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<br />Engineers and chemists can useMINTEQA1 and
<br />similar models to help plan and evaluate corrosion
<br />control programs.
<br />
<br />In water samples that have been in contact with
<br />the pipe for prolonged periods (first-draw samples, for
<br />example) lead levels may approach the concentra-
<br />tions these equilibrium models predict Since first-
<br />
<br />Table 3. Temperature Dependent Constants for
<br />LSI Calculation
<br />
<br />Temperature
<br />
<br />OC OF K A
<br />5 41 2,16 0.494
<br />10 50 2,08 0.498
<br />15 59 2,00 0.502
<br />20 68 1.93 0.506
<br />25 77 1.85 0,511
<br />30 86 L78 0.515
<br />
<br />draw samples are required for compliance monitoring,
<br />the reader can appreciate the value of equlibrium
<br />calculations. Lead concentrations will generally be
<br />lower in flow water samples.
<br />
<br />Calculated results can only approximate results
<br />obtained in the field, however. Several uncertainties
<br />may reduce the accuracy of a model's calculation.
<br />Among these are inconsistencies in thermodynamic
<br />data, inexact knowledge about water composition in
<br />the immediate vicinity of the actively corroding areas
<br />as compared to bulk water composition, and uncer-
<br />tainties about the identities of solids that control con-
<br />stituent solubility. Therefore, water treatment plant
<br />managers should verify any treatment schemes sug-
<br />gested by model calculations with pipe loop, labora-
<br />tory, pilot scale, or field tests.
<br />
<br />References
<br />
<br />1, Drinking Water Regulations: Maximum Contami-
<br />nant Level Goals and National Primary Drinking
<br />Water Regulations for Lead and Copper, Pro-
<br />posed Rule, U.S. Environmental Protection
<br />Agency. 1988. Federal Register, Vol. 53,
<br />No,160 (53FR31516-31578, August 18, 1988.)
<br />
<br />2, Merrill, D, T. 1978, "Chemical Conditioning for
<br />Water Softening and Corrosion Control." In R.
<br />L. Sanks, ed., Water Treatment Plant Design.
<br />
<br />Definitions
<br />
<br />pH. A measure of the intensity of the acid or alkaline
<br />condition of a solution. It is calculated as the negative
<br />log (base 10) of the hydrogen ion activity and ranges
<br />generally from 0 for very strong acids to 14 for very
<br />strong bases, The pH of drinking water is typically
<br />between 6 and 8.5.
<br />
<br />Acid. A compound containing more hydrogen ions
<br />than hydroxide ions, An acid has a pH below 7,
<br />
<br />Base. A compound containing more hydroxide ions
<br />than hydrogen ions. A base has a pH above 7.
<br />
<br />Alkalinity. A measure of a water's capacity to neu-
<br />tralize acids. To1a1 alkalinity is composed primarily of
<br />hydroxide (OR), carbonate (CO,"), and bicarbonate
<br />(HCO,-) ions. The proportion of each of these alkalin-
<br />ity forms is dictated primarily by pH. Alkalinity is ex-
<br />pressed in mg/! as calcium carbonate equivalent
<br />(CaCO,).
<br />
<br />Acidity. A measure of the capacity of a solution to
<br />neutralize a strong base, Total acidity is composed
<br />mostly of hydrogen (H+) and bicarbonate ions, car-
<br />bonic acid (H,CO,), and carbon dioxide (C02),
<br />
<br />ButTer Capacity. The compounds thai cause alkalin-
<br />ity also act as buffers to resist a change in pH due to
<br />acid or base addition. Therefore, buffer capacity and
<br />alkalinity are related, Buffer capacity is reported 10 be
<br />a positive factor in reducing corrosion rates.
<br />
<br />Stability. A stable water is saturated with respect to a
<br />solid, tending neither to dissolve nor deposit it
<br />
<br />Saturation. When water is saturated with respect to a
<br />solid it is in equilibrium with it, tending neither to
<br />dissolve nor precipitate it,
<br />
<br />Corrosion. Corrosion occurs when water contacts
<br />any metal. The rate of that reaction depends on the
<br />interaction of many variables such as pH, alkalinity,
<br />dissolved oxygen, chloride, temperature, and the
<br />electrical p01ential of the corroding metal surface,
<br />
<br />Langelier Saturation Index (LSI). One of several
<br />indices that assess the tendency of a water either to
<br />dissolve (a negative LSI) or deposit (a positive LSI)
<br />CaCO,.
<br />
<br />Hardness. A hard water is caused by polyvalent
<br />metallic ca1ions, usually calcium and magnesium, Hard
<br />waters tend to be less corrosive than soft waters. Hard-
<br />ness is measured in terms of mg/! as CaCO"
<br />
<br />6
<br />
<br />WATER STABILITY
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