Water treatment products help control scale deposits, minimize corrosion, and in some applications fight unwanted microbiological activity. Not only is selecting the proper products vital for a successful water treatment program but understanding how they behave and how that relates to controlling them is equally as important.
The total dosage of a water treatment product is composed of two key elements, as shown in the equation below. These two elements (demand and residual) must be evaluated together when controlling and troubleshooting product dosages.
Dosage = Demand + Residual
The total amount of water treatment product added to a water system is the “dosage.” How much product needs to be added will depend upon the reactions taking place within the water environment (DEMAND) and the actives residual (RESIDUAL) required to control the target problems and provide a buffer for system upsets.
Various reactions will occur between the actives in the product added and the components in the water being treated. These DEMAND reactions are typically by design (but not always). Examples include: Oxygen reacting with oxygen scavengers (e.g., sulfite), bacteria reacting with oxidizing biocides (e.g., chlorine), calcium reacting with scale inhibitors (e.g., phosphate), or oxidizing biocides degrading organic inhibitors (e.g., phosphonate and azole).
Once the DEMAND reactions are met, any excess chemistry left over is the RESIDUAL. A minimum actives residual may be necessary to provide corrosion and scale control. This residual also serves as a buffer to react with new DEMANDS as they enter the system (e.g., increased oxygen levels or a hardness excursion). In some applications, the residual may be used for testing to control the water treatment program (e.g., sulfite, free chlorine, and phosphate testing).
Water systems are dynamic. Their water chemistry and needs can change by the hour, day, and season. Failures in pretreatment equipment, process contamination, fouled probes, malfunctioning feed and control equipment, etc. can also have major impacts upon the dosage of water treatment products required and maintained. Understanding the behavior of a water treatment product helps the user properly respond when residuals go away, are too high, or require adjustment.
For example, what if the free chlorine RESIDUAL in a cooling tower is half as high as it needs to be. The user may be tempted to double the dosage of the sodium hypochlorite feed by doubling the chemical feed pump setting. However, doubling the feed will also double the dosage that was fed to meet the system DEMAND. This could result in a free chlorine RESIDUAL that is MUCH higher than anticipated.
The following table describes possible DEMANDS and the purposes for RESIDUALS for common water treatment product categories.
| PRODUCT | POSSIBLE DEMANDS | RESIDUAL |
| Corrosion Inhibitor | Passivate bare metal surfaces. React with corrosion byproducts in water. Degradation due to reaction with oxidizers.Precipitation with other water components. | Buffer to repair breaks in passivated surfaces.Testing for control. |
| Scale Inhibitor | Reaction with scale-forming minerals (e.g., calcium).Degradation due to reaction with oxidizers. | Buffer to handle upset conditions (e.g., water softener leaking hardness).Testing for control. |
| Oxidizing Biocide | Reaction with unwanted microbiological activity.Offgassing from system.UV degradation.Reaction with other water components (e.g., ammonia).Corrosive reactions with system components. | Buffer to maintain toxic environment for microbes.Testing for control. |
| Non-Oxidizing Biocide | Reaction with unwanted microbiological activity.Degradation over time or by oxidizer. | Buffer to maintain toxic environment.Testing for control, although many non-oxidizing biocides do not have a test available. |
| Oxygen Scavenger | React with Oxygen. React with corrosion byproducts in water. | Buffer to maintain oxygen-free environment.Testing for control. |
| Dechlorination | React with halogens (e.g., chlorine). | Buffer to maintain a halogen-free environment.Testing for control. |
| Dispersant | React with solids (e.g., calcium phosphate, iron, silt) to keep them suspended in the water. Degradation due to reaction with oxidizers. | Buffer to handle upset conditions (e.g., hardness excursion in calcium phosphate programs, iron corrosion byproducts being returned with condensate). |
Successful water treatment requires more than just adding water treatment products to a water system. To respond to the dynamic nature of these systems, it is important to understand how the water treatment products interact with the water, what’s in the water, and what the water contacts. To learn more about water treatment products and their dosages, contact Chem-Aqua today.