Before you take a gulp of water, try to mentally trace where that water that just gushed out of your taps has been: How did it go from that weird-tasting raindrop to the clear, odorless water that is sitting in your glass now?

Safe drinking water is a privilege Americans often take for granted — until a health crisis like the one in Flint, Mich., happens that makes us think about where it comes from and how we get it.

Our drinking water comes from lakes, rivers and groundwater. For most Americans, the water then flows from intake points to a treatment plant, a storage tank, and then to our houses through various pipe systems.

The most common steps in water treatment used by nearly every utility company:

  1. Coagulation and flocculation - Chemicals are added to the water. They bind with the dirt and dissolved particles, forming larger particles called floc.
  2. Sedimentation - The floc is heavy, so it settles to the bottom of the tank.
  3. Filtration - The clear water on top passes through filters composed of sand, gravel and charcoal to remove dissolved particles such as dust, parasites, bacteria, viruses and chemicals.
  4. Disinfection - Chlorine or chloramine is added to kill parasites, bacteria, viruses and germs. Fluorine is added to prevent tooth decay.

Various other chemicals can be added to adjust for hardness and pH levels or to prevent corrosion, based on the water source. But depending on where you are in the United States, there can be different challenges, and corresponding methods of treating drinking water. For example:

Lead Pipes Are Common In The Northeast And Midwest

Lead pipes or fittings are a culprit in the current water crises in Flint, Baltimore and other cities. Many old water pipes are made of lead, which may leach into the water supply if preventive measures aren't taken. According to EPA, even low levels of lead can cause behavior problems, slow growth and affect IQ levels.

While the most effective solution might be to replace lead pipes entirely, water utilities usually add some form of phosphate to the water supply. That forms a protective film between the lead pipe and the water flowing through it.

Flint failed to add orthophosphate to control corrosion when it switched water sources from the city of Detroit to the Flint River; water from the Flint River has eight times more chloride than Detroit's, which is highly corrosive to the pipe.

Agricultural States May Suffer From High Levels Of Nitrate

Nitrate runoff in rivers and groundwater can be common in places with high levels of farming activities. Fertilizers, manure storage and septic systems are sources of this pollution. High levels of nitrate in drinking water can cause "blue-baby syndrome," where infants younger than 6 months suffer from shortness of breath. If untreated, it might lead to death.

Des Moines often has to deal with treating high levels of nitrate in its rivers. The Des Moines Water Works utility removes it through an ion exchange process in one of its treatment plants.

Many Western States Drink Saltier Water

Water with high salinity is prevalent in the western part of the United States. In some places, the water can be too salty for drinking or other uses and needs to undergo desalination. These saline water sources include seawater and brackish groundwater.

Turning seawater into drinking water is a relatively new concept. The Carlsbad plant in California that opened last year is the largest seawater desalination plant, and some see it as a possible solution to the statewide drought.

Brackish groundwater has high levels of salt but not as much as seawater. Texas relies heavily on brackish groundwater as a water source.

There are two methods plants may use for desalination. One, used by Carlsbad, is called reverse osmosis; it forces water through semipermeable membranes under very high pressure.

The other is a thermal process that heats the water to form water vapor, which is then condensed and collected as freshwater, leaving the salt behind.

Waterborne Diseases Can Happen Anywhere

According to the Centers for Disease Control and Prevention, there were 32 cases of drinking-water associated outbreaks from 2011 to 2012, the most recent time period for which they have been reported.

Most of it was Legionellosis, a disease typically spread by water droplets in the air. The remaining cases were associated with bacteria and viruses that can be killed by chlorine. To prevent such outbreaks, CDC emphasized the importance of ensuring a sufficient level of disinfectant, such as chlorine, is present in the water from the time it leaves the treatment center to when it arrives in our pipes.

Some utilities use ozone as a disinfectant to kill bacteria and viruses, a method some say is more effective than the usual route of using chlorine. Ozone is bubbled into the water in huge tanks, destroying illness-causing microorganisms. It also gets rid of taste and odor in the water.

Milwaukee, Wis., started using ozonation after an outbreak of Cryptosporidium in 1993 killed 69 people and sickened up to 403,000 residents. It was one of the largest outbreaks caused by a contaminated public water source, according to the CDC.

Some States Try To Protect Their Water At The Source

Watersheds are the areas where rivers, lakes and ponds drain into, and they're a source of drinking water. Some cities, such as Seattle and New York, are famous for the regulations and programs they put in place to protect their watersheds.

In fact, New York City's watershed protection is so good that it's one of the five large cities in the country where the drinking water supply does not need to undergo filtration.

The city works with farmers and landowners upstream to reduce pollution and manage land. The Conservation Easement Program sells or donates land to conservation organizations, limiting the type of development that can occur on it permanently.

Although the water doesn't go through filtration, it is still disinfected with chlorine and ultraviolet light, with the usual sludge of chemicals added to control pH and prevent corrosion.

The EPA regulates approximately 155,000 public water systems in the country, requiring utilities to conduct tests according to schedule and submit water quality data. On the other hand, over 15 million Americans rely on private wells — water quality from this source is not regulated by the EPA, but might be under state rules.

From the lake to the tap, water goes through many steps to become safe for us to drink. It is a crucial process that requires constant monitoring, and — as both history and current events show — it's one that can be easily threatened by bacterial outbreaks, natural disasters and human activity.

Zhai Yun Tan is a digital news intern.

Copyright 2016 NPR. To see more, visit NPR.

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