The only solids dewatering unit to operate under constant high compression is:

The plate and frame filter press operates under constant high compression, which is a significant factor in its effectiveness for solids dewatering. In this equipment, the feed slurry is pumped into the filter chambers formed by the plates, where the solids are separated from the liquids. The pressing action compresses the filter cake formed, maximizing the solid-liquid separation and extracting water from the solid material.

This high-pressure operation allows for a more efficient dewatering process compared to other types of dewatering equipment, which may use gravity or other mechanisms that do not exert the same level of compressive force. Consequently, the plate and frame filter press is particularly valuable in applications requiring rigorous solids removal and maximized moisture extraction from sludges and slurries.

Other options, while effective in their own right, employ different mechanisms. For instance, the belt filter press uses a combination of gravity and mechanical pressure to dewater material, while vacuum filters operate by drawing liquid through a permeable medium with vacuum assistance. Pressure sand filters primarily aim to remove suspended solids from liquids rather than focusing specifically on high compression dewatering. Thus, the plate and frame filter press is unique in its operational capability of constant high compression, making it the correct answer.