Corrosion, Saturation Index, Balanced Water in Drinking Water Systems- Corrosion Control Training for Professionals
 

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Corrosion is a complex series of reactions between the water and metal surfaces (metal piping) and materials in which the water is stored or transported.

The corrosion process is an oxidation/reduction reaction that returns refined or processed metal to their more stable ore state. With respect to the corrosion potential of YOUR drinking water, the primary concerns include the potential presence of TOXIC Metals , such as aluminum, copper, nickel, lead and zinc; deterioration and damage to the household plumbing, and aesthetic problems such as: stained laundry, bitter taste, and greenish-blue stains around basins and drains.

The primary health concern is the potential for the presence of elevated levels of lead and copper or other metals in the water. The primary source of the lead includes the use of lead pipes, lead lined tanks, and use of 50/50 lead/tin solder. Because of the concern with lead, the EPA banned the use of high lead solders in 1986. The primary source of copper is the leaching of copper from the household piping used to convey the water throughout the home. In some cases, the water is so corrosive that the interior plumbing system needs to be changed and completely replaced with PEX piping or CPVC (NSF 61).

Corrosion will occur anywhere a galvanic cell or field can be or has established. To establish the field all that is needed is two dissimilar metals that are connected directly or indirectly by an electrolyte, such as water. This is the same chemical reaction that occurs within a battery.

Nearly all metals will corrode to some degree. The rate and extent of the corrosion depends on the degree of dissimilarity of the metals and the physical and chemical characteristics of the media, metal, and environment. In water that is soft, corrosion occurs because of the lack of dissolved cations, such as calcium and magnesium in the water.

In scale forming water, a precipitate or coating of calcium or magnesium carbonate forms on the inside of the piping. This coating can inhibit the corrosion of the pipe because it acts as a barrier, but it can also cause the pipe to clog. Water with high levels of sodium, chloride, or other ions will increase the conductivity of the water and may promote corrosion.

Corrosion can also be accelerated by:

1) low pH (acidic water) and high pH (alkaline water)- For high alkalinity water - it is possible that a chemical scale may form that would help to protect against corrosion, but if a bacteria becomes established the scale, such as SRB (sulfur reducing bacteria), you may experience a problem related to Microbiologically Induced Corrosion or MIC;

2) high flow rate within the piping can cause physical corrosion;

3) high water temperature can increase biological rate of growth and chemical corrosion;

4) oxygen and dissolved CO2 or other gasses can induce corrosion;

5) high dissolved solids, such as salts and sulfates, can induce chemical or bio-chemical corrosion;

6)If the mass ratio (CMSR) of chloride to sulfate is > 0.2, but < 0.5 there is an elevated concern, but if the CMSR is > 0.5 and the alkalinity of the water is less than 50 mg CaCO3/L the concern should be significant;

7) corrosion related bacteria, high standard plate counts, and electrochemical corrosion can result in pinhole leaks and isolated corrosion and aesthetic water quality problems,  and

8) presence of suspended solids, such as sand, sediment, corrosion by-products, and rust can aid in physical corrosion and damage and facilitate chemical and biochemical corrosion.

If it is necessary to flush or run your cold water in the morning for a few minutes before you drink because the water has a bitter taste or discolored, YOUR Water is probably CORROSIVE. If you see blue-green stains in your basins or some staining along the joints of your copper piping, YOUR Water is probably CORROSIVE. As corrosive water stands or seats in pipes or tanks, it leaches metals from the piping, tanks, well casing, or other metal surfaces that water is in contact.

If you see pink staining along the shower tile or tub or as a ring in the toilet - this may not be corrosion, but pink bacteria.  Pink bacteria is an airborne bacteria.

The cost of corrosion can be expensive. Corrosion can impact you and your families’ health, aesthetic quality of your water, waste money, and damage your household piping and fixtures. Corrosive water costs you in a number of ways:

1 ) decreases the efficiency of water heaters, heat exchange units,  and may cause premature failure to the heater or violate the warranty of a water-related appliance;

2 ) corrodes and causes premature failure of household plumbing and plumbing fixtures;

3 ) imparts a bitter taste to your water because of elevated levels of metals, which causes you to purchase bottled water;

4 ) results in the formation of red water or greenish-blue stains on drains; and

5 ) consumption of water with elevated levels of toxic metals, such as lead and copper, have been shown to cause both acute and chronic health problems.

Besides the aesthetic concerns, the corrosion process can result in the presence of toxic metals in your drinking water. These metals include: aluminum chromium, copper, lead, and zinc. The following are the recommended maximum contaminant levels for regulated public water supplies for the aforementioned metals: chromium (0.05 ppm), copper (1 ppm), lead (0.05 ppm), and zinc (5 ppm).

