Water
On a long-distance hiking trip, practical water treatment options come in two varieties: filtration, and chemical treatment. Some hikers even choose not to treat their water at all, though they are in the great minority. The topic of treating water can be as simple or as complicated as you make it. If you are interested in the science of water treatment you can check out the links in the 'More Information' section at the bottom of this page. Otherwise we'll keep it simple.
Filters vs Chemicals
There are two kinds of contaminants that can make water unsafe to drink: chemical contaminants and biological contaminants. Chemical contaminants include gasoline, chemical herbicides, oil, etc. Biological contaminants include E. Coli, giardia, cryptosporidium, etc.
A good filter will filter out both chemical and biological contaminants. Filters don't alter the taste of the water like some chemical treatment options do. However, filters are slow, laborsome, and you must replace the filters on time for them to continue being effective.
Good chemical treatments (like Aqua Mira) will neutralize biological contaminants, but not chemical contaminants. They are generally easier to use: you just add the chemical to the water, toss the bottle in your pack, and continue hiking down the trail as your water treats. Chemical treatments are also lighter in weight.
Since chemical contaminants are uncommon along most of the PCT (Southern California has some exceptions) most PCT thru-hikers use some kind of chemical water treatment.
Lightest Treatment Option
Chemical treatments are always lighter than filter options. Chlorine-based crystals or tablets are the lightest. One example is Potable Aqua. Chemical treatments that come in a liquid state, like Aqua Mira, are only slightly heavier.
Cheapest Treatment Option
If you already own a water filter, then replacing the filter a few times along the PCT may be cheaper than buying chemical water treatment. Otherwise, buying chemicals for treating your water will likely be cheaper. All chemical treatment options are in the same price range, and you should be able to chemically treat all your drinking water during your entire Pacific Crest Trail thru-hike for a total of about $50 - $100 (depending on your trip length and assuming you don't treat cooking water that you'll boil anyway). You may reduce this cost if you learn to identify safe water sources (snow melt, springs, etc) and only treat your water when the source is questionable.
Water Containers
You can carry your water in water bags/bladders, water bottles, or canteens. Water bags and bottles are the most common. Most water bladders have sipping tube attachments available, and many hikers find them convenient. Bladders take up very little space when empty. That's a nice feature - especially for the dessert where you'll need a couple of two liter containers to carry enough water. Plastic soda bottles are also nice because they can be replaced cheaply and often. That means you never have to deal with water mold building up inside them. Heavy plastic water bottles (such as Nalgene bottles) are unnecessarily heavy for long distance hikers.
Lightest Carrying Option
A 2.4L Platypus Bag weighs just 1.2 oz (that's 60% less than an empty 2L soda bottle). A 1L bag weighs even less. To learn more about Platypus Bags, click here.
Cheapest Carrying Option
Empty soda bottles are very light weight, very inexpensive, and easy to replace. Why pay $20 for a heavy Nalgene bottle when you can pay $1 at a vending machine at a trail town and get a new plastic bottle (with a free soda)?
More Information
If you want to delve a little deeper into the subject of water purification, here are some places you mght start: