Misleading Asda Water!

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It's also handy to have a few 1/4" push-fit taps if you haven't already got some lying around, so you can shut the input and output of the filter off when you're not using it (as I said above, you want to keep it full of water when you're not using it, or the membrane will dry out).
 
If instead of buying the 3/4" adapter separately, you get one of these sets - then you get a couple of taps and a few other bits and bobs thrown in: Tspkey Ro Water Filter Fitting, 1/4" Pushfit Straight Connector for Water Pipe, Push in to Connect Water Tube Fitting Set Pack of 12 : Amazon.co.uk: DIY & Tools

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In defence of Asda et al, they are selling water in their drinks section, it's drinking water, not brewing water and the composition is listed as typical values. So they're not really misleading anyone as their product is fit for its purpose.
 
Just as a footnote, those RO filters look a bit intimidating and technical with all the pipes and so on - but in fact they are really simple. Here’s what’s actually going on:
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The first two stages just remove small particles of sediment and any ‘organic’ odours; but the main action takes place in the larger vessel which is where the RO membrane lives, wound up in a spiral so it takes up less space.
The membrane lets pure water through but not dissolved minerals. So as the water flows past, a bit of pure water ends up on the other side (and flows out for you to collect) and the majority of the water just flows past and comes out as ‘bypass water’.
Note that a decent flow rate of bypass water is required to ensure the minerals are continuously washed away (otherwise they would very quickly block up the membrane and stop it working.
Finally there is a flow restrictor that stops the bypass water flowing TOO fast. There is also a special tap called the ‘flushing tap’ that you can use to temporarily get a faster flow of water past the membrane - which you do for 5 mins or so when the filter hasn’t been used for more than a few days.
 
In defence of Asda et al, they are selling water in their drinks section, it's drinking water, not brewing water and the composition is listed as typical values. So they're not really misleading anyone as their product is fit for its purpose.

100% disagree. The values are wildly different to what is advertised. I only bought it because of the values it stated to me, which was not what I got. That's wrong. If the values are different to what was originally posted on the website, then the website should be changed. That gives me the choice if I want to buy it or not!
 
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As @Hazelwood Brewery says, you can optionally add a fourth stage called a “DI”
(de-ionising) filter that uses a special kind of charged resin to remove any stray mineral ions that sneak past the RO membrane - but for brewing you probably don’t need to worry about this because the grain will add plenty of minerals back during the mash :-)
 
I was going to ask - been researching. And wondered if a 3, 4 or 5 stage system is better?

Also, do I need the alkaline filter thing to add some minerals back in? Seems pointless to me adding stuff back in? And supposedly it makes it more acidic if you don't use it. They don't say how acidic though. Ugh.
 
Three stage is absolutely fine for brewing.
A “four stage” just adds a DI filter as mentioned above (I do use one because I have it for other things, but it‘s unnecessary for brewing in my opinion).
A “five stage” is unsuitable for brewing: as you correctly say, it adds some minerals back in because pure filtered water tastes really flat if you are drinking it for domestic use. But you do NOT want want that for brewing water :-)
 
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