Corrosion Treatment

Water Testing for Corrosion

To determine the corrosion potential for the water, the “Langelier Saturation Index” is an old tool, but not the best tool. 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. The saturation index is then determined based on a particular water temperature, typically 25 C.

In addition, it is recommend to have the water checked for evidence of testing the water for 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 is collected and then a second sample is collected after the line has been flushed, typically three to five minutes. 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. Corrosive or Aggressive water could result in aesthetic problems, increased levels of toxic metals, and deterioration of household plumbing and fixtures. In one case that the University was involved, the homeowner’s water was so corrosive that the interior household plumbing was changed twice prior to the University becoming involved and evaluating the wells water quality. The homeowner spent nearly $ 9000.00 to replace and repair plumbing, fix damage interior and only after this expense and 15 year of living in the home the water was finally tested. The testing indicated that the Saturation Index was -4.25, which indicates that the water is very corrosive. Following the testing, it was determined that a neutralizing filter was needed. The total cost to install the filtration system was only $ 850.00.

If the homeowner would have gone through the expense of a $ 125.00 to $ 150.00 water analysis 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.

Corrosion Treatment

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 hotwater temperature, and replacing copper piping with PVC or CPVC. The pretreatment process treats the corrosivity of the water by changing the Saturation Index through an increase or decrease in the pH, hardness, and/or alkalinity. The resultant Saturation Index 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 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. 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” and results in the partial dissolution of the media. Therefore, the neutralizing filter actually increases the hardness of the water and raises the pH. 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) Increase water hardness to more than 120 mg as calcium carbonate or > 7 grains per gallon, i.e., Hardwater.

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.

The caustic feed system 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 feed 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.

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 hotwater temperatures, chlorinating the water and improper matching of metal pipes. 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 into the water. 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.

Over the next few years, all regulated community water systems will be 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.

Дата добавления: 2015-11-13