Non-Oxidising Biocides vs Bio-Dispersants: Which Is Better for Cooling Tower Water Treatment?

Maintaining clean, safe water in cooling towers is crucial for preventing the growth of harmful bacteria, including Legionella. Biocides and bio-dispersants both play important roles in this process, but there is growing interest in whether the application of non-oxidising biocides might offer a more effective solution than traditional bio-dispersants alone. By combining oxidising and non-oxidising biocides, many experts suggest that water systems can be treated more thoroughly, targeting both bacterial load and the biofilms that protect microorganisms.

This article explores the differences between oxidising and non-oxidising biocides, the role of bio-dispersants, and how these treatments interact in cooling tower environments. We’ll also examine current guidance from the Health and Safety Executive (HSG274 and ACOP L8), and the importance of correct dosing and timing when applying these treatments. Ultimately, we aim to clarify whether a shift towards non-oxidising biocides can improve water safety and system efficiency, or whether bio-dispersants are still necessary for cooling tower water treatment.

Cooling tower biocides and bio-dispersants

Biocides are regularly used to help clean and maintain water quality in cooling towers, evaporative condensers, and various other water systems. They help keep these water systems safe and free from harmful bacteria and other microorganisms that could potentially colonise it.

Cooling towers are often treated with bio-dispersants alongside oxidising biocides. However, there is an argument for replacing the bio-dispersants with non-oxidising biocides. This would mean the water treatment programme would involve both oxidising and non-oxidising biocides working hand in hand.

What are biocides?

Biocides are special chemical compounds used in the treatment of water to eliminate or reduce bacterial load. In cooling towers the control of Legionella bacteria is essentential. Examples of biocides includes compounds of chlorine and bromine, hypochlorous acid, chlorine dioxide and hydrogen peroxide.

What do biocides do in the treatment of water?

Biocides will work to kill harmful bacteria in a water supply. However, to do this, they need to be added in sufficient quantities for the specific water supply in question. They must also be present for long enough to complete the job, sometimes referred to as “kill time”.

There are two types of biocides which can both play an important role in this task:

• Oxidising biocides
• Non-oxidising biocides

Oxidising biocides are usually faster acting and don’t need to be dosed at as high a level as the non-oxidising version. However, both can combine to provide an effective treatment for cooling towers and other water systems.

What is a bio-dispersant?

Bio-dispersants are special chemical compounds used to treat water systems. They disrupt and disperse bacteria and biofilms which are slimy layers of microorganisms (bacteria, algae, fungi) that adhere to surfaces such as pipes, cooling towers, or heat exchangers.

What do bio-dispersants do?

With bio-dispersants the clue is in the name. They do not kill harmful bacteria or microorganisms in the water supply. Instead, they work as surfactants. This means they allow other water treatment chemicals – such as biocides – to more easily penetrate biofilms in the water system. They also help them reach and treat deposits more easily. The biofilm and deposits are then more readily dispersed in the water, allowing the biocides to act more effectively in destroying them.

Some non-oxidising biocides have a bio-dispersant included, which can help ensure the best approach to treating the water within a cooling tower. To this end, the combination of an oxidising biocide with a non-oxidising biocide can, when carefully chosen, work well in maintaining the safe operation of the tower.

The Health and Safety Executives ACOP L8 and supporting guidance HSG274 provides extensive advice and guidance on reducing risks associated with Legionella bacteria. It comes in three parts, covering evaporative cooling systems, hot and cold water systems, and other systems, respectively.

Correct dosing and timing are essential

It’s not enough to simply combine these two oxidising and non-oxidising treatments to ensure an ACOP L8 safe, operational cooling tower. It is essential to make sure the correct dose of each chemical product is added at the right time. It would be easy to assume the biocides are working as they should be, and yet there could be too low a dose in the system for too short a time to make a significant difference. This raises the risk of bacteria, biofilms and debris remaining in the water supply and potentially raising the risks from Legionella and other dangerous waterborne pathogens being present as well.

By enlisting the services of a competent water treatment specialist who is qualified to determine the dosage rates of non-oxidising and oxidising biocides to add to the cooling tower or water system, you are ensuring the water systems operate safely and efficiently. Bio dispersants do have a role to play, as we’ve seen, and they can be combined within a non-oxidising biocide treatment. This ensures that biofilms can be dispersed and removed from the system, thereby reducing the risk of waterborne bacteria colonising the cooling tower. Get in touch for further information regarding such treatments and how we can help ensure your cooling tower is always safe for use.

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