Corrosion control is a complex science. 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:

• Increase water hardness to more than 120 mg as calcium carbonate or > 7 grains per gallon , i.e., Hardwater.
• The use of finely ground limestone may result in the reduction of water pressure and some fine media may enter the system.
• The system requires weekly backwashing. This is typically automated, but it does put a strain on any on-lot disposal systems.
• 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.

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.

Treatment
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.

Infomation From- http://www.water-research.net