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Biosolids Recycling

For more information about the Biosolids Recycling Program, please send us an email message or contact us at: 

King County Wastewater Treatment Division
Resource Recovery
201 S. Jackson Street
Mail Stop: KSC-NR-0512
Seattle, WA 98104
Phone: 206-477-5557
Fax: 206-684-2057

Loop Biosolids Content Analysis

Nutrients

Loop biosolids contains nitrogen and other nutrients that are essential for plant growth. Because nitrogen is naturally deficient in the glacial soils of the Pacific Northwest, plant growth typically improves with addition of nitrogen.

Loop contains 6 to 7 percent total nitrogen. Ammonia nitrogen is an inorganic form of nitrogen, readily taken up by plants, and averages about 1.2 % percent in biosolids. The remainder of the nitrogen is bound in organic compounds. This 'organic' nitrogen is sometimes referred to as slow-release nitrogen, because the organic matter must decompose over time before nitrogen becomes available for plants. Organic nitrogen averages 5 to 6% in biosolids.

Application rates are based on assessing the amount of nitrogen needed by each crop, then calculating the amount and predicted availability of the different forms of nitrogen in biosolids. For more information on nitrogen, see Managing Nitrogen from Biosolids (external link) at the Department of Ecology Website.

Nutrients and Conventionals, 2013 Averages
  West Point
Treatment Plant
South
Treatment Plant
Brightwater
Treatment Plant

Total Solids

28.2%

23.8%

 20.4%

Ammonia Nitrogen

0.8%

1.2%

 0.9%

Organic Nitrogen

5.2%

5.2%

 6.6%

Phosphorus

1.6%

2.5%

 1.3

Potassium

0.1%

0.2%

 0.1

Sulfur

1%

1%

 1.1

Metals

Biosolids contain trace amounts of metals, as do natural soils, manures and commercial fertilizers. The metals in biosolids come from human wastes, metal pipes, household products, businesses and industries. Many are micronutrients required for healthy plant and animal growth, and it's the presence of these micronutrients that help to make biosolids more effective than chemical fertilizers. Other metals, sometimes called "heavy metals," are of no value to plants or animals, but are nontoxic at the small quantities found in Loop. EPA and the Department of Ecology regulate metals in biosolids, and concentrations of metals in Loop fall far below the regulatory limits.

Metals in biosolids are tightly bound to organic matter and other elements in biosolids, making them unavailable to be absorbed by plants or animals. Because of the unique ability of biosolids to bind metals, University of Washington researchers are studying the use of biosolids to restore vegetation on soils contaminated by heavy metals and other chemicals.

Recently, King County focused on reducing levels of mercury in biosolids. Although Loop is not a significant source of mercury, it is important to us to reduce contaminants as much as possible. Thanks to cooperation from local businesses, Loop biosolids have a mercury content less than 6% of EPA's regulatory limit for safe recycling. Most dental offices have installed equipment to collect amalgam waste, thereby reducing a significant source of mercury in wastewater. As King County continues to work with businesses and industries to reduce releases of chemicals into the sewer system, the level of these substances in biosolids should continue to fall.

Concentrations in Loop meet EPA's most stringent regulatory limits. The chart below shows metals concentrations for a typical year (data are presented on a dry weight basis).

2013 Average Trace Metals Concentrations (ppm)
  West Point Treatment Plant South Treatment Plant  Brightwater   Treatment
Plant
National and State Regulatory Standards
Arsenic
5.51
5.58
3.07
 41
Cadmium
2.2
2.6
1.0
 39
Copper
402
406
309
 1500
Lead
89.2
27.2
14.1
 300
Mercury
0.97
0.94
0.75
 17
Molybdenum
8.2
8.5
6.3
 under reconsideration
Nickel
22.9
16.7
15.3
 420
Selenium
5.2
5.7
5.9
 100
Zinc
863
878
657
 2800

Pathogens

Microbes such as bacteria and yeast are a vital component of the natural world. Most are beneficial, performing such functions as decomposition and nitrogen fixation. Microbes that can cause disease are called pathogens. During treatment, beneficial bacteria and other tiny organisms break wastewater solids down into simpler, harmless organic matter. The process kills about 95% of the pathogens. Salmonella bacteria are typically found in concentrations of less than 1 or organisms per 4 grams of biosolids, while viruses and parasites are only rarely detected. Because some pathogens may survive this treatment however, they are classified as Class B, and site management precautions are taken to minimize risk when they are applied to land.

Most pathogens in Class B biosolids die quickly when exposed to environmental conditions after land application. Temperature changes, sunlight, variable moisture (drying), and competition from other organisms in the soil all greatly decrease the chance of pathogen survival. To ensure that pathogens do not reach the water supply, application sites are set back from surface waters and wells and steep slopes are avoided. Long term monitoring has shown no impact from our biosolids application activities on nearby waters.

With additional treatment to eliminate pathogens, biosolids are classified as Class A, which means they safe and can be used with no more restrictions than other fertilizers or soil amendments. About 5% of King County biosolids is treated further by composting to produce a Class A, pathogen-free biosolids compost called GroCo. GroCo is available to the public for use in home gardens, parks, and landscaping.

Trace organics

Because wastewater is collected from many different sources, including homes, and industries, biosolids can contain small amounts of a variety of organic compounds. Source control programs limit the release of chemicals into wastewater from industries and other large sources, and provide consumer education on finding less toxic alternatives to common household products.

Organic compounds found in biosolids are present at such low concentrations (near the lowest detectable limits) that studies conducted by EPA found negligible risk to public health and the environment. For this reason, EPA did not include organic compounds, including PCBs and dioxins, in biosolids regulations. King County regularly monitors biosolids for the organic compound identified by EPA as priority pollutants. Only a few of these compounds are detected in Loop, and monitoring has confirmed they are present at very low concentrations.

In addition to monitoring, King County participates in and closely follows research on organic chemicals that are now possible to detect because of new analytical technologies. For example, recent University of Washington research studies have focused on the fate of organic "micro-constituents" such as nonylphenol (a detergent metabolite), triclosan (an anti-microbial), and estrogen. Removing these compounds during wastewater treatment prevents their input to the aquatic environment. Research on biosolids shows these compounds decompose when applied to soils, do not get taken up by plants, and do not leach into groundwater or streams. These results, along with measures of soil and plant quality, provide a new level of assurance on the benefits of biosolids.

Find more information on King County's Biosolids program, including: