Some drugs are not metabolized to a great extent before they exit the body.<p>For instance, an arctic shaman would eat poisonous mushrooms, taking the hit for the dangerous toxins in the fungus, then other people would drink the shaman&#x27;s urine, which still contained the active hallucinogenic chemical, and go on a much safer trip.  The hallucinogen would persist in this fashion through multiple kidneys before it was no longer worth drinking someone else&#x27;s piss.<p>Now imagine that instead of pissing into a cup, you&#x27;re pissing into a creek, and someone downstream will be taking a lower dose of something that was prescribed to you.  That&#x27;s one of the current problems with just running the wastewater output to the water treatment input.  The treatment systems can sterilize and flocculate, but that makes it <i>biologically</i> sterile, not <i>chemically</i> sterile.<p>It&#x27;s similar to the difference between <i>distilled</i> water, <i>deionized</i> water, and <i>pure</i> water.  There will be some things that your specific purification process does not remove.  If you have no outflows, your water system will become like a salt lake, where soluble chemicals enter via the tributaries and are left behind when the water evaporates, thus making the lake saltier and saltier the longer it exists, until it is so salty that otherwise soluble salts precipitate out in certain weather conditions.<p>Essentially, you would need a desalinator plant between your wastewater plant and the water treatment plant, and those are somewhat more expensive than just diluting your more problematic waste into the whole ocean.