Water sources such as lakes, rivers, and ground water supply much of the water for domestic use. Some of the water that reaches our household faucets has also been used for other purposes. Water from these various sources is treated to remove impurities and to make it suitable for human consumption. Several steps can form the treatment process. Large items and particles can be filtered out using screens. Some particles may be allowed to settle out. This can be aided by first adding lime (CaO) to produce a basic solution. Alum (KAl(SO₄)₂) is then added, which reacts with hydroxide ions (OH^– ) to form Al(OH)₃, a precipitate which settles out of solution, taking small particles with it. Additional particles can be removed by filtration through sand. Activated charcoal has been treated to increase its porosity and surface area, and is used to adsorb odors and some colored substances. Certain impurities, including carbon-based particles and chlorine, are attracted to the charcoal and remain trapped in the pores. Water is also disinfected using chemical treatments, commonly chlorine. In this Activity, students make a water filtration column using a two-liter plastic bottle that contains layers of gravel, sand, and activated charcoal. They contaminate a sample of water and examine the filtration ability of the column.

See related Classroom Activity Connection.