Why Reliable Dechlorination is Even More Important in Pharmaceutical Applications
A typical reverse osmosis membrane is rated with a 500 to 1,000 PPM-hour chlorine tolerance but irreversible degradation progresses linearly as the membrane is exposed to oxidizing agents.
In validated systems, RO permeate quality can drop below acceptable levels very quickly as a membrane starts to break down through oxidation. While the final purified water may still be usable, this unexplained degradation in RO permeate quality will trigger a lengthy and costly root cause analysis, corrective action and revalidation process. This makes the reliability of the dechlorination step particularly important in pharmaceutical purified water systems.
Summary of Dechlorination Technologies
By far the most frequently used dechlorination technologies are activated carbon and sodium metabisulfite. More recently, some manufacturers have started to use ultraviolet irradiation for dechlorination. All three options have their merits, so let’s quickly review how they work.
Activated Carbon
Activated carbon dechlorinates water essentially by using up the chlorine’s oxidation potential. Initially the oxidation reactions occur on the activated carbon’s surface as water flows through the carbon media. Left alone, carbon beds actually get more effective at removing chlorine over time because they also trap organics and become a breeding ground for bacteria, all of which reduce chlorine on contact.
Granular activated carbon used for dechlorination
In the pharmaceutical industry it is fairly common to see carbon filters designed to be steam sterilizable, in an attempt to reduce the bacterial load sent downstream to the RO system. This choice has a major impact on the capital cost of activated carbon filtration as a pre-treatment step. The use of ultraviolet disinfection (not to be confused with UV dechlorination) as a last step in pre-treatment can also be used to slow the rate of biological fouling on the RO membranes.
Standard activated carbon is not particularly effective at removing chloramines, mostly since chloramines have a much lower oxidation potential than chlorine. Specialized activated carbons known as catalytic carbons offer much better chloramine removal. Activated carbon used for chloramine removal will need to be replaced more frequently than carbon that is simply removing chlorine.
Sodium Metabisulfite (SMBS)
Sodium metabisulfite is a common reducing agent, often used as an oxygen scavenger. In reverse osmosis pre-treatment, it is regularly used to directly reduce free chlorine and chloramines through the following chemical reactions:
For Free Chlorine: Na2S2O5 + 3H2O + 2Cl2 –> NaHSO4 + 2HCl
For Chloramines: Na2S2O5 + 9H2O + 2NH3 +6Cl2 –> 6NaHSO4 + 10HCl + 2NH4Cl
Sodium metabisulfite is a common reducing agent
While the reaction of SMBS with chlorine and chloramines is nearly instantaneous, care must be taken to ensure that the dosage is sufficient for complete removal. This can be a challenge when feed water chlorine concentrations are variable as in many municipalities. SMBS also acts as an oxygen scavenger, creating an anaerobic environment and accelerating biological fouling of RO membranes when dosage is too high. Consequently, RO cleaning frequency is typically increased when using SMBS in comparison to carbon filtration. Membrane life tends to be lower as well due to the combined effect of free chlorine “leakage” and more frequent cleanings.
Note: other reducing agents such as sodium thiosulfate, hydrogen peroxide and ascorbic acid are sometimes used but SMBS is by far the most common due to its low cost.
Ultraviolet Irradiation
A lesser-known alternative for chlorine and chloramine removal, Ultraviolet Irradiation (UV) is now being used successfully in pharmaceutical applications. Requiring a dose about 40 times larger than the standard disinfection dose, chlorine and chloramine removal by UV will also effectively disinfect water, reducing the potential for biological fouling of the RO membranes. Of course, UV is a chemical-free alternative and the maintenance of UV systems is typically quite simple, making it an interesting option in many cases.
If UV is being used as the only dechlorination method, great care must be taken both in sizing and maintaining the UV system. The sizing of the unit must be large enough to ensure complete dechlorination at peak chlorine concentrations and measures must be taken to protect the RO membranes in case of UV system failure. In many cases it is worth actively monitoring RO feed chlorine levels with on-line or at-line instrumentation.
It is also worth noting that at the UV doses required for dechlorination, significant heat will be generated and must be considered in the overall system design.
Selecting the Right Option for Your Application
The table below summarizes the trade-offs to consider when selecting a dechlorination technology for a pharmaceutical purified water system. We recommend printing this table as a PDF to fill out. For each question, circle the “yes” or “no” responses in each of the four columns to the right, then add up the total number of circled responses for each column. The technology with the most circled responses is likely to be the best fit for your application. Of course, we recommend that you consult with trustworthy experts before making your final decision.
| Activated Carbon (FRP) | Sterilizable Activated Carbon | SMBS | UV | |
| Does the feed total chlorine concentration vary by more than 1mg/L over the course of a year? | Yes | Yes | No | Yes |
| Is a quarterly 8-hour shutdown for chemical sanitization and cleaning acceptable? | Yes | No | Yes | No |
| Will purified water regularly be circulated back to the system feed? | Yes | Yes | No | No |
| Does your company have a policy requiring sanitary design for all system components? | No | Yes | No | Yes |
| Is low capital cost a priority (provided final water quality is not impacted)? | Yes | No | Yes | No |
| Is floor space very limited? | No | No | Yes | Yes |
| Answer this question only if chloramines are present in the feed water. Is a shutdown for carbon media replacement every 6 months acceptable? | Yes | Yes | No | No |
| Is clean power available? | No | No | No | Yes |