RODI unit output slowed

[jaytizzle]

I noticed a few days ago that the output rate from my RODI unit has slowed dramatically. I used to get good output but now it's down to maybe 10 gallons per day. I assume that the RO membrane is shot. Does this sound about right?

I have a 5-stage 75 gpd BRS unit with carbon and 5-micron particulate pre-filters, RO membrane, then dual DI filters. I'm still getting 0 TDS out and around 130-150 ppm in with 62-63 psi on the first stage.

I'm thinking about replacing the RO membrane. I've also been considering adding a second membrane (upgrading to the 150 gpd setup), and adding a 1-micron pre-filter or carbon block stage. This would make it 5-micron prefilter, 5-micron carbon block, 1-micron carbon or prefilter, dual RO membrane, dual DI resin. Anybody see any issues with this? It may just be overkill as 0 TDS water is 0 TDS water regardless of how you get there, right? It

 

[SmokeyCFH]

I've read that once the PSI gets to 80 it's about time to change out the ro and carbon blocks. My PSI reads 60-65 and they're brand new and my TDS reads 34 in and 0 out. Obviously there is going to be A LOT more waste than purified water, maybe you should flush your system, there could be a build up somewhere in the tubes.

 

[Conrad]

Being from LA, I'm guessing you don't have a cold water problem. But if the temperature of the water is too cold, it will not produce water very fast at all..... During the winter I eiether have to put a bunch of RO line coiled into a bucket with water/heater so it brings it up to temperature. Just something to check...

 

[jaytizzle]

Thanks, Conrad. I'm thinking it could be temp related as well. It's been quite cold down here lately (below 50°F the last couple of times I tried to run the unit, it's in my garage, duh!). I had a friend recommend trying to put a heating pad on the RO membrane housing. I'll see if that affects things. We should be getting warm here in the next few days so I'll try again then as well.

 

[AZDesertRat]

What are your water pressure at the RO membrane, water temperature, exact measured waste ratio (not what they claim), tap water TDS, RO only TDS before the DI and final RO/DI TDS numbers. What is the difference between your static water pressure with the treated and waste lines valved off or crimped and the flowing pressure at the membrane? Does it go up more than 2 or 3 psi?
How old are the sediment and carbon block filters? Did you disinfect the system when changing filters?

0 TDS is not 0 TDS. Read the articles in the FAQ section at www.spectrapure.com for a better explaination.

You do not want any more filters in front of the membrane than necessary, everything you add has an associated head loss which reduces efficiency and GPD. You want better filters, not more filters. I run 0.2 or 0.5 micron absolute rated sediment filters in my systems and a single 0.5 micron 20,000 gallon carbon block so the RO membrane is well protected.

If you want a dual membrane system you need a minimum of 65 psi for it to function at all and 80 or more is preferred.

DO NOT, let me say that again, DO NOT try to warm the water or membrane! That is the fastest way there is to ruin a membrane in a heartbeat. They cannot take anything over 113 degrees or the glues that hold the fabric melts and it is shot. 113 is not all that hot when you think about it. Besides, colder water treats better than warmer water since it is more dense and wil give you lower TDS and longer DI life. Ig temperature and GPD are an issue them look at a RO booster pump which is better bang for your buck and will overcome any cold water or low pressure problems. You should be able to easily push a 75 GPD RO membrane to 110-120 GPD even at 55-60 degree water temp. I get 120GPD at 95 psi and 58 degree water temp with 630 TDS at the tap.

 

[jaytizzle]

Hmmm... a lot of questions and info there. I'll be able to shift measurement devices around and check on all of those parameters this weekend when I've got time to play with the unit. I doubt anybody keeps enough pressure gauges, temp gauges, and TDS meters in their system to monitor all of those parameters all the time.

I'll look into the tighter micron prefilters as well. I don't really know what got me on this kick other than I can. I guess a booster pump would be a better investment but I've yet to have any problems with my RODI so not sure why I want to go changing things.

RO pressure = Unknown
Water temp = Unknown
Measured waste ratio = Huge right now
Tap TDS = 130-150ppm
RO only TDS = Unknown
Final TDS = 0 ppm
RO pressure with lines crimped = Unknown
Prefilter age = Replaced around September 2010, currently 1.5 years old
Disinfect = Huh?

Guess I need to do some more reading... didn't realize this was or needed to be so complicated.

