The water cycle is the perfect example of The Theory of Conservation of Matter and Energy, It is the cycle that water goes through on Earth. Starting from the ocean, it evaporates from the suns energy. Then it builds up in the clouds. Eventually it becomes too heavy for the clouds and it falls down as rain. Some of it falls back into the ocean, but some falls onto the land and gets absorbed into the plant and the ground. Also some rain goes into lakes and rivers that eventually go back into the ocean....were the process starts back over again.
Every day 2 million tons of sewage and industrial and agricultural waste are discharged into the worlds water. The UN estimates that the amount of waste water produced annually is about 1.5 thousand km, six times more than the water that exists in all the rivers in the world. World wide 2.5 billion people live without improved sanitation. Over 70% of these people who lack sanitation or 1.8 billion live in Asia. Africa is slowest improving sanitation region in the world about 31% of the residents have access to improved sanitation in south Asia 63% of rural people- 778 million people practice open defecation.