RO/DI TDS Creep - is it worthy of concern?

Reverse osmosis de-ionizing (RO/DI) systems are an affordable piece of equipment for hobbyists everywhere, allowing them to amass and store pure water for their aquariums' needs. A RO/DI system can produce both drinking water and water for our reef tanks, eliminating the need to haul jugs from the local store for daily top-off and for those larger water changes.

If you have an RO or RO/DI system plumbed directly to your tank's sump, you have created a dangerous risk to all of your hard work - it only has to fail once to end years of hard work in a matter of hours. I've written more on this topic in this article, which I strongly encourage you to read, digest, and to reconsider your current method. It may not be as convenient to have to occasionally fill up a reservoir, but it provides a measure of protection that can't be emphasized enough. Additionally, a system that is topped off automatically with a float valve or float switch suffers from a problem called TDS creep.

TDS stands for Total Dissolved Solids. The lower the TDS, the better. Most hobbyists endeavor to ensure that their RO/DI systems are producing 0 TDS after the final stage, using two DI cartridges rather than one specifically to assure that every last bit of solids has been removed with the cati & anion resin beads that make up the DI stage.

The RO membrane is the workhorse of the entire system, pulling out 92% to 98% of the dissolved solids before the water ever enters the DI stage. The membrane is tightly rolled and semi-permeable, and works best at 40 PSI or greater. Many mistakenly feel that if TDS is rising, they need to replace the prefilters (sediment and carbon filters), but this is simply not true. They need to be replaced on a regular schedule - most choose to do so every six months. When the prefilters clog up with particulates, the pressure drops off and the membrane is less efficient, and when the carbon filters are spent, chlorine can begin to eat away at the membrane. The simplest solution is to determine the period it will take to collect 600-gallons of pure water, and change the filters according to that timeline. Using a label maker, print and affix a sticker on the RO system stating when the filters were changed last as a constant reminder that can't be overlooked.

So what is TDS creep? When water sits in the membrane housing for a duration, TDS rises as it wicks out of the membrane. The initial 60 to 90 seconds when the system turns on will send this burst of TDS into the DI resin, or into the sump if it was plumbed directly (without a DI stage to pass through first). Each time the RO system turns on, this occurs. When one is plumbed directly to the tank, the constant on / off process day-in and day-out reduces the membrane's lifespan and efficiency.

Does TDS even matter if I'm getting 0 out of the DI stage? My reply would be "How would you like to save some money?" Membranes work best when they run for longer periods of time instead of short bursts. Some LFS (local fish stores) opt to never turn off their water purification systems to maintain the lowest TDS possible from the constancy of water passing through the membrane(s) at a consistent PSI, choosing instead to doing massive water changes in the selling systems to use up any excess water that would otherwise overflow their storage tanks. As hobbyists, we tend to save anywhere we can, including water usage. We care about every drop of water, what filters cost, and how we can avoid being wasteful.

What's the solution? By installing a tee fitting in the spot between the RO membrane and the DI stage, a short section of tubing with a ball valve is all that is needed to start saving DI resin. The systems I sell from Melev's Reef come this way so the owner can collect drinking water for the family to enjoy in addition to making water for their aquariums. This same feature can be used to prolong the usable lifespan of the DI resin, and it literally only takes a minute of your time. When it is time to start making RO/DI water, open up this valve and let it run for 60 seconds or longer to extract the TDS-laden water, then close the valve again. Open the valve after the DI and the water will now be entering the DI stage at the lowest TDS level which avoids wasting the resin. It's that simple.

Real life example: When I need to start collecting more water to refill the top off reservoirs in my fishroom, I open the valve before the DI section and collect that water in some clean empty Gatorade bottles. When those are filled, I put them in the fridge, ready for when I'm thirsty. That TDS won't hurt me; it came out of the tap water in the first place, and it only happened in the first 60 seconds anyway. Now that my drinking water is collected, I close the valve and begin filling up the 5-gallon Ozarka jugs. With my current 150gpd Boosted RO/DI system, the TDS coming out of the membrane is 1 before the DI stage. Instead of sending 100 TDS into the resin for even a minute, I physically divert it out of the system. The DI polishes off that last TDS easily; with 1 TDS going into the resin, I've been able to use the same DI cartridge since August 2009. Even though I make about 225g of RO/DI water per month for my tank, I'm expecting this cartridge to last a full 12 months because I didn't waste any of the resin with TDS creep. With my previous system (100gpd at 60 PSI and a TDS of 6), the DI resin lasted for five months. True, the booster pump helps make the membrane more efficient and thus the lowered TDS output after the membrane; I would still burn off the first minute or two of water to help make the DI resin last longer.

Instead of running a RO/DI system daily for 30 minutes, it would be better to run it once or twice a week for several hours in a row. (To avoid stagnation issues, once a week is best.) The membrane works better during these longer production runs, and it will last longer. As I said, this will help save you money as you don't have to replace it as often. For those of you using the RO system to top off your tank directly, the system turns on and off multiple times per day - which is the worst thing you could do.

Here's some video proof of what TDS creep looks like during the first minute. I have an inline dual TDS meter, and the reading you are watching is the RO water's TDS coming out of the membrane over the first minute. Surely you have a minute to spare, which will spare the resin, and help keep some spare change in your pocket.




Additional reading:
Wikipedia - Reverse Osmosis
Why should you use RO/DI water?