Water is added to make transport to a central treatment facility easy.
Water must be separated from the fertilizer etc. Too much fertilizer will cause rampant growth of the natural bacteria and plants in the environment. If fertilized growth of the natural bacteria consumes all of the available oxygen, fish etc. will die. Bacteria that does not need oxygen will consume the rest of the fertilizer and as a byproduct, produce hydrogen sulfide gas with a rotten egg smell. Pathogens will cause disease or death in animals exposed to the wastewater.