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Pure water problems


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Hi Everyone.

Having a problem with my water purification system which is as follows:


Typical 3 stage pre filter (5 micron, carbon block, carbon GAC)

Typical 3 stage RO system which produces 300 litre per day

20" resin filter using high quality Tulision resin

This feeds a 1000litre storage tank

Tape water varies between 175 and 275ppm


I wouldn't say my system is used heavily, maybe about 1000 litres per week.

I back flush my system regularly (monthly)

I been using the pure water system for about 5 years

I change the pre filters approx. every 6 months.

Last summer I changed my RO units for the first time.

Change my resin as and when ppm raises above 12-15ppm




My water quality has has dropped off a cliff measuring about 50ppm. (4 days ago) So I immediately changed my resin which worked, bringing the reading back to zero. But within 36 hours to was right back up. I have just (today) changed all three pre filters and re-connected the system but without the resin filter. It measures 55ppm (tape water 200ppm). Which seems high, so I have also measured the water quality before the RO section - so just the pre filters - and that measures 200ppm, same as the raw tape water. Indicating to me that the pre-filters are not doing anything.


So my overall question is - Where does my problem lie?


Approx. what amount of impurities should the pre filters alone remove?


Approx. What amount should the RO units remove?


Approx. What amount should be left for the resin to remove?


I'm at a loss as to which part or parts of the system are failing and why...


Thanks to all who read this, hope it's understandable and thanks in advance for any help.




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I've just been checking up on that, thought it was last summer but it was acually the summer before, so 18 months old. Not sure how long they should last. I bought the unit second hand and didn't need to replace them for the fist 3 years, so thought they would last longer than 18 months. I'm I wrong?


Seems like you back flush alot more than me. Was always told that every month would be sufficient. Maybe not?


Who should I view the pre filter figures? Is it normal to see that they do not lower the tds readings?

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Prefilters won't make much of a difference to the tds of the tap water passing through them.


The carbon block and GAC filters remove chlorine from the water. Chlorine damages r/o membranes.


Without knowing what carbon block filter you are using, it becomes difficult to make a helpful suggestion. However GAC filters only have a service life of 2500 US gallons according to the label included in my last order. An American gallon is slightly smaller than our Imperial gallon.


Using your stats as a guide, you are using around 4000 litres a week to process 1000 liters of pure if your pure to waste ratio is 3 to 1 which most are that have an inline restrictor fitted. If you were using a CAG filter alone then it would need to be replaced every 2 weeks or so.


A Fiberdyne 10" carbon block filter is good for around 38000 litres. I believe these are the best on the market. At your usage, you would need to replace a Fiberdyne C/B filter every 10 weeks.


But in the early days I only used GAC filters and our r/o membranes lasted 6 years with a 3 month prefilter replacement cycle. This experience of ours probably shows how the makers recommended filter replacement schedule and our experience can be vastly different - unless they have changed to spec of the GAC filters. (The first r/o I purchased was a year old. It was part of a system being sold by a local lad. He changed the prefilters for the first time just before I bought it. Those membranes lasted me another 6 months and they were shot. Changing prefilters sooner is always recommended.)


However, there are cleaners on this and other forums who only change their prefilters every 6 months and process much more water than you do. I guess the amount of chlorine added by the water companies is different from area to area.

One thing I do watch for is when the water companies flush the pipes. I try not to use my r/o at this time.


I would try to find if you have a rogue membrane. I would disconnect the pure on each membrane housing and test the output with my tds meter. (As per Smurf's post - wait about 10 minutes after you start the r/o to let it settle down.) Test after membrane and before di.


Years back RoMan told us to only buy quality membranes. They said that the cheap Chinese copies weren't very good. That was 8 years ago so I don't know if the same advise still applies.


Personally I would only purchase prefilters and membranes from trusted suppliers, Gardiners, Daqua, Gaps Water, RoMan to name a few. Ebay isn't the best place for quality stuff IMHO.

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Quite a bit for me to consider there. And thanks spruce & smurf for your imput.


I did buy my replacement RO units from ebay. £70 for 3. Will look at the suppliers you have recommended this time round.


I am now going to test the individual membrane's as you suggest.


Just one last question. Typically what tds reading should I expect after pre filter and RO but before resin?

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Presume your tds is identical on either side of the prefilters. I think mine has about a 2 ppm difference.


