Skip to main content
King County logo

Since 1973, our biosolids program has worked with local universities to develop and test biosolids recycling methods. Research has included effects of Loop biosolids on soils, crops, wildlife and water quality, as well as developing new markets and testing application methods. Results give us the technical basis for site management and environmental monitoring, assist in the development of regulations, build public acceptance, and provide quality assurance for landowners. University scientists continue to be the technical advisors for our projects, providing ongoing third party review and oversight.

Carbon Sequestration & Climate Change

Using biosolids to store carbon in soil is an important strategy for building soil health and fighting climate change. King County has participated in studies to measure the potential for soil carbon sequestration and examine the carbon impacts of different biosolids management strategies.

A carbon accounting of our program examined the greenhouse gas emissions and carbon sequestration potential of the options available for biosolids use compared to disposal (land application, incineration, and landfilling), quantifying the implications of each for climate change. The greenhouse gas analysis included emissions from transporting biosolids, emissions from biosolids incineration, energy generation from methane capture (either during biosolids production or at a landfill site), and soil carbon sequestration. While landfilling and incineration both result in net emissions of greenhouse gases, land application captures all the fertilizer benefits of biosolids and sequesters more carbon in the soil than is released during the recycling process (Brown & Leonard, 2004a&b).

A study done in partnership with UW and WSU verified these carbon benefits by soil testing at biosolids application sites around Washington State, including farm fields, orchards, vineyards, commercial landscapes, and vegetable gardens. Every site showed increases in soil carbon, nitrogen, and phosphorus compared to unfertilized controls, indicating that biosolids were effectively adding plant-essential nutrients and increasing soil carbon sequestration (Brown et al., 2011). Soils amended with organic matter appear richer in color than soils without added organic amendments (left).

Another study examined the greenhouse gas implications of different land-use alternatives for former mining sites. The authors compared restoration of the sites to forest, with or without the use of biosolids, or conversion to low-density suburban housing. The results clearly showed that by using biosolids to restore land to forest and focusing housing in the already-developed urban core, soil carbon storage and forest health can be maximized while avoiding the emissions involved in building and inhabiting new areas (Trlica & Brown, 2013).


carbon_compareSoils amended with organic matter appear richer in color than unamended soils.

Dryland Farming: improved yields, reduced erosion, and soil carbon sequestration

Since 1994, WSU scientists have maintained research plots at the Boulder Park agriculture site to answer questions of interest to farmers, scientists, and biosolids producers. This site has been home to short- and long-term studies of crop and soil responses to biosolids. Current projects at Boulder Park and other sites are focused on enhancing soil carbon storage and reducing erosion.

Studies are typically conducted by applying various rates of biosolids or synthetic fertilizer to designated research plots, then planting and harvesting according to normal farming practices for the area. Researchers can then take samples of soils and crops to observe the plant response to different fertilizers. Since the establishment of research plots in 1994, numerous studies have confirmed that agronomic application of biosolids (applying amounts based on the nitrogen needs of the crop) produces grain yields equal to or greater than synthetic fertilizers. 

The amount of soil carbon has increased over time as carbon from both the organic matter in biosolids and from increased crop residues helped replace what was lost due to decades of farming. Monitoring has shown no appreciable trace metals accumulation in biosolids-amended soils or in wheat grain. 

Results from these long-term studies have affirmed that biosolids are both safe and highly effective for fertilizing crops in dryland farming systems.

ManWheatCompare_300Comparing wheat plants grown with Loop (left) to unfertilized wheat (middle) and wheat grown with conventional fertilizer (anhydrous ammonia).

Stormwater Bioretention Research

Researchers from the University of Washington have teamed up with King County to investigate compost made with biosolids for use in stormwater bioretention systems. Using plants, soil, and compost, these systems are engineered to mimic nature’s ability to clean stormwater by absorbing it into soil, filtering out pollutants and reducing pollution in waterways. Stormwater bioretention systems and rain gardens are being implemented on an increasingly wide scale. Current research is focused on ways to optimize performance. UW researchers are examining the suitability of composts made with biosolids for bioretention and working to develop performance-based standards for soil/compost mixes.

BioretentionResearch_300Testing the ability of bioretention soil mixtures to remove pollutants from stormwater


King County participated in a nationwide study by the University of Arizona concerning bioaerosols: airborne pathogens that could affect people exposed to them. By placing special sensors downwind during biosolids loading and application (see photo), researchers were able to study the concentration of airborne bacteria or viruses that could be a source of exposure for neighbors or biosolids workers.

Samples were taken during operations at a biosolids forestry site and at one of our partner farm sites. Pathogen levels detected during forest operations were indistinguishable from ordinary background levels. While a small increase in bacterial concentrations was noted at the farm site, this difference was only detectable within close range of the loading equipment.

Field sampling for bioaerosols during biosolids applications in the forest.


Brown, S. & P. Leonard. 2004a. Biosolids and global warming: evaluating the management impacts, Part 1. BioCycle, August. 6 pps.

Brown, S. & P. Leonard. 2004b. Building carbon credits with biosolids recycling, Part 2. BioCycle, September, p. 25-29.

Brown, S., K. Kurtz, A. Bary, & C. Cogger. 2011. Quantifying benefits associated with land application of organic residuals in Washington State. Environ. Sci. Technol. 45(17): 7451-7458.

Brown, S., D. Devin-Clarke, M. Doubrava, and G. O’Connor. 2009, Fate of 4-nonylphenol in a biosolids amended soil. Chemosphere 75:549-554.

Trlica, A. and S. Brown. 2013. Greenhouse gas emissions and the interrelation of urban and forest sectors in reclaiming one hectare of land in the pacific northwest. Environ. Sci. Technol. 47(13): 7250-9.