RO/DI - What's best bang for the buck?

[feh]

Acroholic;678782 wrote: The advantage of RODI for FW aquaria is that it allows you to manage the water chemistry to your specifications, and also eliminates things like nitrate and phosphate, which can be algae food in a FW tank.

I'm dumping 2 1/4 tsp of KN03 and 3/4 tsp of KH2PO4 in my freshwater tank weekly and I don't have any algae issues. Algae is a result of imbalance or poor husbandry in freshwater tanks. Things like too much light, poor flow, and dirty filters are more likely to lead to algae blooms. RO/DI water in freshwater tanks does have its advantages, but I wouldn't use pure RO/DI.

 

[acroholic]

feh;681945 wrote: I'm dumping 2 1/4 tsp of KN03 and 3/4 tsp of KH2PO4 in my freshwater tank weekly and I don't have any algae issues. Algae is a result of imbalance or poor husbandry in freshwater tanks. Things like too much light, poor flow, and dirty filters are more likely to lead to algae blooms. RO/DI water in freshwater tanks does have its advantages, but I wouldn't use pure RO/DI.

Sorry, I was not recommending using pure RODI in a FW tank. That would be incredibly bad for the fish in there. In a FW tank you need to reconstitute the water hardness, but you can manage the chemistry more to your needs with RODI as a base.

You are adding KN03 and KH2PO4 to your tank, but you failed to mention that this is a planted tank and the two chemicals are food for your plants.

Using RODI in a saltwater/reef tank, we reconstitute the water chemistry with our salt mix.
Dave

 

[feh]

Acroholic;681999 wrote: Sorry, I was not recommending using pure RODI in a FW tank. That would be incredibly bad for the fish in there. In a FW tank you need to reconstitute the water hardness, but you can manage the chemistry more to your needs with RODI as a base.

You are adding KN03 and KH2PO4 to your tank, but you failed to mention that this is a planted tank and the two chemicals are food for your plants.

Using RODI in a saltwater/reef tank, we reconstitute the water chemistry with our salt mix.
Dave

I know you weren't suggesting pure RO/DI in a FW tank. Pure RO/DI screws up the kH for fresh water tanks and can cause pH issues for one thing, you can theoretically fix this with buffers. My point is nitrates and phosphates aren't the cause of the algae whether its a planted tank or not. Algae is opportunistic. You don't need to have a lot of nitrate or phosphates or traces for a bloom to occur. It can be as simple as too much light, not enough flow, dirty filters, low O2, etc. All of which can happen in a fish only FW tank. Stripping nitrates, phosphates or any other elements from the water isn't necessarily going to fix it because algae can survive in some pretty harsh conditions unlike plants.

The reason for RO/DI in saltwater tanks is fairly straight forward though. You want to strip everything from the water because the salt mixes contain everything you need to get as close to real sea water as humanly possible and not removing whats already in the water throws the chemistry off which leads to problems.

 

[acroholic]

Buckeye Field Supply;679537 wrote: It is misleading to tell people they can cut down on waste water by adding a second membrane. Here's why.

First - remember that what folks call "waste water" really would be better thought of as "flush water" in that this water serves the important purpose of internally flushing the surface of the semipermeable membrane to keep the membrane from fouling/scaling.

When you configure a system with two membranes in series (the waste from the first membrane going to the "in" port on the second membrane), for this discussion let's say it's two 75 gpd membranes, the system behaves like you have a single long (75 gpd x 2) 150 gpd membrane.

Now - if you use a proper flow restrictor, that is, one for a 150 gpd membrane, you'll have about a 4:1 waste to product ratio. Sounds familiar, right?

If however you don't change the flow restrictor - meaning you keep using the same restrictor you were using when you just had one 75 gpd membrane, then you'll see a waste to product ratio much lower than 4:1. But remember that the recommendation for a ~4:1 ratio comes from the membrane manufacturer. They are telling you that you need about a 4:1 ratio to keep the membrane flushed and keep the membrane from fouling or building up scale. Run the system with a lower ratio and you will foul/scale the membrane(s) quicker than would have otherwise been the case.

