Beer is mostly water and the substances dissolved in it can make a difference to the brew especially to the mash and the hop/malt flavor balance. This is mainly of concern to All Grain brewers but all brewers need at least to be sure that there isn’t too much Chlorine in the water. The addition of 1/2 a Campden tablet will usually be enough to remove it from 25 liters or so of water.
What I do:-
I measure out the water I’m going to use for the mash and sparge then use a Salafert Alkalinity test kit to determine the alkalinity of that water. I enter this result and the details from my water suppliers online water report into a water treatment calculator, I use the one at
and then treat the water as instructed.
The calculator can look imposing but if you read the notes with it all should become clear, it isn’t actually hard to use. The Brewers Friend website also has very usefull notes on water treatment.
Lots of experienced brewers have their water supply independently analyzed but I use the water suppliers report it’s an average so it’s wrong but being an average it’s likely to be close enough. Independent analysis of a spot sample will only tell you what was in the water you sent to be tested and that might not be the same as the water coming out of your tap a week, a month or a year later.
You can avoid this by using water from a Reverse Osmosis system, if it's working correctly the water should be empty of other substances and you can use the water treatment calculator to determine what to add to it.
In the UK it should not be necessary to use RO, treating your water should be enough and even hard water can produce good Lagers.
If you want to gain real understanding the book :- Water: A Comprehensive Guide For Brewers by John Palmer and Colin Kaminski. covers the subject in depth.
However it’s not going to get onto any "good read" lists and you may prefer simply to poke your eyes out. I found it dreadfully unreadable and my background is in Analytical Chemistry so I know boring when I see it.