Explainer: How is water cleaned up for drinking | Science News for Students

Explainer: How is water cleaned up for drinking

Treatment plants focus on removing germs and debris, often overlook industrial chemicals
Nov 29, 2018 — 6:30 am EST
a photo of pipes used to transport water in a sewage treatement plant

Most people get their drinking water from lakes, rivers or groundwater aquifers. But before it flows out the tap, cities will typically treat the water in big industrial plants, as here, to purify it.


People are used to turning the handle on a sink and seeing a stream of clear water pour forth. But where does this water come from? Typically, a town will pump it from a river, lake or groundwater aquifer. But this water can host an array of germs and solids — waterborne dirt, rotting plant bits and more. That’s why a community will typically process that water — clean it — through a series of steps before sending on to your faucet.

The steps of water treatment

The first step is usually to add coagulants (Koh-AG-yu-lunts). These are chemicals that cause those solid bits to clump together. Even if those solids didn’t hurt you, they could cloud water and give it a funny taste. By making these bits clump, they become bigger — and easier to remove. A gentle shaking or spinning of the water — called flocculation (FLOK-yu-LAY-shun) — helps those clumps to form (1).

Next, the water flows into big tanks where it will sit for a while. During this settling period, the solid sediments begin to fall to the bottom (2). The cleaner water atop it then moves through membranes. Like a sieve, they filter out smaller contaminants (3). Then the water is treated with chemicals or ultraviolet light to kill harmful bacteria and viruses (4). Following this disinfection step, the water now is ready to flow through pipes to homes throughout a community (5).

Different communities may tweak this process in some way. They may add chemicals at different stages to trigger reactions that break down chunky, toxic organic molecules into less harmful bits. Some may install an ion-exchange system. This can separate contaminants by their electric charge to remove ions. These include magnesium or calcium, which can make water “hard” and leave a scaly deposit on faucets and pipe. It may also take out heavy metals, such as lead and arsenic, or nitrates from fertilizer runoff. Cities mix and match different processes. They also vary the chemicals used, based on the qualities (chemical recipe) of the incoming local water.

Some water companies are streamlining their treatment process even more by installing technologies such as reverse osmosis (Oz-MOH-sis). This technique removes nearly every contaminant in water by forcing the water molecules through a selectively permeable membrane — one with really tiny holes. Reverse osmosis can replace a number of steps in the water treatment process or reduce the number of chemicals added to water. But it’s expensive — out of reach for many cities.

Well owners are on their own

More than one in every seven U.S. residents gets water from wells and other private sources. These are not regulated by a federal law known as the Safe Drinking Water Act. These people face the same contamination challenges as municipal water systems. The difference, individual families have to worry about their own cleanup and treatment — without help or funding from other community members.

“When it comes to lead in private wells … you’re on your own. Nobody is going to help you,” says Marc Edwards. He’s the Virginia Tech engineer who helped uncover the Flint, Mich., water crisis. Edwards and Virginia Tech colleague Kelsey Pieper collected water-quality data from more than 2,000 wells across Virginia in 2012 and 2013. Some were fine. Others had lead levels of more than 100 parts per billion. When levels are higher than EPA’s 15 ppb threshold, the government requires that cities take steps to control corrosion and notify the public. Homeowners are unlikely to ever realize they have such a problem with their own well. The researchers reported those findings in 2015 in the Journal of Water and Health.

To remove lead and other contaminants, well users often rely on point-of-use treatments. This is usually some type of filter. It’s placed at or near the faucet to remove most — but not all — pollutants. Some people may spring for the gold-standard treatment at home: a costly reverse osmosis system.

Power Words

(more about Power Words)

aquifer     Rock that can contain or transmit groundwater.

array     A broad and organized group of objects. Sometimes they are instruments placed in a systematic fashion to collect information in a coordinated way. Other times, an array can refer to things that are laid out or displayed in a way that can make a broad range of related things, such as colors, visible at once. The term can even apply to a range of options or choices.

arsenic     A highly poisonous metallic element. It occurs in three chemically different forms, which also vary by color (yellow, black and gray). The brittle, crystalline (gray) form is the most common. Some manufacturers tap its toxicity by adding it to insecticides.

bacteria     (singular: bacterium) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals).

calcium     A chemical element which is common in minerals of the Earth’s crust and in sea salt. It is also found in bone mineral and teeth, and can play a role in the movement of certain substances into and out of cells.

chemical     A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

colleague     Someone who works with another; a co-worker or team member.

contaminant     Pollutant; a chemical, biological or other substance that is unwanted or unnatural in an environment (such as water, soil, air, the body or food). Some contaminants may be harmful in the amounts at which they occur or if they are allowed to build up in the body or environment over time.

corrosion     (v. to corrode; adj. corrosive) A chemical reaction in which metals react with gases or other materials in their environment and undergo a type of degradation. The rusting of iron is one example of corrosion that is driven by exposure to moisture. These reactions normally are enhanced in an environment that is strongly acidic or strongly alkaline.  

electric charge     The physical property responsible for electric force; it can be negative or positive.

engineer     A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.

