Sand filtration, both slow and rapid, is becoming yearly more popular for water purification and methods yof washing the sand to maintain the filter up to the highest efficiency are being constantly improved.

As generally practiced, however, this sand washing process is still the most expensive part of filtration.

Several times a year the upper layer of sand has to be laboriously scraped off by hand, carted away, and washed in machines of greater or less complexity and costliness. Moreover, the shovels or hand tools used for this scraping cannot be so manipulated as to remove the upper sand layer with uniformity and much more sand has to be washed than is necessary. A considerable amount of sand is washed away and lost, and the washing itself, left to ordinary workmen, must often be imperfectly done.

Devices for stirring up the sand in place under water, and removing the dirty water by suction as it is formed have been tried with more or less success. But these machines have seemed to the writer, as far as he is informed about them, to be unnecessarily complicated and expensive in operation. He has designed the device shown in Figs. 639 to 642 for a quick automatic scraping and washing of a layer of sand of uniform and minimum thickness. It has been designed especially for a plant for filtering the water of a brook for a small community. The cleaning is to be done while the filter is in use so that no shut down and consequent duplication of the plant is required.

Sand Filtration 664

The filter is planned to be built about three hundred feet from the brook and its water level is substantially constant within a few inches, because it enters a lake just below the filter. This maintains the ground water at a permanent level and simplifies the problem of filtration by providing a uniform head throughout the year. This level is indicated by dotted line on the drawing. The water is partially filtered in passing from the brook through gravel from eight to ten feet deep, and enters first a sediment reservoir and thence through a regulating valve into the filter.

Fig. 639. Longitudinal Section of Writer's Slow Sand Filter.

Fig. 639. Longitudinal Section of Writer's Slow Sand Filter.

The filtration may be accomplished in this plant either on the usual plan of constant contact, or by intermittent contact or on the sprinkling plan recently advocated by Miquel as far more effective than either. The drawings show modifications and details of my own.

The sprinkling filter allows of constant and much more abundant aeration throughout the entire bed than is possible with other systems, while the water slowly descends. The water is distributed over the sand at the speed of about one drop a second for every square inch of filter area by means of small enamel covered troughs, half round in section, and supplied by a main pipe running the whole length of the filter bed.

The sand is washed in place without interrupting the operation of the filter by running a two-edged galvanized iron scraping knife rapidly from one end of the bed to the other under the water by means of a machine motor, the ends of the knife being furnished with small wheels or rollers which run on tracks on each side of the bed. To permit of this the beds are made long and narrow, the width being limited by the practical length of the knife, which in this case has sufficient strength with a length of twelve feet. The length of the bed may be as great as desired. In this case it measures one hundred and seventy feet. Any number of knives and beds may, of course, be built side by side, and operated by the same motor and a single set of shafting. The knives have prongs cast upon their upper surfaces as shown, to aid in breaking up the "schmutzdecke," as the dirty surface of the sand to be scraped off is sometimes called. This surface or skin to be removed is usually very thin, and sometimes matted together almost like a carpet when vegetable growths or algae develop in it, and in such cases considerable agitation of the skin or schmutzdecke is necessary to properly break it up. The prongs, together with a specially rapid travel of the knives accomplish this, and the knives may be run back and forth several times if necessary to secure sufficient agitation and cleansing.

Fig. 640. Transverse Section of Filter Bed.

Fig. 640. Transverse Section of Filter Bed.

Fig. 641. Transverse Section of Trough and Knives.

Fig. 641. Transverse Section of Trough and Knives.

Fig. 642. Detail of Knife and Troughs.

Fig. 642. Detail of Knife and Troughs.

The dirty water is then pumped out by a centrifugal pump of size sufficient to remove all the water before the sediment to be removed has had time to deposit itself. A six-inch suction pump is used in this instance. The water is removed through the same tile pipe, with its evenly distributed openings, which is used to feed the filter, so that no violent suction is brought to bear on any one part of the bed. This feed pipe is supported by brackets built into the concrete walls of the filter, and is laid eight or ten inches above the surface of the sand so as to give room for the small transverse secondary feed pipes or troughs below it which are supplied with water through small round openings left in the cement joints of the main feed pipe.

The troughs are in their turn placed high enough above the bed to permit of the passage under them of the scraping and stirring knife. The feed water is distributed by the troughs over the bed by means of small drip points cast on their under surfaces as shown in the details, which alternate with the knife prongs and aid them in breaking up the schmutzdecke.