Killing the germs: disinfection and why home filters are not enough

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.

Dose and time: C·t. How well a disinfectant works depends on two things you control: its concentration C (in mg/L) and the contact time t (in minutes) it has with the water. Multiply them to get C·t, in mg·min/L. Roughly, doubling either one doubles the effect. Plants therefore pass water through winding contact tanks with baffles, so every drop gets enough minutes. In the scene's teaching model the number of log reductions (each log is a factor of 10 fewer germs) is about k × C × t, with a made-up constant k. Real design uses measured values for each microbe, temperature and pH.

There is a trade-off. Chlorine reacts with natural organic matter left in the water and forms by-products such as trihalomethanes, which are regulated. The best way to limit them is to remove organic matter earlier (coagulation, carbon) and not to over-dose. Too little chlorine risks germs; too much costs taste and creates more by-products. Plant operators balance both with frequent tests.

Why a home-made filter is not enough. Layers of sand, charcoal and gravel in a bottle are a classic school demonstration. They clear the water, and that is all they do reliably: they reduce cloudiness, but they do not remove or kill the germs and chemicals that make water unsafe. A clear result looks just like safe water, which is exactly why it is dangerous. Please treat that demonstration as a picture of filtration, not as a way to prepare drinking water.

When the supply is unsafe. Health authorities say what to do. A common instruction is to bring water to a rolling boil, for at least 1 minute, or 3 minutes at high altitude (above about 2,000 m, or 6,500 ft), and let it cool. Boiling kills germs but does not remove chemicals: it can even make nitrate slightly more concentrated as water boils away. Certified household filters and treatment products exist, and the local health authority can say which kind fits a given hazard. Always follow your water authority and health authority for drinking-water advice.