The purification of the water of swimming baths.

  • Great Britain. Ministry of Health
Date:
1929
Catalogue details

Licence: Public Domain Mark

Credit: The purification of the water of swimming baths. Source: Wellcome Collection.

    39/72 (page 29)
    Washing the Filter. Rapid filters differ from slow sand filters in that they are provided with a speedy method of washing sand. This is necessary, because washing may be required at intervals of a few hours, whereas the slow sand filter for waterworks frequently runs for months, before it needs cleaning. To wash a rapid filter, two distinct processes are necessary :— (1) Breaking up of the solidified sand bed by some form of agitation ; (2) Washing of the sand by means of an upward flow of water. - Many devices have been introduced to break up the sand bed, and there has been keen controversy as to the respective merits of such systems. In the early days of rapid filters for waterworks purposes the use of rotating rakes worked by power was general, but this necessitated a circular design of filter of limited size, and the demand for large rectangular masonry filter units brought in the method of agitation by means of air admitted under pressure through the collecting orifices at the bottom of the bed. Air agitation has been replaced by ‘‘high velocity wash’’ in the U.S.A., but, in this country, high pressure water for breaking up the sand, and for washing, has not found favour, as it is con- sidered to be uneconomical in water consumption, and is attended with the risk of carrying away sand. While the use of rotating rakes for circular filters is effective, the apparatus is costly and, in this country, most manufacturers have continued to develop the equally efficient method of air agitation. A third method of breaking up the sand is found in a horizontal filter, which is rocked backwards and forwards on its axis. Different makers of filter plant naturally praise the merits of their own particular designs, and it will perhaps suffice to say that the Ministry of Health does not discriminate between air and mechanical methods of agitation, both being effective for breaking up the sand bed. For installations of any size, a power unit is required to work the agitation system, and in the case of rakes, the provision of shafting, gear and belts is a costly item. ‘. Air for agitation is supplied by a compressor or blower according to the size of the plant, and, in plants where steam is available, a simple form of Korting air injector worked by a steam jet is effective. Where compressors are used it is essential to place an oil separator on the compressed air pipe to retain any waste oil from the compressor cylinder which might otherwise form an un- sightly film on the bath water. To wash a filter, the unit is isolated, and a connection at the top leading to waste opened.