Reverse osmosis operation

[Ctrip]

I am looking for others experience in the manner which they operate their RO systems. I am at a new facility which has completely different operating philosophy than I am familiar with. They are having problems with membrane fouling and are needing frequent CIPS performed. I ran a previous RO plant at a 75% recovery and monitored reject conductivity keeping it around 3200-3500us. I was wanting to know if running with a higher reject conductivity, around 4000us, would contribute to the problem? I also traditionally ran all RO banks on continously and modulated flow to maintain requirements, whereas here they do not adjust flow and cycle banks on and off to meet demand. I suspect this laying up may be encouraging microbiological growth on the membranes and contributing as well to the fouling. I have been told the vendor instucted not to adjust flows and stay around 350gpm, whereas the RO's I previously operated ran from 225-450gpm. Anyone know of why they would of been told this?

 

[GHartmann]

First, do you have a vendor that supports your RO unit? If so, I would ask them for help.

You have to go back and try and find the original performance parameters to see if your 4000 us is high. Each system is somewhat unique due the varying inlet water quality.

Try these links for some ideas:

http://watertreatmentguide.com/maintenance_&_troubleshooting.htm

http://www.separationprocesses.com/Membrane/MT_ChpRO-7.htm

http://www.gewater.com/handbook/ext_treatment/ch_9_membrane.jsp

http://www.avistatech.com/Solutions/trouble_shooting_guide.htm

http://www.environmentalpurewatersolutions.com/Industrial/TechnicalResources/ROTroubleshooting.aspx

As you can see after reading the various guides, you need to do some more troubleshooting / water analysis to determine the cause of your fouling.

Typically RO units run on relatively constant flow in order to maintain a constant rate of flux (flow/unit area) that maintains the necessary pressure drop for the membranes to work properly. Your operating range for your previous system is extremely unusual.

I had a hard time coming to grasps with the on/off constant flow idea with my experience in continuous process systems. If you are also working on the refinery processes you may also find the concept unusual.

Good luck

 

[bimr]

If shut down correctly, membrane fouling will be a problem.

There are different types of fouling. One is due to the saturation of salts when concentrated in the RO membrane. This will vary depending on the water quality. If you have a water high in calcium and sulfate, then it is more likely to saturate and foul than a water which is mainly sodium and chloride. The cure for fouling from saturated salts is to install a water softener in the feedwater to the RO unit.

Operating at 75% recovery is typically a conservative design. Some systems are designed for +90% recovery and obviously will have a less conservative design.

The second major source of fouling is suspended matter in the influent to the RO. To prevent this type of fouling, you need adequate pretreatment ie filter of some kind.

RO units are designed to operate on/off as your present system is designed. Modulating the flow is not recommended. Modulating the flow will reduce the cross-flow velocity of the feed and concentrate streams and cause fouling. The flow is also directly related to pressure. Reducing the pressure is counterproductive as you will get less flow out. Every RO system that I have been associated with has operated in the on/off mode.

http://www.membranes.com/docs/trc/TROUBLES.PDF

Laying up must be done correctly to prevent fouling from salts and also to prevent microbiological growth. This should be outlined in your operating manual.

 

[dfmorvan]

Hi Ctrip

Let's take the problem globally. Each water is different, so it is not always wise to make simple comparisons.

First of all do you have access to a water analysis? The scaling potential is a function of the type and concentration of ions, pH of the water and plant recovery and hydraulics.

Also there is the question of raw water origin and pretreatment.
Is the raw water a well water, mains water or surface water ?

did you have access to a Silt density index result. SDI 15 mn is the standard industry test to know the fouling potential of your water on the membranes.

What are the "symptoms" leading to cleaning (high conductivity, high pressure drop (or reduced output)) or acceptable conductivity and reduced outputwith high feed pressure ?

Do you do a flowrate standardization program? Do you have some plant performance logging (flowrate, temp, conductivity of raw water and permeate, membrane pressure drop...)

Normally, you run an Ro with a rather constant flow. Membranes do not like variable flows and have flow limitations (high and low). 75 % is the standard recovery. Going higher means you know what you are doing in terms of water chemistry, pretreatment and plant hydraulics.

send me the water analysis and origin and we'll begin to discuss your plant problems

Best regards

 

[omjee]

Hi Ctrip,

Can you give us information about the pre-treatment for this RO? There are many types of fouling so you might need to identify the type of fouling here to pinpoint the root of the problem.

Higher RO reject conductivity could mean that you have higher feed TDS as RO operates by rejecting a certain percentage of this TDS. You need to work out the recovery that you are running at for this plant and compare it to the RO projection. Running your RO at higher recovery than your design could lead to high scaling tendency.

Regards,
Wan