To protect the public, the EPA requires public water supplies to be non-corrosive and the “Lead and Copper Rule” has set new action levels for lead and copper of 0.015 ppm and 1.3 ppm, respectively. Because of the toxicity of lead to children, the EPA has established a recommended goal maximum contaminant level of 0 ppm for lead, but the regulatory limit is still 0.005 mg/L (Source) and 0.015 mg/L (Point-of-Use). If a public water supply is corrosive, the state requires that the water be treated to make the water non-corrosive.

There are no regulations in Pennsylvania or many other states that require private water wells or unregulated individual water wells or springs to be tested or treated for corrosive water.  In many states, there is no requirement to test private wells for toxic metal, pathogenic organisms, or organic chemicals.   IT is up to YOU to monitor the Safety of your own private water well – Get Your Water Tested.

The Langelier Saturation Index is a means of evaluating water quality data to determine if the water has a tendency to form a chemical scale.  In order to use this index, the following laboratory analysis is needed: pH, conductivity, total dissolved solids, alkalinity, and total hardness.

In manipulating the data, the actual pH of the water is compared to the theoretical pH (pHs) based on the chemical analysis. The Saturation Index =

SI = pH - pHs

The Saturation Index is typically either negative or positive and rarely 0. A Saturation Index of zero indicates that the water is “balanced” and is less likely not to cause scale formation. A negative SI suggests that the water is would be under-saturated with respect to carbonate equilibrium and the water may be more likely to have a greater corrosive potential problem or more likely vulnerable to corrosion.

A corrosive water can react with the household plumbing and metal fixtures resulting in the deterioration of the pipes and increased metal content of the water. This reaction could result in aesthetic problems, such as bitter water and stains around basins/sinks, and in many cases elevated levels of toxic metals.  A positive SI suggests that water may be scale forming. The scale, typically a carbonate residue, could clog or reduce the flow in pipes, cause buildup on hot water heaters, impart an alkali taste to the water, reduce the efficiency of the water heaters, and cause other aesthetic problems. Table 1 presents a typical range of SI that may be encountered in a drinking water and a description of the nature of the water and general recommendations regarding treatment.

Testing Options:
DIY Water Screening Tests
Well Water and City Water Testing

Table 1.
SI Values and Recommended Treatment
Recommendations Based on Professional Observation - 
In most cases, additional testing or evaluation is needed to rule out other 
forms of corrosion (MIC - microbiological induced corrosion, Galvanic Corrosion, etc)

Please Note- SI Index is not a reliable means of evaluating
corrosion potential, but it can be used as a guide.

other indices that may be helpful

To determine the corrosion potential for the water, the “Langelier Saturation Index” is an old tool, but not the best tool in all cases, but it is a good FIRST STEP. To calculate the saturation, it is necessary to determine the alkalinity, pH, calcium hardness (or total hardness), conductivity and total dissolved solids content of the water, but it is also wish to test for the level of chloride, other salts, and sulfate. The saturation index is then determined based on a particular water temperature, typically 25 C, but it may be advisable to run at a higher water temperature.

It is recommended to have the water checked for elevated levels of lead and copper. This is conducted by determining the lead and copper content of the water after the water has been left in the piping overnight.  The first draw or first flush sample is collected and then a second sample is collected after the line has been flushed.   The first draw sample is the first one liter of water collected from a cold water tap which has been shut off for at least six hours. This is the sampling procedure EPA is requiring community water systems to use to determine compliance with the new action levels. Samples are then analyzed by atomic absorption spectrophotometry for lead and copper.

It is strongly recommended that a homeowner or new homeowner have the corrosivity of the water tested at least once every few years.  For this test, we typically recommend the “Corrosion Check Kit”.  Corrosive or aggressive water could result in aesthetic problems, increased levels of toxic metals, and deterioration of household plumbing and fixtures. In one case, the homeowner’s water was so corrosive that the interior household plumbing was changed twice prior to the Center becoming involved and evaluating the well's water quality. The homeowner spent nearly $ 12,000.00 to replace and repair plumbing and fix damaged interior.   It was only after this expense and 15 years of living in the home did the homeowner finally test their water. The testing indicated that the Saturation Index was -4.25, which indicates that the water is very corrosive, and the first and flush testing showed elevated levels of copper, lead, and zinc in the water. Following the testing, it was determined that a neutralizing filter was needed. The total cost to install the filtration system was only $ 2850.00.

If the homeowner would have gone through the expense of a $ 150.00  corrosion check prior to purchasing the home, it is possible that the cost of installing the system could have been included in the real estate transaction and it would have prevented the expense and frustration of having to change the household plumbing twice within 15 years, plus prevented his family exposure to elevated levels of lead, copper, and zinc.

