Help with TDS levels!

[nbgen12]

I have a bulk reef supply 4 stage ro/di unit. right now my tap water reading is 150ppm and after going through the ro/di it is still reading 25ppm. I am brand new to this hobby and got this ro/di used. could anyone point me into the right direction as to what to do? I dont know much of anything about rodi units...

[Shane Hoffman]

The filters may be exhausted. Do you know how long the guy you bought it from used it?

[Shane Hoffman]

I would replace the filters if you have no idea how old they are. Filters are only good for a certain amount of time or gallons. You cant run hot water through them, it will ruin them. You cant let them dry out, that too will ruin them. I keep mine full of water when not in use and if your going to go months in between using the RODI remove the filters from the unit put them in a zip lock baggie add a cup of water and store them in the fridge.

[balto777]

I would think that anytime you buy a used RO/DI unit, you should consider the cost of all new filters as part of the price you pay unless the seller says the filters are new. A 4 stage unit should have two prestages, a RO stage and a DI stage. These should not be too much for your particular model and will help you know where the filters are at so you can replace them with confidence in the future.

[zachfishman]

And don't forget to run it without the post-membrane DI cartridge until all the preservative in the new membrane is washed clear. Almost made that mistake once - would've cost me a DI cartridge.

[carolina reefer]

As stated, get a new set of filters from Bulk Reef Supply. You may or may not need the RO membrane but for sure need the others. Get the color changing DI resin so you will know when it is used up. Should get 0 TDS from new filters. If not, RO may need replacement.

[nbgen12]

I just ran some water through the unit for 20 minutes and then collected some water which tested at 0ppm when the tap reading was 150. I have been working on getting the tank cycled for a couple weeks....do you think the water could possibly be too contaminated to cycle? i still have no nitrites or nitrates and i added about 15 pounds of live rock about 2 weeks ago...

[Shane Hoffman]

I replace my filters when my tds hits 20. A lot of people spazz or are much more anal than myself who replace there filters when they see 5 or 10 TDS. IMO thats not necessary. I wouldnt be concerned using water that is 25tds to set up a tank. I wouldnt want to run long term with 25 tds water especially when you get corals. If your wanting to order some filters and get your tank set up in the mean time I would be comfortable with that. Just do a couple of large water changes with 0TDS water when you get your new filters.

[nbgen12]

should i wait for the tank to go through its cycle first before doing these water changes I assume?

[Buckeye Hydro]

I just ran some water through the unit for 20 minutes and then collected some water which tested at 0ppm when the tap reading was 150. I have been working on getting the tank cycled for a couple weeks....do you think the water could possibly be too contaminated to cycle? i still have no nitrites or nitrates and i added about 15 pounds of live rock about 2 weeks ago...

If your DI water measures 0 ppm, that's your goal - use the DI water!

Use your TDS meter to also check the TDS in your RO Water - the water coming from the RO membrane that feeds the DI housing.

Re when to change your filters, this (from our FAQs) may help:

A good rule of thumb is to replace your sediment filter and carbon block after six months. A more precise way to maximize the useable life of these two filters is to use a pressure gauge to identify when pressure reaching the membrane starts to decline. This is your indication one or both of the filters is beginning to clog.

Also be cognizant of the chlorine capacity of the carbon block. A good 0.5 micron carbon block for example will remove 99% of chlorine from 20,000 gallons of tap water presented at 1 gpm. Some original equipment suppliers commonly provide carbon cartridges rated at 2,000 to 6,000 gallons.

Regarding your RO membrane and DI resin, use your TDS meter to measure, record, and track the TDS (expressed in parts per million) in three places:
1. Tap water
2. After the RO but before the DI
3. After the DI.

The TDS in your tap water will likely range from about 50 ppm to upwards of 1000 parts per million (ppm). Common readings are 100 to 400 ppm. So for sake of discussion, let's say your tap water reads 400 ppm. That means that for every million parts of water, you have 400 parts of dissolved solids. How do we go about getting that TDS reading down to somewhere near zero?

If you do some experimenting with your TDS meter, you'll note that your sediment filter and carbon block filter (collectively called “prefilters”) do very little to remove dissolved solids. So with your tap water at 400 ppm, you can measure the water at the “in” port on your RO housing and you'll see it is still approximately 400 ppm.

The RO membrane is really the workhorse of the system. It removes most of the TDS, some membranes to a greater extent than others. For instance, 100 gpd Filmtec membranes have a rejection rate of 90% (i.e., they reject 90% of the dissolved solids in feed water). So the purified water coming from your 100 gpd membrane would be about 40 ppm (a 90% reduction). Filmtec 75 gpd (and below) membranes produce less purified water (aka “permeate”), but have a higher rejection rate (96 to 98%). The life span of a RO membrane is dependant upon how much water you run through it, and how dirty the water is. Membranes can function well for a year, two years, or more. To test the membrane, measure the total dissolved solids (TDS) in the water coming in to the membrane, and in the purified water (permeate) produced by the membrane. Compare that to the membrane’s advertised rejection rate, and to the same reading you recorded when the membrane was new. Membranes also commonly produce less water as their function declines.

After the RO membrane, water will flow to your DI housing. DI resin in good condition will reduce the 40 ppm water down to 0 or 1 ppm. When the DI output starts creeping up from 0 or 1 ppm, you know that your resin needs to be replaced. Sometimes people complain that their DI resin didn't last very long. Often the culprit is a malfunctioning RO membrane sending the DI resin “dirty” water. This will exhaust the resin quicker than would otherwise have been the case. Sometimes the problem is poor quality resin – remember that all resins are not created equal!

Russ @
Buckeye Field Supply