 

[Conrad]

There is nothing wrong with putting a bunch of ro line coiled into a bucket of water with a small heater set at 60. Tons of people do it including myself and its very safe as the water won't be getting too warm. Now putting heat directly on the membrane could easily damage it.

Conrad

 

[AZDesertRat]

Plastic tubing is a horrible conductor of heat and cold so is a waste of money as far as a heat exchanger. Take a look at how many BTU's are required to het one gallon of water even one degree and you will see why this is foolish and does not work well when you have water passing through in the tube. Its similar to the people why try to chill water with a dorm refrigerator, the power costs far outweigh the benefit.

 

[jaytizzle]

Okay... took some measurements today...

Pressure into unit = 62 psi
Pressure into membrane = 66 psi
TDS into unit = 123 ppm
TDS into membrane = 103 ppm
TDS out of membrane = 25 ppm @ startup, 5 ppm after running a few seconds
TDS out of dual DI = 0 at all times
Water temp = 66F
Clean water production rate = 100mL in 3:15
Waste production rate = 100mL in 0:10
Waste ratio = 19.5:1 (not sure I did that right?)

One thing, I think my dual TDS meter may have a bias. I swapped the probes from the input and output and found:
TDS IN = 110 ppm (using what is typically OUT probe)
TDS OUT = 7 ppm (using what is typically IN probe)

Is it possible to get 7 ppm TDS OUT when I'm using a dual DI setup with color change resin and I can clearly see that I have plenty of resin blue resin (~90%)?

 

[twinspots_goby]

I have the same very low output problem with my BRS ro/di unit. When I first bought it, it gave me a fairly good output. 120 gpd and my unit is 150 gpd. 1.5 years later, now it's just 50 gpd. I changed membranes and filters, and they didn't help. I wonder somewhere was clogged inside the tube or somewhere in my unit.

 

[AZDesertRat]

Some of your numbers don't make sense to me.

You say TDS into unit=123 then TDS into membrane=103? Where are you measuring those numbers and how? Do you have access to a handheld TDS meter so you can get more accurate numbers? The TDS from the tap and the TDS after the carbon block or into the membrane should be near identical since the sediment filter only removes suspended solids, not dissolved solids, and the carbon may at most remove 1-2 or so TDS which is actually residual chlorine and a few organic chemicals.
The TDS out at startup is normal, this is called TDS creep and is inherent w=to all forms of membrane treatment. What it is is when you shut a membrane off, the water inside the housing osmoses or migrates from the tap water side to the agressive treated side and when you start it back up the initial water contains that TDS, often as high as half the tap water TDS for a few ounces.

If you have 5-7 TDS in the treated RO water and 103-110 in the tap that gives you a rejection rate of about 93-95% which could be a little better. I strive for 98% or better myself. For every 2% you increase the RO efficiency, you double the life of your DI resin.

A dual inline TDS meter only has two probes, where are your placed? It should be post RO and final RO/DI so how are you measuring the tap water TDS? Do you have two dual inline TDS meters or a handheld or ?

Swapping the probes should give you similar results +/- a few TDS. Make sure the probes are fully inserted in the tees and the probes are rotated in the correct orientation of direction of flow for best results. Inline probes cannot be stuck in a glass of water or used portable like a handheld so have limited capabilities plus they are not temperature compensated so have some built in inaccuracies if the water and air temperatures are not exactly the same. I much prefer a handheld such as the HM Digital TDS-3, TDS-4TM or COM-100 for accuracy and ease of use.

 

[rvela]

You may need to replace your flow restrictor. I was having the same issue and that fixed the problem right up.

My issues were slow output and my DI resin was getting used up very quickly. There are two types of restrictors...the small inline and the others are the quick disconnect. The latter is what I have.

Try this if you have a quick disconnect...pull it off and inspect the small screen inside. I soaked mine in hot water/vinager mix for a couple hrs and reconnected...sure enough my waste water increased significantly, which in turned sped up my water production. You can find replacements for about $5 online. Make sure you get one that is rated for your r/o unit water production(e.g. 75 gal per day).

 

[AZDesertRat]

Time the treated and waste both for one minute.

 

[AZDesertRat]

When you increase the waste, the product or treated water should drop not increase. You are allowing more volume and pressure to escape out the waste line thus reducing the pressure available to drive the remaining water through the membrane.

