RO Membrane Cleaning, New Technologies available?

The usual technique in preventing seawater membranes from fouling is some form of particulate filtration upstream (screening, then sand filter?) to take the sharks out followed by free chlorine removal (carbon) followed by safety filter (cartridge) then squeeze it through the RO, usually with a positive displacement pump. Things to avoid are increases in concentration of horribles like calcium and magnesium carbonates, barium sulphate and a couple of nasty fluorides, and these can be kept moving by dosing antiscalants.

MF/UF, both being solid-in-liquid removal techniques, are unlikely to affect TDS [Total Dissolved Solids].

Most autopsies on RO membranes show a lack of attention to the detail of antiscalant dosing and of cleaning maintenance. Many RO packages are shipped with a Clean-In-Place [CIP] system attached, which enables the equipment to be cleaned without dismantling. On a larger installation, some redundancy is built into the equipment so that sections of it can have CIP while the rest of the plant is kept running; 2 x 100% streams is routine, as is 3 x 50%, and 6 x 20% is not entirely unknown; the configuration depends on the end user's preferences for operation and investment.

There is a risk to the membranes arising from the possibility of mechanical damage by passing air though them; were the membrane units to end up at one end of the pressure vessel due to a broken interconnector caused by vibration one would not be very popular, and the cost of chemicals is small in comparison...

When using the standard Sulfuric acid / sodium hydroxide CIP method care needs to be given when your raw water (Feed water) contains plant material which often happens. The standard method can cause the plant material to form a hard concrete like film that is almost impossible to remove.

Pre-treatment by injecting a coagulant prior to the Multi-media filters (MMF's) and injection of anti-scalants prior to the RO is the best way to keep the cost of operating a large RO system down. Pre-treatment is the key to healthy membranes and cost reduction on large systems.

The system I work with use to require an RO array to be taken offline and cleaned every 5 days or so, with four first pass arrays the crews were cleaning RO's almost non-stop. Once we tweaked the pre-treatment and changed out the anti-scalants to a new mix our cleaning cycle time was reduced to once every 6-8 months. We also chose to send the membranes off site for cleaning were after cleaning each membrane could have a flow performance test done. In the end the overall operating cost was slashed by at least 30% even though pre-treatment costs increased.

As for new technologies in cleaning RO membranes I have not seen any published recently but then again I haven't read every tech brief so something could have been missed.

Richard L,

Thanks for you comments. Coagulant that gets through forms that sticky film as well in that first stage. I was curious if there were any ultrasound or ultrasonic technologies out there that could scour or shake loose or prevent the deposit of the hard biofilms and other films collected w/o destroying the membrane and/or having to replace them. As you mentioned these films are almost impossible to remove and cause flow problems on the membrane leaf leading to channeling that destroys the integrity of the membrane even if you are able to clean it.

Which leads to the question, would you be able to run an ultrasonic or ultrasound pulse frequency through the vessel during operation that would prevent collection of film on the feed spacer of the membrane during operation without destroying the membrane? I was wondering if any research had been performed in this area. I have heard that a few people out there are currently experimenting with this.

Respectfully...Gizmo78

My interest is peaked so I put a few calls out to a couple vendors and my companies R&D. Hopefully I will hear something back within the week that is of use.

Not much information about the use of Ultra-sonic for cleaning in any form as far as RO membranes are concerned. Several companies have looked into it but keep coming across power issues with even a single membrane so they have dropped research in favor of advancing chemical solutions.

Dear Mr. PWSlack,

Thank you for your comments, MF/UF do affect TDS. A silt test is taken after MF/UF membrane to determine this. Please take David H. Paul courses online to bring yourself up to date on this technology or attend his RO seminars. David H. Paul also offers an online correspondence course leading to a degree in RO membrane technology. Vendors who sell MF/UF also provide instructional videos concerning this. Thank you for your contributions.

Respectfully...Gizmo787

Never heard of him.

And PWSlack is not a 'Mr.'. How presumptuous!

Dear Ms. PWSlack,

Please accept my deepest apologies. You are quite correct. One should not assume that the originator of the message is a male.

The David H. Paul website is as follows.

http://www.dhptraining.com/

Thanks again for your contributions. I enjoy reading all your input on every forum.

Respectfully...Gizmo787

PWSlack is not a Ms. either. How presumptuous!

PWSlack is not a Ms. either. How presumptuous!

******"PWSlack is a nom-de-plume for an unnamed, genderless individual that has a strong interest in things chemical, a moderate tolerance for things in other fields and a peculiar perversion towards railway engineering that is lifelong, chronic and terminal. While remaining a shy mystery, the location and national characteristics of this individual are prevalent.

Never a solo trumpeter, this individual is highly averse to the use of personal pronouns and has a wicked sense of humour that ebbs and flows like the tide on a moonlit night."********

Off topic a little but I found the definition of "PWSlack" well I think it is the definition

So there.

Its possible to remove bio-fouling in RO membranes using rapid oxidation. This can be achieved in many ways like hydrogen peroxide dosing, ozone, chlorine dioxide etc. However, its most important to know the tolerance of membranes to oxidants and chlorine ions. If the membrane is tolerant then, its easy to dose these products along with the water to be filtered through the membrane and the biofouling will be removed very quickly. I know of one product known as Micro2 which is used by some friends who sasy that the product removes slime, biofilms and fouling very quickly and does not damage the membranes either.

A word of caution: RO membranes are generally intolerant to free chlorine. In most plants one will find a carbon filter upstream of the RO to take it out before it gets there. Once it has beome chloride there is no problem.