Biosolids are the fertilizing and soil building products created by wastewater facilities around the world. Wastewater facilities can do more than just clean the water, they can also turn the valuable resources in the water into useful products like biosolids, biogas (renewable energy) and recycled water. This is what King County does. King County’s biosolids are called Loop®, and Loop is rich in essential nutrients and organic matter that plants love.

All the water that comes to the plant from homes and business is first separated into “solid” and “liquid” streams using gravity. Then, the “liquid” (the water) goes to its own part of the plant to be cleaned and to be turned into recycled water for things like irrigation and street cleaning. The “solids” (essentially the poop and the food) are put into a big heated tank called an anaerobic digester. In the digester, beneficial microorganisms, the same ones that live in our own stomachs and digestive system, break down the material until it is biologically transformed, all while creating biogas (renewable energy). When this process is complete, extra water is removed, leaving a fertilizer and soil builder that is full of the essential nutrients and organic matter.

No. No more than bread dough is the same as a loaf of bread. Biosolids are the end-result of regulated treatment processes, such as anaerobic digestion at warm temperatures. Anaerobic digestion uses big heated tanks. Just like your stomach, microorganisms break down the poop and the food, leaving organic matter and nutrients. The solids that are separated and cleaned only become biosolids after treatment is complete.

Loop looks like black mud. It’s somewhat crumbly but also sort of spongy, much like Play-Doh. Sometimes Loop sparkles with struvite, a beneficial phosphorus and nitrogen-rich mineral.

Freshly applied Loop can have an odor, usually described as “earthy” or “organic,” similar in some respects to bone meal or manure; people often describe a hint of “ammonia” to the odor as well. After being worked into the soil the odor typically goes away within a short time.

Compost or soil mixes that contain biosolids have much less odor — usually quite similar to potting soil or any other organic mulch. People often describe compost made with biosolids as having an earthy aroma, similar to a forest floor.

Loop was available in the Puget Sound area as an ingredient in the commercial compost product GroCo, but the family-owned business closed in 2020. Visit the Gardens & Landscapes page to see where you can get similar products and find out what’s next for Loop compost.

Commercial growers in Douglas or Yakima counties can contact Boulder Park Inc. or Natural Selection Farms respectively. Visit the Our Partners page  for more information on these customers, and read their stories on the Testimonials page.

Yes! Compost made with Loop, or similar products such as the City of Tacoma’s TAGRO soil blends can be used by anyone to grow the biggest, most delicious veggies and all edible crops. The process to make these products kills disease causing organisms (pathogens), using high heat and microorganisms to transform it into garden products.

Loop can be used to grow some types of food, like wheat or canola. Certain crops have waiting times before harvest, so the environment has enough time to do its job and kill any lingering pathogens. Studies have found that food grown with biosolids has higher protein content than food grown with commercial fertilizer.

Visit the Research & Testing and Environmental Benefits pages for more information.

GroCo compost was a natural, soil building compost. For almost 50 years, a small amount of Loop was sent to a private composter, who made and sold GroCo compost. GroCo compost was produced by composting 3 parts sawdust with 1 part Loop. It was dark, rich, and odor-free. In 2020 our compost partner, GroCo Inc. and Sawdust Supply, closed their business. This family-owned business has been around since the early 1900s, but the family is no longer interested in continuing their business and has sold the property.

Stay tuned for more product availability in the future via a potential King County owned Loop Compost.

Check out our video or visit the Garden & Landscape page for more information.

The nutrients found in Loop are also found in compost made with Loop. It contains both macro- and micronutrients beneficial for plant growth. On a dry weight basis compost made with Loop typically contained 1.24% N (total nitrogen), 2.04% P (as P2O5), and 0.2% K (as K2O). Visit the Gardens & Landscapes page for more information.

Add it to annual flower and vegetable beds, turf, perennial shrubs and trees, even indoor plants!

