APPLICATIONS AND SOURCES OF ULTRAVIOLET 



79 



regardless of its previous condition, water-disinfecting equipment must 

 he designed to ensure adequate treatment of the most absorptive water, 

 with some provision for effective use also with water of considerably less 

 absorption. Because of these factors, water disinfection must be based 

 on exposures of the order of 700-1000 ultraviolet /xw-min/cm^. 



Water-disinfecting Devices. Ultraviolet water-disinfecting devices fall 

 into three general classes: (1) devices in which water under pressure 

 is disinfected by lamps either immersed in the water or separated from it 

 by a concentric jacket, (2) devices in which the lamps are isolated in an 

 air-pressure chamber over the surface of the water, and (3) devices which 



IP OP G 



Fig. 2-17. Basic types of ultraviolet water-disinfecting devices. I. P., source immersed 

 in water under pressure. O.P., source in air over water, subject to air pressure and 

 gravity flow. O.G., open gravity type. 



provide for disinfection of free-flowing water in a trough over which ultra- 

 violet sources may be suspended. The open gravity type is used in fac- 

 tory processes involving small quantities of water and in isolated camp 

 and farm installations where there may already have been a routine bac- 

 teriological control. A variety of pressure-type units have been devel- 

 oped commercially, usually with considerable difficulty, where the ultra- 

 violet source has been entirely surrounded by water but isolated from it 

 by a concentric ultraviolet-transmitting jacket because of ultraviolet- 

 absorbing deposits on the lamp or its surrounding jacket. Complete 

 immersion of the ultraviolet source is usually impractical also because of 

 the excessive cooling of the mercury arc. The more complicated but 

 more promising method isolates the ultraviolet source under air pressure 

 and over the water in the remaining half or two-thirds of the tank. 

 Examples of these typical devices are shown schematically in Fig. 2-17. 



ABSORPTIVE LICjUIDS 



Since water was first disinfected by ultraviolet with commercial equip- 

 ment, reported by Recklinghausen (1914) and the entire subject reviewed 

 in a U.S. Public Health Service report (1920), attempts have been made 

 to disinfect more absorptive liciuids such as wine, beer, and milk. The 



