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crocodile

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RO and silica fouling
Aug 30^th, 2012, 2:16am

Hi Everybody,

Hopefully someone with more knowledge than me may be able to point me in the right direction.

It appears that we now have clogged membranes on one of our RO plants. Water test results today show a presence of silica. These were not present when the system was first installed. Feedwater is from a bore with pH = 7.6, Cond = 860 uS, Turb = 0.8 NTU.

I realise that we now have to deal with silica and pre-treat the water before it hits the RO but I'm just wondering how we go about cleaning and descaling the membranes. Is it feasible to do this on-site.

Thanks for all your past help guys,

Glenn

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JACKMANGAL

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Re: RO and silica fouling
Reply #1 - Aug 30^th, 2012, 11:02am

Hi glen
you can operate your plant on low recovery to get rid of the silica scaling. You have to make sure that the concentration of silica in the reject side is less than the solubility limit of silica 120mg/lit at 25 degree celsius and 7 pH. Ofcourse if you use a good antiscalant the limit can be taken to 240 mg/lit. By the way what is your silica level and recovery of RO plant? If you need to continue with the current recovery rate try pretreatment methods involving using a lime, dolomite dosing followed by poly dosing in a clariflocculator

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crocodile

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Re: RO and silica fouling
Reply #2 - Aug 30^th, 2012, 3:58pm

Hi Jack,

Thanks very much for your time. I apologise for not being clear. What I'm after is how do I clean the membranes after the silica has fouled them. That is, what do I need to do to them now before putting them back in service with appropriate measures of prevention of course. Unfortunately, I have little knowledge in this area. My role is to design the control systems and electronics. The guy that used to look after this area is not with us anymore so I get lumbered with the solution.

Regards,

Glenn

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sobisch God Member Offline Posts: 1777 Gender: Company or Organization: L.U.M. GmbH	Re: RO and silica fouling Reply #3 - Aug 31^st, 2012, 1:30am You should consult the supplier of the membranes for best procedure. By the way I am curious whether your problem might be due to fine silica, silt or clay particles. Do you have a filter in place to protect the RO membranes. Kind regards T. Sobisch
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crocodile

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Re: RO and silica fouling
Reply #4 - Aug 31^st, 2012, 1:49am

Thank you Sobisch,

Yes, we have pretreatment to reduce insolubles, chlorine and VOC. These are in the form of activated carbon, 2 micron aquazol and 0.2 micron ultra. Unfortunately, the original manufacturer is based in China now and tech support is not so good. Once the membranes have been cleaned I will be looking into anti-scaling and adjusting the reject rate

Regards,

Glenn

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JACKMANGAL YaBB Newbies Offline Posts: 13 mumbai Gender: Company or Organization: ROCHEMINDIA	Re: RO and silica fouling Reply #5 - Aug 31^st, 2012, 10:57pm As sobisch pointed out the manufacturer might help you into this. As far as i know silica fouling permanently damages the membrane if found in large quantity. What is the silica ppm in feed? and recovery rate?
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The Muck Slinger

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Re: RO and silica fouling
Reply #6 - Sep 1^st, 2012, 5:52am

"Never smile at a Crocodile???"

I do lots of rehabilitating/redesigning systems after the 'experts' have got their money and run.

Before we get to that...

We have developed some incredible oil removal chemistry of late, and if your early processe aren't perfect, your UF & RO are just ticking timebombs.

We have an Australian-made & designed set of membrane chemistry products.

We can also help with remedial CIP, and even give advice on PLC logic.

And we have a department in Perth.

Please feel free to PM me if you want more commercial-based help, or throw around specific questions here on the forum.

May I ask, what level of Hardness are you seeing in the Raw water, and what is the pH out of the RO that you are spraying your nice new equipment with??

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crocodile

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Re: RO and silica fouling
Reply #7 - Sep 2^nd, 2012, 6:21am

The Muck Slinger wrote on Sep 1^st, 2012, 5:52am:

"Never smile at a Crocodile???"

I do lots of rehabilitating/redesigning systems after the 'experts' have got their money and run.
It is actually one of our own systems so we can't run. The system is well built and has been trouble free up until now but I can see where the problem is re silica presence. Unfortunately, the organisation that we sourced the membranes from is difficult to deal with. I was after information on the remediation steps for the membranes. Perhaps your organisation can help.

Before we get to that...

We have developed some incredible oil removal chemistry of late, and if your early processe aren't perfect, your UF & RO are just ticking timebombs.
Please direct me to your website. Always looking for products like these.

We have an Australian-made & designed set of membrane chemistry products.

We can also help with remedial CIP, and even give advice on PLC logic.
PLC logic not required but certainly interested in any info on your CIP products.

And we have a department in Perth.
Anything on the east coast ?

Please feel free to PM me if you want more commercial-based help, or throw around specific questions here on the forum.
Much appreciated

May I ask, what level of Hardness are you seeing in the Raw water, and what is the pH out of the RO that you are spraying your nice new equipment with??
Plant is not new. Bore water specs as shown on top post. After treatment, pH = 7.3, Cond = 40 uS, Turb < 0.01 or lower than the sensitivity of the instruments. Post treatment also involves chlorination to 1ppm and UV. Water is for potable purposes. Haven't done a hardness test on the product water as yet.

Thank you and I'd be happy to at least have your info on file for future use.

Glenn

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crocodile

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Re: RO and silica fouling
Reply #8 - Sep 9^th, 2012, 7:38am

Hi Everyone,

Thanks to all those who have offered opinions. Much appreciated. Some more info has come to light. Firstly, the silica has been identified as 'reactive silica' a 25ppm on the inlet side and 2.6ppm on the permeate. That's not all. It appears that someone has closed off the reject stream valve, reducing the reject rate to less than 5% of permeate. I suspect that many salts may have reached their saturation points and precipitated out of the reject stream and fouled the system. Think that silica may not be the only problem. Since silica has a very low solubility I was just wondering if anyone could suggest a practical reject rate to use.

Thanks,

Croc

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JACKMANGAL

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Re: RO and silica fouling
Reply #9 - Sep 9^th, 2012, 9:25am

The solubility of silica is depended upon various parameters like temperature, pH. however considering 25 degree celsius and 7pH the solubility is 120. As pointed out by me earlier you can keep the reject rate maximum at 240(Considering you are using a good antiscalant) Since feed is having 25 ppm reactive silica and 240 at the reject side(considered) the recovery of the RO plant can be kept max 85 %. Anything above that can actually convert the reactive silica to colloidal and membrane fouling can start to occur. Also since you pointed out that somebody had closed the valve causing the recovery to go more than 95%, it can also be a fouling of calcium carbonate(the common scale formation) combined with other salts. Try finding the LSI and SDI index for your concentrate water to find that out.

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