Which method below is not used to stabilize municipal wastewater treatment plant sludge?

Contact stabilization is a method primarily associated with the biological treatment processes in wastewater treatment rather than directly stabilizing sludge. It involves aerating the biological mass to enhance the decomposition of organic matter and is associated with the treatment of wastewater to improve effluent quality. In contrast, methods such as composting, thermal drying, and digestion are specifically employed to stabilize sludge by reducing its volume, pathogens, and odor, thereby enhancing its potential for reuse or safe disposal.

Composting involves the aerobic decomposition of organic matter, which stabilizes the sludge, while thermal drying reduces moisture content and further stabilizes it. Digestion, whether anaerobic or aerobic, breaks down organic solids and reduces the mass of sludge, stabilizing it for subsequent processing. Thus, contact stabilization does not play a direct role in sludge stabilization, making it the correct choice in this context.