Let's Talk RO & RODI

Buckeye Hydro · May 1, 2019

I wanted to introduce, or re-introduce some common jargon pertaining to RO and RODI systems that few people in the hobby commonly use.

PERMEATE - the water that has made its way through the RO membrane. Often called "RO water." In most situations, the TDS of permeate should be 5% or less of the feedwater TDS.

CONCENTRATE - best thought of as "flush water," this is the water + contaminants that DID NOT make it through the RO membrane to become permeate. The concentrate flushes the contaminants from the surface of the RO membrane and out of the system. Concentrate is often mislabeled "waste water."

RECOVERY - the percentage of the feedwater that becomes permeate. So if your system has a 20% Recovery, for every 5 gallons of feedwater, 1 gallon becomes permeate, and 4 gallons become concentrate. 1 / 5 = 0.20, or 20%.

REJECTION RATE - the percentage of the feedwater TDS that DOES NOT make it through to the permeate. The higher the Rejection Rate, the more pure the permeate.

HOUSING - The most often clear (in this hobby) "containers" that hold the filter cartridges. Want to show you're a noob in a discussion with professional water treatment folks? Call a housing a "canister."

If you have any water treatment jargon you'd like clarified, jump in and we'll do our best.

Russ

Ron Reefman · May 2, 2019

I have always hated that so many refer to the water that is rejected by the membrane as 'waste' water. I use it in our washing machine as it's been through a couple other filters before being 'rejected' and is just about as clean (TDS) or cleaner than the water from before the RO/DI system.

Buckeye Hydro · May 2, 2019

Agreed. And calling it "waste water" sort of negates its important function in keeping the membrane from scaling or fouling.

KStatefan · May 2, 2019

You should add flush and bypass to that list.

another one flow restrictor

Buckeye Hydro · May 2, 2019

PERMEATE - the water that has made its way through the RO membrane. Often called "RO water." In most situations, the TDS of permeate should be 5% or less of the feedwater TDS.

CONCENTRATE - best thought of as "flush water," this is the water + contaminants that DID NOT make it through the RO membrane to become permeate. The concentrate flushes the contaminants from the surface of the RO membrane and out of the system. Concentrate is often mislabeled "waste water."

RECOVERY - the percentage of the feedwater that becomes permeate. So if your system has a 20% Recovery, for every 5 gallons of feedwater, 1 gallon becomes permeate, and 4 gallons become concentrate. 1 / 5 = 0.20, or 20%.

REJECTION RATE - the percentage of the feedwater TDS that DOES NOT make it through to the permeate. The higher the Rejection Rate, the more pure the permeate.

HOUSING - The most often clear (in this hobby) "containers" that hold the filter cartridges. Want to show you're a noob in a discussion with professional water treatment folks? Call a housing a "canister."

FLUSH VALVE - A flush valves serves to allow water moving through the RO element and the RO housing to bypass the flow restrictor. This flow flushes the RO element and the housing. Used to help maximize the life span of the RO membrane. The flush valve is installed on the concentrate line. A flush valve is a very inefficient/ineffective way to address TDS Creep.

DI BYPASS - Typically made from a 3-way valve (one "in", and two "outs"), a DI Bypass allows the user to temporarily direct RO water away from the DI stage. A DI bypass can be used to get easy access to RO water, if for example you want to fill a drinking water jug; or more commonly, to avoid delivering TDS Creep water to the DI resin when you turn the RO system on.

FLOW RESTRICTOR - A partial plug that limits the flow through the concentrate outlet tube. It is this restriction that causes water pressure to rise inside the RO membrane housing which forces water through the membrane to be purified. There are a number of different types of flow restrictors - some (capillary flow restrictors) go INSIDE the concentrate tube, some are small inline cartridges where concentrate flows in one end and out the other, and there are restrictors that go inside the fitting screwed into the concentrate port of the RO membrane housing. For several reasons, we prefer the capillary restrictors.

Mjl714 · May 13, 2019

Flush valve, why do you say “A flush valve is a very inefficient/ineffective way to address TDS Creep.”?

Buckeye Hydro · May 13, 2019

Mjl714 said:
...why do you say “A flush valve is a very inefficient/ineffective way to address TDS Creep.”?

Good question

The answer is a little complicated but I'll give it a go.

Think of the RO membrane as a sheet of filter cloth. On one side of the cloth we have pressurized, contaminated feedwater, and on the other side we have very low pressure purified water (permeate, or "RO water"). When we remove the feedwater pressure from one side of the cloth, over time, we get a natural movement of contaminants from the feedwater side to the purified water side (a.k.a. "TDS Creep"). When we reapply the pressure by turning the feedwater on again, these contaminants on the clean side of the membrane show up for a short time in the permeate as high tds permeate. So to address TDS creep, you need to temporarily, upon startup, route the permeate to the drain, or at least away from your DI resin. THAT is exactly what a DI BYPASS does.

A flush valve is on the concentrate ("waste water") tube, and it allows the concentrate to bypass the flow restrictor. All well and good, but it does nothing for that high TDS water that made its way to the purified water side of our filter cloth.

Does that make sense?

Russ