Combat water supply detachments employ Reverse Osmosis Water Purification Units (ROWPUs) to treat raw freshwater and seawater sources in austere field environments. Prior to reaching the high-pressure semipermeable membranes, raw water must pass through a multimedia filter containing layered anthracite coal, fine garnet, and graded silica sand. This multimedia stage traps suspended particulates larger than thirty microns to protect downstream cartridge filters from premature clogging. A differential pressure gauge continuously monitors resistance across the multimedia bed; when the pressure drop across the vessel exceeds fifteen pounds per square inch, operators must initiate a twelve-minute backwash cycle to dislodge accumulated silt and restore proper flow.
According to the passage, operators must initiate a backwash cycle on the multimedia filter when the pressure drop exceeds fifteen pounds per square inch.
Answer: Answer