Which of the following is NOT a characteristic of reverse osmosis systems?

In reverse osmosis systems, the primary driving force is not gravity but rather pressure. The process involves applying pressure to the contaminated water, which pushes the water molecules through a semi-permeable membrane. This membrane allows only certain molecules, such as water, to pass while blocking larger molecules and contaminants.

The use of pressure distinguishes reverse osmosis systems from methods that rely on gravity, such as sedimentation or basic filtration. Therefore, describing reverse osmosis as using gravity as a primary force is inaccurate, making it the correct answer for identifying a non-characteristic of these systems. The other aspects, such as requiring skilled operation, producing high-efficiency water treatment, and removing a large variety of contaminants, accurately reflect the capabilities and operational requirements of reverse osmosis technology.