Lake or river
Drinking water can come from many sources, including large reservoirs, lakes, rivers, and groundwater aquifers. In many cities, water comes from ore than one source, and it goes through pipelines that can be miles long before it reaches the municipal water-treatment plant. The water-purification process undergoes seven steps: 1) Coagulation: Chemicals are added to the water to cause the suspended solids present in the water to clump together, or coagulate; 2) Flocculation: Large paddles gently mix the water, which causes the coagulated particles to join so they become larger and denser; 3) Sedimentation: The flocculated water travels into primary settling basins where the dense particles settle and the clarified water is separated and sent through pipes to secondary settling basins; 4) Filtration: The water goes through filters that remove any remaining suspended particles in the water; 5) Disinfection: Chlorine or ozone is added to the clarified water to disinfect it; 6) pH Adjustment: Lime, also known as calcium oxide (CaO), is added to ensure the pH of the water remains alkaline, or basic, enough so it doesn’t corrode water-distribution pipes; and 7) Fluoridation: After water leaves the secondary settling basins, it is treated with fluorosilicic acid (H2SiF6), which adds fluoride to drinking water to help prevent tooth decay. Finally, the water goes through the water storage and distribution system.
DWAINE EUBANKS; THINKSTOCK
Heavy metals such as arsenic, mercury,
and lead are a serious health concern because
they are toxic to humans and animals. Heavy
metals can naturally find their way into water,
which is one reason why water needs to be
treated before it can be piped into your home.
For example, in many parts of the world, the
groundwater comes into contact with soil
or rocks that contain high concentrations
of arsenic. The water dissolves some of the
metal, or leaches it, and the water becomes
If heavy metals are present at the source
of the drinking water, the water-treatment
facility can use chemicals that react with the
metals and precipitate them out of the water.
SHUTTERSTOCK; THINKSTOCK; ISTOCK
Chlorine: Unintended Hazards
A precipitation reaction is a reaction in which
soluble ions in separate solutions are mixed
together to form an insoluble compound that
settles out of solution as a solid. That insoluble compound is called a precipitate. Soluble
heavy-metal ions can be removed by converting them into insoluble metal hydroxides or
carbonates. This is typically done by the addition of compounds, such as sodium hydroxide
(NaOH) or calcium oxide (CaO).
THE USE OF CHLORINE AS A DISINFECTION AGENT CAN CREATE BY-PRODUCTS, which can be difficult to remove and can cause health hazards. One common
group of disinfection by-products are trihalomethanes—compounds in which three of
the four hydrogen atoms of methane (CH4) are replaced with halogen atoms (
specifically chlorine or bromine). Chloroform (CHCl3) is one well-known trihalomethane.
These chemicals are suspected carcinogens at low concentrations. Trihalomethanes
are typically removed using large amounts of activated carbon—a form of carbon that
has small pores. The pores increase its surface area and make it more reactive with
trihalomethanes. The carbon and adsorbed by-products are then removed by settling
and filtration (see Filtration, step 4, in the above illustration). Other chlorine by-products
are not as easily removed, however.
For example, soluble nickel ions can be
converted to an insoluble hydroxide by the
Ni(NO3)2(aq) + 2 NaOH(aq) ➞
Ni(OH)2(s) + 2 NaNO3(aq)
The solubility of the metal compounds
depends on pH—most are least soluble when
pH is above 7 (basic solutions). The optimal
pH for precipitation depends on the metal to
be removed and anion to be used, so the best
ChemMatters | OCTOBER/NOVEMBER 2017 15