Composting is a high heat process (greater than 131⁰F) that transforms biosolids into a stable, pathogen free soil conditioner that releases nutrients slowly. Biosolids composts are considered Class A biosolids, and they can be used like any other compost product; they are used freely (without permitting) by farmers, landscapers, and gardeners alike. To compost biosolids, they are mixed with coarse woody materials and stacked into large piles. The piles then self-heat due to the metabolic activity of the naturally-occurring microorganisms eating and stabilizing the material. To ensure that this process kills all pathogens, the compost piles are closely monitored, and they must meet minimum time and temperature requirements. Laboratory testing is used to confirm pathogen destruction. Since composting requires the biosolids to be diluted with woody, carbon-rich materials, and the composting process reduces the availability of nutrients, and therefore compost typically contains a lower concentration of nutrients than other forms of biosolids. This compost, however, is very well suited as a soil conditioner, and where slow-release nutrients are desired. Studies have found that biosolids compost is not only excellent for adding slow-release nutrients, but also for increasing the water-holding capacity of the soil, reduce soil erosion and compaction, and for sequestering carbon in the soil.
See, WEF Fact Sheet on Land Application and Composting of Biosolids.