Alkalinity serves to control the buffer intensity of most water systems; therefore, a minimum amount of alkalinity is necessary to provide a stable pH throughout the distribution system for corrosion control of lead,
copper and iron and for the stability of cement-based linings and pipes.
Alkalinity of the finished water is affected by the use of reverse osmosis and nanofiltration processes. These processes remove sodium, sulphate, chloride, calcium and bicarbonate ions and result in a corrosive finished water (Taylor and Wiesner, 1999). This underlines the importance of process adjustments such as addition of base and aeration of the permeate stream to recover alkalinity prior to distribution.
According to thermodynamic models, the minimum lead solubility occurs at relatively high pH (9.8) and low alkalinity (30-50 mg/L as calcium carbonate) (Schock, 1980, 1989; Schock and Gardels, 1983; U.S. EPA, 1992; Leroy, 1993; Schock et al., 1996). These models show that the degree to which alkalinity affects lead solubility depends on the form of lead carbonate present on the pipe surface. When cerussite is stable, increasing alkalinity reduces lead solubility; when hydrocerussite is stable, increasing alkalinity increases lead solubility (Sheiham and Jackson, 1981; Boffardi, 1988, 1990). Cerussite is less stable at pH values where hydrocerussite is stable and may form. Eventually, hydrocerussite will be converted to cerussite, which is found in many lead pipe deposits. Higher lead release was observed in pipes where cerussite was expected to be stable given the pH/alkalinity conditions. However, when these conditions are adjusted so that hydrocerussite is thermodynamically stable, lead release will be lower than in any place where cerussite is stable (Schock, 1990a).
Laboratory experiments also revealed that, at pH 7-9.5, optimal alkalinity for lead control is between 30 and 45 mg/L as calcium carbonate and that adjustments to increase alkalinity beyond this range yield little additional benefit (Schock, 1980; Sheiham and Jackson, 1981; Schock and Gardels, 1983; Edwards and McNeill, 2002) and can be detrimental in some cases (Sheiham and Jackson, 1981).
Schock et al. (1996) reported the existence of significant amounts of insoluble tetravalent lead dioxide in lead pipe deposits from several different water systems. However, the alkalinity relationship for lead dioxide solubility is not known, as no complexes or carbonate solids have been reported. The existence of significant amounts of insoluble lead dioxide in lead pipe deposits may explain the erratic lead release from lead service lines and poor relationship between total lead and alkalinity (Lytle and Schock, 2005).
Alkalinity is not expected to influence the release of lead from leaded solders, since this release is mostly dependent on the galvanic corrosion of the leaded solders as opposed to the solubility of the corrosion by-products formed (Oliphant, 1983a). However, Dudi and Edwards (2004) predicted that alkalinity could play a role in the leaching of lead from galvanic connections between lead- and copper-bearing plumbing. A clear relationship between alkalinity and lead solubility based on utility experience remains to be established. Trends in field data of 47 U.S. municipalities indicated that the most promising water chemistry targets for lead control were a pH level of 8-10 with an alkalinity of 30-150 mg/L as calcium carbonate (Schock et al., 1996). A subsequent survey of 94 U.S. water companies and districts revealed no relationship between lead solubility and alkalinity (Lee et al., 1989). In a survey of 365 utilities under the U.S. EPA Lead and Copper Rule, lead release was significantly lower when alkalinity was between 30 and 74 mg/L as calcium carbonate than when alkalinity was < 30 mg/L as calcium carbonate. Lower lead levels were also observed in utilities with alkalinities between 74 and 174 mg/L and greater than 174 mg/L when the pH was 8.4 or lower (Dodrill and Edwards, 1995).
Laboratory and utility experience demonstrated that copper corrosion releases are worse at higher alkalinity (Edwards et al., 1994b, 1996; Schock et al., 1995; Ferguson et al., 1996; Broo et al., 1998) and are likely due to the formation of soluble cupric bicarbonate and carbonate complexes (Schock et al., 1995; Edwards et al., 1996).
Examination of utility data for copper levels, obtained from 361 utilities under the U.S. EPA Lead and Copper Rule, also revealed the adverse effects of alkalinity and estimated that they were approximately linear and more significant at lower pH: a combination of low pH (< 7.8) and high alkalinity (> 74 mg/L as calcium carbonate) produced the worst-case 90th-percentile copper levels (Edwards et al., 1999).
However, low alkalinity (< 25 mg/L as calcium carbonate) also proved to be problematic under utility experience (Schock et al., 1995). For high-alkalinity waters, the only practical solutions to reduced cuprosolvency are lime softening, removal of bicarbonate or addition of rather large amounts of orthophosphate (U.S. EPA, 2003).
Lower copper concentrations can be associated with higher alkalinity when the formation of the less soluble malachite and tenorite has been favoured (Schock et al., 1995). A laboratory experiment conducted by Edwards et al. (2002) revealed the possible dual effect of high alkalinity. For relatively new pipes, at pH 7.2, the maximum concentration of copper released was nearly a linear function of alkalinity. However, as the pipes aged, lower releases of copper were measured at an alkalinity of 300 mg/L as calcium carbonate, at which malachite had formed, than at alkalinities of 15 and 45 mg/L as calcium carbonate, at which the relatively soluble cupric hydroxide prevailed.
Lower iron corrosion rates (Stumm, 1960; Pisigan and Singley, 1987; Hedberg and Johansson, 1987; Kashinkunti et al., 1999) and iron concentrations (Horsley et al., 1998; Sarin et al., 2003) in distribution systems have been associated with higher alkalinities.
Experiments using a pipe loop system built from 90- to 100-year-old unlined cast iron pipes taken from a Boston distribution system showed that decreases in alkalinity from 30-35 mg/L to 10-15 mg/L as calcium carbonate at a constant pH resulted in an immediate increase of 50-250% in iron release. Changes in alkalinity from 30-35 mg/L to 58-60 mg/L as calcium carbonate and then back to 30-35 mg/L also showed that higher alkalinity resulted in lower iron release, but the change in iron release was not as dramatic as the changes in the lower alkalinity range (Sarin et al., 2003). An analysis of treated water quality parameters (pH, alkalinity, hardness, temperature, chloride and sulphate) and red water consumer complaints was conducted in the City of Topeka, Kansas (Horsley et al., 1998). Data from the period 1989-1998 were used for the analysis. The majority of red water problems were found in unlined cast iron pipes that were 50-70 years old. From 1989 to 1998, the annual average pH of the distributed water ranged from 9.1 to 9.7, its alkalinity ranged from 47 to 76 mg/L as calcium carbonate and its total hardness ranged from 118 to 158 mg/L as calcium carbonate. The authors concluded that the strongest and most useful relationship was between alkalinity and red water complaints and that maintaining finished water with an alkalinity greater than 60 mg/L as calcium carbonate substantially reduced the number of consumer complaints.
Alkalinity is a key parameter in the deterioration of water quality by cement materials. When poorly buffered water comes into contact with cement materials, the soluble alkaline components of the cement pass rapidly into the drinking water. Conroy et al. (1994) observed that alkalinity played a major role in the deterioration of the quality of the water from in situ mortar lining in dead-end mains with low-flow conditions. When the alkalinity was around 10 mg/L as calcium carbonate, pH levels remained above 9.5 for up to 2 years, and aluminum concentrations were above 0.2 mg/L for 1-2 months following the lining process. However, when alkalinity was around 35 mg/L as calcium carbonate, the water quality problem was restricted to an increase in pH level above 9.5 for 1-2 months following the lining process. When the alkalinity was greater than 55 mg/L as calcium carbonate, no water quality problems were observed.