Rapid Filter with Automatic Wash Gear
The filter illustrated is designed to effect the cleaning of the bed automatically and to utilize the flow of the water for effecting this without the intervention of any mechanical appliance. The water is, of course, treated in the first place with chemical coagulants. The main difference between this continuous washing or drifting sand filter and those with mechanical or compressed air wash gear is that, while the latter usually require washing at least every day the former will operate continuously for from 3 to 30 days according to the state of the raw water. Hence a plant of this kind can be looked upon as largely automatic, with correspondingly low operating costs. The illustration shows the general arrangement of the filter. After the usual treatment with sulphate of alumina the water enters the filter partly by standpipe A and partly by by-pass B. The automatic sand washer is shown at C. Within this sand washer the raw water pipe is constructed on the Venturi principle and fresh sand therefrom is induced into A, passes up therein with the water and over the top where it settles on the bed in a sugar loaf formation. The sand thus deposited is continually drifting away. In the meantime the water is passing through the sand and being filtered eventually passing away through the collector D. Simultaneously the sand washing is going on all the time, the sand being taken away to be cleaned through the extractors E. A portion of the sand bed is stationary, but part of it, as shown, is in the form of a hollow cone of continuously drifting sand hence the name given to the filter. This drifting sand is being continuously cleaned in the sand washer from which the dirt is expelled. The stationary body of sand is fairly hard and compact but the drifting sand is buoyant and spongy.
The former is occasionally washed by a reverse flow of the water, a process common to other types of rapid filter, but assisted in the latter by, as stated, mechanical means or compressed air. The filter under consideration has a nominal depth of sand of about 9 feet and it rests upon a 10-inch bed of gravel, F. The sandwasher. C is of special design, hut really resolves itself into a series of jets which impinge on the sand as it falls into the washer to the base of which it gravitates and is then drawn into pipe, A, as stated by Venturi action. The by-pass arrangement permits of the rate at which the sand is moved, which in turn, is dependent upon the state of the raw water, the dirtier the water the more washing does the sand require, hence the less water bypassed via B. The system is applicable to both closed and open filters, while it may be added that in a large filter several vertical pipes with their attendant sand drifting pipes and washing gears may he installed. An operating head of about 30 feet is necessary and a filtering rate of 140 gallons (British) per square foot of bed area per hour can be worked to.
“No city can grow or prosper unless it possesses an abundant supply of pure water. Lacking this, a city is exposed to conflagrations, inconvenience and disease. Davenport has been well served for fifty years by its water company. The management of this company believes that it is charged with a very important public duty and that it is responsible to the public for its stewardship.”—Davenport, Ia., Water Company.