Why don't they just? The answer is usually the same: it would cost more. Drilling operations regularly flare (burn) natural gas, because it would cost too much to transport it.
I misread your question, but I'll write my answer anyway:
Wastewater treatment plants do produce fertilizer. There is some concern about using on plants destined to be human food, due to the bacteria present in human waste, so it is often used on other crops
Human waste contains medicines, hormones, heavy metals etc. which the waste water treatment plants can’t generally remove. You don’t want these things back into the human food chain, without further treatment.
Comments
Why don't they just? The answer is usually the same: it would cost more. Drilling operations regularly flare (burn) natural gas, because it would cost too much to transport it.
I misread your question, but I'll write my answer anyway:
Wastewater treatment plants do produce fertilizer. There is some concern about using on plants destined to be human food, due to the bacteria present in human waste, so it is often used on other crops
Human waste contains medicines, hormones, heavy metals etc. which the waste water treatment plants can’t generally remove. You don’t want these things back into the human food chain, without further treatment.