Many manufacturing processes require ultrapure water to ensure a sterile environment and remove very small traces of impurity from products. Continuous electro-deionisation (CEDI or EDI) units are routinely installed in manufacturing plants to produce this ultrapure deionised water. These units remove common contaminants, such as silica, carbon dioxide, bacteria, pyrogens (bacterial fragments) and metal ions, which are often found in the feed water supplied to manufacturing plants.
Many parts of the world add chlorine to drinking water as part of their disinfection process and this can prove extremely damaging to CEDI units in ultrapure water loops. Resins utilised in CEDI units have very low tolerance to chlorine in water and if chlorine enters a CEDI module it can be very quickly oxidised. As the cost to rebuild these units is extremely high, due to the rapid effects of chlorine oxidation, it has become a requirement for CEDI-based ultrapure water systems to incorporate a chlorine removal stage before the water reaches the CEDI modules as well as prior to the reverse osmosis system.
There are currently a number of ways to ensure the dechlorination of water in ultrapure water loops and the optimal method remains a topic of much debate.
Dechlorination
Chlorine removal from ultrapure water often takes place by adding a solution of sodium bisulphite or sodium sulphite into the water flow. This reacts with chlorine residuals and chlorine removal is guaranteed as the resulting sulphite ion is a strong reducing agent.
A typical method for monitoring chlorine removal is the use of a residual chlorine monitor, which acts as a final line of defence before the water flow reaches the CEDI unit. However this method has a number of limitations. Perhaps most notably, chlorine monitors do not perform well when left to run at 0.000 parts per million for extended periods and are often slow to respond when chlorine does enter the module. Chlorine monitors can also be misleading as a failed sensor will often read zero and therefore it is impossible to differentiate between instrument failure and zero chlorine.
In addition, most plants in need of dechlorination currently run relatively high sulphite residuals to ensure complete chlorine removal at all times. Many dosing systems have a fixed sulphite dose or a simple flow proportional dose. However this can result in excessive chemical consumption which can prove extremely costly. Another challenge of traditional chlorine monitors is that they do not indicate how much bisulphite is present in excess and therefore can lead to more chemicals than necessary being consumed in the water purification process.
Other methods of protecting continuous deionisation units in ultrapure water loops include redox potential (ORP). ORP measurement is used to indicate the presence of oxidising or reducing agents such as chlorine and bisulphite in water. This is important as deionisation resins present in CEDI units can be damaged by oxidation. This method warns if the bisulphite solution is not properly removing chlorine as the presence of oxidising agents such as chlorine in the water flow raises the ORP level (positive values), whilst the existence of reducing agents such as bisulphite cause a drop in ORP (negative values).
This method is simple but suffers from limitations in sensitivity, linearity and selectivity. ORP measurement detects the change caused by moving from extremely small parts per billion (ppb) concentration. However, the reliability of this system is questionable as there are other variables which could affect the results, such as pH, temperature and other oxidising or reducing agents that may be present in the water. Large concentration changes of sulphite are not always reflected in the same size change of ORP signal.
New technologies, such as sulphite monitoring systems, are currently being developed with the aim of eliminating many of the challenges associated with traditional approaches. Sulphite monitors are made up of three separate components: a chemistry module where the sample is pH adjusted for measurement, an inlet overflow assembly where raw sample is delivered to the system, and an electronic readout containing the sulphite concentration display, analogue output and alarm contacts.
Recent developments mean it is now possible, with the use of sulphite monitors, to continuously measure excess bisulphite over ranges of as low as 0-200ppb up to parts per million (ppm), keeping chemical consumption to a minimum. This is beneficial as controlling bisulphite excess significantly lowers operating costs. Early sulphite monitors used a direct measuring membraned sensor. These systems were beset with fouling problem caused by the presence of sulphur reducing bacteria.
Some innovative sulphite monitoring systems take a unique approach to the measurement of sulphite ion concentration.
First a small amount of sample water is mixed with acid, converting the sulphite ion into sulfur dioxide. The mixed sample then flows into a chamber where the sulphur dioxide is removed and a sensor located in the gas stream measures the released sulphur dioxide concentration, displaying the results in terms of equivalent sulphite ion concentration.
The main advantage of this process is that the sensor never comes into contact with the water sample. As a result, the system is able to continue functioning regardless of the quality of the water or the presence of sulphur reducing bacteria that can thrive in water containing excess sulphite.
Sulphite monitoring systems are more complex than ORP systems and similar in complexity to chlorine monitors. These systems also require regular maintenance. For example, peristaltic pumps needed for sample and acid delivery require around ten minutes of maintenance to replace both tubes once every six months. In addition, the sulphite gas sensor requires occasional visual inspection to ensure no deposits have collected on the sensing membrane. This type of system also requires reagents and acid usage for pH adjustment in the chemistry module is around four litres per month.
Monitoring sulphites in solution using the gas stripping technique provides extremely high measurement sensitivity, and good accuracy at the cost of more maintenance and consumables. Calibration of sulphite monitors is similar to calibration of ORP and chlorine systems. A standard sulphite solution is prepared and introduced to the monitor, which is then adjusted. A typical calibration period is a monthly check of zero and span.
Conclusion
To properly protect CEDI modules and to greatly reduce the risk of irreversible damage, the need for a continual supply of deionised water in many manufacturing processes, the dechlorination of water before it reaches these electrodeionization system is essential.
Methods of monitoring dechlorination, such as the use of chlorine monitors and ORP monitors, do not match the sensitivity, selectivity and overall accuracy of newly developed sulphite monitors. Sulphite monitors are slightly more complex and require more maintenance than ORP monitors and about the same as chlorine monitors; they are however an overall more cost-effective way of ensuring water is chemically safe to enter CEDI units. The fact they are able to maintain an appropriate, safe but not too excessive residual sulphite concentration, a problem associated with chlorine monitors, also reduces chemical consumption and keeps expense to a minimum.
The disadvantages of higher running costs and higher maintenance (compared to ORP) are offset by the advantages of direct, specific, sulphite only measurement.
Using this emerging technique, water samples can be kept down to low ppb levels of excess sulphite guaranteeing that no chlorine is present and keeping sulphite costs to a minimum.
Due to this method's advantages over traditional techniques, sulphite monitoring is increasingly becoming the dechlorination method of choice among manufacturers utilising ultrapure water. Its flexibility and continuous development mean that sulphite monitoring can deal with challenging dechlorination processes in a range of applications.
Dr Michael Strahand is General Manager Europe, Analytical Technology, Mossley, Ashton-U-Lyne, UK. www.analyticaltechnology.com