Your membranes should be working to an efficiency of between 94 and 99%. They generally refer to this percentage as the membrane's rejection rate. Our old r/o like yours without booster and with a 50 psi water pressure reduced a tap tds of 254 to 4.


Now I have a 4040 and our water quality has improved due to pipe relining. Our tds is between 90 and 125. My membrane is over 2 years old and we still have a tds of 2 before the di. This was the same with the aforementioned r/o with membranes of around 5 years old.


At 275 your membranes working at 98% efficiency should produce a pure output of around 5 - 6 ppm. It was once suggested that letting your membrane efficiency drop below 96% was a waste as your resin costs would soon negate the cost savings of new membranes.

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dont put pure water into your water container untill its 0.00 i used to put it straight through the di then expect it to be 0.00, but sometimes it used to be 0.05, then i realised i was putting it into water storage container too soon, now i check water coming out of the d i before i put ro tube into the butt i store it. in

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................. it always worries me when the label tells you it will reduce the amount of chlorine rather than using the word REMOVE.


The trouble with some of these products is that they are used to make tap water more palatable. Having a little chlorine left in the water after its passed through isn't exactly a bad thing in drinking water. To expand their sales marketing areas they (Retailers) will include other uses, such as fish keeping and window cleaning. After all, we all use a standard prefilter housing. A reduction in chlorine isn't good enough for our application though.


On another forum there is a window cleaner in Southampton that uses a large di vessel as a carbon block filter. He had issues with reduced membrane life and since reverting to this 3 or 4 years ago he has solved his problems. June at GAPS Water suggested it.




I found his post on this. He posted;


"I use a 35" DI vessel and get a year out of a fill (about a third of the £120 bag) filtering about 430,000 litres last year. So that's about 11,000 litres per £, much cheaper than any 20" filter I've seen. Between the DI and the RO I have a sediment filter in case any charcoal dust gets out of the charcoal - it's acid washed so very little dust seems to get out. Comes out pretty much spotless after a year, as the charcoal also filters out sediment in the water as well as the chlorine."


I asked him how he knows when the activated carbon needs changing. His reply;



"Bluntly, I don't know when the charcoal's spent. I ran it and watched my RO output TDS like a hawk. After about 13 months the TDS hadn't increased (so I assume no chlorine getting through) but I lost my bottle and changed the charcoal.


One of the main reasons I like it is just that I don't have to mess about with changing it more than once a year. I'm all for an easy life. The fact that it's cheaper is just a bonus. It's easy to change as the old stuff goes into a plant border and the new stuff pours in like dry sand.


One thing to note is that if you do it, you're only meant to fill the DI to 2/3 full, rather less than you would with resin."

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Sort of @Duncs but that is not back flushing as all he is doing there is running the ro without it going through the di first which will stop the resin getting hit by tds creep.

I prefer to disconnect the di then back flush the ro beforehand each time I use the ro so the membranes work as efficiently as they can. Meaning the ro output is the lowest tds reading I can get. Then afterwards whilst the di is still disconnected I set it back to produce pure and run the ro for a few mins until the tds has dropped again as low as I can get it before reconnecting the di.



smurf just watched that video do you do that disconnect before your DI until pure

then reconnect great bit of advice

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Hi Everyone.... Thx again for all your input. I am certainly going to take it all onboard.


I have just been out and purchased a pressure gauge that fits on my out side tap. It measures 55psi


So inline with Spruce's experience, and my tap water tdi readings, I should be getting around 5 to 6ppm before DI.


I tested the individual membranes with results as follows 39, 79 & 85.... So they are obviously not working. They were made, or at least supplied by a company called Greenfilter. They are rated at 100gpd


I'm going to give Gapswater a ring for some new membranes and maybe a better carbon pre filter.


To quote from Gapswater:


"Emphasis should also be given to pre-filtration. Reverse osmosis membranes cannot tolerate chlorine so a carbon filter is needed to remove the chlorine. Carbon blocks are the most effective but GAC (granular activated carbon) have less of a pressure drop. We usually advise using a carbon block filter but if your pressure is particularly low you could use a GAC filter although it is advised to change it more frequently and to use a good quality filter".


I've a couple of questions on this.


Is a mains pressure of 55psi considered to be low? Indicating that GAC filters should be used. Or is is high enough to use a Block Carbon filter? Up till now I have been using one of each as recommended by the unit manufacturer - Ro-man.


If you use one of each does the order they are placed in the process matter?

Currently Iv'e been placing them in the following order - Sediment - Block - GAC.




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