RO Feed Pressure spikes

[CSTS]

What is normally the causes that spikes the RO feed pressure? CIP cleaning is done regularly.
The NDP (Normalised pressure differential [Feed – Reject]) for 2 stages RO had hit CIP trigger limit for 2 months since operation. RO CIP was done after 4 months only.
Is the current observation (normalized data) and practice is healthy and normal? Is the current CIP regime effective? If not what is the usual practice and what dosage required? How we could restore our membranes back to normal health and control the rise in feed pressure and normalized delta P?
Appreciate help please.

e.g. RO feed pressure spikes to 2 bar in 4 months operation compared to 0.1-0.2 bar in a year

 

[georgeverghese]

The biggest enemy of an RO unit is solids in the feed, so have we done a thorough check on the prefiltration - is there a small leak through some of the filter elements gaskets, for example?

Another cause for high feed - reject stream dp is a high concentration of dissolved salts in the reject stream, so is there a change in feed quality, or have you reduced the reject to feed flow ratio ?

 

[CSTS]

Thanks for responding georgeverghese. Pre-filtration system has no problem at the moment. There is no leaks as well. Will check the feed-reject stream. What is the suggested reject to feed flow ratio?

 

[georgeverghese]

As long as the feed stream salt concentration is not changed from design, the reject to feed ratio stated in the operating manual should be followed. If you go lower, the reject stream salt concentration may rise and crystallise in the RO membranes.

Presume you havent changed the type of RO membranes either all this while, or increased the feed stream flowrate per module beyond recommended values.

 

[ashtree]

What you are experiencing is hardly a spike.
Given that you have said that the pressure increase is (Normalised pressure differential [Feed – Reject]) there is an excellent chance that you have a bio-fouling issue. Given that previously you have had a much lower and slower rise in pressure but now you have a faster rise i would say that you have always had a slight biofoul problem but now you are growing the biofilms within your pre-treatment area and membranes.

You have not said anything about your CIP regime othere than that its about 4 monthly which is still good. However you will find that the frequency and instensity needed will increase with biofouling.

You should check for your specific membranes what the pH and temp limits are but typically you will need to do CIPs at about pH12 and preferably in warm water at about 35 degrees to remove biofoulant. Caustic soda is typically used but your membrane manufacturer might have other suggestions.

You will probably also need to do an antimicrobial treatment of the whole plant including the pre-treatment with a non oxidising biocide eg DBNPA.

You should make sure that you do the high pH CIP before you do a low pH CIP.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[ashtree]

I will just clarify something.(Normalised pressure differential [Feed – Reject])refers to differential pressure. High differential pressure is normally an indicator of biofouling.

This assumes that nothing else has changed eg: re-circulation rates (if you have re-circulation)

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[CSTS]

Hi Ashtree thanks for the illuminating response. Yes it does have the re-circulation and still operating at normal flows. We will try the method above and see what will be the effect. Hopefully it can go back to original feed pressure since its a new system which is just less than a year.

 

[ashtree]

RO membranes tighten up a little over the first six to 12 months as well and causes the pressure to gradually rise.. This relates mainly to feed pressure rather than differential. This pressure rise is seperate to anything related to fouling or scaling but plateaus out after a while. So it is unlikely to return to the same pressure as when new but if biofouling is present you will get some improvement in differentials.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"