Case Study: Catalytic Chlorine Dioxide Biocide Wash of CHW System

The CHW system at the new headquarters of a prestigious company in London had a resistant Pseudomonas biofilm problem from the build and fit-out of the building. Catalytically generated chlorine dioxide was used for the first time as part of balanced flushing process to eradicate the biofilm and leave a clean system.

The system was a two port valve fancoil system using commissioning modules from the main flow and return legs to run a group of fancoils. This system allows for rapid installation and fast easy commissioning but leaves whole areas of the system isolated from flow when there is no need for cooling.

This coupled with the bypasses in the modules led to a number of issues in achieving sufficient circulation for effective water treatment.

Problems with circulation were seen throughout the early fit-out and a night time circulation exercise to run all plant and fancoils for two hours to 100% overnight was implemented.

The system was built and commissioned in stages, with the base build completed in 2003 and final stages of fit out completed in 2004. During the fit-out pseudomonas levels began to rise despite BSRIA guidelines supposedly being followed as various sections were added to the base build. The consultants for the fit out recommended biocide screening and non-oxidising biocide treatment. This was initially effective but gradually the organism became resistant to the biocides used.

The current building occupiers moved in during 2004 and Clearwater Technology Ltd were employed as water treatment specialists (in consultation with the original consultants). It was concluded that the instances of the organism becoming resistant to non-oxidising biocides in the past were due to poor circulation and dead legs in the system. These dead legs allowed the bacteria to thrive, unaffected by the biocides used.

A plan was developed whereby further biocide screening and applications of high concentrations of non-oxidising biocides (up to 5000ppm) with a tracer chemical would prove biocide circulation. Potassium Sulphate as a tracer in conjunction with a blend of Isothiazolin and Gluteraldehyde biocide was used and initial bacteria results were good. However areas of poor circulation were found and the bacteria levels soon came back up as the organism adapted to the biocide.

The circulation issues were addressed and a further tracer test and non-oxidising biocide was used. This time Lithium Bromide was used as the tracer and a blend of Methylene-bis-thiocyanate and Dodecylguanidine Hydrochloride biocide. Again initial bacteria results were good but pseudomonas came back within weeks. Re-dosing with alternating biocides on a weekly basis gave good results but as soon as dosing stopped, levels increased.

At this stage there were no further non-oxidising biocides available that had not been tried or eliminated in screening. The biofilm within the system was extremely resistant and our efforts so far had only been completely effective against motile bacteria. Also at this point dead biofilm and biocide residue (MBT) were beginning to be deposited in strainers and the decision was taken to flush the system and carry out a biocide wash with Chlorine Dioxide.

Health and safety – traditional chlorine dioxide is created by mixing acid and chlorite onsite resulting in a high concentration product which is prone to gassing.
Corrosion issues - as an oxidising biocide, chlorine dioxide will also oxidise metal, however over acidification and residual chlorite inherent in the acid/chlorite mixing production method is far more corrosive than chlorine dioxide alone.

These two issues were overcome by using Cloxide TM via a catalytic generating system.

The production method utilises an ion exchange resin bed and a catalyst to convert a chlorite solution into chlorine dioxide at 700ppm instantly. The resulting product stream has no residual chlorite and no residual acid; as no acid is used in production.

Chlorine dioxide is a highly effective biocide in closed systems because it is a gas dissolved in water; making it highly mobile within the system. The mobility and penetrative capability of chlorine dioxide is far greater than other oxidising and non-oxidising biocides and allows sterilisation of areas such as valve seats, short dead legs and pump seals without damage to the system fabric. Action on biofilm and pseudomonas is well documented from previous work in open and domestic systems. Biofilm is stripped rapidly and oxidised away leaving little or no debris in strainers.

Prior to any flushing taking place the maintenance subcontractor undertook a full clean of all commissioning module 300 micron strainers and all other system strainers. Some of these strainers were heavily fouled with biofilm and biocide residue and there were concerns over fouling between cleaning and flushing. A week of careful monitoring showed the strainers to be clear enough for flushing.

The CHW system was balanced pre-flushed with boosted mains cold water until TDS was within 10% of mains water. This was to remove as much as possible of the previous biocide additions and motile bacteria to enable the chlorine dioxide to act without excessive organic loading. During the flushing all parts of the system were opened in turn to 100% to ensure a good system water change. All commissioning bypasses and other bypasses were manually opened to allow flow across these areas. Strainers were checked at various points to ensure the flushing process was not releasing any debris that could potentially cause blockages.

A CloxCat R catalytic chlorine dioxide generator was fitted to the system via two Grundfos DME60 digital dosing pumps. These pumps dosed 120 litres per hour of 700-1000ppm chlorine dioxide into the CHW system, balanced with a discharge from the return header of the system. The system was constantly circulated to allow biocide flow to all areas.

After several hours a residual of chlorine dioxide was tested in the plant room area and then monitoring was commenced at the distal points and sentinel plant. Residual levels were found on all floors and at all plant areas of between 0.5 and 1.0ppm. The system was continually dosed and tested for a further four hours and all commissioning bypasses and other bypasses were opened again to allow biocide circulation.

Soluble iron levels were monitored onsite continuously during the chlorine dioxide dosing and were not seen to rise above 0.2ppm. This may be due to rapid oxidation of soluble iron to insoluble iron during the process but following the process soluble and insoluble iron levels have remained very low.

The system was then balanced post-flushed to remove any dead biofilm and entrained debris with chlorine dioxide again being dosed into the flushing water. The final phase of flushing was completed with raw water before dosing of inhibitors and non-oxidising biocides.

Clearwater Systemcare B84 was dosed into the system to provide corrosion control and maintain optimum heat exchange efficiency.

The system was sampled for TVC and Pseudomonas on the final day of flushing and every day for the following week from several points. At the time of writing this article, no pseudomonas re-growth has been observed.

Catalytically generated Cloxide is the safest form of chlorine dioxide generation currently available.

The system used is a regenerating CloxCat R unit from a family of similar units capable of treating any volume of water.
We also offer CloxCat X generation units which require the input of only one chemical. Ion exchange vessels for these units are regenerated off-site.
Chlorine dioxid is generated directly at the point of use - no storage prior to use, no possibility of spillage or gassing.
The system is highly efficient with fewer corrosive or toxic by-products than seen in other systems.
The system produces no hydrogen gas (like electrolytic systems can) - no risk of explosion.