After filtration the water is clear but may still hold microbes, so it is disinfected. There are three main methods.

• Chlorine (as chlorine gas, bleach solution or related compounds) reacts with microbes and inactivates them. It is cheap and, crucially, leaves a small residual that keeps working in the pipes on the way to your home. Some suppliers use chloramine (chlorine plus ammonia), which lasts longer in long networks.
• Ultraviolet (UV) light damages the genetic material of microbes so they cannot reproduce. It is excellent against parasites such as Cryptosporidium that chlorine handles poorly, but it leaves no residual.
• Ozone is a very strong oxidiser made on site from oxygen. It works fast and improves taste, but it also leaves no residual and is more costly.

Many plants use two methods together, for example UV at the plant and a little chlorine to protect the pipes.