Most drinking water comes from a surface water source, such as a lake or river, or a groundwater source, such as a well or spring. This water requires treatment before it can be safely consumed; the level to which the water is treated depends on the source of the water and also on federal regulations. In the United States, for example, the Safe Drinking Water Act was created by the Environmental Protection Agency to provide standards for tap water and public water systems. Although drinking water treatment practices vary, most water treatment plants follow a six-step process to remove undesirable contaminants.
The first step in the treatment process is the removal of harsh grit particles (sand, gravel, cinder, etc.) and other large objects (cans, bottles, tree limbs, etc.). This step is necessary because it prevents damage to the pumps, which are used to transport the water from step to step in the treatment process.
After any large objects are removed from the water, chlorination chemicals are added to control algae and other biological growth. Aeration, or the circulation of air through a liquid substance, also takes place in this step so that any dissolved gases can be dispelled.
Once a majority of the gases are removed from the water, suspended solids must be removed as well. In order to collect them, it is essential that they settle to the basin floor, but these particles are often so small that they cannot settle without assistance. Therefore, to advance the settling process, coagulating compounds are added to the water. The suspended solids stick to these compounds and create heavy clumps called floc particles.
The sedimentation basin is often located within close proximity of the flocculation basin so that the particles do not have to travel far and will not have a chance to break apart. When they reach the sedimentation basin, the water’s velocity is slowed down so the floc particles can sink to the basin floor. This process is enhanced by tube settlers, which increase the settling capacity of basins and clarifiers by reducing the distance a particle must fall before reaching a flat surface. The particles collect and agglomerate within the tubes, forming heavy floc particles that can sink to the bottom of the basin where a sludge collector system will scrape them off.
Any remaining particles are removed during the filtration stage when the water is run through a series of filters, often including media, screen, and sand filters.
Although the water is now largely free of contaminated particles and microorganisms, disinfectants must be added to destroy any remaining disease-causing pathogens. This is commonly done with chlorination and makes the water safe to drink.
Once the treatment process is complete, the water can be pumped through public water systems to your home where it will flow from your faucet.