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August, 1930

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While the treatment of water by lime-soda is by no means novel, the plant under consideration, which we are enabled to describe through Horden Collieries Ltd., is of special interest on account of its large size and the fact that steam is used in the softening process.

The plant treats the feed water for three Babcock & Wilcox latest type of water-tube boilers fired with pulverised fuel, and for two smaller Babcock boilers and the colliery locomotives. It consists of a heater-softener of the lime-soda solution type with self-contained, mechanically cleansed, gravity Silex filter combined with continuous blow-down and heat recovery equipment. The water supply, which is pumped from the workings, is treated by the softener at the rate of 200,000 lb. per hour, its analysis being as follows :—

Calcium Carbonate	17.10
Magnesium Carbonate	3.69
Magnesium Sulphate	7.05
Sodium Sulphate	1.48
Sodium Chloride	6.96
Free Carbonic Acid	0.88
Total hardness — 27.3 English degrees.

A general view of the plant, which gives some idea of its dimensions, is reproduced in Fig. 1, and detailed drawings showing the general arrangement are given in Fig. 2, Fig. 3 showing diagrammatically the general layout scheme.

Crude water is fed by gravity to the softener, the inflow being automatically controlled by a float valve operated by the rise and fall of the water above the filter at the top. This valve shuts off the admission of water when the large reserve reservoir and the hot-well are full. On entering the water flows to a Boby Simplex distributing apparatus by which it is automatically measured and dosed with the necessary amount of chemical solution. No water is by-passed, the whole of it being dealt with by the Simplex apparatus, in which a feature is that the reagents are introduced into the "tipper" at each of its oscillations before the crude water enters, and are there diluted and thoroughly mixed by the vigorous discharge of the crude water from the jets of the sparging pipe. By this means an excess of reagents is maintained during the mixing and until the moment for discharge from the tipper, when chemical balance is achieved. Some idea of the size of the Simplex apparatus may be gained when it is known that at each oscillation the tipper, at the time of falling, contains 2,000 lb. of water, yet so perfect is the balance that there is no appreciable shock. A steel framed house covered with Robertson's protected metal sheeting is provided over the apparatus.

After dosing, mixing, and agitation, the water passes into the steam mixing chamber, where it is heated by exhaust steam supplemented, as necessary, by live steam, both supplies being admitted by means of silent heating nozzles. The steam mixing chamber is fitted with baffles which ensure an intimate mixing between steam and water. Vents are provided so that excess vapour can escape to atmosphere. A considerable amount of sludge forms immediately when the dosed water is heated, and provision is made for the ejection of this material by means of geared rousers and valves worked from a platform.

From the steam mixing chamber the heated water passes to a downtake tube which is external to the main reaction tank and enters the latter near its bottom. On entering the main tank the water becomes quiescent and rises to the top ; reaction is completed and the great bulk of suspended lime and magnesia formed by the softening action is precipitated to the bottom. When the water reaches the top of the reaction and sedimentation tank it is decanted over weirs into a trough system and thence distributed evenly over the top of the gravity filter. This is placed at the top of the main tank, and takes the form of a self-contained compartment supported on joists carried by the main shell. On the bottom of this compartment is disposed the manifold collector system, designed to draw water evenly from the whole area of the filter during normal operation and to distribute the air-scour and wash-water evenly in the reverse direction when the filter is being cleansed. More than 800 brass collection and distribution nozzles are fitted in the manifold. Disposed above the collector system is a graded and washed bed of gravel upon which rests the filter bed.

The trough system which receives the water from the weirs is situated at a suitable distance above the top of the filter bed and distributes the water in thin films over its edges, thus avoiding furrowing of the surface of the filter bed as would be the case if the water were allowed to cascade directly from the weirs. The last traces of precipitate are, removed by the filter bed, and the water, now perfectly clear, is discharged through the collector system into the outlet pipe.

The condition of the filter bed, in addition to being protected by the trough system, is further preserved from disturbance by a depth of water which is never allowed to be less than a predetermined minimum, this minimum being maintained by means of a float-operated valve upon the outlet of soft water. An inspection box is fitted in the outlet pipe, having a glass top and sides, and a white tiled bottom ; it is further provided with electric lights and switches, enabling the condition of the water as it issues from the filter to be observed at any time, independently of daylight.

Ejection of Sludge.

Sludge is formed in the main sedimentation tank and also upon the top of the filter bed. The sludge from the tank is ejected by means of the makers' cantilever sludge expeller, which has the advantage that it does not require an opening through the side of the tank. Sludge from the top of the filter bed is removed by means of an air-scour followed by a water flush. This scour, which thoroughly agitates the bed and breaks up the deposit, is obtained by means of a pair of steam operated air injectors.

Wash water at the high rate required is supplied by means of a motor-driven centrifugal pump, and the collector system previously mentioned ensures a distribution of air and water evenly throughout the bed. The water flush carries the deposit upwards and decants it into the trough system above the bed, whence it is led away to the drain. The whole operation of cleansing the bed can be carried out in less than fifteen minutes. For the first one or two minutes after cleansing the bed, the water issuing from the outlet is cloudy and is by-passed through the inspection box to the drain until the attendant sees it again to be clear.

All the valves for operating the plant are so grouped as to be accessible from a platform. A building adjacent to the plant accommodates the chemical mixing mills, and wash-water pumps, together with their motor and starting panels, and the heat interchanger; the testing laboratory and a storage for the chemical reagents are also lodged under the same roof. The testing laboratory is equipped with apparatus which enables the chemist to make his routine examinations of the water on the spot. We understand that satisfactory results have been obtained from the date of starting work.

The whole plant was supplied and erected by William Boby & Co. Ltd., of London, who also have in hand a further softener for the locomotives at the Horden Collieries Ltd.'s Blackhall Colliery.

Drawings and Photographs accompanying the article