Filtration and Circulation
The circulation loop
Water leaves the pool through surface skimmers (which should carry most of the flow and catch floating debris) and the main drains, then passes through the pump strainer, pump, filter, heater and chemical injection point before returning through the inlets. Everything is sized around turnover time - the minutes needed to move one full pool volume through the filter. GPM = pool volume ÷ (turnover hours × 60). Wading pools and spas turn over far faster than a lap pool because of bather load per gallon.
Filters and their signals
Sand, high-rate sand, diatomaceous earth and cartridge filters all trap particles and all eventually load up. A pressure gauge rising 8–10 psi above the clean, just-backwashed reading is the standard backwash trigger; cartridges are cleaned instead. Record the clean pressure immediately after every backwash so you always have a baseline.
Traps
Questions love to blur filter type with filter media, or to hand you a flow rate when you need turnover time. Total dynamic head and the pump curve determine actual flow: a dirty filter or a partly closed valve pushes the operating point down the curve and drops GPM. Never dead-head a pump by closing both suction and discharge, and never assume the installed flow meter is accurate until it has been verified.
Worked example
A 30,000-gallon pool on an 8-hour turnover needs 30,000 ÷ (8 × 60) = 62.5 GPM. Tightening turnover to 6 hours requires 83.3 GPM - check the pump curve and filter rating before promising that to anyone.