Environmental Protection Agency (or EPA)     A national government agency charged with helping create a cleaner, safer and healthier environment in the United States. Created on Dec. 2, 1970, it reviews data on the possible toxicity of new chemicals (other than foods or drugs, which are regulated by other agencies) before they are approved for sale and use. Where such chemicals may be toxic, it sets limits or guidelines on how much of them may be released into (or allowed to build up in) the air, water or soil.

federal     Of or related to a country’s national government (not to any state or local government within that nation). For instance, the National Science Foundation and National Institutes of Health are both agencies of the U.S. federal government.

fertilizer     Nitrogen, phosphorus and other plant nutrients added to soil, water or foliage to boost crop growth or to replenish nutrients that were lost earlier as they were used by plant roots or leaves.

filter     (in chemistry and environmental science) A device or system that allows some materials to pass through but not others, based on their size or some other feature. (in physics) A screen, plate or layer of a substance that absorbs light or other radiation or selectively prevents the transmission of some of its components.

germ     Any one-celled microorganism, such as a bacterium or fungal species, or a virus particle. Some germs cause disease. Others can promote the health of more complex organisms, including birds and mammals. The health effects of most germs, however, remain unknown.

groundwater     Water that is held underground in the soil or in pores and crevices in rock.

ion     (adj. ionized) An atom or molecule with an electric charge due to the loss or gain of one or more electrons. An ionized gas, or plasma, is where all of the electrons have been separated from their parent atoms.

journal     (in science) A publication in which scientists share their research findings with experts (and sometimes even the public). Some journals publish papers from all fields of science, technology, engineering and math, while others are specific to a single subject.

lead      A toxic heavy metal (abbreviated as Pb) that in the body moves to where calcium wants to go (such as bones and teeth). The metal is particularly toxic to the brain. In a child’s developing brain, it can permanently impair IQ, even at relatively low levels.

magnesium     A metallic element that is number 12 on the periodic table. It burns with a white light and is the eighth most abundant element in Earth’s crust.

membrane     A barrier which blocks the passage (or flow through) of some materials depending on their size or other features. Membranes are an integral part of filtration systems. Many serve that same function as the outer covering of cells or organs of a body.

metal     Something that conducts electricity well, tends to be shiny (reflective) and malleable (meaning it can be reshaped with heat and not too much force or pressure). 

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

nitrate     An ion formed by the combination of a nitrogen atom bound to three oxygen atoms. The term is also used as a general name for any of various related compounds formed by the combination of such atoms.

organic     (in chemistry) An adjective that indicates something is carbon-containing; a term that relates to the chemicals that make up living organisms.

osmosis     The movement of certain molecules within a solution across a membrane. The movement is always from the solution where the concentration of some chemical is higher to the solution where the concentration of that chemical is lower. This movement tends to continue until concentrations on each side of the membrane are the same.

permeable     Having pores or openings that permit liquids or gases to pass through. Sometimes materials can be permeable for one particular type of liquid or gas (water, for example) but block others (such as oil).

pollutant     A substance that taints something — such as the air, water, our bodies or products. Some pollutants are chemicals, such as pesticides. Others may be radiation, including excess heat or light. Even weeds and other invasive species can be considered a type of biological pollution.

resident     Some member of a community of organisms that lives in a particular place. (Antonym: visitor)

runoff     The rainwater that runs off of land into rivers, lakes and the seas. As that water travels through soils, it picks up bits of dirt and chemicals that it will later deposit as pollutants in streams, lakes and seas.

sediment     Material (such as stones and sand) deposited by water, wind or glaciers.

threshold     A lower limit; or the lowest level at which something occurs.

toxic     Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.

ultraviolet light     A type of electromagnetic radiation with a wavelength from 10 nanometers to 380 nanometers. The wavelengths are shorter than that of visible light but longer than X-rays.