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

Wastewater Treatment Process

From collection system or pumping station o o

Bar screen

Grit chamber

Primary Treatment Sedimentation basin

Secondary Treatment

Aeration basin Sedimentation basin

Seed sludge

Sludge Processing

Stabilization

Thickening

'Dewatering=

Wastewater products to beneficial use

Wastewater products to beneficial use

Advanced Treatment

Chlorination Filtration

Sedimentation basin

Nutrient removal

Advanced Treatment

Chlorination Filtration

Sedimentation basin

Nutrient removal

Effluent discharge or reuse

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