Substances that are removed during the process include parasites (such as Giardia or Cryptosporidium) , bacteria, algae, viruses, fungi, minerals (including toxic metals such as Lead, Copper etc.), and man-made chemical pollutants.
Many contaminants can be dangerous—but depending on the quality standards, others are removed to improve the water's smell, taste, and appearance.
A small amount of disinfectant is usually intentionally left in the water at the end of the treatment process to reduce the risk of re-contamination in the distribution system.
Many environmental and cost considerations affect the location and design of water purification plants.
Groundwater is cheaper to treat, but aquifers usually have limited output and can take thousands of years to recharge.
Surface water sources should be carefully monitored for the presence of unusual types or levels of microbial/disease causing contaminants.
The treatment plant itself must be kept secure from vandalism and terrorism.
It is not possible to tell whether water is safe to drink just by looking at it.
Simple procedures such as boiling or the use of a household charcoal filter are not sufficient for treating water from an unknown source.
Even natural spring water - considered safe for all practical purposes in the 1800s - must now be tested before determining what kind of treatment is needed.