Biosolids are nutrient-rich organic materials that result from the treatment of wastewater. They are commonly recycled as a fertilizer for crops and as a soil amendment to improve depleted soils. However, because biosolids have low levels of pollutants and pathogenic organisms, their use in the U.S. is regulated by the Environmental Protection Agency (EPA). Managing biosolids safely and effectively is an important issue for communities because of the quantities that are produced. The EPA estimates that the annual U.S. production of biosolids, recorded at seven million tons in 2000, will continue to increase.
In developed countries, biosolids are produced at treatment facilities that receive wastewater from homes, businesses, and industries. Domestic wastewater carries organic matter from food preparation, cleaning of clothes and cookware, and human waste. Industrial wastewater may contain organic material, oils, metals, and chemical compounds, but it is usually pretreated at the industrial facility to reduce the concentration of pollutants. Raw materials pumped from rural septic systems are often transported to treatment plants.
At the wastewater treatment facility, the solids from these various sources are first concentrated by settling out (primary treatment). Then they are biologically degraded (secondary treatment) by bacteria and other microorganisms feeding on the organic matter. To encourage the growth of the bacteria, the wastewater is aerated. As the microbes consume the dissolved and suspended organic matter, it is incorporated into their cells. Most diseasecausing organisms (pathogens) are destroyed during this process. After further digestion or another equivalent treatment, the living and dead microbes form a stable residual material, biosolids.
In developing countries, lack of wastewater treatment is a serious problem. Raw sewage and other untreated organic wastes should not be confused with biosolids, whose treatment and use are regulated by environmental laws.
Using or Disposing of Biosolids
Biosolids must be recycled or disposed of somewhere in the environment. The 1993 Federal Sewage Sludge (biosolids) Standards define and regulate the three legal ways to manage biosolids: They can be incinerated, buried in a landfill, or recycled on land.
- Incineration—This is the method of disposal preferred by some eastern U.S. cities. Energy produced from burning biosolids can be captured and converted to electricity. Incinerators require technology to prevent the release of particulates and pollutants to the atmosphere. The biosolids are reduced to a small amount of ash, which is usually landfilled. About 22 percent of the biosolids in the United States are incinerated.
- Landfill Disposal—Biosolids can be mixed or layered with municipal solid waste and buried. Landfilling of biosolids usually occurs where agricultural lands for recycling are not readily available or the quality of the biosolids does not meet the strict EPA standard for recycling. Approximately 15 percent of the nation's biosolids are landfilled.
- Ocean Disposal—Prior to 1990, ocean disposal was the preferred method of disposing of biosolids in the world's coastal cites. The United States outlawed the ocean disposal of biosolids with the Ocean Dumping Ban Act of 1988. Europe and Australia then enacted similar bans.
|element||biosolids1 (mg/kg dry)||u.s. regulations 2 (mg/kg dry)|
|1biosolids from king county (seattle, wa) south plant, 2000 data, annual means. mg/kg dry = parts per million.|
|21993 federal sewage sludge standards 40 code of federal regulations (cfr) part 503. pollutant concentration limits for exceptional quality biosolids.|
- Beneficial Use—Biosolids can be used to fertilize agricultural crops and forests, reclaim mines and disturbed lands, cover landfills, and make compost for soil amendment and landscaping. Most of the biosolids in this country, about 63 percent, are put to beneficial use. The EPA predicts that this will increase to 70 percent by 2010.
Biosolids are desirable soil amendments because they add nutrients and organic matter. All the elements essential for plant growth are found in biosolids, including the macronutrients (nutrients needed in large amounts) nitrogen, calcium, phosphorus, and sulfur and micronutrients such as boron, manganese, zinc, and copper. Organic matter benefits the soil in many ways. It improves water infiltration and helps hold water and nutrients for use by plants, thereby reducing runoff and erosion.
Evaluating Risks and Benefits
U.S. biosolids quality standards are based on risk assessments conducted by scientists at the EPA and the Department of Agriculture. After evaluating the pollutants in biosolids, scientists selected nine elements that had the greatest potential to harm humans, livestock, wildlife, or the environment. They used risk assessments to calculate the permissible increases in soil and crop pollutant levels from repeated applications of biosolids. The regulatory standards (see table) were then set below the level that would cause harm.
However, not all scientists agree with the U.S. risk assessments and standards. They caution against allowing soil pollutant concentrations to rise above background levels. Several European countries use this precautionary philosophy as the basis for their regulations. Periodically, the EPA has asked the U.S. National Academy of Sciences (NAS) to review the federal regulations for biosolids. The NAS review in 1995 concluded that "the use of biosolids in the production of crops for human consumption when practiced in accordance with existing federal guidelines and regulations, presents negligible risk to the consumer, to crop production and to the environment." The NAS reviewed the regulations again in 2002 and concluded, "There is no documented scientific evidence that the federal regulations have failed to protect public health."
Most researchers agree that the effects of organic compounds, metals, and microorganisms in biosolids are not harmful to humans or the environment if managed carefully. Many studies have shown that metals in biosolids are chemically bound in stable compounds and will not easily move into ground and surface waters.
Still, some land application projects are controversial, especially if they release odors. Odors at an application site can cause neighbors to raise questions about the safety and adequacy of regulations for biosolids recycling.
In response to the need for accurate and consistent information, the National Biosolids Partnership was established in 1997 by the federal EPA, Water Environmental Federation, and the Association of Metropolitan Sewerage Agencies. One of their goals is to encourage safe biosolids management practices in local communities through the use of environmental management systems.
National Biosolids Partnership Web site. Available from http://www.biosolids.org.
Peter S. Machno and Peggy Leonard
BIOSOLIDS REMEDIATE METAL-CONTAMINATED SOILS
The mining and processing of metal ores have contaminated soils in many countries. In the vicinity of lead (Pb) and zinc (Zn) mines and smelters, soils may have Pb and Zn concentrations as high as 20,000 mg/kg. These soils—with their high metals, low pH, and lack of nutrients and organic matter—are toxic to plants. Land around the mines is acidic and barren, often with blowing dust and metals leaching into ground and surface waters.
Three such sites on EPA's Superfund list—Palmerton, Pennsylvania; Leadville, Colorado; and Bunker Hill, Idaho—have demonstrated that biosolids mixtures can restore soils and vegetation. Biosolids combined with a calcium carbonate material such as lime or wood ash create a fertile soil and vigorous, self-sustaining plant growth. Iron and phosphates in biosolids adsorb lead and convert it to an insoluble compound, chloropyromorphite. Wood ash raises soil pH and prevents Zn from being taken up by plants or leached. Biosolids supply nutrients and organic matter for rebuilding soil and soil microbial communities.
Similar results have been reported in Upper Silesia, Poland, where lands have been contaminated by toxic coal and smelter wastes. New secondary treatment plants in the region will be producing a supply of biosolids for future restoration projects.