Water Stabilazation (ir.ahmad Jusoh)

WATER STABILAZATION

Water is considered to be stable when it neither dissolves nor deposit calcium carbonate i.e., the calcium carbonate is in equilibrium with the hydrogen-ion concentration (Eq. 10-39). If the pH is raised from the equilibrium point, water becomes scale-forming, depositing calcium carbonate. Water turns corrosive if the pH is lowered. CaCO3 + H+



Ca2+ + HCO3-

A thin coating of calcium carbonate on the pipe interior protects the metal against excessive corrosion. Such a covering can be maintained permanently if the water is held at a proper level of calcium carbonate saturation.

Saturation Index Langelier9 developed an index, in the pH range 6.5-9.5, that makes it possible to predict whether a given water will deposit or dissolve calcium carbonate. The saturation index is calculated by the following equation : SI = pH – pHs = pH – [(pK2 – pKs) + pCa2+ + pAlk] where

pH pHs (pK2 - pKs ) pCa2+

= = = =

pAIk

=

actual pH of the water pH at saturation empirical constants negative logarithm of the calciumion concentration, moles/liter negative logarithm of the total alkalinity equivalents/liter

A positive value for the index signifies the water is oversaturated and will precipitate calcium carbonate. A negative number indicates that the water is corrosive. The saturated index serves as a measure of the water’s tendency to dissolve or precipitate calcium carbonate but

IR. AHMAD JUSOH/UMT/2009

does not give either the rate at which stability is attained or the capacity. The value of (pK2’ -pKs , ) based on temperature and ionic strength can be determined from Table 10-3 by Larson and Buswell.10 Ionic strength is calculated using the following equation : Ionic strength = ½ ( C1 Z12 + C2 Z22 +….. + Cn Zn2 ) where C = concentration, moles/1000 g of water Z = valence of the individual ions

 EXAMPLE Calculate the saturation index (SI) for water based on the following information :

component Ca2+ Mg2+ Na+ K+ CO3 2HCO3 SO4 2Cl-

mg/l 63.3 14.8 19.5 10.1 7.8 94.4 80.0 17.0

pH = 7.9; Temperature = 150C

IR. AHMAD JUSOH/UMT/2009

Molecular Weight 40.1 24.3 23.0 39.1 60.0 61.0 96.0 35.5

Moles/l 0.00158 0.00061 0.00085 0.00026 0.00013 0.00155 0.00083 0.00048

Solution Using Eq. 10-41’ Ionic strength (Ca2+) (M2+) (Na+) (K+) (CO32-) (HCO3 -) (SO4 2-) (Cl - )

= = = = = = = =

0.5 X 0.00158 X 4 0.5 X 0.00061 X 4 0.5 X 0.00085 X 1 0.5 X 0.00026 X 1 0.5 X 0.00013 X 4 0.5 X 0.00155 X 1 0.5 X 0.00083 X 4 0.5 X 0.00048 X 1

= 0.00316 = 0.00122 = 0.00042 = 0.00013 = 0.00026 = 0.00078 = 0.00166 = 0.00024 ________ 0.00786

Using Table 10-3, with an ionic strength of 0.008 at a temperature of 150C : (pK2’ -pKs , ) = 2.455 pCa2+ = - log 0.00158 = log 634 = 2.80 pAlk = p[C O32- + HCO3 -) = - log[0.00013 X 2 + 0.00155] = 2.74 Substitution in Eq. 10-5 yields

SI = 7.90 - 2.46 - 2.80 - 2.74 = - 0.10

The negative value of the saturation index indicates that the water is slightly corrosive and will dissolve calcium carbonate. IR. AHMAD JUSOH/UMT/2009

PH Adjustment

The stability of softened water is established by controlled recarbonation. If the water processing does not include softening, the method most commonly used in corrosion control is upward adjustment of pH, and addition of metaphosphates. Bringing the water pH above its calcium carbonate saturation value preserves a thin protective coating on the pipe interior. Metaphosphates sequester the slight excess of calcium and carbonate ions, preventing crystal formation of calcium carbonate scale.

Either lime or soda ash can be applied to treat corrosive waters having a hardness exceeding 35 mg/l. The former is preferred in treating soft water to provide the needed calcium as well as pH adjustment, although lime for calcium and soda ash to provide carbonate may be needed.

IR. AHMAD JUSOH/UMT/2009