Recycling organic waste.

• Author: Gardner, Gary

Most of the world's cultivated food passes through human beings, so it is no surprise that human waste is a trove of nutrients and organic matter. Harvesting this material for agriculture is a natural way to close an organic loop; indeed, Chinese farming thrived on recycled excreta for thousands of years. However, as more cities process these wastes using technologies designed to dispose of, rather than reuse, them, safe recycling of human waste becomes much more difficult.

Safe reuse best is ensured by shifting away from disposal technologies--such as conventional treatment plants or sewers that mix industrial and domestic waste-toward methods engineered to produce a clean fertilizer. For countries not yet committed to expensive disposal systems, this shift can occur more quickly than for those that are. Until such a shift takes place, the reuse of human excrete can be practiced safely only by observing the strictest standards.

In developing countries, where 72% of the population has access to adequate sanitation, sewers, septic systems, and pit latrines are the dominant disposal systems. Sewers and septic tanks predominate in Latin America and the Middle East, while Africans and Asians rely heavily on pit latrines. Most sewers flow to the nearest river, bay, or ocean, and a mere 10% of this sewage receives treatment. Where pit latrines are used, waste material typically remains buried. Except for parts of Asia, which has a long history of excrete reuse, and some arid regions, where sewage water (often untreated) commonly is used for irrigation, human waste widely is regarded as unwanted debris.

Industrial countries long have had the same perspective, but this is changing. Many now encourage reuse of sewage sludge on farmland, and the practice is growing. European countries applied roughly one-third of their sewage to agricultural land in the early 1990s, while the U.S. did so to 28%. The interest in reuse may reflect dwindling options for cheap disposal, rather than a strong interest in building farm soils. Traditional dumping sites--landfills, incinerators, and oceans--are less available, more costly to use, or legally off-limits today, while farmland often is an inexpensive alternative disposal site. However, just as sewers and treatment facilities are not designed for recycling, farmland is not suited to absorb the chemicals and heavy metals often contained in the sewage stream.

If human wastes are made safe for use on farmland, though, their reuse can help reduce applications of chemical fertilizer. In many developing countries, the nutrient content of human waste is equal to a substantial share of the nutrients applied from fertilizer, even after losses of nitrogen to volatilization (passing off in vapor) are taken into account. For Organization for Economic Cooperation and Development (OECD) countries, nutrients in human waste that is not spread on land already equal roughly eight percent of the nutrients applied as fertilizer. As with municipal organic waste, this figure understates the potential contribution of nutrients in human waste. If fertilizer use in OECD countries were cut by a third, nutrients in human waste would amount to 12% of nutrients applied as fertilizer.

Recycling human waste will require new technologies or different ways of using existing ones. Modern methods for disposing of human waste are not designed for reusing it. Sewers, for instance, commonly serve residences and industry together, a practice that often contaminates organic matter with heavy metals or toxic chemicals. Conventional treatment plants are designed to remove nutrients (and other matter) from wastewater, thus lowering the enrichment level of effluent used for irrigation. Moreover...