Water is one of the most abundant resources we have in the world. It is considered the ultimate sustainable resource due to the hydrologic cycle that returns water back to nature for reuse in the form of precipitation. This precipitation, when immediately formed, will be water in its purest form. As it falls through the sky and travels to the different water sources (e.g., ground water, surface water, etc.) it will pick up a variety of impurities. Water is therefore commonly called the universal solvent since it has the ability to dissolve, to a certain extent, almost every substance found in nature. However, not all water qualities are the same and will vary across geographic regions.
In industrial water treatment, there are several impurities and water properties that are more problem causing than others, leading to corrosion, deposits, scale, etc. Examples of the most common include:
Conductivity measures the ability of water to “conduct” an electrical current. It is often used to estimate the TDS content of the water. The higher the conductivity, the greater the dissolved solids/minerals content. Some of these dissolved minerals may then precipitate onto system heat transfer surfaces, if they are allowed to concentrate above recommended limits. They may also contribute to the potential for increased corrosion.
pH is a measure of the hydrogen ion concentration in water, which is related to acidity and basicity. Water is considered “neutral” at a pH of 7.0. In general, the lower the pH, the greater the potential for corrosion to occur, and the higher the pH, the greater the potential for scale, but there are exceptions to this rule.
Alkalinity measures the acid neutralizing capacity of the water. The value of pH is loosely related to alkalinity. Waters that have low alkalinity will typically have a relatively low pH, and those that have a high alkalinity will typically have a relatively high pH.
Hardness measures the amount of calcium and magnesium in the water. Calcium hardness can combine with alkalinity to form calcium carbonate or “lime” scale. This is the most common scale deposit that forms in heat exchange equipment and domestic water systems. It has an inverse solubility with temperature. As the temperature increases, the solubility of calcium carbonate decreases. It is very insulating causing loss of heat transfer and increased energy costs. Calcium-based scale can also lead to increased corrosion potential due to under-deposit corrosion.
Silica (SiO2) is another impurity that has solubility limits in water. Silica can form very hard and difficult to remove deposits. These deposits, if formed, can also be very insulating and require hazardous chemicals to remove. Silica deposits can form with magnesium hardness if levels are high enough.
Chlorides and Sulfates
Chlorides and sulfates can lead to severe corrosion issues. High levels of these anions can lead to different types of corrosion including general corrosion, pitting, and stress corrosion cracking. Even at low levels, they can become especially detrimental under deposits.
Bacteria can be found in water and may lead to waterside problems. An example would be a cooling tower system. A cooling tower continuously scrubs dust, nutrients, and microbes from the air. As they begin to concentrate in the tower system, these contaminants can lead to problems such as biofilm formation and/or corrosion. The cooling tower is at ideal temperature and pH, is continuously aerated, and is exposed to sunlight, which provides excellent conditions for bacteria to thrive. Closed loop systems and domestic water can also have bacteria problems if the right conditions exist.
Dissolved gases can lead to potential problems in steam boiler systems and hot water closed loops. The dissolved gases that can typically be problematic are oxygen and carbon dioxide. Both of these gases have an inverse solubility with temperature. As the temperature goes up, both oxygen and carbon dioxide become less soluble. They are especially problematic in steam boiler systems where high levels of oxygen can lead to oxygen pitting on the water surfaces of boiler tubes and feedwater tanks. Carbon dioxide can also form carbonic acid when steam condenses that will cause corrosion of the steam and condensate lines. Hot water closed loops with high amounts of makeup can also have corrosion issues from oxygen being released.
With all of the impurities that can be found in water, it is important to understand that if managed and controlled they can become less problematic. The proper chemical treatment, water quality balance of each system, and appropriate equipment can protect your systems from the damaging effects that each impurity can cause.
Consult with your Chem-Aqua representative for additional guidance to determine if any of the impurities discussed and more can cause problems for your systems.
Written by: Matt Schnepf