Which solids treatment method processes solids in a sealed unit?

Anaerobic digestion is the correct answer because it is a solids treatment method that occurs in a sealed unit, known as an anaerobic digester. This process involves the decomposition of organic matter by microorganisms in an oxygen-free environment. The sealed nature of the digester allows for the control of temperature, pH, and other conditions essential for microbial activity, thereby effectively breaking down organic solids and reducing the volume of waste while producing biogas that can be used for energy.

In contrast, composting takes place in an open or semi-open environment where aerobic conditions dominate, allowing for the breakdown of organic matter with the help of oxygen. Sand drying beds and vacuum filters are methods used for dewatering sludge and do not involve the sealed processing characteristic of anaerobic digestion.