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. The Weyerhaeuser Company began
using biosolids routinely in 1987 on its Snoqualmie Tree Farm. 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 recycling
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 with several public and private partners: the state
Department of Natural Resources, King County, the Weyerhaeuser Company and the
University of Washington (UW).
You can tell the age of a tree by looking at a cross section of the tree and counting the growth rings. 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.
Using biosolids can lead to impressive growth response in trees, especially in areas of the Pacific Northwest, where lack of nitrogen in native soils limits tree growth. Biosolids increases growth rates to match those in trees found on more fertile soils.
The other major limiting factor for tree growth is water. The organic matter in biosolids helps improve the ability of the soil to hold water and keep it available for the forest vegetation during the summer.
How Biosolids are Applied
Treated biosolids from King County's wastewater treatment plants are
delivered to the forest in a semi-solid state (about 22% solids). Haul trucks
unload the biosolids into an open-topped metal box, located adjacent to
application sites. The applicator vehicle is a converted log forwarder with a
'throw spreader' mounted on the rear of the chassis. After self-loading with a
modified log grapple/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 prescribe site-specific amounts of nitrogen for each area. 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 the other nutrients essential for plant growth, including phosphorus, zinc, boron, manganese and chromium.
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The applicator vehicle can fling dewatered biosolids more than 200 feet out into the 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. |
Environmental Safeguards
King County, Weyerhaeuser and the state DNR (on state lands) select
sites in consultation with scientists from the University of Washington College
of Forest Resources. Streams and other water bodies are protected by setbacks
(buffer areas that are not fertilized). Depending on site-specific conditions,
setbacks from surface waters are equal to or greater than the minimum of 33 feet
required by federal and state rules.
Biosolids use is regulated under a statewide general permit for biosolids recycling - part of the Washington Department of Ecology biosolids program implemented in 1998.
Stream Monitoring
Since 1986,
surface waters downstream of application 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 as a
control. Storm events are also sampled to verify that the site has been buffered
adequately. Two major storms, one in the spring and one in the fall, are
sampled. Data are then compared to background levels and to control
samples.
More than 11 years of monitoring surface waters at Weyerhaeuser's Snoqualmie Tree Farm has shown no changes to surface water quality from biosolids.
On selected sites, suction lysimeters are used to monitor nitrates in soil water just below the rooting zone. This research tool provides feedback about application rates to the project's scientific advisors.
Other Concerns
Prescribing
the right amount of nitrogen is the most important technical issue for 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 nearby 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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Biosolids Management
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