Recycling organic waste

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, conventional treatment methods (with the exception of disinfection which rarely is practiced in developing countries) reduce pathogens by too little for safe reuse in agriculture. Thus, many of today's disposal technologies are not suited to produce fertilizing products.

Where sewers and treatment plants have been fumed to waste reuse, there have been mixed results at best. Even in countries considered successful with reuse--Israel, for example, which diverts treated wastewater to irrigation--caution is warranted. Israel began large-scale reuse of sewage effluent in 1972, and today recycles 65% of its wastewater to crops. No excessive rates of illness have been linked to its use. Nevertheless, cadmium levels have been shown to increase by five to 10% annually in Israeli effluent-fed soils, and heavy metals were found to have accumulated in an aquifer below land that was irrigated with effluent for 30 years. If industrial wastes were not dumped in sewers, the country could apply sewage effluent to crops more safely. Better yet, if human wastes were managed utilizing dry (non-sewered) methods such as composting toilets, the water currently used to carry sewage would be available to agriculture as clean water.

Where sewers are little more than feeder lines to irrigation canals and the sewage they carry is untreated, risks to human health are much greater. Raw sewage used to irrigate vegetables and salad crops is blamed for the spread of worm-related diseases in Berlin in 1949, typhoid fever in Santiago, Chile, in the early 1980s, and cholera in Jerusalem in 1970 and western South America in 1991. Even so, the risky use of wastewater continues in many developing countries. In the Mexican state of Hidalgo, wastewater from Mexico City is utilized in the world's largest wastewater irrigation scheme, covering some 80,000 hectares. (A hectare equals 2.47 acres.) The effluent, 55-80% raw sewage (the balance is storm water), is barred from use on some salad crops, but other foods--including corn, wheat, beans, and certain vegetables--are irrigated with sewage water.

In contrast to wastewater reuse, application of sludge to farmland carries a different set of risks, especially where industrial wastes or household chemicals are part of the sewage flow. Researchers from Cornell University and the American Society of Civil Engineers have found more than 60,000 toxic substances and chemical compounds in U.S. sewage sludge, and they report that 700-1,000 new substances are developed every year, some of which enter the sewage stream. These substances include PCBs, pesticides, dioxins, heavy metals, asbestos, petroleum products, and industrial solvents, many linked to ailments ranging from cancer to reproductive abnormalities. They also are a threat to soils. Once introduced to cropland, heavy metals persist for decades (as in the case of cadmium) or even centuries (lead). Because little control is exercised over what enters sewers, the contents of a given load of sewage sludge can be highly unpredictable and potentially dangerous to people and soils.

Although industrialized nations maintain standards for sludge reuse, these may be lax. Such standards in the U.S. are the least stringent of any in the industrialized world, with allowable levels of heavy metals an average eight times higher than in Canada and most of Europe. Indeed, Cornell University researchers have recommended that U.S. farmers apply sludge at no more than 10% of the levels permitted by the Environmental Protection Agency. Moreover, testing in the U.S. is required infrequently--as seldom as once a year for the smallest applied amounts--even though the contents of sludge can vary greatly from load to load.

Alternative technologies

Clearly, reliance on mixed-waste sewers and treatment plants does not guarantee output that is safe for use in agriculture. Other technologies, most of which are simpler and cheaper than sewers and treatment plants, may offer greater possibilities for recycling wastes. Indeed, opportunities exist for developing countries to leapfrog past industrial nations by adopting cutting-edge technologies that are affordable, environmentally sound, and can help to close the organic loop by safely resuming human wastes to agriculture.

