ABSTRACT
Hardness of natural waters results from dissolution of geologic formations containing calcium, magnesium, and silica. It has been explained traditionally by the occurrence of calcium and magnesium compounds: bicarbonates, carbonates, sulfates, chlorides and nitrates in water. The calcium and magnesium contents of natural waters do, of course, vary considerably. Consequently, scale-forming tendencies also vary. Ferric hydroxide is precipitated from ferric sulfate by sodium or calcium hydroxide for silica removal. The most efficient technique for silica removal is the "hot-process" softening. Current water softening processes by the lime-soda ash method are traced to Thomas Clark, a Scottish professor of chemistry at Aberdeen University. The term "conventional type" is commonly used to designate a continuous cold lime-soda water softener that does not employ a sludge blanket, but allows sludge to settle to the bottom of a softener. A process for recovery of lime and magnesium carbonate from softening sludges was developed at Dayton, Ohio.
