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<br />The pH of a water also affects the solubilities of
<br />metals such as lead, copper, zinc, and iron. For
<br />example, an alkaline pH will cause the precipitation of
<br />insoluble PbCO, or Pb,(CO,),(OH), species. Studies
<br />have indicated that the optimum pH for achieving lead
<br />control with these species is between 9 and 1 0.' At a
<br />pH of 8 to 8.5, more acceptable for distribution sys-
<br />tems, the evidence shows that significant lead leach-
<br />ing still occurs,
<br />
<br />However, the adjustment of pH to values exceed-
<br />ing 9 for lead control may introduce other problems,'
<br />Most seriously, a rise in pH could cause reductions in
<br />disinfection effectiveness and increases in trihalo-
<br />methane (THM) formation following chlorination. Addi-
<br />tion of pH control chemicals such as lime may also
<br />increase turbidity. Furthermore, a higher pH could
<br />cause excessive CaCO, scaling. Finally, pH adjust-
<br />ments can diminish the performance of some coagu-
<br />lants, particularly alum, and reduce filtration efficiency
<br />unless these adjustments are made at the end of the
<br />treatment system, just before the water enters the
<br />distribution system
<br />
<br />Intuition suggests that carbonate addition would
<br />decrease lead solubility by encouraging deposition of
<br />lead carbonate scales, However, experience and cal-
<br />culations do not support this idea. In fact, inorganic
<br />carbon levels in excess of 50 mg/I, as CaCO" appear
<br />to increase lead solubility through formation of soluble
<br />lead carbonate complexes. Some investigators have
<br />suggested that decarbonation may be needed for very
<br />alkaline waters. However, water managers should
<br />consider alternative corrosion inhibitors before under-
<br />taking costly decarbonation programs.
<br />
<br />Orthophosphate is one such inhibitor, Lead ap-
<br />pears to form at least one insoluble orthophosphate
<br />solid (Pb.(PO.),DH) under realistic drinking water con-
<br />ditions. Calculations indicate that orthophosphate can
<br />reduce lead solubility at a lower pH than that required
<br />to reduce lead solubility with pH and carbonate
<br />adjustment. The advantages of operating at a lower
<br />pH are mentioned above. Calculations also suggest
<br />that, in many waters, orthophosphate addition can
<br />reduce lead to lower levels than can pH-carbonate
<br />adjustments. Use of orthophosphate to control lead
<br />corrosion may also reduce corrosion of other plumbing
<br />materials in the distribution system,
<br />
<br />Water Quality Models and Corrosion
<br />Chemical scales affect water composition not only by
<br />sealing off corroding surfaces but also through the
<br />laws governing chemical mass action. As these laws
<br />state, waters in contact with chemical scales contain
<br />
<br />the same components that make up the scale. For
<br />example, waters containing Pb.(PO,),DH films must
<br />contain Pb, PO" and OH Ions; waters containing
<br />CaCO, films must contain Ca and CO, ions, The
<br />mass action relationships can be used to predict
<br />component concentrations when the scales and water
<br />are in equilibrium.
<br />
<br />The mass action laws have been incorporated into
<br />models that chemists and engineers can use to
<br />estimate the tendencies and capacities of water to
<br />deposit protective films. Some of these models also
<br />predict the concentrations of dissolved species once
<br />equilibrium is established. The models range from
<br />single equations that predict the behavior of one com-
<br />pound to highly sophisticated computer programs that
<br />simulate the reactions of hundreds of species. The
<br />more sophisticated models give more realistic results
<br />because they account for many more of the factors
<br />controlling constituent solubility,
<br />
<br />The LSI (see box on next page) is an example of
<br />a simple model. it estimates whether or not CaCO,
<br />will tend to deposit from a given water. It provides no
<br />information about how much CaCO, could be depos-
<br />ited or about the concentrations of lead that might
<br />occur once CaCO, films are established. Neverthe,
<br />less, operators must frequently estimate whether
<br />CaCO, will form, and they can use the LSI for this
<br />purpose.
<br />
<br />The calcium carbonate precipitation potential
<br />(CCPP)1O is a more useful model than the LSI be-
<br />cause it determines not only a water's CaCO, deposi-
<br />tion or dissolution tendencies, but the amount of
<br />CaCO, that can be precipitated. Information about
<br />CaCO, deposition potential is important because ex-
<br />cessive deposition can clog water pipes. However,
<br />the CCPP is more difficult to calculate than the LSI,
<br />and neither model gives information about lead
<br />concentrations that might occur once CaCO, films are
<br />established.
<br />
<br />The lead carbonate saturation index enables
<br />engineers and chemists to estimate the water quality
<br />conditions that promote formation of lead carbonate
<br />films and the equilibrium lead concentration that would
<br />occur if lead solubility were controlled by lead carbon-
<br />ate films, A complete water chemistry model like
<br />EPA's MINTEQA 1" allows calculation of the satura-
<br />tion indices for more than 50 solids, including lead
<br />carbonate and other lead solids that might control lead
<br />concentrations. It also determines saturation indices
<br />and precipitation potentials for compounds that might
<br />form large amounts of scaie (CaCO" for example).
<br />
<br />4
<br />
<br />WATER STABILITY
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