One simple--and ancient--alternative to sewage treatment plants is waste stabilization ponds, a series of holding areas where sewage is retained for 10 days to a few weeks. Bacteria and algae work to convert the effluent to a stable form as it passes from pond to pond. Stabilization ponds require more land than conventional treatment plants, but are much cheaper, simpler to build and maintain, and, best of all from a recycling perspective, more effective at producing safe irrigation water. A conventional treatment plant can reduce the number of fecal coliforms in a milliliter (.061 cubic inches) of water from 100,000,000 to 1,000,000--a 99% reduction, but not enough for use on crops. For unrestricted irrigation use, the World Health Organization recommends a fecal coliform level 1,000 times lower--no greater than 1,000 per milliliter--and waste stabilization ponds can achieve this.

One variant of the waste stabilization pond is a wetland modified to process wastes, the showcase example being the one in Calcutta. For more than half a century, sewage has been channeled to a wetland east of the city, where multiple ponds are used not only to process waste, but to raise fish and provide nutrient-rich irrigation water for farmers. The system works by mimicking the interconnectedness of a natural ecosystem. Nutrients in the waste feed fish, plants, and organisms in the ponds. The fish, in turn, greatly reduce or eliminate algal blooms, making the final wastewater product more useful for agriculture. Water hyacinth cultivated at the ponds' edges further purifies the water and protects the banks from erosion. Moreover, the hyacinth either is harvested for animal feed or composted. These multiple benefits, combined with a cost less than 25% that of a conventional sewage treatment plant, have made the area a valuable municipal resource.

A constructed micro-version of the Calcutta wetlands system could provide wasteprocessing capacity for some industries, thereby preventing) their wastes from entering the sewer system. Complete with plants, microorganisms, and even fish, these facilities consist of a series of pools and constructed wetlands, often built in a garden-like setting, that progressively treat industrial wastes. One U.S. firm has found a robust market for these facilities, with 20 projects built or under construction since 1992 at businesses and institutions as diverse as the M&M/Mars Company in Brazil and Oberlin College in Ohio.

For all their advantages, these natural filtering systems are land-intensive. Stabilization ponds are estimated to require 30 hectares for every 100,000 people served. The industry-level facilities also require an extensive area that may prove prohibitive in crowded cities. Where land is tight, other choices are available, some of which can avoid the expense of sewage infrastructure.

One of the more promising options for processing sewage safely is Sistema Integral de Reciclamiento de Desechos Organicos (Integral System for Recycling Organic Waste), a series of simple technologies developed and patented in Mexico and known collectively by their Spanish acronym. SIRDO systems build on the double-vault waste treatment concept developed in Vietnam, whereby one chamber collects current deposits of waste, while the other is closed for several months as previously deposited material composts. Solar heating and bacteria transform wastes and other carbon matter into a safe and odorless "big-fertilizer" that is sold to nearby farms.

SIRDO technology is applied in diverse ways Some designs are "dry," requiring no water--and no sewage infrastructure--for their operation. Dry units are self-contained structures that are detached from a house and serve one or two families. They compost household organic matter together with human waste, thereby easing pressure on landfills and sewage treatment plants. "Wet" SIRDO units are neighborhood-level mini-plants that biologically process the wastes of up to 1,000 people, operating in conjunction with existing flush toilets and local sewer lines. Even these wet systems are water savers, be cause they separate gray-water from solids and percolate it through a bed of sand and gravel until it is purified enough to reuse on gardens or to irrigate non-food crops. These systems are simple enough to maintain and operate that they do not require constant oversight by an engineer. A trained layperson can handle day-to-day operations, with occasional assistance from a SIRDO specialist. Several of the wet units in Mexico City are maintained by the gardeners of the condominium complexes the units are located in.

SIRDO's advantages extend beyond fertilizer production and water savings. As an effective sanitation technology, it improves the level of public health by reducing illnesses caused by pathogen-tainted water supplies. In the warm climates where SIRDOs currently are utilized, the unit's solar-heated waste chambers generate higher temperatures, over longer periods, than are needed to ensure that pathogens are killed. In the town of Tres Marias, Mexico, introduction of SIRDO technology and a new potable water system are credited with cutting the rate of gastrointestinal illness from 25 cases per person in 1986 to less than one case per person in 1990. Since contaminated water is a major cause of sickness and death among children in developing countries, the technology's success in sterilizing wastes is a welcome advance.

