Ro Membrane for 500+ TDS tap?

Bustyraker · Jun 30, 2015

As of right now I use two 75 gpd membranes and needless to say I burn through a ton of DI resin. I use a booster pump and the tds before DI is usually around 12. Would a 50 gpd membrane be better or should I even go with a 35?

booth2010 · Jun 30, 2015

@AZDesertRat

FoD · Jun 30, 2015

I would move.

cdness · Jun 30, 2015

I am in the same boat as you where my TDS ranges from 450 - 650 depending on the time of the year. I burn through DI resin as well, but it is due to the CO2 content of the water, not the TDS of the post-membrane water. I would suggest testing the CO2 content of the water, especially if you are receiving water originating from a well.

It isn't the GPD that matters but the rejection ratio. I have a 90 GPD 99%+ rejection ratio certified membrane from Spectrapure. It has helped the TDS of the post-membrane water, but it didn't help the burning through DI. Here is what I am using: http://spectrapure.com/FILTERS-MEMB...-SpectraSelect-Plus-Tested-90-GPD-RO-Membrane

Bustyraker · Jun 30, 2015

That makes sense. I do have well water. I guess DI resin isn't that expensive lol

Opus · Jun 30, 2015

Based on the formula I've seen before, the DI usage is tds divided by 17.1 equals your DI grain usage. The average cartridge is 260 grains so divide that by the grain usage to get the estimated gallons the DI cartridge will process before depleted. So using that formula your 12 tds would get around 370 gallons from the di before depletion. Move up to the spectrapure which should get you to around 6 tds and you should get around 740 gallons from the di.

Hope my math is correct.

Bustyraker · Jun 30, 2015

I'm getting closer to 200-250 gallon from a dual DI unit

Opus · Jun 30, 2015

Then the CO2 mentioned above must be coming into play. I'm not sure how that formula takes CO2 into consideration if at all. Hopefully AZDezertRat replies today, he is the expert around here. May not be anything you can really do other than moving over to the Spectrapure RO membrane. Are you using a booster pump?

Bustyraker · Jun 30, 2015

Yeah I'm using a booster pump. I've heard about people running their water into a 30 gallon brute can and let it run with a power head for a few hours and then pumping it through their rodi. But I'd rather just spend money on DI to save the trouble

AZDesertRat · Jun 30, 2015

If you know your waters pH and alkalinity you can use this nomograph to calculate your CO2 level
http://spectrapure.com/frequently-asked-questions#calcc02

The first questions I always as a RO or RO/DI owner are the basics. What is your tap water TDS? What is your water pressure and your water temperature? Do you use softened water or do you know your hardness? What is your exact measured waste ratio? What is your RO only TDS? What is your final RO/DI TDS? What DI resin are you using, is it fresh and if it is bulk has it been properly vacuum sealed so it stays moist and stored in a cool dark place such as the back of the refrigerator? How much do you buy at a time and how long does it last?
You say you have two 75 GPD membranes? How are they plumbed and do you have the correct flow restrictor for them to match your water conditions, usually 4:1 waste ratio but this can be reduced a little with soft water and lower than normal TDS. Dual membranes ARE NOT water savers! I hate it when vendors promote them as such as they are uninformed and doing you a disservice, your membranes will die prematurely since they are not being flushed via the brine or waste line as they have to be. Flush kits are a waste of money and have no proven value and do nothing for you here.

You can pretty much figure a full pound or 16 oz of fresh nuclear grade or semiconductor grade mixed bed DI resin will remove 3000 to 6000 total TDS. So if you know your TDS going in is 12 you can calculate the range it should last you. If you have CO2 present it will be on the low side. Again this applies to fresh resin only and not old, improperly stored or recharged resin which you can never actually recharge back to 100% at home. At best fresh resin has a shelf life of 6 months when factory sealed in mylar foil bags in the original wetting solution and it goes downhill from there once opened.