RO/DI Unit - Water Waste?


I've heard the RO/DI units acutally have a good amount of water waste when producing purified water. Is this true? If so, approx how much water is wasted per 1 purified gallon?
Manufacturer sites do not seem to mention this.


the one i bought said the average is 3-4gal to 1gal, meaning for every 1 gal of water made 3-4 is wasted.


Active Member
About 3 to 4 gallons of waste per gallon is the norm if your water temperature is in the 70's, your pressure is around 50 and your water is not too hard. If conditions are not ideal, waste ratios can be as high as 10 gallons.


when i bought my ro unit i didn't know it had a "waste water" line. I was shocked to find out it does have a lot of waste water like 3-4 gallons per 1 gallon... this i think is lame but it's worth it. I was using tap water before with the fish... i can actually see a difference in their behavior! wierd huh... sorry about the waste water's a shocker on da' water bill!!!!!!!!


Active Member
Looking at my water bill
96 days of usage = 1600 cubic feet of water ( actual reading )
1600 cubic feet of water = 11968.83 gallons of water
The bill was $42.03
$42.03 / 11968.83 = $.0035 per gallon.
I use about 2.5 gallons per day of RO/DI water.
I estimate around 125 gallons per month for both tanks for top offs and water changes.
I'll be generous and say I use 200 gallons per month of RO/DI for my tanks.
If my RO/DI unit is producing a 5:1 bad to good water ratio - I guess my total water usage for hobby would be around 1200 gallons per month.
1200 gallons times $.0035 = $4.20 per month for water.
I may have made a mistake with the math - but I do feel that showering/bathing/cooking/drinking/cleaning/flushing/car washing/laundry are the what attributes to most of "our" water usage here.
If I made a huge error in trying to figure this out - that's okay.
Even if I used twice this amount of tapwater - I'd be looking at less than $10 bucks per month for the hobby. That's not too bad compared to some other expenses - at least for me.
It's worth it to me to have RO/DI water at home.
I've got too much invested in the tanks to skimp on freshwater.


Everyone refers to it as "waste water".
Once the tap water has gone through the RO/DI process and produces this "waste water" does this mean that this wasted water is totally useless?
Someone once told me that the waste water produced has been RO purified by the time it exits the RO/DI unit. Is this right? Or is this waste water possibly worse than the original tap water?
I have been using the "waste water" from my Kent Marine RO/DI unit in my freshwater Discus tank for about 6 months now and I have had no problem thus far. Would it be harmful for me to continue to use this waste water in a FOWLR setup?
very perplexed novice


Active Member
I think the waste water is probably better than what comes out of the tap. It has gone through a sediment filter and one or two carbon filters. It just takes a little time a pressure for water to go through the membrane and the majority of it gets passed out as waste water.
You've got me curious now. The next time I make up water, I'll collect a glass of waste water and test it with my TDS meter and compare it to the tap.


I would not use it for a Discus tank they like soft water and the water that comes out is too hard. it has all the stuff that the ro rejected. i would test it


Active Member
The waste water is water that has "NOT" passed through the RO membrane.
It's reject water, as javatech mentioned - water than did not make it through the membrane and will contain some of the minerals/sediments that made it through the prior mechanical/carbon filtration cartridges ( if you have them installed ).
You can use it for anything you want - but it's going to be hard water - containing some degree of minerals and such.
The amount naturally depending on the quality of the tapwater.
I would imagine that house plants would dig it.


Active Member

Originally posted by broomer5
The waste water is water that has "NOT" passed through the RO membrane.

That's what I was trying to say, I just worded it poorly.
I actually made some water tonight and tested it with the TDS meter. I was suprised.
Tap - 434
RO - 15
RO/DI - 0
Waste Water LIne - 551
The waste water actually has more disolved solids than the tap. I guess it makes sense if you think about it. All of it comes in distributed evenly among the water. If 1/4 of the water passes through the membrane, the solids become more saturated in the waste water.
The waste water is good for other things like letting it water your lawn.


Well then, I guess the "waste water" really is waste water.
It's funny you mention the house plants because I have been using it for them since I've had this RO/DI. I figure it may not be totally purified but it has had the chlorine removed........correct? Where I live the smell of Zest soap isn't what wakes me up in the morning shower, it's the smell of the chlorine! I have alot of money invested in my Bonsai Trees so this is important to me that they have good water.
Thank you for testing the waste water. Based on your results I will refrain from using it for anything except plants.
Speaking of testing water, what is a good test kit for a beginner starting a FOWLR? I know there is a million to choose from but that's why I am asking so I can narrow it down to one that's more accurate than the rest. I trust the opinion of people on this forum more than that of my LFS.
Thanks again


New Member
MAN, that sounds like a lot of waste water!! if their is that much waste how long will my filters last, 6 months, 1 year, etc.? does any body have an idea?? :confused: :confused:


I have a Kent Marine Maxxima Hi-S 60PD RO/DI
They suggest replacing the filters like this:
1st canister: Micron Fiber Sediment filter - Every 6 months
2nd canister: Carbon Block filter - every 3,500 gallons or 6 months
3rd canister: Resin filter - every 400 gallons or 6 months
Top canister: RO membrane - every 3 years
I'm sure these replacement time periods vary with different units but I thought this might give you an idea of how often.


i was just wondering if you could use a PUR water filter that you hook up to the faucet. will it work that same or what


I'm just speculating, but I don't think that the "PUR" faucet water filter will do the same as an RO/DI Unit.
The RO/DI unit removes 99.9% of all contaminants from the tap water. Meaning it even strips the water of oxygen it's so pure.
I'm not familiar with the PUR filter but I'm pretty sure it's not quite as intense a filtration process as the RO/DI units. My local grocery store sells the PUR faucet water filter for $25. My RO/DI costed $200+. I'd like to think I got what I paid for. If you find out that the PUR filter does the same as my RO/DI unit don't tell me because I will cry myself to sleep that night.


Active Member
I have an RODI that I am going to set up this weekend. BUT I also have a Rain Soft water purifer on the house. We put in about 80lbs of salt each month. And replacethe filters. I am wondering if I would have less waste water with this?

reef dude

I also have a Kent Marine RO/DI Hi-S Maxxima 35gpd and i just tested it for phosphates and it read about .2 to .3 ppm Is this supposed to happen???
On Kent's instruction manual it says that nitrate and phosphate readings will be incorrect, but why?


I'm just guessing, but I doubt the "PUR" faucet water filter will be as effective as a RO/DI unit.
The RO/DI system eliminates 99.9% of all pollutants from the tap water. It's so clean, in fact, that it actually depletes the water of its oxygen.
PUR filters are new to me, however I'm guessing they don't filter water as well as RO/DI devices do. The PUR faucet water filter costs $25 at my local grocery shop. My RO/DI system set me back well over $1000.00. I'd like to think that I received value for my money. If you discover that the PUR filter performs the same function as my RO/DI unit, please do not inform me, as I shall weep all night.