their furnaces are fitted

with steam-jet forced draught, to facilitate the use of inferior fuel.

It is proposed to provide boilers of higher pressure, worked by refuse destructors, so as to save the cost of fuel.

The sewage pump is delivered into a mixing chamber at the east end of the tanks; about 10 ft. cube, with a mixing screw 6 ft. diameter on a vertical shaft. Here is connected the gravitation flow; and the milk of lime precipitant is added.

There are sluice valves to provide for a by-pass flow (during repairs, etc.) of all the sewage along the gravitation channel to the west end of the tanks; but the usual flow is through two silt pits, one about 100 ft., the other 50 ft. long, each 10 ft. wide and 14 ft. deep, each with an 18-in. valve and silt discharge pipe to the sludge tanks.

Between the two silt pits is a weir, where the flow of sewage is gauged by an automatic recorder; and below this weir the dissolved salts of iron precipitant is added. (The preparation of the precipitants is described

later.)

The sewage then enters the central channel, 10 ft. wide, about 4 ft. deep, and 550 ft. long, from which it is passed into the tanks on each side by large sluices. There is a system of large sluices between the several tanks, also outlet sluices to the tank effluent channels, one on each side of the range of tanks, so that the flow can be run through all the ten tanks in series; or through groups of five, three, or two tanks; or through single tanks in parallel, any tank being shut off, emptied, and cleansed when required.

The ten precipitation tanks have a total capacity of about 5,000,000 gallons, each tank being about 115 ft. by 82 ft., and from 7 to 10 ft. deep. (The disposal of sewage sludge from these tanks is described later.)

The tank effluent channels lead to a cross channel at the east end of the tanks, in which are 20-in. valves for supplying each of the roughing filters, six in all, which adjoin this channel. Each roughing filter has a row of inlet holes along its west end, supplied from a chamber in the wall below the cross channel. The floor is of perforated tiles on short legs, thus forming an outlet channel leading to an outlet pipe 20 in. diameter, controlled by a valve in the valve chamber; and from this chamber three 30-in. pipes convey the effluent to the aerating filters or bacteria beds.

The roughing filter valves are arranged so as to provide for reversing the

current in any one of the six filters, and thus "upward washing" it, the washing passing to the sludge tanks or to the sewer, as preferred.

Also, for cleansing these roughing filters, each of the six is divided by screen walls into three bays, thus forming together eighteen bays, which can be "upward washed" one by one. Each bay of about 105 square yards area has a system of air-blowing pipes fixed near its floor, through which air is blown at a pressure of 5 lbs. per sq. in., through 4800 holes of -in. bore, thereby disturbing the gravel and speeding the upward flow of the washing

water.

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These roughing filters are made of 3 ft. depth of fine gravel between and diameter. They have to be "upward washed" and blown through once a day, or oftener when the flow of sewage is great. Their purpose is to remove matter in suspension and colloidal matter.

The three 30-in. pipes above mentioned are along the north or upper end of the bacteria beds, with a 15-in. branch and control valve to each of the 15 beds first constructed, and with an 18-in. branch and valve to each pair of the beds subsequently formed. The 30-in. pipes have also valves at their inlets, so as to control the beds in three groups.

When the first 15 beds were designed in 1894 there was but little experience as to the durability of bacteria beds, so they were arranged with a strong tram rail supported on cast-iron pillars at each 31-ft. width, each alternate line of pillars also carrying a light tram rail at 2-ft. gauge from the strong tram rail, so that tram trucks of 2-ft. gauge might thereby bring the filtering material to its place. A light portable bridge with a turntable then carried the trucks across the width of a bed, so that the material could be dropped exactly where it was wanted.

The strong rails were intended to carry a machine something like a steampower travelling crane, by which the surface of the beds could be raked, or the material refilled into tram trucks and carried away for cleansing or renewal when required.

The supply of clarified sewage to each bed passes through a 15-in. pipe at the base of the cast-iron pillars, and each pillar forms a rising branch up to a horizontal pipe 4-in. diameter, fixed above the surface of the bed, at 10-ft. centres, with six spray-jets across the 31-ft. breadth of beds, each spray-jet serving an area 10 ft. by 5 ft. 2 in.

In the later constructed beds the tram rails are omitted, and the supply pipes are carried above the surface on brick pillars, with 3-in. branches right and left, so as to cover a double bed of 62-ft. breadth. All the distributing pipes have emptying valves and pipes leading to a sewer, to avoid risk of frost when resting.

The filter floors are of concrete, with culverts at 32-ft. spaces for conveying the filtrate to the 4-ft. final outfall culvert at the south end of the beds. The concrete floors are covered with perforated tiles on legs 4 in. long, so as to provide ample ventilation below the beds. The earlier beds have ventilating manholes at each end of each culvert, and the later beds have vent shafts through the brick piers carrying their supply pipes; they have also open sides allowing free air-way to or from their raised tile floors, but the earlier beds are enclosed with walls. There are no dividing walls between the adjoining beds; they form really one continuous bulk.

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The filtering material is crushed cinders and clinkers, from about to in. in the earlier beds, and from 1 to 1 in the later beds, with some coarser