Looking to increase RO (reverse osmosis) output. Will these system modifications work?

[ajz]

Hello,

Hoping to find someone with a background or experience with RO systems to provide some input.

In brief, we have a RO system that we run in year round at 25% duty cycle. It filters municipal drinking water to a water quality suitable for injection into a gas turbine. Our operations staff is comfortable with the system operating at the 25% duty cycle, however we are experiencing duty cycle increase driven by two reasons: 1) INCREASED DEMAND - we have installed new equipment that draws RO water during the summer (50% increased demand -- call it 37.5% duty cycle). 2) DECLINING OUTPUT - As the changing seasons bring colder inlet water in the winter, the duty cycle factor can rise quite substantially (from 25% to 50% duty cycle in winter).

Thus, our operations staff has expressed desire to increase system output to keep that duty cycle lower.

Short of spec'ing and installing a new unit, I am looking for methods to increase this system's output. I have a few ideas, and was looking for...
[ul][li]input to confirm that an idea will/wont work[/li]
[li]a ballpark estimate of the magnitude of effect (e.g. 10% increase, 20% increase, etc.)[/li]
[/ul]
Here is a diagram of our system...

As you can see, it's a 2:1:1 configuration, however the 4th membrane sleeve DOES NOT have a membrane in it (i.e. we operate as a 2:1 configuration).

Additionally, the system has a "Recycle Flow Control" valve that diverts some of the concentrate back the the system intake.

My ideas:
[ul]
[li]Install a membrane in the 4th sleeve to gain an additional stage of filtration. I would guess that this would increase output by about 10-20% [/li]
[li]Play with the recycle knob. Could doing so increase output?[/li]
[/ul]

I recognize that installing the 4th stage comes at a consequence. The 4th stage will be filtering the greatest concentration of contaminants, and thus would be the first membrane to foul. Would we need to implement a membrane rotation schedule (i.e. put elements 3 and 4 in the positions of 1 and 2 every few months).

Can anybody offer input or recommendations on what to do?

Would it be ridiculous to consider preheating the inlet stream to maintain a relatively constant temperature so that we don't get the massive output drops in the winter time?

Here is a messy diagram that has some ranges of pressures and flows based on real world observation.

 

[bimr]

It all comes down to the raw water analysis which you have not presented. Have the system manufacturer run a projection on your proposal.

The maximum percent water recovery will be based on the scaling tendencies of the reject in the last stage. That is what you will learn from the projection.

Also, what do you mean by duty cycle?

 

[ashtree]

Bimr is correct in that many of the system limitations can only be decided by looking at the feed water analysis and then determining if the solubility limits will be exceeded.

However there are some other technical issues that will need to be resolved. The fourth vessel is empty for the reason that it is unlikely to work in the current configuration and as bimr suggests probably scales badly. It is probably not staged correctly and there is a need for either stage boosting or permeate throttling valves neither of which are shown.

There is lots of ways possible to increase the output but don't just put a 4 th element in.

Increasing recycle flow on its own won't necessarily increase your output but perhaps changes to the recycle flow as part of an overall process change might be okay. The feed water quality would be required to know for certain.

I cant read the membrane type on the drawing.

Can we assume that you use an antiscalant.

It maybe a dumb question but is it worth increasing your storage tank size for the permeate to allow the current RO to chug along just keeping the tank topped up. ROs certainly like to run continuously rather than stop/start so a tank increase might achieve all your aims.

Heating the water may give you more flow but it depends on what your flow is now and what the membrane limits are. Generally warming the water would be used to give the same flow at less operating pressure. Whether or not it is worth doing depends on a lot of factors such as energy source for warming , current temperature etc. RO membranes have a temperature limit around 40 degrees C.

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

 

[georgeverghese]

Given that this system is also pressure drop limited, would suggest
a)Install the elements back into the 4th unit and run it in parallel with the 3rd element
AND
b)Zero recycle of high TDS concentrate back to pump suction.

The water injection requirement at the GT seems too low for feed air precooling, so this must be for online turbine water wash??

 

[dfmorvan]

Hi Ajz

To determine what you can get out of the unit, you need to provide a detailed (as much as possible) water analysis.
Please add any pretreatment you may use before the RO (sand filter, decantation, softener)
This will help determine the scaling potential of the feed water and we could calculate the maximum recovery (hydraulic yield = Permeat water flow/feed water flow). For the moment your recovery is 54 to 57 %
Usually one tries to get to a recovery of 75 to 80 % with brackish water but that depends on the scaling potential of the feed water.

The other important parameter is the membrane flux (flow of permeate/surface of the elements).
This parameter will depend on the fouling potential of the feed water and is a function of the origin (surface water will imply lower fluxes than well water, because surface water is usually more fouling to the membranes).
To determine what flux can be applied, you need to do a SDI15 mn test (Silt density index, 15 mn). Depending on the results the acceptable flux can vary widely, or you may need a more sophisticated pretreatment.

The concentrate recirculation you are speaking about is probably needed to respect some hydraulic paramaters on the elements

to finish checking the design we also need the type of module you are using and the number per pressure vessel - it is not possible to read the small script.