Moreover, the SIRDO systems are affordable and even generate modest flows of revenue. A cost-benefit analysis by the National Wildlife Federation (NWF) found that all five SIRDO models studied--three wet and two dry--offered net financial gains under Mexican market conditions for water, labor, and big-fertilizer. The simplest dry design, for example, costs $307 for set-up and $20 per year for maintenance--a total of $607 over 15 years--but earns the owner $2,088 in fertilizer revenues in the same period. The net income for user families is modest--about $30-60 per year--but nonetheless meaningful for people living on the economic margin.

Significantly, the NWF analysis was limited to private costs and benefits. It did not consider the technology's social benefits, which include the reduced need for sewage treatment, boosted levels of public health, and improved soil structure and fertility on farms that use the big-fertilizer. SIRDO's multiple advantages to users and society have spurred its adoption in Guatemala, Chile, and nine states in Mexico.

Other approaches

Another non-sewer approach to waste processing doubles as a source of energy. Since the 1970s, China has installed more than 5,000,000 anaerobic digesters--large chambers, mostly underground, that break down a rural family's organic waste, including manure, human excrete, and crop residues, producing gas in the process. Toilets and pigsties drain directly into the digester, yielding enough biogas to meet 60% of a family's energy needs, mostly for cooking and for fueling gas lamps. The unit produces an odorless dark slurry, used primarily for fertilizer, but also viable as feed for livestock or fish. The digesters are inexpensive--$80 covers the cost of materials and the help of a technician in construction.

In cities that already have sewers and whose populations are accustomed to flush toilets, separatiOn of human and industrial wastes will be more challenging, and may need to be viewed as a medium- to long-term goal. Nevertheless, current technologies suggest several possible approaches. Dry composting toilets can be installed in the bathrooms of many suburban homes. They look like standard flush models--without the water tank--and can hold up to several years' worth of excrete. They require some maintenance, including occasional additions of carbon material, such as sawdust or leaves, and periodic inspection of the equipment and the compost itself. Service contracts, though, can minimize the burden on homeowners. Other non-sewer technologies include micro-flush toilets, which use as little as one pint of water per flush, and vacuum-powered toilets similar to those in aircraft lavatories.

All of these systems create a fertilizing product that can be applied to home gardens or, where economically feasible, collected and sold to farmers. Because the excrete is segregated from the flow of detergents, cleaning products, solvents, and other chemicals used in many households, the composted material is clean. The systems are not cheap, however, ranging in price from $1,000 to $6,000 per unit.

Large buildings, such as multi-story apartment complexes, could be served with different technologies. (Composting toilets usually require that the holding chamber be located directly below the toilet, making their use in multi-story buildings impractical.) Constructed wetlands are one possibility for buildings that have plenty of land.

A more viable option is the use of biogas digesters, similar in concept to those utilized in China, but built on a larger scale. Located in the building's basement, the digester would collect wastes from standard low-flush toilets and produce two products: methane, which could provide part of the building's power; and uncontaminated sludge, which could be collected and applied to farmland. Digesters offer a glimpse of the multiple benefits possible from full exploitation of human waste.

The nutrients in human waste constitute a vast, untapped agricultural resource. Getting them safely back to farmland would help to build up soil and reduce the need for additional nutrients from fertilizers. Separating human excrete from industrial wastes--the prerequisite for safe recycling--will require imagination and commitment. Ironically, unsewered cities may be in the best position to capitalize on new technologies for excrete management, technologies designed to produce an uncontaminated fertilizer product.

Mr. Gardner, a research assistant with Worldwatch Institute, Washington, D.C., is the author of Recycling Organic Waste: From Urban Pollutant to Farm Resource.

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