If you presently have a fixed type restrictor, a capillary tube is much better since you can fine tune it to an exact 4:1 waste ratio which is what you want.

 

[jaytizzle]

I did time the treated and waste. Instead of gathering volume over 1 minute I took time to reach 100mL. It was 3:15 for the treated and 0:10 for the waste.

I am using a dual in-line TDS meter. I typically have it with one on the inlet to the unit and one on the outlet. I can easily move around the probes, though, so that's how I took all of the readings.

As far as the 20+/- TDS difference across the sediment and carbon blocks, that's what I'm getting. I can't offer an explanation other than that is exactly what the meter told me and it was the same probe that I measured the inlet with so it should be at least precise even if it's not accurate.

I should be able to get my hands on a handheld TDS meter through work. I'll see if I get any different readings going this route.

 

[AZDesertRat]

The formula for figuring waste ratio works much easier if you time the two flows for exactly the same amount of time rather than volume, just divide one into the other with no conversions needed. You want 4:1 waste to good so eithe rway you need a new flow restrictor and a capillary tube type will allow you to adjust it correctly. I would do the testing and trimming with the treated water flowing to an open container with no restriction personally but if you normally run the system with a valve restricting the treated water then do it that way, restricting the treated drives the waste up tremendously, often well over 25:1, thats why if you had a pressure tank a permeate pump is a good investment to help reduce excess waste due to backpressure.

I would eliminate the valve on the treated line and let the float valve or solenoid valve and float switches do their thing unrestricted. Your RO/DI system will thank you.

I have two dual inlines so 4 probes but honestly rarely tun them on since they never agree with the COM-100 handheld. I consider them like a litmus paper test, a rough indicator or guide only.

 

[jaytizzle]

Where do I get a capillary tube type flow restrictor? I've looked for them briefly but not exactly sure what I'm looking at.

There is no valve on the treated water line. I have a solenoid on the feed line to the system that is triggered by floats in my ATO container (ATO is metered to the sump by a Tunze Osmolator). When the water level gets very low (~1 gallon), the solenoid opens and the unit starts running. When it gets full (~12 gallons), the solenoid shuts off.

Based on a rate of 100mL in 3:10, I'm looking at a production rate of 11.7 gallons per day (waste is 227.73 gpd!). That is terribly low considering that I'm getting appropriate pressure to the membrane. Will a capillary tube flow restrictor help fix this? Do I need to go ahead and get a booster pump? Should I replace the membrane? Should I replace the prefilters?

I guess where I'm at now... I see that there's a problem... what should I do to fix it?

 

[AZDesertRat]

No, you do not need a booster pump, you have 62 to 66 psi which is more than sufficient. Your problem is you are sending the pressure down the drain via the flow restrictor.
You can get a capillary tube flow restrictor here:
RO MEMBRANES AND FLOW RESTRICTORS
if you have a 75 GPD membrane get the FR-90, same thing.

Or here:
http://www.buckeyefieldsupply.com/showproducts.asp?Category=197&Sub=109

I prefer to trim my restrictor myself so I get it exactly where I want it.

Just for giggles, until you get a new restrictor you can stick a ball valve or needle valve on the waste line and adjust it down to 4:1 ratio temporarily, you should see a big improvement in GPD. At 66 psi, 4:1 waste ratio and 66 degree water temp you should be seeing around 83 GPD out of a 75 GPD membrane. is your water really 66 degrees? Even in Phoenix mine is only 58 degrees much of the year. I hope you are not heating the water or blending hot and cold?

 

[jaytizzle]

No heating or blending... that was the water temp as of yesterday afternoon... oddly enough it was 67F outside. I'll double-check that temp reading to be sure. It's the same cold water line in my house it's just outside on a hose bib.

Thanks for the link to the restrictor. I'll get one ordered ASAP and revisit this thread then.

EDIT - just looked at the restrictors... should I order a certain one and trim it down or should I order the exact one (75/90gpd)? If I want to trim my own down, which one should I order? I am unfamiliar with this so I don't know which one is longer (lower gpd or higher gpd) so I can trim it down properly.

 

[jaytizzle]

Okay... ordered the FR-90 flow restrictor. Also got myself a new sediment filter and carbon block while I was at it since shipping was the same.Thanks for all the help, AZDesertRat! Hopefully this solves the problems.