What Works Best For Pretreatment Biocontrol?
Compiled by Mike Henley
(Editor’s note: This column is based on recent discussions in the LinkedIn Ultrapure Water Group. This column seeks to accurately reflect comments from each contributor. On occasion, there may be the need to edit contributor comments for clarity or length. Readers are invited to join the Ultrapure Water Group and to participate in discussions. An important purpose of the group is to provide a forum for practical examination of issues facing endusers of high-purity water.)
Mike: “For controlling microbials in pretreatment, what do you find works best? Chlorine? Chlorine dioxide? Bromine? Ozone? One of these combined with say MF or UF? Something else?”
Shlomo: “There is a simple combination of hydrolysis for scale reduction that both serves to stop precipitation in RO and also generates free chlorine. Combined with a very powerful UV for dechlorination, we have found zero bacteria all the time.”
Nissan: “Sanitization by definition is a 3-log reduction of microbials. Which sanitization method is more robust at 0.1 ppm concentration ClO2, NH2Cl, HOCl, NaClO, Ca(ClO)2, or O3?
Sodium hypochlorite and calcium hypochlorite are the least effective and need more than 500 hours of contact to perform sanitization at 0.1 ppm. Hypochlorous acid is slightly better but still ineffectual as it would take over 400 plus hours to sanitize at 0.1 ppm concentration. Chlorine dioxide will be slightly better at 400 plus hours to sanitize at 0.1 ppm. Chloramines are better than some of the other compounds, but the residual ammonia or ammonium is hard to reject from the water unless it forms ammonium hydroxide, or an ammonium compound rejected by the ROs.
Ozone is obviously the best option at 0.1 ppm concentration. Ozone will sanitize microbials to a 3-log reduction almost instantaneously.
When introducing chemistry compounds into a water system, be knowledgeable of efficacy, time of application.”
Nikhilesh: “Very interesting. Is it just chlorination of water, Cl2 + H2O ⇌ HClO + HCl? To push this reaction forward, one needs excess Cl2.”
Mike: “What about the use of microfilters and ultrafilters for microbial control on the pretreatment side? Do they have merit? Or, is it best to stick to treatments that have been discussed?
Ashfaq: “Raw water from deep wells is usually chlorinated with sodium hypochlorite solution. The recommended value is 0.3 to 0.5 ppm as free chlorine. Some standards mention keeping the ORP value in range 650 mV since ORP is also dependent on water pH. Does anyone have experience with what standard will be followed if ClO2 is dosed in the system?”
Sergey: “The proper choice of disinfection method strongly depends on the raw water quality, application field, and treatment technology. Sometimes strong oxidizing agents (ozone is the most powerful) could be the best choice, sometimes non-oxidizing biocides perform better. In other cases, membranes (UF either NF) are preferable. There is no general rule.”
Jean-Francois: “For lab water systems, there has been no attempt so far to keep bacteria levels low upstream of RO or DI purification steps; it is more efficient to get rid of bacteria downstream of the purification unit, using UV at 254 nm or membrane microfiltration (0.22 µm), or a combination of both.”
Zeeshan: “Chlorination is the best possible option.”
Maxime: “In my experience, an important factor to keep in mind is the dispersive nature (or not) of the treatment. UV or ozone can be good choices, but if anything gets though alive, then you could have a proliferation problem downstream (hence the recirculation solution mentioned by Nikhilesh. Chemical solutions like chlorine or chlorine dioxide will treat the entire system (providing the dosage is suitably controlled). So, as a rule of thumb, use point-of-use treatments close to the usage point, and dispersive treatments to treat the bulk of your water system.
If we're talking pretreatment prior to a UPW production plant, then membrane filtration is a simple, cost-effective way to reduce bacterial, mineral, and organic load. The choice of RO/UF largely depends on your raw water quality.”
Nikhilesh: “Fortunately, we have many treatment options both for pre-and post-stages and they work. What we need is a good understanding of the water chemistry and also good knowledge about the pros and cons of every technology. No technology is perfect, and none would give same results under all conditions. If we meet the above two requirements, we can optimally configure the best option that work in the given situation.”
Gareth: “The other issue to consider is the material of construction of the wetted parts, including the piping and the potential for by-products such as trihalomethanes.”
What are your thoughts? You may join this or other discussions in the Ultrapure Water group. We welcome new members.
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