How Pool Filters Work: Sand, Cartridge & DE Filter Guide

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How Pool Filters Work: The Basics

Your pool filter is the heart of your water circulation system, working continuously to remove contaminants and keep your water crystal clear. Understanding how your specific filter type works helps you maintain it properly and troubleshoot issues when they arise.

All pool filters operate on the same basic principle: your pool pump pulls water from the pool through the skimmer and main drain, pushes it through the filter media where contaminants are trapped, then returns clean water to the pool through the return jets. The key difference lies in what type of filter media is used to capture debris.

Sand Filter Operation

Sand filters are the most common type, using a bed of specially graded silica sand as the filter media. Here's how they work:

1. Water Entry: Pool water enters the top of the sand filter tank through the multiport valve
2. Filtration Process: Water flows down through 18-24 inches of sand, which traps particles as small as 20-40 microns
3. Clean Water Collection: Filtered water collects at the bottom of the tank via lateral pipes with tiny slots
4. Return to Pool: Clean water flows back to the pool through the return lines

The sand bed becomes more effective over time as trapped debris creates smaller pathways, improving filtration. However, this also increases pressure, requiring regular backwashing when pressure rises 8-10 PSI above the clean starting pressure.

Sand Filter Maintenance

Sand filters require backwashing every 1-2 weeks during swimming season. The sand itself should be replaced every 5-7 years, or sooner if you notice consistently cloudy water despite proper chemical balance. Use only #20 silica sand specifically designed for pool filters.

Cartridge Filter Systems

Cartridge filters use pleated polyester fabric or paper elements to trap debris. They're increasingly popular for smaller pools and spas because they're easy to maintain and filter particles down to 10-15 microns.

1. Water Flow: Water enters the cartridge housing and flows from outside to inside of the pleated cartridge
2. Physical Filtration: The pleated fabric traps particles in its fibers, including oils and some bacteria
3. Clean Water Exit: Filtered water exits through the center core and returns to the pool

Cartridge filters don't require backwashing, making them more water-efficient than sand filters. Instead, you remove and rinse the cartridges every 2-4 weeks, depending on bather load and environmental factors.

Cartridge Filter Maintenance

Rotate between two sets of cartridges for best results. When pressure rises 8-10 PSI, remove cartridges and rinse with a garden hose from inside out. Monthly, soak cartridges overnight in a cartridge cleaner solution to remove oils and embedded debris. Replace cartridges annually or when the pleated fabric becomes gray or damaged.

Diatomaceous Earth (DE) Filters

DE filters provide the finest filtration, capturing particles as small as 2-5 microns using diatomaceous earth powder made from fossilized diatoms.

1. DE Coating: DE powder coats fabric grids inside the filter tank, creating the actual filter medium
2. Ultra-Fine Filtration: Water passes through the DE coating, which traps even microscopic particles
3. Grid Collection: Clean water passes through the grids and returns to the pool

DE filters require adding fresh DE powder after each backwash. Use 1 pound of DE per 10 square feet of filter area. Always pre-mix DE in a bucket of water before adding through the skimmer to prevent clogging.

DE Filter Maintenance

Backwash when pressure rises 8-10 PSI above starting pressure, then add fresh DE. Every 3-4 months, disassemble the filter for thorough cleaning of the grids with muriatic acid solution (1 part acid to 10 parts water). Always add acid to water, never water to acid, and wear protective equipment.

Optimizing Filter Performance

Regardless of filter type, proper operation requires adequate circulation time. Run your filter system 8-12 hours daily during swimming season, or use the formula: Pool volume ÷ pump flow rate × 1.5 for minimum daily runtime.

Maintain proper water chemistry to support filtration effectiveness:

• Free Chlorine: 1-3 ppm (based on CYA level using FC/CYA chart)
• pH: 7.4-7.6 for optimal sanitizer effectiveness
• Total Alkalinity: 80-120 ppm (60-80 for salt water pools)
• Calcium Hardness: 250-350 ppm for plaster pools

Common Filter Problems and Solutions

Poor filtration despite clean filter media often indicates inadequate circulation time, improper water chemistry, or pump issues. Cloudy water that won't clear may require a SLAM (Shock Level And Maintain) process using liquid chlorine, maintaining shock level FC until water clears completely.

Short cycles between cleanings suggest high bather load, inadequate sanitization, or the need for filter media replacement. Consider upgrading to a larger filter or adding a secondary cartridge filter for problem pools.

Remember that filters only remove physical debris - they don't sanitize water. Proper chlorination remains essential for killing bacteria and algae that filters cannot capture.