One of the oldest active methods of water purification is distillation. While not very advanced by any means, it set the standard for water purification technology. The process is simple. Water is brought to a boil in a sealed container with a tube leading to another container. Steam condenses in the tube, and drips into the second container. The problem is anything with a boiling point over 100°C is indiscriminately left behind and anything with a boiling point under 100°C is also collected in the second container. Meaning healthy minerals are lost and chemicals are kept.
More efficient methods of water purification are always in demand, especially designs capable of desalination. Reverse osmosis was developed as an answer to this need. Touting the same strengths and weaknesses as distillation, it justified its existence by needing far less maintenance and human interaction than a still, and producing far more water. While obsolete outside of desalination, you will still find people trying to sell this relic of water purification technology.
Developed to remove contaminates without altering mineral quantity, a water purification technology known as the micro-porous membrane was developed, basically a three part sieve. Each part is designed to block smaller infiltrates than the last. Unfortunately, these methods of water purification are unable to deal with chemicals or bacteria.
Ultra-filtration is a highly advanced and extremely customizable water purification technology. Membranes are chemically constructed to block infiltrates of specific sizes. The problem is you want all the infiltrates removed, not just some or one of them. While custom methods of water purification have vast commercial and industrial use, they're not intended for drinking water.
To remove chemicals, a water purification technology called electrodeionization is available. Unfortunately, that's all it can do. Any other infiltrates will either be unaffected or reduce the effectiveness of ion exchange based methods of water purification. Meaning water to be electrodeionized needs to first be filtered of all organic infiltrates.
If sieves and membranes aren't your thing, water can be bombarded with UV radiation. Involving nothing more than a large flat tank and a series of low density mercury lamps, it is a very effective and efficient germicidal technique.
The Answer to These Problems
Despite all the new and interesting methods of water purification, the most effective water purification technology was developed to counter chemical warfare in WWI. Known commonly as activated carbon filtration, it involves simply passing water through a filter constructed of special charcoal. Activated carbon will block pretty much everything harmful, but leave healthy minerals. A small problem is any blocked bacteria will remain in the filter. This problem was corrected by the addition of silver to the design. Referred to as granulated active carbon (GAC), the life of such a filter is greatly extended.
While there are more methods of water purification, here we've covered all actual water purification technology. The rest revolve around pseudoscience and false claims.
