Measurement of chlorine dioxide and bromine used simultaneously in the disinfection of cooling system water

October 24, 2018 0 Comments

Palintest was invited to Faltec’s Boldon facility to work alongside Tim Wafer (Consultant, H2O Solutions) and Kevin Morten (Sales Manager, Hydro-X). We were hosted by Senior Health and Safety Manager, Graham Stamp.

Executive Summary

The use of both chlorine dioxide and bromine simultaneously in the treatment of a cooling system has created a challenge in determining the separate concentration of each disinfectant as existing test methods are not specific.

In order to solve this challenge Palintest has proposed and trialled on site, the use of two different technology platforms: A photometer with DPD and a ChlordioX Plus with two types of sensors.

The DPD indicator is normally non-specific but can be used with additional reagents to isolate the disinfectants from each other prior to measurement. However on-site testing discovered a non-characteristic colour development when testing with DPD. It is suspected that this may be because the sample pH is too high for the test. Further work could be carried out to determine and eliminate the effect of high pH on the test.

The second technology using disposable sensors on the ChlordioX Plus product was also trialed. Chlorine dioxide sensors (CDX) are able to determine concentration levels of chlorine dioxide without any additional reagents and without interference from bromine. There is no sensor for bromine however it was suggested to use chlorine sensors (CS) after a degassing stage to remove the chlorine dioxide and apply a mathematical correction to convert the result.

On-site testing seems to support this possibility and work previously carried out by in-house expert, Dr. Birch, allowed us to determine the correct conversion factor.

Based upon the product performance and technical support, Faltec decided to purchase a ChlordioX Plus Kit.

Application Background

Faltec Europe Ltd is a manufacturer that specializes in extrusion and injection molded parts for the automotive industry.

Their factories incorporate a wide variety of injection molding, extruding and bending machinery that require active cooling.

The factories are spread over 3 units each with a dedicated cooling system. This system provides a ‘cooling ring’ around the unit that individual machines can connect to, taking in cool water and returning warm water. These connections are known as drop points and the largest system contains 25,000 L of water feeding over 60 drop points.

The cooling effect is achieved as a result of evaporation of a small amount of water reducing the temperature of the bulk remaining water.

At Faltec they employ a forced drought system which uses a powered fan to force air into the cooling tower. This air moves upwards through the fill pack whilst at the same time the water to be cooled is distributed and ran down through the fill pack. The fill pack is a media typically made out of thin plastic sheets that are closely stacked, causing the water to spread out into thin sheets drastically increasing surface area and thermal efficiency. The opposite flow of air over this large surface area allows exchange of heat from the water and a small amount of water to evaporate, further enhancing the cooling effect. A schematic and photo of this system is shown to the above right and right respectively. Visit Report

Water Treatment

The water in cooling systems can often be conducive to microbial growth due to a number of factors:

 Recirculation and storage of water

 Water at ideal growth temperatures (20-45°C)

 Growth supporting deposits (rust, sludge, scale, organic matter and biofilms)

Because of this it is often necessary to treat water with a disinfectant or biocide to prevent microbial build up that can be hazardous to health. This is typically chlorine at 0.5 – 1 ppm or bromine at 1 – 2 ppm.

Bromine is chosen over chlorine where the system water is run at a higher pH to prevent corrosion due to bromine’s maintained disinfectant power above pH 8. This is typically limited to smaller systems where dosing concentration and pH control are less critical.

In open systems where the cooling water is sprayed onto the fill pack and droplets are formed, the risk of legionellosis must be managed. Legionellosis is the term given to illnesses that are caused by the inhalation of water borne Legionella bacteria (shown above), which include Legionnaires' disease and the milder Pontiac fever.

The use of chlorine of bromine is usually sufficient to manage to microbial risks of cooling tower water, however in instances where there are high organic loadings or if halogen-resistant biofilms have formed in the system, alternative disinfectants may be required.