Instead of adding a second membrane to lower that ratio, you could have just changed out your flow restrictor ($4) instead. A much less expensive approach to get you to the same endpoint in terms of saving on waste water.

Now, to confuse things just a bit. Filmtec specs call for the 4 to 1 ratio on the basis of assumptions about the water that will be supplied to the membrane. If you have very soft water you MAY be able to get a decent service life from the membrane running at a ratio lower than 4 to 1 (e.g., 3 to 1). Remember that the waste water from the first membrane is about 25% harder than your tap water.

Bottom line: If what you are after is reduced waste water, experiment with a different flow restrictor for $4 instead of messing around with a second membrane plumbed in series.

As a side note, you can also lower the ratio by increasing the pressure delivered to the membrane (with a booster pump), because flow restrictors are sized assuming you are providing factory spec conditions (50 psi and 77 degrees for Filmtec membranes). Increase the pressure and you'll drive more water through the membrane and viola - less waste water. But as I mentioned above, if you do this (just like over-restricting a membrane) - the lower the waste to product ratio, the shorter the lifespan on the membrane.

Makes sense?

Russ

I have read these arguments many times before, and they may have merit for a lot of the country with higher TDS in their water, but around here the average TDS is about 40, so in the case of a two membrane unit, the second membrane would only be receiving 50 TDS water per your information. And we have ARC Members here reporting a current use life of 12 years for a single RO Membrane. This upgrade works in this area.

A second phenomenal benefit about the dual membrane upgrade is the actual rate of RODI production. I went from 96 GPD (4 gallons/hr) actual production on my former 100 GPD unit (w booster pump) to 264 GPD (11 gallons/hr) with the upgrade, allowing me to run my RODI much less, which saves a ton of water as well, all the while with a measured (by me) waste water : product water ratio of .98 : 1. The TDS of the product water with this setup is 1 before entering the DI canister.

So, IME, this upgrade is a tremendous benefit in terms of waste water reduction and water production rate.

 

[buckeye field supply]

With feedwater at 40 ppm, yes - you could get away with all sorts of things most others couldn't. If your primary goal is reducing waste water, the easiest/least expensive way is to change your flow restrictor.

Russ

 

[acroholic]

Buckeye Field Supply;682018 wrote: With feedwater at 40 ppm, yes - you could get away with all sorts of things most others couldn't. If your primary goal is reducing waste water, the easiest/least expensive way is to change your flow restrictor.

Russ

But if you can do the upgrade and make 275% more RO water while also reducing waste to a 1:1 ratio, seems like a no brainer to me.

Edit:

feh;682003 wrote: My point is nitrates and phosphates aren't the cause of the algae whether its a planted tank or not. Algae is opportunistic. You don't need to have a lot of nitrate or phosphates or traces for a bloom to occur. It can be as simple as too much light, not enough flow, dirty filters, low O2, etc.

Algae, like plants, in a FW tank needs three things to grow: light, micro/macro nutrients, and a carbon source. Without adequate amounts of each it will not grow, so excess light, without enough micros/macros or a carbon source, will not cause algae to grow. Each of the necessary components not supplied by your above examples becomes the limiting factor in growth if it is not present in adequate amounts, regardless of the excess of the other two. Give a FW planted tank a ton of KNO3 and excess carbon, but inadequate light to power photosynthesis, and you will not see excess algae growth. Or give a FW planted tank excess light, but inadequate micros/macros and/or an adequate carbon source, and you will not see excess algae growth.

Certainly algae is opportunistic, what algae is not, but it is subject to the same rules as plants regarding growth.

feh;682003 wrote: All of which can happen in a fish only FW tank. Stripping nitrates, phosphates or any other elements from the water isn't necessarily going to fix it because algae can survive in some pretty harsh conditions unlike plants.

The purpose of RODI in a FW tank, particularly a FW planted tank, is to remove potential food sources for algae, like nitrates and phosphates, not prevent algae growth. Why add it in the beginning when you don't have to?