The orchards, too, and the gardens should be fenced all around and should lie close by, in a place to which there may flow all manure-laden sewage from barnyard and baths […]; for both vegetables and trees thrive on nutriment of this sort too.

–Columella, De Re Natura (1st Century AD)

Photograph: Lars Schoebitz
Photograph: Lars Schoebitz

By 2100, there will be over 11 billion people on this planet. Add in climate change, urbanization, and resource scarcity—especially for water—and it’s clear that reconsidering what we do with our waste will be a major priority for the next century.

Since humans have been generating waste since the dawn of civilization, it makes sense to look at history to understand the challenges of linking sustainability across sectors. In this case, human excrement is a unifying force.

An Ancient Roman writer, Columella, gives us a hierarchy for the best manures: pigeon and bird droppings are ideal, followed by human feces, and then cattle dung. Nearly two millennia later, a traveling American agronomist observed that the practice of recycling human wastes for fertilizer, politely called “night soil,” was alive and well in parts of China and Japan.

So recycling isn’t new. But it’s still preferable in many “developed” parts of the world to throw away household waste and water rather than recycle it. In fact, this habit is built in to the history of wastewater treatment, and it’s intimately tied with the story of scientific progress.

Throughout time, civilizations have built drainage systems to carry dirty water away from urban settlements (the ancient Minoans did it, and so did the Indus Civilization). But the modern wastewater management system only emerged in the late 1800s and grew out of improvements in urban planning and modern notions of sanitation, hygiene, and health.

European city-dwellers were living next to dirty rivers for a long time. Even Baron Haussmann’s revitalization of Paris could not stop the river Seine from remaining a polluted dumping-ground. But the waves of cholera that struck Paris and London in the mid-1800s sparked the field of epidemiology and convinced the medical community that illnesses were spread through water-borne organisms. Soon engineers came up with ways to actually clean up wastewater before discharging it, leading to the first biological treatment plants in the 1880s (Rautanen, 2010). With the germ theory of disease established in the 1890s, there was no going back to the old ways of neglect.

Today, municipal wastes are sent to wastewater treatment plants, where a series of filters, biological processes, and settling ponds are used to clean up pollutants before the water is re-released back into the environment. The process leaves behind “sludge”—an organic mass of nutrients containing nitrogen and phosphorous.

Although sludge has been spread on cropland at times, it is for the most part thrown away because of health hazards. Even when U.S. Environmental Protection Agency protocols are followed, resulting in the re-branding, “biosolids,” it’s unclear if levels of some components of sludge like heavy metals and pharmaceuticals are high enough to pose a threat to land and food. The debates around their safety are ongoing and the lawsuits brought in the U.S. alone show they are far from over.

So, it turns out that the waste treatment process still leaves behind—well, waste.

One of the big reasons that sludge has remained a waste is because farmers don’t rely so heavily (or at all) on it for fertilizer anymore. The key ingredients of fertilizer are nitrogen, potassium, and phosphorus and thanks to the discoveries such as the Haber-Bosch process in 1911, synthetic alternatives are available. It was a chemical feat that sidestepped the need for organic nitrogens, like those from manure. It has been wildly successful in increasing crop yields in the years after World War II, thanks also to the Green Revolution.

But can our reliance on chemical fertilizer last forever? Scientists project that global supplies of phosphorous might wane in the coming century. And the Haber-Bosch process relies on hydrocarbon fuels as a resource.

Enter the modern environmental movement: In the 1960s, societies were dealing with the after-effects of their industrial success—rivers were catching fire, and ecosystems were in a crisis. Soon conversations between developing and developed nations led to new phrases like “limits to growth” and “sustainable development.” The Rio Earth Summit of 1992 was a milestone where sustainable waste management became a global policy priority.

Today we know that sustainability cannot be forsaken for development. So where do we go from here? Feeding the world is still an imperative. And countries have a long way to go on perfecting wastewater treatment.

The current trend is focused around innovative sanitation technologies that turn human waste not only into various types of fertilizers but also power cooking stoves, generate electricity, treat to create potable water, etc. and the new testing ground for them appears to be in developing countries. For instance, new techniques for farm-based wastewater treatment and sludge usage are being trialled in rural Ghana.

But what I hope has been clear in this post is that we’ve made breakthrough innovations across time, and it would be smart not to let history go to waste.

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Here’s some additional reading on the topic, if you haven’t had enough:

Alleman, J. E. and Prakasam, T.B.S. Reflections on seven decades of activation sludge history. Journal (Water Pollution Control Federation). Vol. 55, No. 5 (1983): 436-443.

Columella. On Agriculture. Loeb Classical Library, Harvard University Press, 1941. DOI: 10.4159/DLCL.columella-agriculture.1941

Fertilizer Indicators, May 2013. 3rd Edition. Paris: International Fertzilier Industry Association, 2013.

Jones, Richard, ed. Manure Matters: Historical, Archaeological and Ethnographic Perspectives. Farnham, Surrey, GBR: Ashgate Publishing Group, 2012.

King, F.H. Farmers of Forty Centuries, or Permanent Agriculture in China, Korea and Japan. Madison, USA: Mrs. F. H. King, 1911.

Nelson, Lewis B. History of the U.S. Fertilizer Industry. Tennessee Valley Authority, 1990.