One of the most popular disinfectants for use in these instances is chlorine dioxide. Some of the unique properties of chlorine dioxide that make it particularly suited to this application are:

 Disinfection power is unaffected by pH

 Effectively penetrates biofilms

 Does not form as many disinfection-by-products and is less affected by higher organic loadings as a result

However it can be more complex and costly to dose chlorine dioxide and it has a high volatility meaning maintaining a residual can be difficult throughout the entirety of the cooling network.

Because of the difficult residual maintenance dosage can often be controlled through microbial monitoring rather than chemical monitoring.

The problem

In order to comprehensively combat biofilm build-up in their system Faltec added chlorine dioxide to their treatment process. In order to maintain a persistent residual throughout their network they decided to keep their existing bromine dosing and operate the chlorine dioxide dosing alongside.

This created the need to be able to test for, and distinguish between both the bromine and chlorine dioxide residuals in the system.

The common indicators used for the detection of bromine and chlorine dioxide are non-specific meaning both species will positively interfere with each test. Therefore a selective testing procedure for each disinfectant is required to obtain an accurate and clear understanding of the system’s water quality.

In order to meet this challenge Palintest visited Faltec’s facility and trialed two proposed testing methodologies: DPD and ChlordioX Plus.

DPD

DPD stands for N,N diethyl-para-phenylenediamine. It is the most commonly used chemical indicator for measuring disinfectant concentrations in water and the method’s creator A. T. Palin was a founder of Palintest’s modern business.

DPD will react with oxidizing species to produce a distinctive pink color. The intensity of this color is proportional to the concentration of oxidizing species in the sample and this can be quantified visually, or for greater accuracy, photometrically.

DPD is non-specific and will react with any oxidising species in a sample, including both bromine and chlorine dioxide at the same time. However by using DPD Glycine and DPD Nitrite it is possible to isolate the bromine and remove the chlorine dioxide from the sample allowing determination of both values with some basic mathematical calculations and corrections for units

Testing onsite with DPD was carried out using samples taken from the dosing control system on a Palintest 7500 Bluetooth Photometer. Addition of the DPD 1 tablet immediately led to the formation of an unusual red color distinctively different from the typical pink color associated with DPD.

Diluting the samples with deionised water did reduce the intensity of the color but did not alter its shade. It was suspected that the sample pH was too high for the test and was overwhelming the buffering capacity of the tablets.

Acidifying the sample prior to addition of the DPD 1 tablet did generate the typical pink colour formation but further work will need to be carried out before any conclusions and testing procedures are finalized.

ChlordioX Plus

The ChlordioX Plus is based upon the disposable chronoamperometric sensor technology that was originally pioneered on the chlorosense product.

The ChlordioX Plus makes use of the ChloroSense sensor and ChlordioXense sensor, both of which avoid the reagent and end-user issues that can be associated with traditional test methods. Although there is no specific sensor for bromine, the tetramethylbenzene (TMB) impregnated on the ChloroSense sensor surface should react with bromine and produce a positive response.

Onsite the ChlordioX Plus initially showed very little free chlorine dioxide in the system followed by a tenfold increase in concentration once the chlorine dioxide pump was manually activated. This indicated positive interference from bromine is very unlikely.

The sample was degassed to remove chlorine dioxide and the use of a ChloroSense sensor showed a positive response. When retested after addition of glycine the results remained the same (ensuring response was from bromine not chlorine).

By using data from a ballast water study in 2014 (in natural water) it was found that the electrochemical response from bromine was not identical to chlorine and thus a multiplication factor of 3 must be applied to the results. This factors accounts for both the differing electrochemical response and the change in units from mg/L Cl2 to mg/L Br2.

Final recommendation

Whilst the DPD methodology is a viable way of determining both disinfectant residuals, the involved procedure and further required work made it a less attractive alternative to the ChlordioX Plus.

The ChlordioX Plus has allowed the accurate determination of both residuals with sensor technology that reduces the interference and end-user errors that are often associated with reagent based methods. As a result of the proven instrument performance and the support and technical guidance from Palintest, Faltec agreed to adopt the ChlordioX Plus for future testing requirements.

Read original Palintest Ltd document here

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