336 MODERN METHODS OF WATER Pl'RIFICATION. 



pointer worked by a float in a stand pipe connected with the 

 outlet pipe. The reading- of the pointer on the dial shows the 

 " loss of head " or frictional resistance at any time. When the 

 loss of head reaches the intended maximum, an automatic con- 

 tact is made which rings a bell, and turns on an electric light, 

 thus giving warning that it is time to wash the filter. The 

 elegance is attractive, but so far from factories as we are in 

 South Africa the slower spindle is probably better. 



The washing of these filters takes place by reversing the flow 

 of water and running it upwards through the filter at such a 

 rate as will cleanse thoroughly and remove the caught impurity 

 without carrying away the sand with the wash water; but 

 first air is passed through from below upwards. 



In describing the strainers, reference was made to the stem 

 of the strainers and to the small 3-ioth inch air-hole in the neck 

 of the strainer just below the upper wall of the lateral pipe. 

 The wash-water enters the strainer through the bottom end of 

 the strainer stem, and as the air and water are not used to- 

 g"ether, but alternately, the water in the lateral pipe acts as a 

 trap at the lower end of the stem, the air entering above it 

 through the small hole into the strainer. The compressed air 

 comes first. This is used to agitate the sand, and is derived 

 from a machine called Root's Rotary " Blower." worked by 

 electric motor. The 4 inches or so of water left in the filter- 

 bed when the " blower " begins to work is thrown into violent 

 bubbling, and the sand is agitated efficiently by this blowing of 

 air through it, the friction of the bubbling sand grains against 

 each other causes a scrubbing action, and frees the grains 

 easily from the gelatinous film, which the action of the alum 

 has surrounded them with. The air is run through for about 

 two minutes, then turned off; the wash-water valve is opened 

 and filtered zvatcr is run through the filter from below up, 

 floods the filter, and runs away in the wash water gutters to 

 the waste-water pipe, of which the valves have been opened. 

 About 8 gallons per square foot per minute is the rate at which 

 the reverse current of wash-water is run upwards through the- 

 filter, but this varies with the fineness of the sand, and is some- 

 times limited by the height of the gutters to 6 gallons per 

 square foot.* A second application of air. if needed, is now 

 made, and water is always used after air, so if air is twice used, 

 water is twice run through, so the sand is freed from trapped 

 air. 



The washings I saw took from 4 to 8 minutes. The use of 

 hydraulics and electrics to operate the valves is a great time 

 saver. The frequency of the washings varies according to the 

 requirements, rarely less than 12-hourly washings, sometimes 

 much longer runs are made, according to the amount of 

 coagulant used and the turljidity of the water at time of filter- 

 ing. In Fuller's table of the Xew Orleans filtering, the mini- 

 mum was 6 hours, the maximum 29. The amount of filtered 



* Some years ago Hazen suggested that separate systems of air and wash 

 water pipes might give better "results, and at Cohimbus a different air pipe 

 system was introduced. 



