Where do you think all the water, liquid waste, toilet and urine ends up? In many developed communities, this waste water and soluble waste (called sewage) is treated, cleaned and dumped into the sea. Even though they are treated, they are never the same as fresh water.
Click this red button to see how sewage is treated:
In some not-so-developed countries, the sewage is not treated, but quickly dumped into the sea or water bodies. This is VERY dangerous because they contaminate the environment and water bodies and bring many deadly diseases to us.