Potable Water STABILIZATION
Goal: the goal is to produce a "stable," non-corrosive water. Included in this goal is the reduction or control of color, odor, taste, and undesirable deposits or scales.(Which also according to the additions of the Lead & Copper Rule, is one that will not leach out lead or copper from the conveying pipes or pipe fittings.) The treatment process we use to achieve this goal is "stabilization."
The Problems and Their Treatment:
Lead and copper: in the majority of cases, undesirable levels of lead and copper is due primarily to the re-activity of the static water in a copper pipe using solder containing 50 percent lead and 50 percent tin. Lead leaches out of the solder. There is a reduction in the amount of lead leaching from the solder over a period of years. Several studies show a decrease after 5 to 6 years. Low alkalinity, elevated temperatures, and acidic waters accelerate the lead dissolution activity.
These studies show that a major source of the lead in the water is due to water standing overnight, or other long periods of time, which allows for the concentration of lead undergoing dissolution. "First use" or "first draw" sampling is the only true way to quantify this activity.
Increasing the pH of the water to 8.0 or above has been shown to be effective in reducing lead levels. Groundwater's with high alkalinity and calcium values have been shown to be very low in lead values, presumably due to the creation of a calcium carbonate scale on the inner aspects of the pipe. This scale reduces the potential for leaching of lead from the solder or lead pipe and fittings by keeping the water isolated from the metal.
Corrective actions include replacement of lead water pipes, and corrosion control treatment. Lead free solder should be specified.
Other Pipes/Plumbing Problems
Corrosive water has the ability to remove other toxic metals from the water distribution system and plumbing fixtures, such as galvanized pipe and fittings where aggressive water may leach cadmium out of the pipe.
Cast-iron watermain distribution system
The corrosion process described for iron in the previous chapter can create tubercules. These tubercules harbor and protect large populations of microbes. When the tubercules rupture these populations are released into the water supply causing re-growth problems.
The dissolving iron can also create red colored water, which can cause stains on plumbing fixtures and clothing. This iron can also be a source of "food" for a group of bacteria we call "iron bacteria" which create taste and odor problems as a byproduct of this activity.
Copper pipe corrosion:
Aggressive water can attack copper pipe and fittings, and create blue green stains on clothing and plumbing fixtures; and cause a metallic taste.
The undesirable formation of scale in pipes and fittings can easily result in increased energy costs for a hot water heaters, boilers, and other equipment. In extreme cases, pipes and fittings may lose not only flow capacity, but become completely plugged.
Calcium carbonate and magnesium carbonate are the most common scaling problems. Calcium and magnesium found in hard water, combine with other molecules and atoms to form the scale. Generally speaking, as the pH increases, the amount of scaling potential also increases.
The quantity of total dissolved solids (TDS) and water temperature are also factors. As the water temperature increases there is a corresponding increase in the potential for scaling; as the quantity of total dissolved solids increases there is a corresponding increase in the potential for scaling.
To reduce the scaling we usually lower the pH of the water by injecting an acid such as sulfuric or hydrochloric, or carbon dioxide (CO2 ) to place the water in the optimum pH range for stability.
Some of The Basic Chemistry
Generally speaking, lower pH values and alkalinity values are conducive to corrosion; higher pH values are conducive to scale formation.
Langelier Index: the "abbreviated
equation" for this indices is as follows:
LI = pH(actual) - pH(theo. Saturation)
In this calculation we measure the pH of the water and subtract from it the theoretical pH at which the water is saturated with calcium carbonate. (For many operators, this is a challenging indices the first time they use it, as calculating the theoretical pH saturation value involves determining alkalinity, pH, ionic strength and temperature constants, etc . But with some practice, it is not a problem!)
If LI = a negative value calcium carbonate will be dissolved in the water itself, and the water will have a corresponding corrosive property.
If LI = 0 the water is considered neutral or "stable"
If LI = a positive value, then calcium carbonate will precipitate out and possibly form a scale.
and treatment process of the water include:
a) reduction/removal of free carbon dioxide to reduce the acidity and hereby raise the pH of the water
b) pH adjustment: increase the pH of the water by adding an appropriate chemical for your system, such as sodium hydroxide (NaOH). If the water has low alkalinity values and lower pH values we usually add lime to increase the values of these two parameters.
pH adjustment chemicals:
The addition of lime will increase the pH of the water, add calcium, and neutralize free carbon dioxide.
The addition of sodium bicarbonate increases the alkalinity of the water, and adds to the waters buffering capacity. Therefore, if an increase in alkalinity is required without a corresponding increase in pH, sodium bicarbonate is usually used.
The addition of sodium carbonate (soda ash) increases the alkalinity, increases the pH, and neutralizes free carbon dioxide.
Seawater, which has been desalinated, is usually treated with carbon dioxide and lime. In this is done to increase the alkalinity, control the pH, and adjust the calcium concentration.
c) reduction/removal of oxygen to
reduce the corrosivity of the water
d) cathodic protection which is expensive, and used only when necessary.
e) addition of chemicals to create a thin film or coating. Cathodic inhibitors reduce the rate of corrosion by preventing dissolved oxygen from reacting the cathodic areas, thus inhibiting the rate-controlling cathode reactions. Anodic inhibitors are less effective and not commonly used.
Inhibitors that are known to make
certain metals more passive or resistant to corrosion are called
passivators; nitrites and chromates passivate iron.
1) Calcium carbonate.
2) Sodium hexametaphosphates (polyphosphate).
In general, there are two basic approaches
in controlling lead and copper dissolution:
1) create a metal compound film of less soluble material on the pipe surface for protection
2) create a precipitate on the pipe surface for protection
Corrosion inhibitors: silicates, orthophosphate, polyphosphate, and poly-orthophosphate blends. This group forms metal complexes on the inner walls of the pipes reducing corrosion
Polyphosphate addition is not without
Carbonate passivation: alkalinity and/or pH adjustment to create less soluble compounds
Carbonate precipitation: the creation of calcium carbonate precipitate by a modification of the carbonate equilibrium.
Stabilization Process Effectiveness:
The effectiveness of the stabilization process can only be truly measured by inspection of the water distribution system, reviewing customer complaints, and inspection of household and industrial plumbing fixtures.
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