Rodi adding second membrane?

[Jimsreef]

I have a spectrapure rodi with a 60 gpd membrane on it. If I add another 60 gpd membrane to it to make 120 gpd could I add a 100 gpd flow restrictor to the end of the last membranes waste water. Jim

[tkeracer619]

No. You will want an adjustable one. It is just a john guest needle valve.

[Jimsreef]

Like a ball valve that I can adjust the amount of waste water that come out? Jim

[Jimsreef]

Would this work?
http://premiumaquatics.com/aquatic-s...-G0420425.html

[tkeracer619]

nope

14th from the bottom.
http://www.thefilterguys.biz/fittings.htm

[Jimsreef]

Can you use that for any gpd membrane to restrict the flow? Jim

[tkeracer619]

yup. last one you will ever need. Regular restrictors are almost worthless based on temp and pressure differences with every setup. No idea why they don't include these in standard ro setups. It is $3 more...

You have to buy a $1000 unit to get one

[Jimsreef]

Thanks for the help I really appreciate it. One other question actually two.

My pressure gauge is screwed right into the spectrapure membrane housing. Do I need this membrane to be the first inline or second?

And with the two 60 gpd membranes how much good water to waste water ratio do I need? Jim

[tkeracer619]

As far as the gauge goes I would put it on the first membrane. It is a good judge for the status of the prefilters.

I would go 2 to 1.

Your welcome

[allsps40]

Post in the SpectraPure forum. I was just at there warehouse yesterday, they are extremely helpful.

[moonpod]

I have a spectrapure with a piggyback. I think they still recommend 4:1. I got my piggyback from spectrapure and it came with a new flow restrictor that was a bit longer iirc than the original one. Regardless it is tuned to about 4:1 which I am pretty sure is what spectrapure recommended

[tkeracer619]

Does spectrapure recommend a 4 to 1? Yup, they do.

Can a dual filmtech membrane run 2 to 1. Absolutely.

Flush the membranes before each use. You should be doing that regardless.

[Jimsreef]

Why does spectrapure recommend 4 to 1 will the 2 to 1 still be at at 000 tds. Thats %50 diff in wasted water.

[solitude127]

For anyone who's added the 2nd membrane, have you notice if 1 membrane gets worn out faster than the other?

[allsps40]

Originally Posted by moonpod

I have a spectrapure with a piggyback. I think they still recommend 4:1. I got my piggyback from spectrapure and it came with a new flow restrictor that was a bit longer iirc than the original one. Regardless it is tuned to about 4:1 which I am pretty sure is what spectrapure recommended

You are correct sir, 4-1.

[tkeracer619]

RO will not hit 0tds unless the water is great.

The second membrane receives higher tds then your tap water. Generally you will get slightly higher tds out of the unit. DI takes that back to zero as usual.

Spectrapure has a unit that has a 1:1 ratio. It back flushes the membrane with rodi to increase the membranes life.

As far as one wearing out faster then the other I am sure the second one does but after 3 years of running one I still have no increase in TDS. Have been running 2:1 since day 1. I do have it set to auto flush each time it comes on but that was pretty recent.

[thor109]

I'm just curious about this set up. After reading this I hxve to ask, Are you running the second membrane off the waste water from the first membrane?

[moonpod]

yes. to piggyback a second membrane you run the waste from the first membrane into the second membrane. The product lines you can typically join with a Y type connector for simplicity. Final waste line comes off of second membrane and thats where you would install a flow restrictor or as suggested earlier above a valve

[Buckeye Hydro]

Remember that what folks call "waste water" really would be better thought of as "flush water" in that this water serves the important purpose of internally flushing the surface of the semipermeable membrane to keep the membrane from fouling/scaling.

When you configure a system with two membranes in series (the waste from the first membrane going to the "in" port on the second membrane), for this discussion let's say it's two 75 gpd membranes, the system behaves like you have a single long (75 gpd x 2) 150 gpd membrane.

Now - if you use a proper flow restrictor, that is, one for a 150 gpd membrane, you'll have about a 4:1 waste to product ratio. Sounds familiar, right?

If however you don't change the flow restrictor - meaning you keep using the same restrictor you were using when you just had one 75 gpd membrane, then you'll see a waste to product ratio much lower than 4:1. But remember that the recommendation for a ~4:1 ratio comes from the membrane manufacturer. They are telling you that you need about a 4:1 ratio to keep the membrane flushed and keep the membrane from fouling or building up scale. Run the system with a lower ratio and you will foul/scale the membrane(s) quicker than would have otherwise been the case.

Instead of adding a second membrane to lower that ratio, you could have just changed out your flow restrictor ($4) instead. A much less expensive approach to get you to the same endpoint in terms of saving on waste water.

Now, to confuse things just a bit. Filmtec specs call for the 4 to 1 ratio on the basis of assumptions about the water that will be supplied to the membrane. If you have very soft water you MAY be able to get a decent service life from the membrane running at a ratio lower than 4 to 1 (e.g., 3 to 1). Remember that the waste water from the first membrane is about 25% harder than your tap water.

Bottom line: If what you are after is reduced waste water, experiment with a different flow restrictor for $4 instead of messing around with a second membrane plumbed in series.

As a side note, you can also lower the ratio by increasing the pressure delivered to the membrane (with a booster pump), because flow restrictors are sized assuming you are providing factory spec conditions (50 psi and 77 degrees for Filmtec membranes). Increase the pressure and you'll drive more water through the membrane and viola - less waste water. But as I mentioned above, if you do this (just like over-restricting a membrane) - the lower the waste to product ratio, the shorter the lifespan on the membrane.

Makes sense?

Russ