Treatment plant workers add alum and other chemicals to the water, which cause tiny sticky particles, or floc, to form. These floc attract dirt particles, making them eventually heavy enough to sink to the bottom of the water storage tank.
The water and floc flow into a sedimentation basin. As the water sits there, the heavy floc settle to the bottom, where they remain until removal.
Water passes through layers of gravel, sand and perhaps charcoal, which serve to filter out any remaining particles. The gravel layer is often about 1 foot deep and the sand layer about 2½ feet deep.
Water goes into a closed tank or reservoir. Chlorine or other disinfecting chemicals kill any remaining microorganisms or bacteria in the water and help keep the water clean until distribution. If a water treatment facility uses ground water as its only water source, disinfection may be the only step required to sufficiently treat the water. After it is disinfected, the purified water sits in the closed tank or reservoir until it flows through pipes to homes and businesses.
Water is an excellent solvent and can be sourced from almost anywhere on Earth. This property makes it prone to all kinds of contamination.
Particulates: Silt and debris which can be removed by passing water through a 10 to 20 micron filter (or less if necessary).
Microorganisms: Bacterial agents constitute a real challenge for water purification systems. Their growth rate, size and robustness require an efficient design (detection, removal from water inlet, inhibition of growth, etc.). Bacteria are measured in colony forming units per milliliter and can be killed with disinfectants. As a result, their secretions and cellular fragments must also be removed to avoid contamination.
Endotoxins, pyrogens, DNA and RNA: Cellular fragments and bacterial by-products. Harmful to tissue cultures. Can be detected with a Limus Amoebocyte Lysate (LAL) test.
Dissolved inorganic elements: Include phosphates, nitrates, calcium and magnesium, carbon dioxide, silicates, iron, chloride, fluoride, and any other natural or man-made chemicals resulting from exposure to the environment. Electrical conductivity (μSiemens/cm) is used to monitor high concentration of ions, while resistivity (MÙcm) is used to identify ions if present in small concentrations. These contaminants affect water hardness and alkalinity/acidity.
Dissolved organic elements: Pesticides, plant and animal remains or fragments. Total Organic Carbon (TOC) analyzers are used to measure CO2 emitted by organics subjected to oxidization. Organic-free water is mainly used in applications where analysis of organic substances is carried out (e.g. HPLC, chromatography and mass spectrometry).