• For annual flower and vegetable beds add 1-3” to the top of your garden soil and till it in 6-12” deep each spring.
• For turf add 1/2-1” and rake evenly across the grass once a year.
• For perennial shrubs and trees, add 1-3” to the soil surface. As with any compost product or bark mulch, be sure to leave a 3” border around plant stems and trunks.
• Freshen up indoor plants by sprinkling a bit 1/2-2” of compost to the surface of the pot.

Visit the Gardens & Landscapes page for more information.

New innovative technologies are continually reviewed and pilot tested by our technology assessment team and university researchers, because we want to be the leader in our field. But it turns out, what we’ve been doing for decades is still pretty innovative and cost effective for our ratepayers. Not only do we harness renewable energy to power our treatment plants, but using Loop on farms, forests and gardens recycles nutrients, makes plants thrive, and helps us grow food for sustainable communities. Loop even helps us fight climate change.

Because the benefits and safety of using biosolids has been demonstrated for decades, Washington State Department of Ecology mandates that biosolids are used as a soil builder and fertilizer, conserving landfill space for waste materials. If we tried to burn Loop, it would create air pollution and would take a lot of energy because Loop is mostly water.

If we didn’t use Loop on farms, forests and gardens, we would waste all these benefits. Check out how different uses of Loop stack up in the diagram below.

Loop has been used as a fertilizer replacement and soil builder for decades, with a long list of happy customers. The long-term effects of Loop are healthy, fertile soil. Loop improves soil fertility and supports healthy plants. Long-term (20+ years), peer reviewed, academic studies show significant increases in soil organic matter and improved soil structure, both measures of healthy soil.

Yes, Loop is very similar to any other natural fertilizer or soil builder, such as manure. King County has been using Loop on our customer’s farms, forests and landscapes for more than 40 years. Visit the Research & Testing page for more information.

The best way to protect the quality of Loop is to prevent putting certain waste down the drain. The following items should go in the trash, not in the toilet or the sink:

• Stickers from fruits or vegetables
• Plastic and other non-organic materials like condoms or tampons
• Expired/unwanted prescription or over-the-counter drugs.
• Grease and other fat from the kitchen
• Disposable wipes, diapers, and liners, even if they are labelled as “flushable”
• Products labeled “danger”, “hazardous” or “toxic”

Try to use fewer cleaning products and chemicals at home and switch to “biodegradable” or more natural alternatives.

Remember: Don’t flush trouble! Only toilet paper and human waste should get flushed.

Only because there are toxic substances everywhere, from households to the arctic. Using biosolids like Loop is a great way to return organic matter and essential nutrients back to the land, and it makes growing food more sustainable. For a gardener, it would take 77,266 years of using biosolids before you would get one dose of ibuprofen. We’re not planning to live that long!

Visit the Loop Research & Testing page for detailed information on how we protect Loop’s product quality.

Not for long! Almost all the pathogens (95-99%) are eliminated in the cleaning process when we make Loop. The rest die quickly from sunlight, heat, and competing soil microorganisms. Biosolids are managed to keep people and the world we live in safe. The average person will never come into direct contact with Loop, and certain crops have waiting times before harvest, so the environment has enough time to do its job and kill any lingering pathogens. Our operators at the treatment plants and our customers work with Loop every single day – and they are happy and healthy!

Processes like composting kill the remaining pathogens, so products like compost made with Loop can be used by anyone on everything from veggie gardens to landscapes to farms.

Visit the Research & Testing page  to find out more.

No. Decades of research, testing, and monitoring have shown that Loop is a benefit to the environment and our water bodies. Loop is applied at what’s called an agronomic rate. An agronomic rate means that  we apply the exact amount of nutrients the plant needs. Loop is rich in organic matter and nutrients that are released slowly as plants need them, which means Loop’s nutrients stay tightly bound in the soil for plants and away from water bodies. Since what is good for the soil and plants is not always good for the water, Loop is never applied directly next to streams, wetlands, or other water bodies.

Visit the Research & Testing and Environmental Benefits pages for more information.

Visit the King County’s Biosolids Program page for more information.