Loop biosolids is a natural soil builder. Using Loop recycles the nutrients in our food back to the land, the same way nature does. It is an endlessly renewable resource that’s good for people, plants, and Puget Sound.

What is Loop®?

What are biosolids and how are they made? In this video, staff from King County Wastewater Treatment Division show how they transform raw wastewater into Loop® biosolids. After the transformation is complete, this endlessly renewable resource is used to enrich farms, forests, and gardens.

Loop 101

As it has for the past 40 years, King County produces Loop biosolids from solids (the food and poop) recovered during the wastewater treatment process. Gravity separates the solids from the liquids, and then both are treated and cleaned using engineering technology that mimics nature’s cleaning powers. The end result is Loop, a nutrient-rich soil builder that recycles the nutrients in our food back to the land, the same way nature does. Harvested plants take nutrients out of the soil, humans eat those nutrients through our food, and then the nutrients are returned to the soil with Loop. Loop is an endlessly renewable resource restoring carbon and nutrients to the land for the good of plants, people, and Puget Sound.

Held to rigorous standards under the Clean Water Act, Loop is tested and grounded in rigorous science. For nearly 40 years, we’ve built on Environmental Protection Agency and other national data by teaming with university scientists to continuously research and monitor the safety and effectiveness of Loop.

See how Loop is made

Take a tour of King County’s Wastewater Treatment Plants to see for yourself how Loop is made.


Loop has enriched Pacific Northwest landscapes as an ingredient in the commercial product GroCo compost since 1976.