Corrosion control is a complex science, requiring considerable knowledge of corrosion chemistry and of the system being evaluated. Corrosive water can be managed by installing pretreatment systems, installation of non-conductive unions, reducing hot water temperature, and replacing copper piping with PEX or CPVC (NSF61) piping. The pretreatment process treats the corrosivity of the water by reducing the acidity of the water by altering the pH, hardness, and/or alkalinity of the water. The resultant Saturation Index (SI) is typically more positive and preferably the SI is between -0.5 to +0.5.

In addition to changing the SI, one approach includes establishing a thin film of calcium or magnesium carbonate on the inside of the piping which acts as a physiochemical barrier. The pretreatment systems typically used in the application for homeowners or small private water supplies includes either a neutralizing tank filter or caustic liquid treatment feed system. The neutralizing filter is more commonly used approach. The neutralizing filter uses crushed limestone, magnesia or some other mixture and as the water passes through the filter, the filter neutralizes the excess “acid”. Therefore, the neutralizing filter actually increases the hardness of the water and raises the pH while reducing the acidity of the water. The following are the chemical equations for the filter reactions.

H2CO3 + CaCO3 ---------> Ca(HCO)3
2 (H2CO3) + MgO ---------> Mg(HCO3)2 + H20

The main drawbacks of the neutralizing filter include:

1) May increase water hardness to more than 120 mg as calcium carbonate or > 7 grains per gallon, i.e., Hard water.  The system needs to be properly adjusted.

2) The use of finely ground limestone may result in the reduction of water pressure and some fine media may enter the system.

3) The system requires weekly backwashing. This is typically automated, but it does put a strain on any on-lot disposal systems.

4) Neutralizing filters can be used where the raw water pH is 6.0 or greater. A limestone media will raise the pH to only about 6.9 to 7.0. If a higher pH is needed, a magnesia filter media should be used. GET YOUR WATER TESTED before installing a system.

5) Many water treatment professionals may install a water softener after the neutralizing filter.  In some cases, this can make the water even more corrosive.

The caustic feed system  is a dosing system that offers more options and is more flexible than the neutralizing filter, but requires additional safety precautions; more expertise to install, setup and operate, and possibly more extensive testing prior to and following installation. The system can treat waters with a lower pH without adding hardness to the water. Typically a sodium-based solution is used as the caustic source, so the sodium concentration of the water will be increased. Therefore, households that have individuals on a low sodium diet need to make the doctors aware of the treatment system.

The waters with a pH of 4.0 to 6.8 a soda ash (sodium carbonate) is typically used. The soda ash is usually fed into the system at a rate to produce a resultant pH of approximately 7.0. When the raw water pH is less than 4.0, a caustic soda (sodium hydroxide) solution is used. Note: Solutions of sodium hydroxide are extremely aggressive and used only be handled by trained individuals.

Note: Chemical feed systems can be used to increase or reduce pH of the water, disinfect the water,  anti-foulant agents for membrane systems,  and anti-scale formation,

In general, treatment of groundwater can be accomplished by adding some hardness or alkalinity, or both, and adjusting the pH to slightly increase the scale-forming tendency, thus creating a film or barrier to the corrosion. Installation of dielectric (non-conductive) unions between dissimilar metals such as copper pipe and steel hot water heaters can limit corrosion by breaking the galvanic circuit.

One of the more effective methods of controlling corrosion and leaching of toxic metals into the water is preventive, such as using dielectric couplings, installing CPVC piping and stainless steel equipment.  If copper plumbing is used, non-lead solder such as 95/5 tin/antimony solder should be used.

It is important to keep in mind that the corrosiveness of the water can be increased by the installation of water softeners, aeration devices, increasing hot water temperatures, chlorinating the water, turbid or fine sediment, and improper matching of metal pipes thereby facilitating galvanic corrosion. Some water treatment equipment such as softeners and aeration systems can aggravate corrosion. Softeners remove the protective calcium and magnesium and introduce highly conductive sodium or potassium into the water and reduce total hardness. Aeration devices for iron, sulfur or odor removal add oxygen, which is extremely corrosive in water. Higher water temperatures and suspended materials accelerate the rate of corrosion by increasing the reaction rates or causing physical damage to the pipes. Chlorine is a strong powerful oxidant which can promote corrosion.

All regulated community water systems are required to monitor their levels of lead and copper, with samples being taken from cold water taps in consumers' homes. Water systems with levels of lead or copper in excess of the action levels will be required to implement corrosion control procedures. Therefore, it is up to YOU to check and monitor the safety and potability of YOUR own private water supply.

Question Ask? What does blood lead levels mean?
"Experts now use a reference level of 5 micrograms per deciliter to identify children with blood lead levels that are much higher than most children’s levels. This new level is based on the U.S. population of children ages 1-5 years who are in the highest 2.5% of children when tested for lead in their blood." (Source)

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