What method involves mixing biosolids with a bulking agent and allowing them to decompose?

The correct approach described in the question is composting. This method specifically refers to the process of mixing organic materials, such as biosolids, with a bulking agent, which provides structure and aeration to the mixture. The bulking agent can include materials like wood chips, straw, or other carbon-rich substances.

During composting, the mixture undergoes microbial decomposition, allowing beneficial bacteria and fungi to break down the organic matter. This aerobic process generates heat, which can further aid in killing pathogens and breaking down nutrients, ultimately resulting in a stabilized product that can be safely used as a soil amendment.

The other methods mentioned serve different purposes or processes. Aerobic digestion also involves microbial action but is typically a more controlled process focusing on energy production rather than creating a soil amendment. Lime stabilization is a chemical process used primarily to reduce pathogens and odors in biosolids by adding lime, while air drying is a physical process that removes moisture but does not involve biological decomposition in the same way composting does.