I'm sure you've heard it before: Beer is over 90% water. That little adage would lead you to believe that quality water is the most important factor in good beer. I won't go that far, but quality water does help. Aside from temperature controlled fermentations, starting with reverse osmosis water and building back minerals has made the biggest difference in the quality and consistency of my brews. The best part is, it's really not that difficult or confusing. It's easier for me to do this in a Q&A format, so i'll be asking my self questions =)

What the hell is reverse osmosis water?
 
Reverse Osmosis is a selective filtration process that greatly reduces the ion levels in the source water. It also removes many of the nasties found in water that you wouldn't want to drink. You're probably more familiar with R/O water than you think. The water machines that dispense water in front of grocery stores for 25c per gallon, those are R/O machines.

Ok, but why not use tap water?

Like many big urban cities, in Metro-Phoenix, our water is awful for brewing. It's some of the hardest water you'll find, it's loaded with chlorine, and rather high in sodium too. If you filter out the chlorine, it might be decent water for hoppy or roasty beers, but the other issue is the mineral levels vary greatly depending on the water's source. So you're rolling the dice on pretty much every batch. Thankfully my brew-buddy Greg has a R/O system installed under his sink, but even for brewers that don't, you can buy R/O water from those machines in front of the grocery store. Just fill up a fermenter or two.

So what makes R/O water better?

I won't get into too many of the technicalities of reverse osmosis, but there are a few important things to know. First is that using R/O water isn't going to win you the 'Greenest Brewer in the World' award. Typical commercial R/O units waste approx. an equal amount of water than what is recovered. So your 10 gallons of water took 20 gallons to make. Household under-the-sink units are worse, usually wasting water at an 8 to 1 ratio. The next important thing to know is that R/O reduces ion levels in water by roughly 9 to 1. So if your water had 300ppm of calcium, after R/O it would have ~ 33ppm; it's not quite as pure as distillation. Because of this, it's important to know the mineral content of the base water, since it will affect how you build up water for styles that require very soft water (e.g. Czech Pils).

Huh?

Basically, R/O strips all the minerals in your water down to very low numbers we can consider a 'baseline'. From that baseline, we can easily add back what we need to make our water ideal for brewing.

That makes sense, but why not just use straight R/O water for brewing? 

Well if you're an extract brewer you certainly can, but you'll most likely find that your beers are missing something that made the flavors 'pop'. If you're an all-grain brewer, you'll have the same issue, but you'll also suffer some efficiency issues, as your mash pH won't be in the ideal range. As it turns out, we need the minerals that are typically found in our tap water. Starting with R/O water gives us the ability to control the concentration of those minerals.

So how do I start building my water, I've seen brewing water calculators online, they look very complicated?

Assuming your source water isn't off-the-charts hard, post-RO, it'll be relatively devoid of minerals (remember 9 to 1 reduction). For the basic style of beer, I simply:

-Add 1tsp of calcium chloride per 5 gallons of water
-Add 2% acidulated malt to my grain bill (typically between 3 and 5 ounces for a 5 gallon batch.

That's it.

But don't different beer styles require different water?

Yes, that's correct.
Roasty beers (stouts, porters), I skip the acidulated malt.
Hoppy syles (APA, IPA, IIPA), I also add 1tsp of gypsum
Soft water beers (Czech Pils), I cut the calcium chloride down to 1/2tsp
British styles, I double the calcum chloride, and add 1tsp of gypsum.

Those are what work well for me. You may find you need to tweak those numbers some to find what works best for you.

Aren't there other minerals that brewers add like chalk, epsom salts, and baking soda?

Yes.

And......?

I don't find I need them. Acid malt, calcium chloride, and gypsum are all I ever use, and I've made plenty of great beers.

Won't acid malt make my beer taste sour?

Good question. Beers will taste better when their pH is in a certain range. Without acid malt, a mash starting from R/O water will have a pH that's too high, efficiency will suffer, and the beer may taste a little flat. Adding 2-3% acid malt will help lower the mash pH by .2-.3, and I find that it helps accentuate flavors. If you use too much acid malt(5-6%) you will start to taste the acidity, so keep it under 4%.

Lastly I need to give credit and many thanks to ajdelange on Homebrewtalk. Much of this information is from his excellent water chemistry primer.Over the past 18 months, I've made some great beer following his techniques for building water, and I've very thankful for the good advice!