Page 309 - Water and wastewater engineering

7-26 WATER AND WASTEWATER ENGINEERING

soda: the cost is four to six times higher than lime, the potential for hazardous chemical release is
greater because it is a liquid, and freezing problems occur for 50 percent solutions at temperatures
below 13 C (Kawamura, 2000). The choice of caustic over lime will fundamentally be driven by
economic evaluation of the cost of caustic, the feed system, and sludge treatment and disposal.
The stoichiometric reactions may be derived by replacing Ca(OH) 2 with NaOH in Equations
2
7-12 through 7-15 and rebalancing the reactions. Because Ca hardness is not substituted for
2
Mg , the reactions shown in Equations 7-16 and 7-18 are not required. The sodium carbonate
formed in the reactions of caustic with carbonate hardness is available to precipitate calcium
noncarbonate hardness.

7 -5 CONCURRENT REMOVAL OF OTHER CONSTITUENTS
Arsenic
Arsenic removal ranging from 60 to 90 percent have been observed in softening plants that use
2
2
excess lime for Mg treatment. For single-stage softening plants that remove only Ca , 0 to 40
percent removal has been observed (MWH, 2005). Removal effectiveness is highly dependent on
the oxidation state of the arsenic. Arsenate ( 5) is more readily removed than arsenite ( 3). The
major removal mechanism is by adsorption to the precipitate (MWH, 2005).

Iron and Manganese
The solubility diagrams shown in Figures 7-11 and 7-12 reveal that ferrous hydroxide [Fe(OH) 2 (s)]

and manganese hydroxide [Mn(OH) 2 (s)] precipitate at high pH. Softening processes that achieve a
pH greater than 9.6 remove 100 percent of the iron. Manganese is more difficult to remove. The pH
must be greater than 9.8 to remove 100 percent of the manganese (Kawamura, 2000). Because the
desired control pH for softening processes is 10.3 or greater, iron and manganese are effectively
removed concurrently. The extra mass of lime added in the softening process is sufficiently great to
provide an excess over the stoichiometric requirements to remove the iron and manganese.

Natural Organic Matter (NOM)
The concurrent removal of NOM in the softening process is of importance in preventing the for-
mation of trihalomethanes (THM) and haloacetic acids (HAA5) when chlorine is used as a dis-
infectant. The effectiveness of lime-soda softening in reducing NOM is different for each water
source. However, some generalizations may be made (Benefield and Morgan, 1999):

• Calcium carbonate precipitation generally removes from 10 to 30 percent of the color, total
organic carbon, and disinfection byproduct precursors.

• Magnesium hydroxide precipitation generally removes from 30 to 60 percent of the total
organic carbon and disinfection byproduct precursors, and 50 to 80 percent of the color.
• Addition of iron in the form of ferric sulfate generally removes an additional 5 to 15 percent
of the color, total organic carbon, and disinfection byproduct precursors in either calcium
or magnesium precipitation.

Alum hydroxide is an amphoteric hydroxide. That is, it is soluble at both low and high pH. Thus,
at pH values normally encountered in lime-soda softening it is dissolved and is not effective in
enhancing the removal of NOM.