🌊 Calplas AFM-520 Semi-High Bed Sand Filter – 48,000 Litre Capacity

Advanced AFM-optimised filtration for crystal clear pool water, engineered in Spain since 1970

💧 Product Overview

The Calplas AFM-520 is a premium semi-high bed filter specifically engineered for use with Activated Filter Media (AFM) and traditional sand filtration. Manufactured in Spain by Calplas, one of the world’s most experienced filter manufacturers since 1970, this 520mm diameter filter delivers exceptional water clarity for residential swimming pools up to 48,000 litres.

Built from hand-laminated glass-fibre reinforced polyester (GRP) with internal steel reinforcement, the AFM-520 features a generous 900mm filter bed height for superior filtration depth. The filter includes a special AFM diffuser that ensures ideal water distribution across the filter bed, preventing crater formation and channelling that can compromise filtration performance.

Designed for compatibility with the Swiss-engineered Besgo automatic backwash valve system, this filter offers both manual and fully automated backwash options. The transparent acrylic inspection cover and integrated sight glass provide easy monitoring of filter operation and media condition.

⭐ Key Features

✅ Semi-High Bed Design – 900mm filter bed height for superior filtration depth and water contact time

✅ AFM-Optimised Diffuser – Special diffuser system ensures even water distribution, preventing crater formation in the filter media

✅ Premium GRP Construction – Hand-laminated glass-fibre reinforced polyester with internal steel reinforcement at high-stress points

✅ Transparent Inspection Cover – Acrylic top lid allows visual monitoring of filter media condition without opening

✅ Integrated Sight Glass – 2½” sight glass for monitoring backwash water clarity

✅ Service Manhole – ø215mm lateral access manhole for easy media inspection and replacement

✅ Besgo Valve Compatible – Designed for use with Besgo automatic or manual backwash valve systems

✅ Complete Package – Includes pressure gauge, drain plug, and air vent as standard

✅ Dual Media Compatibility – Optimised for both AFM activated glass media and traditional silica sand

✅ 5-Year Tank Warranty – Manufacturer-backed warranty on the filter tank

✅ EN 13121-3 Certified – Manufactured and tested to European pressure vessel standards

✅ UV-Resistant Finish – External gelcoat in blue RAL 5015 resists UV degradation (also available in white)

🔧 Technical Specifications

Specification	Value
Filter Type	Semi-high bed sand/AFM filter
Tank Diameter	520 mm
Filter Area	0.20 m²
Filter Bed Height	900 mm
Overall Height	1400 mm
Distance from Ground (A)	405 mm
Connection Spacing (B)	190 mm
Inlet/Outlet Connections	2″ Female thread (GAS)
Filter Opening (Top)	ø190 mm
Service Manhole	ø215 mm
Sight Glass	2½”
Working Pressure	2.5 bar
Test Pressure	3.75 bar
Maximum Temperature	40°C
Tank Material	Hand-laminated GRP with steel reinforcement
Tank Colour	Blue RAL 5015 (white available on request)
Weight (Empty)	58 kg
Flow Rate @ 30 m³/h/m²	6 m³/h
Backwash Flow Rate @ 50 m³/h/m²	10 m³/h
AFM Media Capacity	11 bags (6× Grade 1 + 5× Grade 2, 21kg bags)
Sand Media Capacity	10 bags (5× fine + 5× coarse, 25kg bags)
Warranty	5 years (tank)

⚠️ Note: Higher working pressure versions (up to 7 bar) available on request from manufacturer.

📐 Pool Sizing Guide

Sizing Principle: The filter flow rate should turn over the entire pool volume within 6-8 hours for optimal water quality.

Formula:

Required Flow Rate (m³/h) = Pool Volume (m³) ÷ Turnover Time (hours)

This Filter:

📍 Standard Flow Rate: 6 m³/h (at 30 m³/h/m²)
📍 Backwash Flow Rate: 10 m³/h (at 50 m³/h/m²)

Pool Size Compatibility:

Pool Type	Volume Range	Suitability
🏠 Small above-ground	Up to 20,000 L	✅ Excellent – generous capacity margin
🏠 Medium above-ground	20,000-36,000 L	✅ Ideal match with 6-hour turnover
🏠 Large above-ground	36,000-48,000 L	✅ Suitable with 8-hour turnover
🏊 Small in-ground	Up to 30,000 L	✅ Well suited
🏊 Medium in-ground	30,000-48,000 L	✅ Suitable with appropriate pump
🏊 Large in-ground	48,000+ L	⚠️ Consider AFM-640 or larger

Turnover Calculations:

Pool Volume	Turnover Time	Suitability
36,000 L (36 m³)	6 hours	✅ Optimal
42,000 L (42 m³)	7 hours	✅ Good
48,000 L (48 m³)	8 hours	✅ Acceptable maximum

🔌 Installation Requirements

Plumbing:

Requirement	Specification
Inlet Connection	2″ Female thread (GAS)
Outlet Connection	2″ Female thread (GAS)
Valve Position	Side-mounted (Besgo compatible)
Connection Spacing	190 mm between inlet/outlet

