What is pH?
The pH of your drinking water reflects how acidic or alkaline it is. pH stands for "potential of hydrogen", referring to the amount of hydrogen present in water, and it is measured on a logarithmic scale between 0 and 14. This means that a water with a pH value of 5.0 is 10 times more acidic than a pH of 6.0, and water with a pH of 4.0 is 100 times more acidic than water with a pH of 6.0. A pH value of 7 is considered neutral, and represents a balance between the amount of acid and base in the water. Under the EPA's drinking water standards, there is no primary standard for pH, but it is recommended that drinking water pH fall between 6.5 and 8.5. Within the water treatment industry, the goal is typically to achieve a pH value of around 7.5 for corrosion control and prevention.
Both low and high pH values can cause corrosion issues within a home. Water with a low pH, below 7.0, can cause a sour or metallic taste, and cause corrosion of pipes and fixtures, resulting in blue green and orange brown staining. Water with a high pH, typically above 8.5, can result in a sour or bitter taste, cause corrosion of certain metals, and result in increased scale build up, potentially leading to inefficient operation of water heaters and other water using appliances. A high pH can also cause the water to feel extremely slippery.
Public water systems are typically treated to ensure that the water delivered to residential homes have a neutral pH. However, for private well owners, they are at the mercy of the groundwater and aquifer system from which they draw their water. The most common regions for acidic water conditions in the United States are New England, the Mid-Atlantic, and the Pacific Northwest. Even within these regions, the pH and water chemistry can vary widely.
The main influence on low pH in these regions is free carbon dioxide, mineral acids, and the lack of sufficient bicarbonate alkalinity. Sources of free carbon dioxide include carbon rich bed rock formations (i.e. coal and black shale) and decaying vegetation. The presence of carbon dioxide results in the creation of carbonic acid in water. Alkalinity is a measure of the water's quantitative ability to buffer itself or neutralize acids. Total alkalinity is a product of the total sum of carbonate (CO3), bicarbonate (HCO3), and hydroxide (OH) ions present in solution. A higher alkalinity results in a greater capacity of the water to resist changes in pH from the addition of acids.
pH Correction and Acid Neutralization
More often than not, the large majority of pH correction issues in the water treatment industry involve neutralizing acidic pH conditions or raising the pH to above 7.0. The two most common practices to do this are:
- Passing the acidic water through a bed of neutralizing media (i.e. calcite or magnesium oxide).
- Feeding a liquid chemical solution directly into the water (i.e. soda ash injection).
It is important to have an accurate laboratory assessment of your water chemistry before any type of pH correction is attempted. Your water chemistry will determine the specific neutralizing media and approach that is most appropriate. Understanding the relationship between pH, total alkalinity, and free CO2 is important when determining which of the two correction techniques is most appropriate. The higher the total alkalinity, the harder it is to change the pH. Higher free CO2 levels will typically require a stronger neutralizing media. Very low pH or strongly acidic water conditions due to the presence of mineral acids can be tougher to correct. In this case, the best approach for pH correction often requires feeding a liquid chemical like soda ash directly into the water. Other influencing factors on pH correction include total hardness, TDS (total dissolved solids), sulfates, and chlorides.
Hardness, TDS and pH Correction
Total hardness and Total Dissolved Solids (TDS) are important factors in determining the appropriate approach for pH correction. The higher the total hardness and TDS, the tougher it can be to correct pH with a neutralizing media like calcite. This is due in part to the common ion effect, which deals with the chemical equilibrium of a given chemical reaction. Further, hardness is related to the phenomenon of neutralizing media solidifying. This phenomenon is related to the Le Chatelier's principle.
Calcium and magnesium are the primary contributors to the total hardness of water. Calcium is one of the major components of calcite. Magnesium is one of the major components of magnesium oxide. Therefore, a sample with a higher influent hardness will likely reduce the effectiveness of a neutralizing media. Higher influent hardness is also typically associated with higher alkalinity, which will also reduce the absorption of neutralizing media. This, however, is a trend and not a rule. Total hardness should never be substituted or assumed-to-be equal to the total alkalinity.
Influence of pH on Other Contaminants
It is also important to understand the relationship between pH correction and the removal of other contaminants from your drinking water. pH Correction is often only one step in water treatment and can change the physical state of other contaminants like carbon dioxide, iron and manganese.
At lower pH:
- Gases tend to be more volatile. In other words, they come out of solution more aggressively.
- Dissolved substances tend to stay in solution and the ferrous form of metals (i.e. iron and manganese) tend to dominate.
- Chlorine is a stronger disinfectant, weaker oxidant, and tends to form more disinfection byproducts (DBP's).
At higher pH:
- Gases tend to more aggressively stay in solution.
- Dissolved substances tend to come out of solution more aggressively and the ferric form of metals (i.e. iron and manganese) tend to dominate.
- Chlorine is a weaker disinfectant, stronger oxidant, and tends to form less disinfection byproducts (DBP's).
