Maximizing cooling tower cycles of concentration offers many benefits that help reduce water consumption and minimize waste generation for large industrial users and power generators. However, increased cycles can also shape a perfect environment for microorganisms to thrive, which can greatly reduce system efficiencies and create health and safety concerns.
Many cooling tower operators use sodium hypochlorite (NaOCl / Industrial Grade Bleach) to control and prevent the growth of microorganisms in their systems. But due to the nature of this halogen product, a circular battle often exists between too little NaOCl treatment, resulting in biofilm formation, or too much NaOCl treatment, which can greatly accelerate corrosion within the system.
Upgrading bleach systems with Aphelion is simple and delivers major operational benefits:
Improves biocidal efficiency of halogens
- Improves halogen biocidal effect by stabilization of chlorine
- Enhances bio-film penetration
- Controlled release of active halogen
Reduces environmental footprint
- Biodegradable in <30 days
- Reduce AOX formation
- Well maintained heat exchanger can reduce energy consumption 10%, reducing cost and C02 emissions
Protects infrastructure by reducing corrosion rates
- Reduces oxidizing halogen dosage while maintaining bio killing efficiency
- Reduction of Bleach leads to less corrosive chloride ions in the cooling water
- Reduces the need for shock chlorination
Lowers your total cost
- Reduces destruction of costly halogen sensitive water treatment chemicals
- Reduces the volume of bio-dispersant
- Reduces energy consumption through increased heat exchanger efficiency
- Decreases replacement costs by lowering corrosion rates
In cooling towers serving large industries and power plants, operators are disinfecting large volumes of water. These systems reuse water, but the volumes are large. Companies treating volumes this high typically lean toward using a relatively inexpensive disinfectant like industrial grade bleach to provide microbial control. It can be effective, but also has its drawbacks.
Bleach, when injected into water, produces hypochlorous acid, which is the key molecule that does the disinfecting. This molecule is very reactive to most everything in the water, such as dirt, silt, leaves, and other material. The resulting hypochlorous acid is consumed before it has a chance to do what you actually want it to do – control biofilm. That’s why adding the halogen stabilizer such as the Aphelion™ Stabilizer should be an important component in cooling water halogen programs, especially in large systems.
Biofilm inhibition after 72 hours growth
Biofilm inhibition on Sphaerotilus Natans after 72 hours
5 ppm AphelionTM Stabilizer: NaOCl
10 ppm NaOCI
5 ppm NaOCI
0 ppm Untreated
Sessile Slime Reduction – Test Conditions
- Mass reduction measured gravimetrically
- Untreated control is used as reference (100%)
- Organism : Sphaerotilus Natans (ATCC 15291)
- Batch test on orbital shaker
- Solution : 5% CGY medium
- Temperature : 22–30°C
*image for illustration purposes
Why is controlling biofilm so important?
In cooling tower water, there are typically planktonic bacteria, which basically float around in the water, and also sessile bacteria, commonly referred to as biofilm. Biofilm grows and attaches to surfaces within the cooling system. When biofilm attaches to vital heat transfer surfaces inside the cooling tower, it impedes the heat transfer process. This decreases the efficiency of the system and, if left unchecked, can lead to system fouling. Another serious problem related to biofilm is Legionella. Legionella grows and harbors in biofilm, presenting a considerable public health and safety concern.
Can’t an operator simply increase the halogen dosing rate to effectively control biofilm?
When you do that you not only increase your bleach costs, you’re also feeding more chlorides into your system, which accelerates corrosion. Besides producing hypochlorous acid, sodium hypochlorite in water also produces chlorides. Most cooling towers have components with stainless steel surfaces. For the most common alloys (304 & 316) the tolerance is less than 1 ppm for temperatures above about 80°C. At 50°C they can withstand about 100 ppm of chlorides.
If you feed too much bleach into a system that’s running at multiple cycles of concentration, you run the risk of the chloride level rising to the point where it can quickly corrode vital parts of the system.
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