|Ask The Expert: More on RO fouling
|Question: I couldn't help but take special notice of the recent "RO fouling in Tanzania" ("Ask the Expert," WC&P, May 2001) query in the magazine. The answer was not acceptable because few in the industry really understand the conditions required for non-fouling of RO membranes. This is a perfect example.
What the operator needs—critically:
1. A review of the operating conditions to determine if they are violating the guidelines established by the manufacturer of his membranes, especially flux. I guarantee they are exceeding them. This is #1 in my article (see Agua Latinoamérica, Julio/Agosto 2001: for a copy of the article in English, contact WC&P).
2. Measure the SDI on an on going basis using a simple test setup specified by ASTM. This is #2 in the article. Then if SDI is >4 he needs to install the correct pretreatment (e.g., special multimedia, absolute rated cartridge filters, colloidal dispersant injection) to get the SDI below 4 and keep it there.
3. A review of the water analysis, subjecting it to a standard industry RO design program, which predicts operating problems using a computer model.
What the operator doesn't need:
1. Activated carbon added to the pretreatment. Wells rarely have organic content and the carbon will just grow bugs, which will add to the problems.
2. Vague answers that cannot be implemented and produce the desired result.
You might consider contacting him. I feel a sense of responsibility.
Robert Slovak, AROMAN Inc., Incline Village, Nev.,
Answer: We agree with most of your recommendations. However, there are a few points you bring up that could use even further clarification.
On Point #1 under "needs" above: The respondent is likely an end-user and, since he refers to suppliers doing maintenance, isn't going to know what flux is let alone how to manipulate it.
Point #2: In Tanzania, he likely isn't going to have an SDI measuring kit either, but there are alternatives. You can measure turbidity and total suspended solids (TSS) to get an idea indirectly of SDI. The trouble with SDI is everything less than 0.45 microns will get through. A water laboratory could measure turbidity or TSS; a typical lab can usually do these, but would be clueless about SDI.
Point #3: He has to know how to understand the water analysis. And even if he has an RO design program, someone has to know how to operate the computer model and interpret it effectively.
As for your suggestion that activated carbon isn't required, we agree. It's doubtful he does have a lot of organic contamination since it is a groundwater source. But on the other hand, with high SDI, he could have almost anything.
Lastly, as to vagueness, where you refer in Point #2 to "correct pretreatment," unless you know what you're doing, this is very vague. The answer provided says the same thing. The way it's worded may not be that clear to the person asking the question. Reducing fouling—you'll never eliminate it—may require more membranes to lessen the load per membrane as well. The simple answer is to find out how to clean the membrane and to put in "pretreatment" to minimize the rate of refouling. This could include sediment filters, cartridge filters and/or a coagulant dispersant. Specific guidelines on handling, application and monitoring results of the last would be mandatory, though, to assure correct usage.
In the end, "Ask the Expert" responses are no substitute for direct professional advice. And in no way should an answer be relied upon exclusively to provide a solution to a reader's point of inquiry. WC&P provides the service using the expertise of its Technical Review Committee and other sources to offer initial guidance only.
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