The natural cycle of water can be divided into 3 steps :

• evaporation - transpiration : on one side, under the effect of th sun, the water of the sea / rivers and lakes evaporates and on the other side plants and living things transpire and release water vapor ;
• condensation : evaporated water rises in the sky and condenses into clouds consisting of thousands of water droplets ;
• runoff and infiltration : water then falls to earth as rain, drizzle, hail, snow etc.., then it flows on the ground, then is absorbed by vegetation, penetrates into groundwater supplies and rivers, lakes and oceans .

The cycle of domestic water in occidental countries follows different steps :

• the welling in the natural environment (pumping groundwater, lakes, rivers) ;
• the water treatment by clarification, filtration and disinfection ;
• transportation and stocking, with an all-time water quality surveillance ;
• distribution to the consumers.

Finally, after use the wastewater is transported through pipes in sewage treatment plants, which clean the water and return it to the natural environment. Sludge is then in most cases treated and recovered.