Loop biosolids is a natural fertilizer and 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 fertilize 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 fertilizer and 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.

Loop is awesome. Choosing to use Loop as a fertilizer doesn’t just help the soil and plants, it also stores carbon while reducing our carbon footprint. Loop helps fight climate change so much, it’s actually one of the main reasons the King County Department of Natural Resources and Parks is carbon neutral. And King County just doesn’t clean the water and make Loop, we also create two more renewable resources: biogas and recycled water.

Loop turns your dirt around. Loop has enriched Pacific Northwest landscapes as an ingredient in the commercial product GroCo compost since 1976. Loop is also used on its own on farms and forests, because Loop delivers all the macro- and micronutrients that plants need, making it a superior source of plant food over synthetic fertilizers. It has a proven ability to improve soil structure, which reduces runoff and erosion and protects local water quality. Using Loop as a fertilizer and soil builder is a cost effective, socially responsible, and environmentally beneficial choice for our community.

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.