Any good environmentalist will tell you that human wastes used to be returned to the land, thus completing the natural cycle. But, with the advent of modern sanitation that cycle was broken. Human wastes are now sequestered in sewage treatment plants where they eventually end up as sludge or "biosolids" as the people in the industry like to call them. The idea of taking that waste and using it for fertilizer isn't new, and it isn't a bad one in principle, provided that certain safeguards are taken to prevent the spread of pathogens into the food supply.
But, the hue and cry over the use of sewage sludge on farm fields is based in part on what actually goes down city drains: a mixture of human wastes, toxic chemicals, and heavy metals from both households and industry. Even the human wastes are riddled with residues of antibiotics and other prescription drugs. It's not exactly what people had in mind when the idea of recycling biosolids was first proposed. And, it points up the virtual impossibility of separating the good and useful from the bad and toxic while living in the chemical soup we call modern industrial society.
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