I have a number of cheap water bottles, and the water in them always tastes horrible at the end of the day. It has been bothering me for some time, so I set out to do some research and come up with a way to fix it. The water bottles in question are all made from low-density polyethylene (or "LDPE" marked with recycle code 4). This type of plastic is free of bisphenol-A (BPA), and is commonly used in squeezable
water bottles.
The plastic taste does not come from the LDPE itself, but rather the products of degradation of the plastic. When the plastic is melted and extruded into the bottle shape, some small part of it degrades to produce a mixture of chemical compunds: ketones, aldehydes, and carboxylic acids. The degradation products are mixed into the bottle itself, and slowly leach out into the water, giving it a "plastic" taste. Some manufacturers have found ways to minimize degradation of the plastic, but chances are if you have a cheap bottle, the plastic taste will make the water almost undrinkable after a long day of riding.
Here is a method that helps to remove a lot of the awful taste, and uses very cheap supplies:
1. Fill the bottle with isopropanol (rubbing alcohol). The rubbing alcohol will dissolve some chemicals that don't dissolve well in water. You can leave the bottle filled with rubbing alcohol as long as you wish, I recommend a few hours or overnight at least. Rubbing alcohol won't dissolve LDPE.
2. Dump out the rubbing alcohol and wash with soap and water. Then, fill with half white vinegar and half hydrogen peroxide. Let that sit overnight or longer.
3. Rinse out the vinegar and hydrogen peroxide, add two tablespoons of baking soda and fill with warm water. Let that sit overnight or longer.
The above process works pretty well at removing nearly all of the bad taste from some ultra cheap bicycle bottles I bought from wal-mart. After doing this, you should avoid heating the bottle, as that will cause more stuff to leach out. I wash mine with soap in cold water. Unfortunately, it only takes part per billion concentrations of some of the chemicals in your water before you notice an off-taste. I don't think there is any way to eliminate 100% of the bad taste from a cheap LDPE bottle, but if anyone has a better method, please let me know.
A rigid polycarbonate (has BPA) or Tritan polyester (BPA free) bottle doesn't have the bad taste, but you can't squueze it and a Tritan bottle will crack if you drop it while riding.