During the first treatment, materials called raw primary solids settle out of the wastewater by gravity. The wastewater then moves on to a second treatment, where tiny organisms called microbes feed on organic materials in the wastewater. After they're done, we're left with clean water and a mixture of organic materials and microbial remains known as raw secondary solids. The work is far from over, though. The raw solids from both stages must be processed further to produce biosolids.
Processing kills disease-causing organisms such as bacteria and viruses, and reduces odors.
A variety of chemical, physical, and biological processes are used to process raw solids. Despite their name, not all biosolids are solid. After all this processing, biosolids are ready to go to work. Good soil tilth means that soils are easy to dig in and work up, easy for seedlings to emerge from, and easy for plant roots to move through. Healthy soil means healthy plants.