Clean water involves a cycle of surface and ground water which may evaporate into the air and rain back down again or it may flow out to the ocean where it also may evaporate and turn into rain or snow again. Humans and other plants and animals need water for growth and bathing or swimming and humans also use water for industrial cooling and for agricultural irrigation of crops.
Water is also used to manage sewage waste from humans and agricultural farm animals. When the sewage is treated and reused as fertilizer it is recycling nutrients that are also essential for growth. When chemical fertilizers are used instead the extra nutrients are lost from the natural life cycle of growth, decomposition, regrowth. For long term sustainability and short term safety of the water supply wastewater treatment plants can be a good investment for communities. Decomposition of waste creates a small amount of natural gas, methane, which can be used to help power the wastewater treatment center, and the solid decomposed waste can be returned to the growing cycle as fertilizer for agriculture or lawn care. Safety for the ground water supply may be increased where flooding is a risk because often sewage is collected in open ponds and just left to gradually decompose instead of being treated to a more rapid decomposition process in a closed treatment center.
For anyone interested in more information about what communities or farmers can do about wastewater treatment a site offers examples and strategies that have been successful: SSWM, Tools for Sustainable Sanitation and Water Management. sswm.info/about_the_toolbox.
Disclaimer: Opinions are my own and the information is provided for educational purposes within the guidelines of fair use.