RO DI pressurized storage tank

[alan mcilvried]

I've searched several forums trying to find out the purpose of the pressure storage tank in an RODI system, and it seems that they are intended to provide a readily available volume of water for drinking (so that when you want a pitcher of water, there is enought available to fill it.)

 

I've also read it helps with "back pressure", but that seems to be anecdotal, as none of the mfr. literature I've seen mentions that aspect.

 

I'm going to purchase a RODI system, and if I collect the water in a large tub, and I'm not intending to drink it, it would seem the pressure tank would not be necessary. I'm curious as to who on this forum uses (or doesn't use) a pressure tank and why.

 

Thanks

Al

[YBeNormal]

I installed an RO faucet in the kitchen for drinking water (tastes great!) and another in the tank room for rinsing test kits, fish food, etc. Having a storage tank give me a ready supply of RO and also prevent the system from kicking on when I use only a cup or glass of RO. They are not necessary for most setups though.

[Guest Bemmer]

I only use my RO/DI unit for my fish tank. I had a pressurized storage tank with my first unit but did not see the value in having it. I do not have one on my second ro/di unit and do not miss it.

[davelin315]

It's used in conjunction with the auto shut off valve in an RO unit. The valve is pressure sensitive and when you pump water into the tank it builds pressure up. When the tank is full, it shuts the valve off and hence the water supply. The tank creates back pressure but I don't know that it impacts your RO membrane as the pressure is post membrane. It's really just for storage when you want a drinking water supply. They sell larger storage systems but they cost a ton. I don't use them with mine as, like you, I use other containers to store it in. I don't find it useful to have the storage tank as I don't use it for drinking or cooking water. Also, when I did have one, the flow out of it was fairly slow because it's passing through the same 1/4 tubing so it's not advantageous when you need it readily available.

[fab]

Alan,

 

A pressure tank is strictly for storing water under pressure so it will deliver itself to a faucet on demand. The normal use is for drinking water. The pressure serves no purpose, whatsoever, beyond that. The backpressure story holds no water, pun intended.

 

An aquarist uses an RO/DI system to produce freshwater for makeup and replacement. Since you will pump water from storage into your display tank, or schlep buckets of water [i hope not], there is no added value for a pressurize storage tank. Further, pressure tanks are very expensive and are usually very small, 5-20 gallons. They do not do much for an aquarist's storage requirements.

 

You need storage because RO/DI units produce water slowly, so you have to produce for a long time then use the freshwater for whatever purpose you have.

 

Beware of 'el cheapo' RO/DI units. They tend to be very expensive to operate. Operating costs are for replacement cartridges and wasted water.

Filtration

Inadequate sediment and charcoal filtration upstream of the RO membrane will reduce the life of the membrane substantially. Good membranes are not cheap. Low end RO/DI systems cut corners on the upstream filtration and on the quality of the cartridges. That results in high operating costs.

 

Membrane efficacy - efficiency

By their very nature RO units waste a lot of water. RO membrane manufacturers generally regard a 10:1 waste water to product water ratio as acceptable. That means it takes 11 gallons to produce 1 gallon of RO product water. The other 10 gallons are waste water. High end units reduce this to around 4:1. One manufacturer, SpectraPure, has a new membrane product coming out that is going to be close to 1:1 ratio of waste water to product water. That is a ten-fold improvement in efficiency. And the membrane will last a lot longer. They also have come out with new DI cartridges that last almost 10 times as long as normal ones. These factors do wonders in reducing operating costs.

 

My performance - 10 years

I have had an RO/DI setup with a drinking water kit in my home for 10 years. By the way, you do not drink water coming out of the DI stage. If you want drinking water, you take the drinking water out of the RO stage before it goes to the DI stage, using a 'Tee' fitting. My RO water quality is incredibly good, as measured in total dissolved solids (TDS) and in actual taste. I have less than 1 ppm TDS at the output of my RO stage, prior to DI processing. My meter is indicated in 1's of ppm, so I can't get a measure of what the fractional value of my TDS is; i.e. no readings between 0 and 1 ppm. In other words, my meter reads 0 ppm after 10 seconds or so of running. Note, after water has been sitting still in the system for a while, i.e., not passing through the RO membrane, you want to let it flow for several seconds to flush through the system, discarding the product water before collecting it for storage or for topping-off. This requires a bit of control in an auto-topoff system.

