RO unit

So what ppm do you let your RO filter get to before it's time to change it?

I'm currently on 128 [emoji848][emoji15]

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@spruce is the man to tell you this but my opinion is if it's not removing at least 90% of the impurities I would be looking to change mine. So as long as your water is 1200ppm then you're fine  :1f603:   :1f44d:

mark lee said:

So what ppm do you let your RO filter get to before it's time to change it?

I'm currently on 128

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Thats far too high!

Not long after I first started wfp the second hand r/o I bought needed replacement membranes due to lack of maintenance  (replacing prefilters). The r/o was just a year old and the prefilters had never been changed.

Back then Peter Fogwill advised that once the membranes are only 94% efficient is when to change them. Now this was a general rule of thumb - if you are in a soft water area then letting it drop a bit further isn't going to eat into your resin too much. But if you are in a hard water area then it will be very costly as you will use a lot more resin.

I found that once your membranes start to deteriorate they then deteriorate very quickly. So that 94% is a pretty good cutoff point IMHO as it could take a few days to get a replacement membrane and fit it.

You haven't stated what your tap water tds is so you will have to work it out for yourself using this example.

Lets say you have a tap water tds of 100 and the pure output from the membrane is 10. To work out your r/o's efficiency as a % is easy.

100 - 10 = 90.

90 divide by 100 = 0.90.

Multiply that by 100 gives you the efficiency of your membrane as a percentage; = 90%.

Its below 94% so the membrane needs replacing.

So lets say your tap water tds is 250 and your output from the r/o is 18.

250  - 18 = 232.

232 divided by 250 = 0.928.

Multiply by 100 to give the figure as a percentage = 92.8%

A membrane running efficiently will remove around 98% of the impurities in the water. So using the second example of tap water tds of 250 your pure should come out of the r/o at 5 ppm which will then be polished off with the di.

It imperative to maintain your r/o regularly. Prefilters are very important to the longevity of your membrane. The carbon filter removes chlorine from your water. Chlorine destroys membranes. Carbon filters have a service life. Once thats exceeded then your membrane is vulnerable to chlorine attack.

I have a 20" GAC carbon block filter under my desk. There is a label inside it telling me its service life is 2500 gals. Thats American gallons and thats 10,000 liters. So that filter is good to remove the chlorine from 10,000 liters of waste and pure going through the membrane. So lets apply this to our 4040.

We were using 26,000 liters of water per month through our r/o. (I have a sub meter on my r/o.) As the r/o is set to a ratio of about 50% waste to 50% pure, we used about 13,000 liters of pure in this example when there were 3 of us drawing water. Using this GAC filter would mean replacing it every 10 days. This is why we use Fiberdyne c/b filters as they have a service life of 76,000 liters meaning I only have to change prefilters once every 3 months.

Now you may ask how the manufacturers know how much chlorine is in our water. They don't, so they have to base the assumptions on the worst case senario. @doug atkinsononce stated that the chlorine in our water becomes less concentrated the further we are from the dosing point. But I have found that as long as I stick to that service life recommendation I get a good service life from my membranes. (My 150gpd membranes lasted 6 years and my 4040 membrane is just over 5 years old as still performing at 98%.) 

I would also suggest that you buy a good quality membrane from recognised suppliers. Axeon is a good quality membrane made in the USA. The chinese membranes are cheaper but are poor quality.

Here's an example

http://www.windowcleaningwarehouse.co.uk/best-value-hf4-4040-membrane.html

WCW tell users that this is best value HF4 4040 membrane for £149.00 excluding VAT.

They also say

Value Performance 80-85% 40" EXTRA Low Pressure Membrane

Now a good quality Axeon membrane will cost you £204 excluding VAT from Daqua.

WCW are telling you that this cheap, good value membrane is 80 - 85% efficient and its for soft to medium water areas. (So they aren't hiding anything. But windies don't tend to read the small print; they are easily persuaded by price.)

So lets say our tap water tds is 100. In the worst case senario your pure from your membrane will show a tds of 20ppm which will have to be removed or polished off by resin. If my water was 100ppm the output would be 2ppm with my efficient Axeon membrane.

If I use 5000 liters of pure a month I will spend about £14.50 on resin according to the calculator using that cheap membrane.

If I use the same example using my Axeon membrane I actually don't need to use resin at all as 2ppm is fine for cleaning windows. But as I prefer to clean with 0ppm my monthly resin cost will be about £1.45.

So in 4 months the additional cost of resin has wiped out the cost saving of buying the cheaper membrane. This doesn't make good commercial sense.

"The bitterness of poor quality is remembered long after the joy of a low price" springs to mind.

So the places to buy good quality membranes are Daqua, Gardiners, GAPS water, etc.

.