MIL-HDBK-1003/6
Coagulation. A coagulant such as sodium aluminate may be added to
7.6.1.4
the raw water ahead of the filter to form a filterable floc by reaction with
the bicarbonates.
7.6.2
Lime-Soda Softening
7.6.2.1
Cold Process. The cold lime or lime-soda process may be used to
reduce hardness and chemical constituents. For this purpose, provide an open
vessel and chemical feeders for adding lime or caustic soda alone (or lime and
soda ash). Lime alone will remove carbonate hardness by precipitating
magnesium hydroxide and calcium carbonate; the addition of soda ash will
remove noncarbonate hardness. The choice of chemicals depends on the chemical
constituents in the water.
Hot Process. In hot-process softening (see Figure 36) impurities
7.6.2.2
are precipitated in a tank at a temperature of 200 to 220 degrees F (93.3 to
104.4 degrees C), or higher. Chemicals are fed to the makeup water in the
upper zone, react with impurities, and precipitate them. Water then flows to
the bottom, rises through the sludge blanket, and discharges to anthracite
filters. Sludge settling in a cone at the bottom of the tank is removed by
blowdown. The process is economical when objectives include the reduction of
hardness and organic matter, dissolved solids, alkalinity, and silica. The
removal of iron and free carbon dioxide are incidental advantages.
141
Previous Page
