[logfromblammo]

Some drugs are not metabolized to a great extent before they exit the body.

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'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's piss.

Now imagine that instead of pissing into a cup, you're pissing into a creek, and someone downstream will be taking a lower dose of something that was prescribed to you. That'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 biologically sterile, not chemically sterile.

It's similar to the difference between distilled water, deionized water, and pure 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.

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.

[ethbro]

I understand the principle (though upvote for the example!), but my question was after leaving a human body whether or not natural processes (e.g. UV) + microbes in the environment would break those chemicals down in a reasonable timescale or if they persist? From s_q_b, it sounds like they do get broken down, albeit with an effect on the flora and fauna.

[logfromblammo]

It depends on the chemical.

Protein-based drugs, such as oxytocin or insulin, are likely to be consumed and recycled into component amino acids.

Most molecules left out in the environment will eventually be oxidized, have an important bond cleaved by UV light with enough energy, or undergo thermal decomposition.

As sunlight, dissolved oxygen concentration, and temperature can vary wildly just by moving a few meters away, and the chemicals themselves have different stability, it is very difficult to predict how long a pill that was flushed down the toilet will persist in the environment.

Generally speaking, certain drugs are only useful because they take longer to degrade inside the body. It wouldn't do you much good to get an injection of a drug if your body's proteases chop it up to uselessness in the first ten minutes, or if it zooms right to the liver and gets methylated, or whatever it is the body does to clear foreign chemicals. In those cases, microbes will also have a hard time turning them into something else.