In water treatment, concentration refers to the increase in the amount of dissolved or suspended solid impurities in a body of water, particularly within recirculating systems where water evaporates.
Understanding Concentration in Evaporative Systems
The concept of concentration is crucial in systems like cooling towers and boilers. These are examples of evaporative systems where water is used and part of it turns into vapor (evaporates). Pure water (H₂O) evaporates, but the impurities (minerals, salts, dissolved solids) contained in the original makeup water (the water added to the system) are left behind.
As more and more water evaporates, the remaining water contains a higher and higher amount of these accumulated impurities. This process leads to an increase in the concentration of these solids in the recirculating water.
Cycles of Concentration
A key metric used to quantify this phenomenon is "Cycles of Concentration". According to the provided reference, "Cycles of concentration" is one of the most important concepts in boiler and cooling water treatment. It measures the degree to which the solid impurities in the makeup water are concentrated in the recirculating water of an evaporative system.
Simply put, it's the ratio of the concentration of a specific substance (like total dissolved solids or chloride) in the recirculating water compared to its concentration in the makeup water.
- Cycles of Concentration = (Concentration in Recirculating Water) / (Concentration in Makeup Water)
For example, if the recirculating water in a cooling tower has 500 ppm (parts per million) of total dissolved solids and the makeup water has 100 ppm, the cycles of concentration would be 500 ppm / 100 ppm = 5 cycles. This means the solids are five times more concentrated in the system water than in the incoming water.
Why is Controlling Concentration Important?
Uncontrolled concentration of impurities can lead to significant problems in water treatment systems:
- Scaling: As minerals become highly concentrated, they can exceed their solubility limits and precipitate out of the water, forming hard deposits (scale) on heat exchange surfaces, pipes, and equipment. This reduces efficiency and can cause blockages.
- Corrosion: High concentrations of certain ions (like chlorides and sulfates) can increase the corrosivity of the water, damaging metal components.
- Fouling: Suspended solids and biological growth can become concentrated, leading to blockages and reduced flow.
- Reduced Efficiency: Scale and fouling reduce the effectiveness of heat transfer (in boilers and cooling towers) and require more energy to operate the system.
Managing Concentration
Effective water treatment strategies aim to manage and control the cycles of concentration to prevent these issues. Common methods include:
- Blowdown: Periodically or continuously draining a portion of the high-concentration recirculating water and replacing it with fresh, low-concentration makeup water. This removes accumulated solids from the system.
- Chemical Treatment: Using chemicals to:
- Inhibit scale formation (scale inhibitors).
- Disperse suspended solids (dispersants).
- Control corrosion (corrosion inhibitors).
- Control biological growth (biocides).
- Pre-treatment: Treating the makeup water before it enters the system to remove some impurities (e.g., softening, filtration, reverse osmosis). This reduces the initial concentration of solids.
Controlling concentration within acceptable limits is a fundamental aspect of maintaining system health, efficiency, and longevity in many industrial and commercial water treatment applications.
