Controlling the growth of microorganisms, particularly bacteria, is critical in various water treatment applications due to the array of problems they can cause. Uncontrolled bacterial growth in building water systems can lead to the development of biofilm and the growth of potentially pathogenic bacteria. Of particular concern is the potential growth of Legionella bacteria and Pseudomonas aeruginosa (in augmented care units).
Water utility companies, in the main, use chlorine as a primary disinfectant to treat mains water ensuring that total bacterial numbers are low and potentially harmful bacteria (indicator organisms) are absent. When water enters the pipework within a building the potential for contamination and proliferation of bacteria is greatly increased. Contamination can enter the water from contaminated cold water storage tanks and pipework. An increase in bacterial numbers can occur due to stagnation and incorrect temperatures in parts of the system. Conducting a comprehensive risk assessment is imperative to identify potential risk factors within a building's water system and recommend necessary corrective actions to minimize these risks. Additionally, implementing a sampling program may be advisable to monitor water quality and ensure continued safety.
If despite implementing all necessary corrective measures, microbiological levels remain a concern, continuous dosing or disinfection of the water system may be recommended. While there are various physical methods for secondary disinfection, such as silver copper ionization and UV treatment, this article focuses solely on chemical dosing.
In the UK, the main chemicals used for continuous dosing of water systems in buildings are Chlorine dioxide, Hypochlorous acid, and silver stabilized hydrogen peroxide. Monochloramine is also utilized in Europe and may have the potential for more extensive use in the UK. Each chemical offers its own set of advantages, and it is advisable to consult with an experienced water treatment company to determine the most appropriate chemical and dosing system for a specific site. Their expertise ensures the selection of the most effective solution tailored to the unique requirements of the facility.
Chlorine dioxide
Chlorine dioxide stands out as a powerful oxidizing biocide renowned for its broad-spectrum effectiveness. Its versatility renders it indispensable across a range of applications, from disinfection and sanitation to water treatment. In comparison to chlorine, chlorine dioxide boasts more than 2.5 times the oxidizing capability and achieves its desired effects in shorter contact times. A significant advantage of chlorine dioxide lies in its efficacy even in alkaline conditions (pH <10), unlike other biocides such as sodium hypochlorite, which lose potency as pH levels increase and are ineffective above pH 8. Furthermore, chlorine dioxide can penetrate biofilms, which are notorious for harboring and shielding microbial populations. This unique capability enhances its effectiveness in combating stubborn microbial contamination.
Another benefit of chlorine dioxide is its minimal reactivity with other water treatment chemicals and nitrogen compounds, including ammonia and naturally occurring organic materials like organic acids.
Due to its effectiveness, chlorine dioxide has garnered widespread approval for numerous applications, including the treatment of potable water, hospital water systems, and sanitation processes in the food industry. Its versatility, combined with its ability to address various microbial challenges, makes chlorine dioxide an ideal choice for many water treatment applications.
Other things that need to be considered when contemplating the installation of chlorine dioxide dosing are the high cost of a properly constructed ‘fail-safe’ chlorine dioxide unit, the temperature stability of chlorine dioxide in hot water systems, and the potential health and safety risk of accidental mixing of the two chemicals used to generate chlorine dioxide. Generators are available that generate chlorine dioxide electrolytically using only one chemical in order to alleviate this potential health and safety consideration. As with any ClO2 dosing system, the residence time of treated water in a drinking water system needs to be minimal to avoid build-up of oxidants that will breach the DWI limits of <0.5ppm total oxidants (as measured by CLO2 and chlorite).
Ultralox 40 and Genox Hypochlorous Acid (HOCl)
Ultralox 40 is a stable active halogen solution produced as a 4000ppm solution of Hypochlorous acid. Ultralox 40 is Approved under Regulation 31 of the Water Supply (Water Quality) Regulations 2016 Approval number 56/4/1530 for microbial control of public and private water supplies. Ultralox 40 is typically dosed to give a residual free halogen level of 0.25 – 0.5 ppm Cl2 and restricted to an application rate of 1.00 ppm residual Cl2 in drinking water.
Ultralox 40 is particularly suited to low – medium water usage systems < 20m3 /day up to 60m3 /day. The cost of the straightforward dosing system, installation, and on-going maintenance are generally low when compared with the installation of a chlorine dioxide generator. Only one chemical is required which eliminates the health and safety considerations of potentially mixing two incompatible chemicals when chlorine dioxide is used. Ultralox 40 is used extensively to continuously dose domestic hot and cold-water systems and it effectively breaks down and inhibits biofilm at recommended dose rates.
Given Ultralox 40's suitability for continuous dosing in small to medium water usage systems and its cost-effectiveness in installation and operation, we plan to delve deeper into this topic with a subsequent blog post dedicated specifically to Ultralox 40. Stay tuned for more insights and information on how Ultralox 40 can optimize water treatment in various settings.
Genox when water usage is above 60m3 /day we would generally advise customers to consider the installation of either a chlorine dioxide unit or a Genox unit. A Genox unit generates Hypochlorous acid from the electrolysis of Brine on demand. The running costs of a Genox unit are extremely low i.e. the treatment cost can be calculated to be as low as 4.0 pence/m3. When using a Genox unit no hazardous chemicals are mixed, stored, or handled, the only chemical used is salt. Genox systems are available in four sizes, depending on the application and water usage.
Silver stabilized Hydrogen peroxide.
The active substance, Hydrogen Peroxide (H₂O₂) is stabilized with very small amounts of ionic silver using a proprietary stabilization technique. This ionic silver stabilization system allows hydrogen peroxide to remain residual for longer in a water system.
The product we supply (Huwa-San / Supersil) is legally compliant and is permitted to be sold in the UK for application classes PT02, 03, 04, and 05. Huwa-San TR50 is also listed as an approved constant dose biocide in the HSE ACOP HSG274 Pt2. When using a silver-stabilized hydrogen peroxide product in drinking water it is essential to ensure that the product is permitted for use under Pt5 of the Biocidal products regulations.
As with Ultralox 40 silver peroxide dosing is particularly suited to low – medium water usage systems < 20m3 /day up to 60m3 /day. The dosing system is also a straightforward cost-effective option although the concentrated silver peroxide solution does present more potential hazards in use than the concentrated Ultralox 40 solution.
Chemical Continuous Dosing Solutions: A Comparative Analysis
Secondary disinfection is not a magic cure-all. Chemical levels in the system should be routinely monitored, dosing equipment requires routine maintenance and little-used outlets still need flushing regularly.
