King County Navigation Bar (text navigation at bottom)
Wastewater Treatment Division image DNR Wastewater Treatment Division

Biosolids Quality

Biosolids samples are analyzed for nutrients, trace metals, trace organic pollutants and microbes, providing information on the physical, chemical and microbial characteristics of biosolids. King County analyzes biosolids monthly in order to characterize quality, examine changes over time, determine biosolids land application rates and to comply with national and state regulations for biosolids recycling.

2006 Annual Biosolids Quality Summary (pdf 284KB):

Nutrients and Conventionals
Biosolids contain 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.

King County biosolids contain 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)

West Point Treatment Plant
2007 Average
 South Treatment Plant
2007 Average
Total Solids
26.7%
 Total Solids
22.9%
Ammonia Nitrogen
0.93%
 Ammonia Nitrogen
1.2%
Organic Nitrogen
5%
 Organic Nitrogen
5.9%
Phosphorus
1.8%
 Phosphorus
2.2%
Potassium
0.2%
 Potassium
0.2%
Sulfur
1.1%
 Sulfur
1.1%

Trace Metals
Regulatory standards for biosolids recycling include limits for nine trace elements in biosolids: arsenic, cadmium, copper, lead, mercury, molybdenum, nickel, selenium and zinc. Concentrations in King County biosolids meet EPA's most stringent regulatory limits.

In 2007, there were statistically significant decreases in cadmium and molybdenum in biosolids from the South Treatment Plant.

Data are presented on a dry weight basis.
2007 Averages
West Point Treatment Plant
South Treatment Plant
 National and State
Regulatory Standards
(dry weight)
mg/kg
mg/kg
mg/kg
Arsenic
6.36
6.21
41
Cadmium
2.95
3.32
39
Copper
523
522
1500
Lead
101
48
300
Mercury
1.42
1.19
17
Molybdenum
11.5
11.3
under reconsideration
Nickel
30.5
27.7
420
Selenium
6.92
7.25
36
Zinc
940
912
2800

Microbes
Biosolids at King County are anaerobically digested; this is a biological treatment process which eliminates about 95% of pathogens and meets Class B pathogen reduction standards. Routine microbiological analyses of biosolids show that viruses are rarely detected and parasites are undetected.


Return to Biosolids Home

For questions about biosolids recycling or this website, please contact us at: biosrecyc@kingcounty.gov

Resource Recovery
201 S. Jackson Street
Mail Stop: KSC-NR-0512
Seattle, WA 98104
Phone: (206) 684-1255
Fax: (206) 684-2057

Updated: May 28, 2008

Home | Natural Resources | Wastewater Treatment | News | Services | Comments | Search

Links to external sites do not constitute endorsements by King County.
By visiting this and other King County web pages,
you expressly agree to be bound by terms and conditions of the site.
The details