Is Forward Osmosis Viable?
Compiled by Mike Henley and James McDonald
(Editor’s note: This article is based on recent discussions in the LinkedIn Ultrapure Water and Industrial Water Treatment groups. 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. An important purpose of each group is to provide a forum for practical examination of issues facing endusers of high-purity and industrial water.)
Ultrapure Water Group Discussion
Compiled by Mike Henley
(Editor, GWi | Ultrapure)
Mike: Any experiences with the use of Forward Osmosis? Do you consider it a present or future option for your treatment system?
Nikolay: “I think it is, but mainly for non-drinking water applications.”
Sina: “Utterly true.”
Ali: “Because of many issues (e.g., very low flux), currently it's just a laboratory curiosity.”
Nikhilesh: “Here is a link to an article I’ve written on forward osmosis: https://www.linkedin.com/pulse/forward-osmosis-overview-nikhilesh-mukherjee.”
Mike: “I'm aware that the Changxing Power Plant in China has a ZLD FO system. Are you aware of other examples, and do you have any comments?”
Lisa: “Forward osmosis and viability is an important topic! A good place to start is to nail down the RO and current membrane operating parameters against different water sources (i.e., salt water versus groundwater) to determine if there is more to forward osmosis than energy savings.”
Go-To Disinfection Agent?
Mike: What is your first choice for a disinfecting agent when treating a water system? Is it based on what is being treated?
Dr. V.K.: “The normal disinfecting agent preferred in India is chlorine. It can be in the form of chlorine gas or sodium hypochlorite solution.”
Roberto: “Chlorine is the cheapest, but not for health reasons itself because of trihalomethanes (THM) compounds. I prefer ozone, but there are high costs with large flows.”
Industrial Water Treatment Discussions
Compiled by James McDonald, PE, Senior Corporate Engineer, Chem-Aqua
Dead Legs and Microbiological Control
James: How quickly can a dead leg re-inoculate a closed-loop system with microbiological activity?
Frank: “Within hours.”
Jan: “Indeed, very fast. I agree.”
Ahmed: “Very fast. Dead legs should be drained and cleaned and a residual biocide should be measured every shift. This is an area where most operators are unaware of and is a frequent cause for surges in microbial activity.”
Jim: “Here is a question for all to ponder: Assume under ideal conditions a single bacteria reproduces every 20 minutes. However, in a closed loop it may not have ideal temperature or nutrients available, so let's double the reproduction time to 40 minutes. In one day (1,440 minutes) that would be 36 reproduction cycles which would result in 6.87x1010 bacteria. If the closed loop has a volume of 10,000 gallons, that would equal 1,815 colony forming units per milliliter (cfu/mL) for each bacteria released into the loop. Any disagreement with that number?
Disturbing the dead leg would certainly release more than a single bacteria. If the dead leg disturbance released 1x104 bacteria into the loop, the resulting population would be 1,815 x 10,000 = 18.15x106 cfu/mL after 24 hours.
Forgive me if my math is wrong— please no snarly replies! But, the more you play with these numbers, the greater appreciation you have for answering the initial question. It's too bad that many of our customers cannot grasp the significance of this!”
Ennio: “Very fast indeed, fast enough to make you want the dead legs as clean as possible AND your closed-system water treated with an effective biocide.”
Steve: “You'd be amazed at the efficacy of chlorine dioxide (ClO2) in large systems and dead legs. After Hurricane Katrina hit New Orleans, I used gas-stripped ClO2 to clean a fouled chill water distribution system of millions of gallons that ran through many of the buildings downtown for equipment and comfort cooling, with many heat exchangers. ClO2, being a true gas in solution that does not react with water, seeks a common concentration everywhere, penetrating biofilm (killing the anaerobes at the bottom that perpetuate the biofilm) and completing the oxidation of only the damaged metal in the heat exchangers, leaving the undamaged metal clean and efficient again. It takes a surprisingly small amount of ClO2 to accomplish this task.”
Ahmed: “Steve, that is impressive!
As an aside, this type of problem/s goes unnoticed in firefighting systems, especially in tank farms of stored liquid hydrocarbons. Refineries/petrochemical plants are a major source for dead legs.”
Microbiological Control in RO Prefilters
James: How is microbiological activity controlled in RO prefilters?
Sina: “After many experiments, I have reached to this point that using ultrafiltration (UF) prior to reverse osmosis (RO) can solve the biofouling up to 99%. Besides, the silt density index (SDI) would be decreased significantly.”
Raj: “Clarifier (polyelectrolyte, sodium hypochlorite, ferric alum, PSF (backwash), ACF (backwash), UF (flushing), and RO). UF backwash, RO cleans in place, and maintains their dosing level. All parameters like total suspended solids, turbidity, chlorine, and chloride should be within limits.”
Javier: “Note that some country regulations do not approve 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.”
Prateek: “Put UV before the feed.”
Jose: “The best way is with a non-oxidizer biocide at 2 to 4 parts per million (ppm) continuously on line or 30 to 40 ppm by shot.”
Steve: “Maintain a free-chlorine residual through the filter housing and dose SBS downstream.”
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