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Disposal of Biosolids and Other Wastewater Residuals


Sewage sludge is a combination of solids and liquid which comes from the treatment of domestic sewage. It contains beneficial nutrients for plants, as well as microorganisms which can cause disease. Untreated sewage sludge when applied to the land becomes a potential cause of diseases in people as well as animals. It contains bacteria, viruses, protozoa, parasites, and other organisms which may cause disease. This problem can be mitigated by various forms of sewage sludge treatment including digestion, drying, composting, lime stabilization, heat treatment, etc. Sewage sludge as well as biosolids can attract vectors, which are a variety of insects and/or rodents who may carry viable microorganisms that can carry disease from the site of disposal to people and animals. Disease can also spread by direct contact with the sewage sludge, walking through the area where it has been deposited, and handling the soil from the fields. It also can be spread through ingestion of food, drinking water, or recreational water which has become contaminated with runoff from the application areas. Airborne contact with the microorganisms can occur when the liquids are either sprayed or splashed on the ground.

Biosolids are treated sewage sludge which after treatment and processing can be recycled by farmers and gardeners to be used as fertilizer containing essential crop nutrients, especially nitrogen and phosphorus, to stimulate plant growth. This material must adhere to the standards of the federal biosolids rule found in 40 CFR, part 503, which establishes numerical limits for metals and pathogens, and other standards for site restrictions, crop harvesting restrictions, and monitoring, record-keeping, and reporting requirements. This helps avoid the use of chemical fertilizers which may readily contaminate the environment. Biosolids are also used in mine site reclamation projects to improve the land, and forestry where it promotes rapid timber growth. Biosolids may also be used in plant nurseries, cemeteries, parks, gardens, and for home and business lawns. If there are too many biosolids to be used in the above ways, then it may also be disposed of in landfills and by incineration at high temperatures.

Biosolids may contain bacteria, viruses, protozoa, and helminths, depending on the amount of pretreatment, which may be pathogenic to people and to animals. Care should be taken not to expose workers or people in surrounding areas to airborne microorganisms at land application or biosolids storage sites.

Dewatering and thickening of the biosolids is important to make the transportation and eventually use of this material easier. This can be accomplished through several innovative processes involving membrane thickening, electro-dewatering, and geotextile dewatering.

Best Practices in Turning Sewage Sludge into Safe Biosolids (See endnotes 46, 47)

• Utilize an aerobic digestion process for the sewage sludge after secondary treatment for 40 days at 68°F and 60 days at 59°F.

  • • Utilize an air drying process where the sewage sludge is dried on beds of sand for at least 3 months and the temperature is above freezing.
  • • Utilize an anaerobic digestion process where the sewage sludge is treated in the absence of air for 15 days at 131°F or 60 days at 68°F.
  • • Utilize a composting technique where the temperature of the sewage sludge is 104°F or higher and stays this way for 5 days. For 4 hours during this time the temperature must be raised to 131°F.
  • • Utilize a lime stabilization technique where there is sufficient lime added to the sewage sludge to raise the pH to 12 for at least 2 hours of contact time.
  • • Utilize the innovative supercritical water oxidation process for thermal oxidation of the organic material within the waste stream by increasing the temperature and pressure in the equipment until the organic compounds are oxidized.
  • • Utilize an innovative sludge-to-fuel process in which a carbon-rich fuel is produced from the sludge under high pressure and heat which after dewatering can be used as a fuel.
  • • Utilize an innovative vitrification process where the minerals in the biosolids are melted and create a glass-like product that can be used as an aggregate.
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