What Are Practical Ways to Control Microbials in RO Prefilters?
Compiled by Mike Henley and James McDonald
(Editor’s note: This article is based on recent discussions in the LinkedIn Industrial Water Treatment group. This column seeks to accurately reflect comments from each contributor. On occasion, there may be the need to edit contributor commentary for clarity or length. An important purpose of each group is to provide a forum for practical examination of issues facing endusers of high-purity and industrial water.)
Industrial Water Treatment Discussions
Compiled by James McDonald, PE
(Senior Corporate Engineer, Chem-Aqua)
James: How is microbiological activity controlled in reverse osmosis (RO) prefilters?
Francisco: “You should control disinfection upstream through chlorination at the well and water lines to maintain a residual (disinfection level) at the cistern, and/or at a storage make-up tank. Filter disinfection should be considered as well.”
Shesharam: “Replace cartridge filters as per requirement, neutralize chlorine just before the cartridge filter. Neutralizing chlorine just before the membrane holds the potential of damaging the RO elements if the neutralization fails.”
Eval: “I would suggest ultraviolet (UV) disinfection as a different approach for protecting RO membranes from biofouling. There are no chemicals, no risk, and high reliability.”
Ali: “Make sure the chlorine dosage is effective for removing bacteria in raw water. That happens best by a rotation test of microbiology. There should be clean-in-place (CIP) for all network connections and tanks before the RO.”
Aqeel: “Regular biocide dosage and disinfecting water upstream of cartridges, along with a regular bacterial activity check.”
Brad: “Similar to some of the other replies, I have used dibromocyanoacetamide (DBNPA) to control microbiological fouling.”
John: “I've seen UV used quite regularly, but I have also experienced membrane failures in these systems. Chlorination/dechlorination seems a much more reliable option.”
Marrack: “Periodic dosing of DBNPA will prevent biofouling, while not requiring the instrumentation and dosing system used for dechlorination of chlorine in the feedwater.”
Sina: “After many experiments, I have decided that using UF prior to RO can solve the biofouling up to 99%. Besides, the silt density index (SDI) would be decreased significantly.”
Raj: “I suggest the following: Clarifier (polyelectrolyte, sodium hypochlorite, ferric alum, PSF (backwash), ACF (backwash), UF (flushing, RO), UF backwash, RO CIP, and maintain dosing levels. All parameters like total suspended solids (TSS), turbidity, and chlorine should be within limits.”
Javier: “Note that some countries’ regulations do not approve the use of DBNPA for drinking water applications. UF pretreatment and control of organic nutrients will do the trick.”
Ashfaq: “Try to control iron and total organic carbon in RO feedwater.”
José: “The best way is with a non-oxidizing biocide at 2 to 4 parts per million (ppm) continuously on-line, or 30 to 40 ppm per dose.”
Ultrapure Water Discussion
Compiled by Mike Henley
(Editor, GWI | Ultrapure)
Mike: Do you find ozone useful for treating UPW systems? Please explain.
Roberto: “Ozone is the solution to most quickly sanitize cold high-purity water loops.”
Nikhilesh: “A good question. I can summarize the facts as follows: High oxidation potential has a very effective disinfection ability and quicker processing time compared to other traditional disinfection techniques. No harmful by-products are a key advantage of ozone. However, the high installation cost and power consumption of ozone technology, and the fact that unsafe exposure to ozone may lead to chronic health problems are some major factors impacting growth of ozone disinfection market. Ozone systems are still uncommon in much of the world. Chlorine continues to be most cost-effective disinfectant in every region of the world.”
Mark: “Trace nitrate can form from N2 in the O2 feed gas. You need to be sure ozone never gets to ion exchange resins, or they can oxidize. One must also ensure every polymer, piping, filter, and connection is compatible with ozone. It can be an excellent disinfectant.”
Mike: Does anyone have tips on the successful operation of ultrafiltration (UF) systems? When problems arise-- are there ways to troubleshoot that have been useful?
Dr. A.K.: “Common problems in UF operation are frequent increases in pressure drop (TMP), which normally occurs because of higher impurities in its feed or irregular cleaning. Even if it is behaving all right, then a chemically enhanced backwash should be carried out on a daily basis, followed by regular cleaning once every three weeks. It would always be better to have free residual chlorine in the UF feedwater in range of 0.5 to 1 ppm to keep the surface clean from organics and algae. During shutdown, special precautions are to be taken by flushing the membranes followed by keeping it in contact with chemicals to avoid scaling and fouling.”
Bala: “I invite individuals to visit the NEWater Visitor Centre in Singapore to learn more about UF maintenance.”
The Ultrapure Water and Industrial Water interest groups may be accessed at www.linkedin.com.
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