How biosolids are applied

Site selection and application rates

King County consults with landowners and scientists from the University of Washington's College of Forest Resources to select sites for biosolids application. Ideal sites have gentle terrain, well-drained gravelly or sandy soils, good road access, are well stocked with trees, and have good understory plant cover. Sites with few streams or drainages are preferable.

A biosolids spreader is loaded with biosolids that are delivered by haul truck.


The spreader self-loads and then drives slowly down access trails.


The spreader flings biosolids evenly to the forest site.

Once a site is selected, the soil is sampled for its nutrient content. Scientists from the UW prescribe site-specific biosolids application rates for each area based on the nitrogen content of the soil and of the biosolids, and the nitrogen needs of the trees and understory. Application rates vary from 3 dry tons/acre for timber to 7 dry tons/acre for young plantations, which corresponds to 150 to 350 pounds of plant-available nitrogen per acre. Visually, the applications look like a sprinkling of biosolids on the ground.

Biosolids application

King County has developed a cost-effective and environmentally-friendly technology for applying dewatered biosolids to forests. First, treated biosolids from King County's wastewater treatment plants are delivered to the forest by haul trucks. Biosolids are delivered to the forest by special haul trucks, and unloaded into an open-topped metal box, located adjacent to application sites.

Biosolids are then applied using a specialized applicator vehicle -- a converted logging vehicle with a "throw spreader" mounted on the rear, which can fling dewatered biosolids more than 200 feet out into the stand of trees. 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.

The biosolids fall to the ground, leaving very little on the trees themselves. The biosolids are very stable, even on slopes. Biosolids applications can be repeated on a site every 3 to 4 years.