What is a primary function of ultrafilters in water treatment?

Ultrafilters play a crucial role in water treatment by functioning as a filtration technology that separates contaminants based on size. Their primary role is to act as sieves, retaining larger particles, such as bacteria, suspended solids, and certain macromolecules while allowing smaller particles and dissolved substances to pass through. This selective filtration process helps improve water quality by effectively reducing turbidity and harmful microorganisms, making the water safer for consumption and use.

The other options involve functions not characteristic of ultrafilters. For example, increasing water temperature does not relate to filtration but rather to heating processes. The removal of volatile organic compounds (VOCs) typically involves activated carbon or other specialized treatment methods rather than ultrafiltration alone. Enhancing the color of the water does not correlate with ultrafiltration's purpose, as it is primarily focused on the physical removal of particles rather than affecting the aesthetic qualities of water. Thus, the correct answer highlights the essential sieving function of ultrafilters in ensuring cleaner, safer water through effective particle retention.