 

fab

Edited May 18, 2007 by fab

[Lee Stearns]

Fab said it all- but to answer your question-

I use a presurized storage tank as well. Just easier to have on demand water from the system for me.

[traveller7]

Fab said it all- but to answer your question-

I use a presurized storage tank as well. Just easier to have on demand water from the system for me.

Yep, no evaporation, dust, less chance for unknown contamination(that is not a garbage can Mr Party Guest!)......

 

14gal pressure tank here.

[fab]

Water on demand for drinking definitely wants to be via a presurized tank so when you use the lever on the drinking water attachment you get water.

 

Water on demand for aquarium usage in my system is available with the flick of a switch that turns on the pump to move the water from storage to wherever.

 

A lidded storage tank ... no dust. A sign on the lid... no garbage!

 

fab

Edited May 18, 2007 by fab

[Gatortailale]

Edit - sorry for photo size, I just linked to spectrapure's site photos.

 

 

We we got our townhouse in 2000 we put in water softener unit that came with a 50gpd RO unit + 3 gallon pressure tank for drinking. I had just set up my 125 prior to getting water filter system. I also talked with Tom at Marine Scene about setting up my tank and controlling water level. He advised I get one of the $15 Spectrapure floats (device in top of photo) and put in sump. It was method he uses on lots of service tanks. So I got it and ran line from my ro unit under sink to float. It worked great for 5+ years (as long as I had the tank up).

 

When I set up my 215 I wanted to make more RO faster so I got 100gpd RO unit and got this kit to control top off and shut off RO waste water when storage tank (trash can) full. Note, no longer user pressure tank.

 

As long as you add the solenoid to your RO unit to shut off waste water when your pressure tank is full, it will work great for top off. Spectrapure recommends their electronic float in combination with float I used this way when Ro unit kicks on, it runs longer and does not short cycle every hour since they say you can get membrane creep or something like that. Anyway, you can learn more here:

 

http://www.spectrapure.com/liquid_level_controllers.htm

[rioreef]

Follow-on to Craigs post, check out Spectrapures hookup diagrams. They are an excellent resource to visualize the equipment and connections necessary.

 

Link Go the diagrams for "Level Control Kits"

[extreme_tooth_decay]

 

RO membrane manufacturers generally regard a 10:1 waste water to product water ratio as acceptable. That means it takes 11 gallons to produce 1 gallon of RO product water.

 

 

I've never heard of a ratio that bad before. I usually hear 3-5:1. In my own non-optimal case

it's about 3:1.

 

tim

[fab]

Once again, the Devil is in the details:

Spectrapure recommends their electronic float in combination with float I used this way when Ro unit kicks on, it runs longer and does not short cycle every hour since they say you can get membrane creep or something like that. Anyway, you can learn more here:

 

http://www.spectrapure.com/liquid_level_controllers.htm

 

BUT NOT FOR controlling high conductivity liquids such as salt water, or very low conductivity water such as deionized water. Do not use the SpectraPure Electronic Tank Level Controller for any saltwater or DI water applications. I must assume that Gatortailale used a SpectaPure Liquid Level Controller, not their Electronic Tank Level Controller.

 

The SpectraPure Electronic Tank Level Controller has metal components that are immersed in the liquid whose level it is controlling. The metal parts are an electrical hazard in salt water. Also, the DI water will corrode the metal parts and bring about a failure. The Electronic Tank Level Controller is the only SpectraPure level controller that is not appropriate for saltwater level control. All of the SpectraPure Liquid Level controllers are fine for saltwater aquarium or storage tank use and for use with DI water.

 

fab

Edited May 18, 2007 by fab

[Oprah Wrangler]

 

Check out www.purewaterclub.com. The units are good, cheap, and replacement filters are inexpensive also. I have a storage tank on mine and it is ideal because my nano is 3 gallons and my storage tank holds 5 gallons. Not to mention the water is great for drinking and cooking with. Check it out. I dont think you will be disappointed.

[YBeNormal]

Not questioning whether that is a good deal or not since I have no experience with the company or the brand but if I had that model I would swap out the horizontal DI chamber with a vertical one. From what I hear the horizontal DI cartridges are prone to channelization. I also prefer clear units over the white ones so I can see when the filters are getting dirty or have trapped air but that's just a personal opinion.

