Water purification

Water purification is the process of removing undesirable substances from water. These substances include chemicals, biological contaminants, gases, and suspended solids.

The primary goal is to produce water that is fit for specific purposes. While most purification is done for human consumption, it is also necessary for medical, chemical, and industrial uses.

Because contaminants like bacteria, viruses, and parasites are often invisible, a visual inspection cannot determine if water is safe. Only chemical and microbiological analysis can provide the data needed to choose the correct treatment method.

Purification begins with several pretreatment steps to protect the facility. First, water is pumped from its source into pipes or holding tanks. This infrastructure must be made of appropriate materials to prevent accidental contamination.

Next, screening removes large debris like sticks, leaves, or rubbish. This prevents damage to the equipment used in later steps. Some plants also use storage reservoirs to hold water for days or months. This allows for natural biological purification, which is helpful for processes like slow sand filtration.

One critical part of pretreatment is pH adjustment. The pH scale measures how acidic or alkaline a liquid is, with 7 being neutral. Fresh water can have widely varying pH values depending on its source. If water is too acidic, chemicals like lime, soda ash, or caustic soda are added to raise the pH. This process is often used for water softening. Raising the pH helps certain chemical processes work better. It also helps prevent the water from dissolving lead or copper from metal pipes.

Most conventional plants use coagulation and flocculation to remove tiny particles. Particles like clay, silt, algae, and bacteria are often too small to sink on their own. To fix this, operators add inorganic coagulants such as aluminum sulfate, also called alum, or iron(III) chloride.

Within seconds, these chemicals neutralize the negative charges on the particles. This allows the particles to begin sticking together. This stage is called coagulation. As the particles grow into larger, heavier clumps, the process is called flocculation. These clumps are known as floc.

Once the floc has formed, the water moves into a sedimentation basin. This is a large tank where the water moves very slowly. Because the water is calm, the heavy floc settles to the bottom of the tank.

These basins can be rectangular or circular. In a circular basin, the water flows from the center outward. The clean water at the top exits the tank over a weir, leaving the sludge at the bottom. This step is vital because it removes the bulk of the solid matter before the water reaches the filters.

After sedimentation, the water undergoes filtration to remove any remaining particles. Common methods include rapid sand filters or slow sand filters.

In some advanced systems, ultraviolet (UV) light is used for disinfection. UV radiation is a form of electromagnetic energy that can stop microorganisms from growing.

This is an effective way to target pathogens without adding more chemicals to the water.

Water can be sourced from many different environments, each requiring different care. Groundwater from deep wells is often very clear because soil and rock act as natural filters. However, it may contain high levels of dissolved solids like calcium or magnesium. Surface waters, like rivers and lakes, usually have a much higher bacterial load.

Even seawater can be used through desalination. This is done via distillation or reverse osmosis to create fresh water. New technologies are even developing ways to generate water by condensing vapor from the air.

Throughout history, our understanding of water safety has changed significantly. In the 19th century, many people believed that natural spring water was safe for everyone. We now know that even natural sources must be tested for contaminants. Today, governments and international organizations set strict standards for water quality. These standards define the maximum and minimum concentrations of contaminants allowed. These rules ensure that the water we use for drinking, cooking, and medicine is safe for all.