Municipal Sewage Sludge Use in Forests of the Pacific Northwest, U.S.a.: Environmental Concerns

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Abstract
Forest applications of dewatered municipal sludge in Washington State, U.S.A. were monitored for heavy metals, pathogens, and nitrates, Heavy metals did not leach from the sludge in significant quantities and remained associated with the sludge for at least four years. Tree seedlings grown in sludge-amended soil and compost accumulated moderate metal levels, but growth was not adversely affected. Populus seedlings took up more metal than coniferous seedlings. Some coniferous seedlings grown directly in unamended sludge had reduced growth and developed symptoms of nutrient imbalances. Fecal coliform and total coliform levels in sludge were initially high, but decreased to background levels in 1.0—1.5 years, suggesting that site access should be limited for a time after application. Bacterial movement in the soil was limited and very few viable bacteria reached the groundwater. Aerosols may be generated during sludge applications. Nitrogen leaching is a major environmental concern or sludge application. Heavy applications resulted in high NO3 - levels in soils and groundwater. A significant fraction of the nitrogen loss was in gaseous form. Sludge applications in forests can be environmentally sound if application rates are matched to site characteristics.