An intermediate process between Ultrafiltration (UF) and RO, often called "membrane softening". It operates at lower pressures, allowing some monovalent ions (e.g., sodium, chloride) to pass while effectively removing multivalent ions (e.g., calcium, magnesium) and larger organic molecules. Key Performance Factors
Fouling, or the accumulation of contaminants on the membrane surface, is the "Achilles' heel" of these systems, requiring robust pre-treatment (e.g., UF, chlorination, pH adjustment).
Recent advancements focus on lowering operating pressures and improving pressure recovery devices (PRDs) to reduce energy consumption.