Using Silver Stabilised Hydrogen Peroxide Biocides post COVID-19

reducing legionella risk when commissioning water systems post covid-19

With the successful roll out of COVID-19 vaccines lockdown restrictions will be progressively removed. This will mean that many buildings will be reopened, perhaps after many weeks standing empty. This post discusses how silver stabilised hydrogen peroxide biocides can provide important help in this process.

It is likely that buildings that have stood empty perhaps for months, will have an increased risk of bacterial contamination and biofilm formation due to stagnation.

Biofilm, consisting of layers of different micro-organisms such as the slime producing bacterium pseudomonas, provides an ideal environment for the growth of legionella.

It is not an option to merely reopen the building and hope for the best.

The UK Health & Safety Executive has published guidance on the risks from Legionella during the coronavirus pandemic, as detailed here.

If a building has been closed or has had reduced occupancy, water stagnation can occur due to lack of use, increasing the risks of Legionnaires disease on re-opening.

The HSE Guidance applies to:

  • Hot & Cold Water Systems
  • Cooling Towers & Evaporative Condensers
  • Air Conditioning Units
  • Commercial Spa Pools & Hot Tubs

The Guidance also references:

  • Appropriate Methods of Control of Legionella
  • Personal protective equipment (PPE) required for cleaning water systems, including safety googles & chemical safety gloves

It is very important to remember that the Health & Safety at Work Act still applies and a buildings Duty Holder must ensure that all reasonable precautions are taken to mitigate the risks associated with the possible contamination of the building hot & cold water systems.

The Legionella Control Association also provides detailed guidance on the reopening of buildings here.

Key Recommendations for hot & cold water system recommissioning

Prior to the reopening of buildings and the recommissioning of hot & cold water systems a legionella risk assessment should be carried out. This is particularly relevant to the Duty Holders of large buildings where the risk of legionella contamination is higher.

The risk assessment must take into consideration the health & safety of personnel carrying out the work of systems recommissioning, especially plans to limit the production of aerosols.

All infrequently used hot & cold water outlets should be flushed weekly to prevent stagnation. If this is not possible then consideration must be given to having the water system cleaned (if necessary) and disinfected.

For smaller buildings with simple hot & cold water systems, flushing with fresh water would be required as a minimum.

For larger buildings with more complex hot & cold water systems, incorporating cold water storage tanks, calorifiers and complex pipe-work leading to multiple outlets including showers, then flushing followed by cleaning and disinfection will be required.

The British Standards Institution guide to the flushing and disinfection of services supplying water for domestic use within building PD 855468:2015, is available here.

Cleaning & disinfection

If cleaning is required, then the system must be isolated and appropriate chemical and mechanical means should be employed to remove the contamination.

All personnel undertaking cleaning and disinfection should be trained and competent (this is an LCA Requirement).

The Cleaning & Disinfection Method Statement and Safety Data Sheets should be read and understood, and appropriate Personal Protection Equipment (PPE) should be available for use, including safety goggles, and chemical safety gloves.

All steps and measurements made during the cleaning and disinfection procedure should be recorded in a cleaning and disinfection log and report sheet.

Methods of Control identified by HSE

The key methods of controlling the proliferation of legionella bacteria in hot & cold water systems identified by HSE are temperature control and the use of appropriate biocides.

Temperature control

Temperature regulation in hot & cold water systems is essential for the control of legionella.

Legionella growth in hot and cold water systems is inhibited if the water temperature is either held below 20oC or above 50oC (55oC in health care establishments).

90% of legionella bacteria will die in two minutes in water at a temperature of 60C. Water temperatures in the range 20oC to 45oC allow legionella bacteria to grow rapidly, so every effort should be made to ensure that water temperatures are maintained outside this danger zone.

The Silver Stabilised Hydrogen Peroxide biocides, Geosil50, Sanosil, Endosan, Huwa-San

HSE also warrants the use of biocides such as chlorine dioxide and copper/silver systems.

The silver systems identified by HSE are products such as Geosil50, Endosan, Sanosil & Huwa-San – the silver stabilised hydrogen peroxide biocides.

All these products are a synergistic blend of hydrogen peroxide and trace amounts of ionic silver, with other stabilising compounds, to provide a fast acting, long lasting, stable and easy to handle biocide.

Using Silver stabilised hydrogen peroxides, such as Geosil50, has proven effective against legionella which along with their long shelf life makes them the ideal biocide in a wide range of application areas.

Silver stabilised hydrogen peroxide biocides are now very commonly used in tank cleaning & disinfection processes and in continuous dosing of hot & cold water systems (albeit at much lower dose rates).

Broad spectrum biocides such as Geosil50, Sanosil, Huwa-San & Endosan, offer significant advantages over more traditional disinfectants such as sodium hypochlorite since they do not require the addition of neutralising chemicals.

But not all silver hydrogen peroxide biocides are created equal.

Compare the Instructions for Use for Geosil50 with the other silver stabilised hydrogen peroxide biocides:

The Geosil50 Instructions for Use are supported by a UKAS Lab report, to BS EN13623, that shows:

A 4.96 log reduction in the Legionella Viable Count at 1000ppm in just 60 minutes. The Statutory Requirement is a > 4 log Reduction in the Legionella Viable Count.

This is a much better performance than Sanosil, Huwa-San and Endosan viz:

  • Endosan – 400ppm for 12 Hours
  • Sanosil – 1000ppm for 4 Hours
  • HuwaSan – 2000ppm for 60 minutes to achieve 4.8 Log Reduction

Sources of Competitive Information:-

  • Endosan – Website Calculator
  • Sanosil – 1000ppm for 4 Hours
  • HuwaSan – Page 22, December edition Waterline Magazine


  • Huwa-San is effectively twice as expensive as Geosil50 & less effective (4.8 log versus 4.96 for Geosil50
  • Sanosil means 3 hours longer spent on site, and offers no information on Efficacy against Legionella.
  • The Endosan Instructions for Use (12 hours contact time) effectively rule it out as a suitable product for any service company offering disinfection services.

Most importantly, you will be able to include the Geosil50 Instructions for Use in your LCA Statement of Compliance which will then be auditable under your LCA Internal Audit.Geosil50 performs best of all the silver stabilised hydrogen peroxide biocides, and so will save you time and money.

Major bonus: Geosil50 is manufactured here in the UK, so no BREXIT related price increases or supply disruption at Dover.

And Geosil50 is the still the only silver hydrogen peroxide biocide to be Authorised by the HSE under UK or EU Biocidal Products Regulation (GB BPR) – Certificate Here.

For more information click here