Softeners Ahead of RO Unit

[swall]

We are running an RO unit with an output of about 15 gpm. We were running softeners ahead of the RO because that was the practice that was recommended.We also have carbon filters ahead of the softeners to keep the chlorine out of the RO. We recently had to quit running the softeners and it seems that our good vs reject RO percentage changed drastically. We had been getting around 70% good RO and 30% reject and that has changed to about 40% good and 60% reject.Could not using the softeners be the cause, or should I be looking for something else?

 

[GarySCWSVI]

There are a number of things that could cause that to happen. Not softening the water is one. Hard water scaling of the RO membrane will reduce product water.

Another is decreasing feed water temperature. Colder water causes reduced product water along with an increase in the quality of the water.

Another is decreasing feed water pressure. Less pressure means less product water and a reduction of the water quality.

It is recommended that you have a qualified RO Service Technician assist you in diagnosing the cause.

Gary Schreiber, CWS VI
The Purolite Co.

 

[quark]

The scaling tendency increases in the RO as the pH is always maintained high. It is good to use a softener. Secondly, you should use chlorine compatible softener resin so that you can take care of bioload in the softener as you may not be sure about the salt microbial quality.

Observe the differential pressure of feed and reject. As a thumb rule, you should go for chemical cleaning of the membranes if there is a drop of 15% in permeate flowrate, and/or permeate quality, and/or a hike of 15% differential pressure.

The fouling gets reduced if you flush the membranes with permeate water.

Here is a good link for details about chemical cleaning.

http://www.hydranautics.com/docs/papers/02_cleaning.pdf

 

[swall]

Thanks for the replies. We currently have a membrane anti-foulant metered into the flow just ahead of the RO units. The change in reject vs good RO was very sudden--seemed to be coincident with shutting down the softeners.Now the reason we shut them down--the calcium chloride discharge was routed into a stainless steel evaporator and the chlorides caused pinlholes in the burner tubes after about a year. A follow up question--Is there any water softener technology that doesn't use salt?

 

[bimr]

The softeners are commonly recommended to minimize fouling of the RO. The fouling can be caused by the precipitation of calcium sulfate if you have high hardness and sulfates in the water or by minor contaminants in the water such as suspended matter that is much harder to identify and quantify.

Think of the water softener as being an insurance policy to avoid fouling of the RO membranes.

The water softener may have fouled suddenly if you have elevated calcium and sulfate. If not, the change in recovery was probably caused by a mechanical change such as pump pressure drop or a valve not fully open.

There is no other type of softeners available. Talk to your RO supplier about the necessity of having the RO. If the RO is absolutely necessary, think about getting an evaporator made out of higher quality materials

 

[UNLengineer]

As bimr states, with the sudden drop in recovery I would suspect a mechanical failure. How are you measuring the recovery rate?

 

[swall]

We measure the recovery rate with flowmeters. One for RO and one for reject. I may try switching the softeners back on and see what happens.

 

[UNLengineer]

Have you checked the flowmeters to insure that you really have lower recovery? I try to always check for measurement inaccuracies before I try to solve a problem that might not exist.

 

[quark]

I don't see any mechanical problems as long as the total flowrate (permeate+reject) is constant. Never the less, check your brine control valve.

You can temporarily increase the feed flow by increasing the feed pressure but chances are that you may plug the membranes. First thing I do is to talk to the supplier.

 

[swall]

Well, when we tried putting the softeners back on line, the RO output went back to where it was. So, taking the softeners off line seems to have been the problem.

 

[ivanhoe374]

Use the softners or alternatively dose an antiscalent. Did the flow come back by itself after you started using the softner again or was a CIP required?

 

[swall]

No, the flow came back right away. Also, we already are using an antiscalent.

 

[ivanhoe374]

How much Ca and Mg are in the feed water?