Temperature might be the most commonly used and arguably most effective means of Legionella management, but that might not always be the case. There are scenarios where it is impractical or potentially problematic to rely on temperature control methods and alternative methods such as Biocides might be considered. Chlorine Dioxide is one of the approved biocides according to HSG 274 part 2.
What is Chlorine Dioxide?
Chlorine dioxide is a chemical compound and is one of many oxides of chlorine, given that it’s a potent oxidising agent, it’s often used as a means of water treatment.
In its raw state, it’s a yellow-green gas and is highly volatile and explosive at concentrations over 10% in the air. This is a double-edged sword for the compound as it can’t be stored in any large quantity and requires generation at the point of use.
Chlorine dioxide is usually generated by reducing sodium chlorite or sodium chlorate using a strong acid to facilitate the reaction.
Use as a Biocide
Chlorine dioxide is a very versatile and effective biocide as it reacts with a wide range of organic material. Even at low levels within water, it can still deactivate bacteria.
In most circumstances, chlorine dioxide is a better disinfectant than chlorine, particularly against waterborne bacteria, viruses and protozoa, including:
- Legionella
- Pseudomonas
- E-coli
- Cryptosporidium
- MRSA
The active dose required will vary depending on the conditions within the system although before introducing it into the water system, a thorough clean should be carried out to remove as much contamination as possible. Generally cleaner systems will require a lower initial dosage whereas more contaminated systems will require a higher concentration.
A good average to try and achieve is 0.1 – 0.5 mg/l at every outlet, this will mean changing the dosing level to suit the size and complexity of the system.
This approach has the added benefits of keeping consistent protection without being compromised by pH levels, water hardness or temperature.
Delivery Method
The most common way of including chlorine dioxide in a system is to install an automatic dosing system. The dosing system will usually have two dosing pumps controlled by a water meter or a conductivity probe that will regulate the level of the chemical within the water. When called for the pumps will dose in unison to deliver chemical into an activation chamber which is proportionally dosed into the main water system.
These systems require regular checks and maintenance to ensure safe and effective operations. Regular checks should cover:
- Weekly – check the system operation and chemical stocks in the reservoir.
- Monthly – test the treated water for both chlorine dioxide and total oxidant/chlorite at an outlet close to the point of injection to verify the dosage rate and conversion yield.
- Monthly – measure the concentration of chlorine dioxide at the sentinel taps – the concentration should be at least 0.1 mg/l and adjust the chlorine dioxide dosage to establish the required residual at the sentinel sample points.
- Annually – test the chlorine dioxide and total oxidant/chlorite concentration at a representative selection of outlets throughout the distribution system – the concentration should be at least 0.1 mg/l chlorine dioxide.
Summary of advantages of Chlorine Dioxide over Chlorine
- Higher yield & greater cost efficiencies – Chlorine Dioxide has a higher oxidation capacity, and a lower oxidation strength than most species of chlorine, making it at least 2.6 times more powerful per ppm according to WHO CT values.
- No carcinogenic by-products & no bad taste occurrences in water – Chlorine Dioxide acts only by oxidation and does not combine with organic compounds to form environmentally hazardous by-products such as Trihalomethane and other chlorinated organic compounds that have been listed as potentially carcinogenic.
- Less corrosive – Chlorine Dioxide has a lower oxidation potential and does not hydrolyse to form an acid, and therefore is less corrosive.
- Works over a wide pH – The effectiveness of chlorine is very pH dependent, and is almost ineffective above pH8. Chlorine Dioxide is effective at all pH’s below 12
- Effective against complex organisms – Chlorine Dioxide has been found to be effective against complex organisms such as cysts and protozoa including Cryptosporidium, Giardia and amoeba. Chlorine is not.
- Destroys biofilm completely at source – by penetrating polysaccharide layers to destroy pseudomonas and other base bacteria within the biofilm. Chlorine does not.
While chlorine dioxide may not always be the best option for controlling Legionella, it does have its uses and situations that play to its strengths, as such careful consideration is required before including it in a control programme.
