There are three filter designs: gravity, pressure, and ________.

The answer identifies membrane filters as the third type of filter design alongside gravity and pressure systems. Membrane filtration is a process that utilizes a semipermeable membrane to separate contaminants from water. This type of filtration is especially effective for removing smaller particles, including bacteria and viruses, due to its fine pore size.

Membrane filters are widely used in various water treatment applications, including reverse osmosis, microfiltration, ultrafiltration, and nanofiltration, which provide a range of filtration capabilities depending on the specific membrane characteristics. The technology is notable for its efficiency and effectiveness in producing high-quality filtered water.

In contrast, other filtering methods mentioned in the incorrect options do not represent the standard classifications in the same context as membrane filtration. Centrifugal filters focus on separating substances based on density using centrifugal force, while cartridge and sand filters are variations that fit into gravity or pressure categories rather than being standalone types. Therefore, recognizing membrane filtration as a distinct system highlights the advanced technology available in modern water treatment.