How is tap water cleaned before it reaches your home?

Water utilities pipe water from its source to a treatment plant, where a series of steps removes harmful germs and chemicals. The CDC describes a common sequence of coagulation, flocculation, sedimentation, filtration and disinfection. Different utilities use different steps depending on the quality of the incoming water. [1]

Coagulation is often the first step. Staff add chemicals, commonly specific salts, aluminum or iron, that bind dirt and other small particles together. In flocculation the water is gently mixed so these particles form larger, heavier clumps called flocs. In sedimentation the flocs settle to the bottom, separating solids from the water. [1]

The clear water on top then passes through filters with different pore sizes, made of materials such as sand, gravel or charcoal. These remove germs including parasites, bacteria and viruses, along with dissolved particles. Activated carbon filters remove bad smells. Singapore's national water agency describes the same pattern, with chemical coagulation followed by rapid gravity sand filters that take out the finer suspended particles. [1] [2]

Disinfection is often the last step. Plants may add chlorine, chloramine or chlorine dioxide to kill remaining germs. Operators keep a low level of disinfectant in the water as it leaves the plant so that it keeps killing germs in the pipes on the way to the tap. Ultraviolet light and ozone also work well inside the plant, but they do not keep acting once the water is in the pipes, so they may be used instead of or alongside chemical disinfectants. [1] [3]

Plants commonly adjust the pH after disinfection, which improves taste, reduces pipe corrosion and helps disinfectants keep working. Many also add fluoride, which keeps teeth strong and reduces cavities. [1]

Water from lakes, rivers and reservoirs typically needs more treatment than groundwater, because it carries more sediment, germs, chemicals and toxins. One utility guide says groundwater is often treated mainly with mechanical filters and, if needed, chlorination or UV, while surface water gets sedimentation, aeration, coagulation, filtration and then disinfection. The same guide notes that chemical contamination of groundwater is an increasing concern in the US. [1] [4]

Some supplies need special methods for substances such as nitrates, radionuclides or toxins from cyanobacteria. Some communities use ion exchange to remove ions such as calcium and magnesium, which cause hard water, and possibly heavy metals like lead and arsenic. Reverse osmosis forces water through a membrane with extremely small holes and removes nearly every contaminant, but it is expensive and out of reach for many cities. It is often used for recycled water or salt water. [1] [5]

Water is tested at the source and at various stages of treatment. Singapore's PUB, for example, sends samples from every waterworks to a central laboratory. Treated water is then stored in tanks or water towers and sent through underground mains to homes and businesses via private service lines. [2] [4]