Fertile ground or toxic legacy?

• Author: Gardner, Gary
• Position: Modern sewage and farming systems

Recycling human waste isn't like recycling newspapers. The use of "night soil" on cropland is ancient, but modern sewage and farming systems have greatly complicated the risks of using our most obvious fertilizer.

In 1997 the U.S. Environmental Protection Agency (EPA) approved a plan that takes sewage recycling to new and bizarre levels. Wastewater from cleanup of the Lowry Landfill near Denver - a Superfund site contaminated with chemicals and heavy metals, including nuclear waste - was to be dumped into local sewers and treated at the local sewage treatment plant. The plant would mix the toxic brew with ordinary sewage, process it, and release the resulting products: water to the South Platte River, and sludge to nearby farms for use as fertilizer. The program was part of the agency's attempt to recycle a greater share of U.S. sludge to "beneficial uses."

The plan, which drew howls of local protest, is an extreme example of a growing trend: the use of human waste to fertilize farms, in spite of unresolved questions about the practice's risks. As greater quantifies of human waste are produced, and as traditional dumping areas are placed off-limits or become increasingly costly, policymakers have resorted to this ancient strategy to dispose of the nutrient-rich material. But because modern waste flows are dirtier than those of centuries past, capturing the benefits of reuse with minimal risk is a growing challenge.

The experience of modern sludge reuse has the qualities of a fable, with valuable lessons for the larger question of recycling. Global materials use is far greater than the planet can sustain; Mathis Wackernagel and William Rees of the University of British Columbia have shown that today's economies already consume one third more resources and ecoservices than nature can deliver sustainably. Reducing our "ecological footprint" will therefore require a far greater level of materials reuse. But as recycling moves well beyond newspapers and aluminum cans, it is important to distinguish between beneficial recycling - the return of materials to advantageous and environmentally benign uses - and careless reuse, which is sometimes little more than dumping under a green label. The lessons of this fable are important for industrialized nations active in recycling human waste. They are even more timely for the many developing country cities that are busy planning and designing sanitation systems for the next century.

An Ancient Practice

Recycling human waste has a long and noble history. Chinese agriculture, for example, was sustained for thousands of years by the "night soil" collected from cities and rural villages. In his classic study Farmers of Forty Centuries, agricultural historian F.H. King reports that farmers would build roadside outhouses and post advertising to entice travelers to use them, so desired was the excrement as a supplemental source of nutrients and organic matter. The city of Shanghai commonly sold the right to collect the city's night soil for sale in the countryside, a concession that in 1908 brought the equivalent of $31,000 in gold to municipal coffers.

Organic recycling was less common outside of Asia, but eventually became more widely practiced as burgeoning cities scrambled to get rid of their waste. By the mid-nineteenth century, as some European cities passed the one million mark in population, sewage was collected by scavengers and delivered to nearby "sewage farms" for use as fertilizer. The practice soon spread to cities in the United States, Australia, and Mexico. Indeed, by the early twentieth century, with sewers commonly in use in more developed countries, land application of sewage was the sole method of disposal in many metropolitan areas.

In this pre-modern era, and in many developing countries today, the chief health risk from sewage came from its pathogen content. Untreated sewage is alive with bacteria, viruses, and parasites, which can spread to people through water supplies, food fertilized with waste, or direct contact. Open-air sewage flows facilitate the spread of sickness, prompting outbreaks of cholera and other infectious diseases, as occurred in urbanized countries in centuries past, and continues to occur in developing countries today. The spouse of a Worldwatch researcher recalls the terrible choice faced by villagers in her native South Korea in the 1960's: use raw human waste in rice paddies to ensure higher production - but at the cost of widespread sickness, as parasites from feces became intestinal worms in nearly all villagers - or protect the village from the health threat of human waste and accept lower yields. Pathogens are a serious and even deadly threat, but they are relatively short-lived, eventually breaking down in the soils to which they are applied.

In this century, underground sewers became increasingly common, and some were eventually connected to treatment plants. Used together, the two technologies greatly reduced the pathogen menace. But new contaminants surfaced in many cities. As industries hooked up to public sewers, their waste flows - often containing toxic chemicals and heavy metals - mixed with human waste. Sewage treatment processes, while somewhat effective at killing pathogens, did not eliminate these other contaminants; instead, the pollutants simply accumulated in the sludge. But because most sludge was destined for disposal at a landfill, in an incinerator, or even on the ocean bottom, authorities saw little reason to worry about these substances.

Over the past quarter century, however, several developments have renewed interest in recycling human waste to land. Urban growth and an increasingly sewered population have concentrated more and more human waste in urban areas. Indeed, U.S. sewage sludge has doubled in quantity since 1972, even though population has grown by only 25 percent, largely because of the 1972 Clean Water Act, which funded a boom in sewer construction. At...