Reverse osmosis: What is the proper waste-water ratio for softened water that is high TDS?

[heybob]

I've seen comments that the brine-to-product-water ratio should be between 3:1 and 4:1 unless the water is softened. I think this is because the waste water flow is needed to flush the membrane and inhibit scale on the membrane.

But what if you run very hard water through a softener so you now have high-TDS soft water? Does this eliminate scale? What is the ideal waste-water ratio in this case where a softener is being used?

In my case, I have 400 TDS and 11 grains/gallon going though a softener, then carbon, then Reverse Osmosis (Filmtec TW30-1812-50HR).

Thanks!

 

[CoralB]

Zero TDS should be your product water . Don’t know of a acceptable ratio you speak of .

 

[Shirak]

I am by no means a professional in the RO system business. I have used them for many years though and currently have a DIY contraption for my salt water tanks.

You are correct about the scale issue with hard water (Ca). The softener will eliminate that issue.

Generally speaking 3:1 for a single membrane with regular tap water which I will guess 200-300ppm dissolved solids. When you go to a second membrane in water saver mode or have soft water or a low TDS source water (which to me means under 100ppm) I think you can run closer to 2:1. Could you go under 2:1? Maybe.. at some point you will start forcing ions through the membrane though and with 400ppm TDS source water, even though it's softened. That may be an issue. Good way to check is to monitor the output of the RO. If your supposed to be getting 98% rejection and your source is 400 then your output should be no more than 8ppm. If it's going over that after running for a little while then you might need to up your waste water output.

On my system I have a needle valve on the waste line and I can adjust the waste water flow.. You can see it on the right side of the RO housing. The small valve with the black knob.

 

[billwill]

It is whatever it is after going through the process. Unless you're writing a research paper or have to provide analysis to someone, install your stuff and move on I'd say. Measure it in a month if you really want to know. Your results aren't going to match any other person except maybe a close neighbor on same water system as you.

 

[CoralB]

Too many variables to give a definitive ratio

I've seen comments that the brine-to-product-water ratio should be between 3:1 and 4:1 unless the water is softened. I think this is because the waste water flow is needed to flush the membrane and inhibit scale on the membrane.

But what if you run very hard water through a softener so you now have high-TDS soft water? Does this eliminate scale? What is the ideal waste-water ratio in this case where a softener is being used?

In my case, I have 400 TDS and 11 grains/gallon going though a softener, then carbon, then Reverse Osmosis (Filmtec TW30-1812-50HR).

Thanks!

 

[Tired]

Zero TDS should be your product water . Don’t know of a acceptable ratio you speak of .

I think they're referring to the ratio of RODI water to wastewater produced, since you get some amount of RODI water and some larger amount of definitely-not-RODI water.