 

I do wonder when a company advertises that they use "most reliable name brand TFC (made in U.S.A.) Reverse Osmosis membrane" rather than actually stating the brand name. If it the most reliable, than why be so vague? Computer companies often make similarly vague comments and IME it usually means that they are buying and installing the cheapest hardware they can find.

[davelin315]

 

Check out www.purewaterclub.com. The units are good, cheap, and replacement filters are inexpensive also. I have a storage tank on mine and it is ideal because my nano is 3 gallons and my storage tank holds 5 gallons. Not to mention the water is great for drinking and cooking with. Check it out. I dont think you will be disappointed.

 

I haven't heard a lot of good things about that company. They sell on e-bay a lot and I have heard that many of them come with dirty filters. I also bought one used here in the club and it had only been used for 30 gallons of water or so, very minimal, but when I cracked it open to see what the problem was with the water supply (TDS was reading in the 150s to 200s, sometimes even above my regular TDS), I found the dirty filters.

 

In my experience, considering that it was a used filter, and from what I've read, the company is unreliable.

 

Oh, and by the way, I believe that there only 2 or 3 companies that actually make the RO membranes that are marketed. If I'm not mistaken, Dow is one of the big ones?

[fab]

Davelin is right about the horizontal DI design, being inferior and problem prone. In fact the only component you should accept in a horizontally installed format is the RO membrane, itself.

 

SpectraPure is high-end, has clear chambers, and is the way to go with RO/DI units to protect the expensive contents of a marine aquarium. If you research water quality technologies you will find that good water quality control doesn't combine well with 'on-the-cheap' equipment, particularly as you move up in how much water you will need to process.

 

An RO/DI product that achieves low recurring cost through the use of high quality filtration coupled with top notch RO membranes is what produces the sweet spot in the total cost of ownership of RO/DI units. A higher up front cost is a necessary hurdle to get to the lower recurring cost systems. Regardless of which brand you choose, the recurring cost of operation is tied directly to how much junk you pass through the filters, the membrane and the DI chambers. That equates in the end to how much water, in gallons, you can process before you have to replace items. For folks with sizeable tanks, not nano tanks, the recurring operation is a sizeable factor in the total cost of ownership of an RO/DI system and needs to be carefully considered when selecting your equipment.

 

Low initial cost units will prove, in the end, to be of lesser quality in terms of performance and recurring cost.

 

BTW, a nice feature is to have a second DI stage with silicone buster capability.

 

fab

Edited May 21, 2007 by fab

[YBeNormal]

BTW, a nice feature is to have a second DI stage with silicone buster capability.

 

fab

 

I understand the concept behind having second DI chamber and maximizing use of DI resin in the first chamber. I plan to add a second DI chamber to my Typhoon III in the future, maybe even with the current AWI group buy. Can you explain the "silicone buster capability" though? Is this a special resin or some other added feature?

[fab]

From the SpectraPure Website:

 

SpectraPure

[Buckeye Field Supply]

I know this is an old thread, but the issue is one that we help customers deal with frequently. There are a number of posts in the thread we wanted to respond to, so here goes.

 

Is a pressurized storage tank a good idea?

It's a fine idea to store RO (not DI) in a pressurized storage container - but only if your intent is to provide a ready supply of RO water - typically to a faucet for drinking. It's not a good idea to send water from a pressurized storage tank on to a DI stage. Back pressure from the tank is a critically important issue here. We can explain why if folks are interested.

 

Re the ratio of waste water (a.k.a. "concentrate") to purified water (a.k.a. "permeate).

Filmtec membranes for instance are tested under conditions of 77 F water, 50 psi, 250 ppm softened feedwater, and about a 5.5:1 ratio. Membranes are often set up in RO systems to run at about 4:1. If a customer came to us with a 10:1 ratio we would look to fix that situation. They would be needlessly sending water to the drain.

 

Re the devoneb system

I would not rank this system as one that's a good deal for anyone in the hobby. Why? No pressure gauge, horizontal DI, GAC stage after the DI, inappropriate use of a pressure tank, etc... We've reworked a lot of these systems once folks understand what they bought.

 

Russ