| Biosolids make an excellent soil amendment and source of nutrients for trees. This has been shown by decades of research in western Washington forests and elsewhere in the U.S. In 1987, we began fertilizing plantations on the Snoqualmie Tree Farm in east King County. The Greenway Biosolids Forestry agreement expanded the program in 1995 to include state forests in the county.
King County's forestry projects are part of a unique program to protect and enhance forests and wildlife habitat along the scenic I-90 corridor east of Seattle. The nonprofit Mountains to Sound Greenway Trust initiated this program which now includes several public and private partners: the state Department of Natural Resources, King County, the Hancock Timber Resources and the University of Washington (UW).
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Tree Growth and Biosolids
Using biosolids can lead to impressive growth response, because it supplies nitrogen not found in native forest soils. Biosolids increases growth rates to match those of trees found on more fertile soils. Water is another factor limiting tree growth during Northwest summers. The organic matter in biosolids helps improve the ability of soils to hold water and keep it available for trees.
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By counting the growth rings on a cross-section of a tree you can tell its age.
This Douglas-fir tree was cut when it was about 30 years old. When it was 20 years old, the tree was fertilized with biosolids, resulting in the wider growth rings.
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How Biosolids are Applied
Biosolids are delivered to the forest by special haul trucks, and unloaded into an open-topped metal box, located adjacent to application sites. The applicator vehicle is a converted logging vehicle with a "throw spreader" mounted on the rear. After self-loading with the clam-shell bucket, the vehicle applies from a system of parallel trails spaced to allow biosolids to reach all parts of the forest stand. This spacing may be 260 feet (between trails) in young plantations and 60 feet in timber stands that have been thinned.
Researchers from the UW calculate site-specific amounts of nitrogen for each forest stand. This prescription determines the number of tons of biosolids to be applied to each acre. Rates vary from 3 dry tons/acre of biosolids for timber to 7dt/ac for young plantations, which corresponds to 150 to 350 pounds of plant-available nitrogen per acre. Along with nitrogen, biosolids also provide all other nutrients essential for plant growth, including phosphorus, zinc, boron, manganese and chromium.
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King County has developed a cost-effective and environmentally-friendly technology for applying dewatered biosolids to forests.
At left, the biosolids are loaded into the spreader. About 30,000 wet tons of biosolids are applied to forests each year.
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The applicator vehicle can fling dewatered biosolids more than 200 feet out into the stand of trees. The biosolids fall to the ground, leaving very little on the trees themselves. This form of biosolids is very stable, even on steeper slopes.
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Environmental Safeguards
King County selects and designs sites in consultation with landowners and with scientists from the University of Washington. Streams and wetlands are protected by setbacks (buffer areas that are not fertilized). Depending on site-specific conditions, setbacks range in size, from the minimum of 33 feet required by federal and state rules up to 200 feet.
Biosolids recycling is regulated under a statewide general permit for biosolids management - part of the Washington Department of Ecology biosolids program that was implemented in 1998.
Stream Monitoring
Since 1986, forest streams units have been sampled and analyzed four time per year for ammonia nitrogen, nitrate-nitrogen, fecal coliform and enterococcus. Wherever possible, water upstream of the units is collected for comparison. Storm events are also sampled to verify that the site has adequate buffers.
More than 16 years of monitoring forest streams near our sites has shown no changes to surface water quality from biosolids.
 Other Concerns
Prescribing the right amount of nitrogen is the most important technical element of these projects. However, people often question the potential for long-term impacts from metals in biosolids. King County's biosolids have very low levels of metals and meet EPA and state standards for use on food crops.
In addition, metals in soils and biosolids are environmentally stable, bound so tightly that they do not leach or move through the soil. Long term local research, even under worst-case conditions, confirms that metals pose no threat to groundwater (wells) or surface waters.
Public Involvement
Neighbors and other interested members of the public are welcome to visit these projects. Your comments are always welcome.
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