pH Adjustment

In water treatment systems, pH adjustment is most often the initial process in a water purification system. It involves raising or lowering the pH of the water so that the standard set point is achieved.

pH Defined


So first, what is pH and how does it affect water purification?

Let’s start with the letters themselves. The H stands for hydrogen, while the actual meaning and origin of “p” in pH is unknown. In different references however, “p” is given various meanings including power, potenz (German for power), puissance (French for power), potential, and percentage.

But whatever may be the more appropriate denotation of p, pH is strictly defined in chemistry as the measurement of the activity of hydrogen ions in an aqueous solution. Mathematically, this is expressed by the equation
pH= -log[H+]

For the purposes of discussing the role of pH in water treatment though, we can use a simpler and more practical definition: pH is the measurement of the acidity or basicity (or alkalinity) of water.

The pH scale ranges from 0 to 14 and pure water is deemed neutral with a pH of about 7 at close to room temperature (250C or 770F). Any solution that registers a pH of below 7 is considered an acid, and anything with a pH over 7 is a base solution.

For an idea of how acidic or basic common solutions are, we’ll mention a few of them. Lemon juice and orange juice are strong acids with pH measurements of 2 and 3 respectively, while soapy water and bleach rank high up in the bases at 12 and 13 pH respectively.

pH Adjustment: The Why and How


In water purification, pH adjustment is a necessary step and there are several reason for this. Bringing the water to as close as possible to a 7.0 pH is the goal in most cases, but there may be other adjustments required depending of the quality of the untreated water.

For one, water that has either a too high or too low pH tends to be corrosive, causing the dissolution of metal pipe materials into the water and scale buildup in pipes. With the pH consistently adjusted to the set point, lead and copper levels are reduced when the water reaches households. This also eliminates the color stain that is bound to be in the water as copper plumbing causes blue-green staining while galvanized plumbing results in red-tinged water.

In addition, pH adjustment is also a method utilized to lessen the occurrences of certain disinfection byproducts (DBPs) in the distribution network. Trihalomethane (THM) concentrations for instance, are effectively reduced when pH is lowered.

Lastly, water adjusted to a slightly alkaline level can facilitate the efficacy of the next processes in water purification - coagulation and flocculation.

The most common method used in pH adjustment is the addition of chemicals, such as lime (calcium hydroxide) and soda ash (sodium carbonate). The general rule of thumb is that acids neutralize base solutions, while bases neutralize acids. The system used to perform pH adjustment in a water purification process may utilize either a neutralizing filter or chemical-feed pump.

Other processes for pH adjustment or acid-base neutralization are aeration and air stripping.


 
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