I think that water treatment has made the biggest impact on the quality of my beers (besides fermentation temperature control and pitching more yeast). It has the greatest impact on very pale or dark beers, which both taste much smoother now that I adjust the various levels of ions in the water. That said if you are either just starting out or simply can't be bothered rest assured that you can still brew some very good beers without worrying too much about your water (read #6 anyway).
Most of the water guides out there in books and online tend to be heavy on the “why” and light on the how. What follows is the water treatment process I go through for most of my batches.
It is aimed at all-grain brewers. Malt extract contain all the minerals that were concentrated from the water that was used to brew it, so ideally you would be using distilled or reverse osmosis (RO) water to reconstitute it. You can still add some salts to the boil for flavor, but in most cases if you are using moderate tap water you probably already have more than enough ions.
In my experience it is not a good idea to copy historic or famous water sources (Dublin, Munich, Burton-on-Trent etc…). The brewers in these areas often go to great lengths to treat their water to make it suitable for brewing, it is much easier to start from your local water and tweak it to make an ideal water for the beer you are brewing. I have made several below par beers simply by dumping water adjustment salts (Epsom Salt, Table Salt, Calcium Chloride, Gypsum, Chalk, and Baking Soda) with no real concept of what levels were right for the type of beer I wanted to come out with, don't let this happen to you.
1. Research your water. The important ion concentrations to find out are calcium, sulfate, sodium, (bi)carbonate, chloride, and magnesium. I have always been able to get this info for my tap water off the city's water department website. ProMash has a very helpful tool that will calculate your Ca, Mg, or Alkalinity based on CaCO3. If your water department doesn't have a site, or the site doesn't have all the ions listed, trying emailing, or calling them. As a last resort you can send your water to a testing company like Ward Labs (Either test W-6 or W-5A).
2. Determine what flavor ions you want in your beer. Calcium, magnesium, and carbonate primarily effect the pH of the mash, the other three are just there for flavor. Here are my suggestions for various types of beer:
Sulfate - Hoppy beers get 175 ppm, moderately hoppy beers get 75 ppm, and low hopped beers get less than 50 ppm.
Chloride – Hoppy beers get less than 50 ppm, moderately hoppy beers get 75 ppm, and low hopped beers get 100 ppm.
Sodium – Hoppy beers get less than 30 ppm, moderate hop/malt beers get 50 ppm, and very sweet/malty beers get 75 ppm.
3. Determine your estimated Residual Alkalinity. To do this John Palmer has a great spreadsheet on his website. Use any brewing recipe software to determine the SRM (color) of your recipe, then input this number into the spreadsheet, the spreadsheet will give you a range of RA, I just take the average of the high and the low.
4. Determine the rest of your water profile. Tweak the target water profile in the Residual Alkalinity spreadsheet until the RA matches the estimate needed for your beer. Adding calcium with lower the RA, adding carbonate will raise it. Magnesium will lower the RA as well, but it is much less effective than calcium and can give an off flavor if there is too much of it so I rarely go above 15 ppm. The spreadsheet also tends to require huge amounts of carbonate to get the RA high enough for very dark beers, but I haven’t found it necessary to go over 300 ppm. All beers need some calcium (it helps enzymes in the mash and promotes yeast health) so even if you need a high RA I would not go below 40 ppm calcium. If you have a lot of carbonate in your water you may want to dilute it with some distilled or reverse osmosis water to lower the number, it is hard to brew a very pale beer with more than 75 ppm carbonate.
5. Determine your salt additions and/or dilution. Enter your water (from step 1) and your target water (from step 4) into the free program BreWater 3.0. The program has a wizard that will automatically determine how much of each salt you need to add to reach your target water profile (a bit of rounding on your part is fine). If you have too much of any ion your best option is to dilute the tap water with store bough distilled or RO water. If you add a significant amount of pure H2O it is always a good idea to add some yeast nutrient blend, near the end of the boil, which will replace the trace elements that tap water contains (copper, zinc etc…) which are used by the yeast.
Steps 1-5 should be completed before you start brewing. Steps 6-10 are done on brew day, although 6 and 7 could be done the night before if you want to have your water and salts prepped.
6. Remove the chlorine/chloramines from your water. A carbon block water filter is the easiest way to accomplish this. In the long run it is much cheaper than a standard Brita/PUR filter and it filters water much faster. There are chemical and heat options as well, but I haven’t had great luck with campden tablets and pre-boiling the water seems like more effort than it is worth. This is also a good time to add any distilled/RO water. This may be the most important step as chlorine compounds can combine with phenols from the yeast or malt and create horrific medicinal chlorophenols.
7. Weigh out the salts on a gram scale. There are volume equivalents available for the weights, but since you are dealing with such small amounts it is best to be as precise as possible. I like to weight out each salt individually and then add it to another bowl. Divide the custom salt blend into two dishes by weight, one for the mash water and one for the sparge divided in the same ratio as the water. For example, if you will be using 3 gallons to mash followed by 6 gallons to sparge you should add 1/3 of the salts to the mash, and the rest to the sparge.
8. Mash in, mixing the malt, hot water, and the mash portion of the brewing salts. Wait 5 minutes for the pH reactions to take place, pull a small sample of wort, cool to room temperature in a bowl, and take a pH reading (I use Color pHast strips, but a pH meter is an option). A pH of 5.5 is optimal at room temp, which is equivalent to a pH of 5.2 at mash temps, but anything within .2 up or down of that is fine.
9. If the mash pH is on target, let the mash rest as normal (this happens for me 90%+ of the time). If the mash pH is too high add ¼ tsp of phosphoric acid, stir and take another reading, repeat until the pH is low enough. If the mash pH is too low, add either chalk or baking soda ¼ tsp at a time until the pH is high enough. It would be a good idea to note how much change each addition causes so you can use this information next time.
10. Add the rest of the salt blend to the sparge water, some salts may not dissolve completely in water so it is worth giving the water a stir occasionally to keep them evenly suspended. I also add phosphoric acid to lower the pH of my spage water below 7 if I am making a very pale beer. The amount of acid you need will depend on the buffering capabilities of your water, it takes about ¼ tsp of 10% phosphoric acid per gallon to lower the pH of my water enough.
If you want a more technical “why” discussion of water treatment, take a look at How to Brew by John Palmer or New Brewing Lager Beer by Greg Noonan, both of which are very in depth and do a good job explaining things like why calcium lowers the mash pH or how to calculate salt additions by hand.