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 waste-water 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 disease-causing 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.

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