Dirty water that contains substances like mud and stones can be separated from water with a sieve. The holes in the sieve will clean the water by allowing the water to pass through while keeping the larger particles behind. The smaller the sieve holes are, the cleaner the water will be after sieving, but it will be still not be safe to drink. Germs, viruses and other harmful organisms are small enough to pass through the sieve holes.
Dirty water is mostly a mixture of water and harmful substances. If dirty water stands for a time, the different substances start to separate. Heavier, solid substances start to drop to the bottom of the container and the liquid stays on top. We say the substance start to settle.
Municipalities supply water to many people and the water must be pure (very clean and free of diseases). We know that our water comes from dams (reservoirs), but it is not safe to drink as it is. It must meet certain standards set for safe drinkable (portable) water. To meet these requirements, water must be purified at a water treatment plant.