Water, as an indispensable element for life, and its purification technologies play a vital role in safeguarding human health and social development. Delving into the principles of water purification technologies is like opening a door to the world of pure water sources, revealing the exquisite scientific mysteries within.

Firstly, let's focus on the principle of coagulation and sedimentation. Coagulants such as polyaluminum chloride or ferrous sulfate are added to raw water. These coagulants rapidly hydrolyze in water to form colloidal substances carrying a positive charge. Since most of the suspended particles and colloidal impurities in water carry a negative charge, according to the principle of opposite charges attracting each other, the coagulant colloids and impurities attract, collide, and aggregate together, gradually forming larger flocs. These flocs slowly sink under the action of gravity, thus separating a large number of impurities from the water and effectively reducing the turbidity of the water and the content of some organic substances.

The principle of activated carbon adsorption is also a crucial part of water purification technologies. Activated carbon has a highly developed porous structure with pores of different sizes inside and a huge surface area. When water flows through activated carbon, dissolved organic substances, odor-causing substances, residual chlorine, and some heavy metal ions in the water will be adsorbed onto the pore surfaces of the activated carbon due to the van der Waals force between molecules. This physical adsorption process is selective and can effectively remove different types and sizes of pollutants, significantly improving the taste and odor of water while reducing the concentration of harmful substances in the water.

Ultrafiltration technology functions based on the principle of membrane separation. An ultrafiltration membrane is a semi-permeable membrane with a specific pore size range (usually between 0.001 and 0.1 micrometers). Driven by the pressure difference, water and small molecular solutes (such as inorganic salts, glucose, etc.) can pass through the ultrafiltration membrane, while macromolecular organic substances (such as proteins, colloids, bacteria, viruses, etc.) are retained. The ultrafiltration process is similar to a fine sieve that only allows substances below a specific size to pass through, thus realizing the purification and separation of water. While retaining the beneficial minerals in water, it removes most of the microorganisms and macromolecular impurities, providing a good foundation for subsequent in-depth purification.

Reverse osmosis technology can be regarded as a representative of high-precision and advanced technologies in the field of water purification. It relies on the unique properties of reverse osmosis membranes. The pore size of these membranes is extremely small, generally on the nanometer scale (about 0.1 - 1 nanometer). In a reverse osmosis system, by applying a pressure higher than the osmotic pressure of the raw water, water molecules are forced to overcome the osmotic pressure and pass through the reverse osmosis membrane, while almost all dissolved salts in the water (such as sodium chloride, magnesium sulfate, etc.), heavy metal ions (such as lead, mercury, cadmium, etc.), microorganisms, and other tiny impurities cannot pass through the membrane pores and are completely retained on one side of the membrane. Reverse osmosis technology can produce water with extremely high purity and is widely used in fields with extremely demanding water quality requirements, such as electronic chip manufacturing, the pharmaceutical industry, and seawater desalination.