Physical Installation:

Requirement	Details
Base	Solid, level concrete pad or paving required
Filter Height	1400 mm total
Ground Clearance	405 mm from floor to connections
Clearance Above	Minimum 500 mm for lid removal and media access
Clearance Around	Minimum 300 mm for valve operation and maintenance

Pump Compatibility:

⚠️ Filter must be matched with appropriate pump.
✅ Recommended pump flow rate: 4-6 m³/h for filtration
✅ Backwash pump capacity: 8-10 m³/h for effective backwashing
❌ Pump must not exceed filter's maximum flow rate of 10 m³/h

Valve Options: The Calplas AFM-520 is designed for use with Besgo backwash valves (wall-mounted, coupling set required separately). Choose from:

• Manual Besgo valve for cost-effective installation
• Automatic Besgo valve with solenoid control for hands-free operation
• Control panel available for scheduled automatic backwashing

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🔄 Maintenance Guide

Routine Maintenance:

Task	When	How
Check pressure gauge	Weekly	Normal operating: 0.5-1.0 bar
Backwash filter	When pressure rises 0.5 bar above clean baseline	Run until sight glass shows clear water
Rinse after backwash	After each backwash	30-60 seconds to settle media
Inspect transparent lid	Monthly	Check for debris, cracks, seal condition
Check sight glass clarity	During backwash	Water should run clear within 2-3 minutes
Inspect media level	Annually	Top up if level has dropped
Full media replacement	Sand: 5-7 years / AFM: 10-15 years	Replace completely

Pressure Gauge Guide:

📊 Clean filter baseline: 0.5-0.8 bar
📊 Backwash needed: When 0.5 bar above baseline
📊 Maximum safe operating: 2.5 bar
⚠️ If pressure exceeds 2.5 bar, stop pump immediately and investigate

Backwash Procedure:

1. Turn off pool pump (unless using Besgo automatic valve)
2. Set valve to backwash position
3. Turn on pump and run until sight glass shows clear water (typically 2-4 minutes)
4. Turn off pump
5. Set valve to rinse position
6. Run pump for 30-60 seconds to settle media
7. Return valve to filter position
8. Resume normal filtration

Winterisation (UK Climate):

• Drain filter tank completely via drain plug
• Remove and safely store drain plug
• Leave tank lid slightly open to prevent pressure build-up
• Cover filter or store in frost-free location if possible
• Never leave water in filter during freezing conditions – ice expansion can crack the tank
• Disconnect and drain all associated pipework

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💎 Filter Media Information

The Calplas AFM-520 is specifically designed for optimal performance with AFM (Activated Filter Media) but also performs excellently with traditional silica sand.

Compatible Media:

Media Type	Filtration Rating	Lifespan	Benefits
AFM (Activated Filter Media)	4-5 microns (without flocculant)	10-15+ years	Superior clarity, self-sterilising, bio-resistant
AFM with APF/ZPM	Down to 1 micron	10-15+ years	Exceptional clarity, removes bacteria
Silica Sand	20-40 microns	5-7 years	Economical, widely available
Glass Sand	10-15 microns	8-10 years	Better than sand, good value

AFM Media Requirements for AFM-520:

Grade	Bag Size	Quantity	Total Weight
AFM Grade 1 (0.4-1.0mm)	21 kg	6 bags	126 kg
AFM Grade 2 (0.7-2.0mm)	21 kg	5 bags	105 kg
Total AFM		11 bags	231 kg

Sand Media Requirements for AFM-520:

Grade	Bag Size	Quantity	Total Weight
Fine Sand (0.4-0.8mm)	25 kg	5 bags	125 kg
Coarse Sand/Gravel (0.7-1.2mm)	25 kg	5 bags	125 kg
Total Sand		10 bags	250 kg

💡 Tip: When filling, always add the coarser grade first (Grade 2 AFM or coarse sand/gravel), then add the finer grade on top.

Signs Media Needs Replacing:

• Poor water clarity despite correct chemical balance
• Very short filter cycles between backwashes
• Cloudy water returning to pool during filtration
• Media visible in pool (indicates damaged laterals)
• Excessive media loss during backwash

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🔬 Filtration Performance

Metric	Value
Filter Area	0.20 m²
Filter Bed Height	900 mm
Standard Flow Rate	6 m³/h
Filtration Rating (AFM)	4-5 microns nominal
Filtration Rating (Sand)	20-40 microns

Why AFM Outperforms Sand:

AFM (Activated Filter Media) is manufactured from specially processed recycled glass with a unique activation process that creates a negatively-charged, self-sterilising surface. This provides:

• 40× greater surface area than sand for superior particle capture
• Bio-resistant properties preventing bacterial colonisation
• Self-sterilising surface that doesn’t support biofilm growth
• Consistent performance without the degradation seen in sand filters after 6-12 months
• Reduced chemical consumption due to cleaner water and no bacterial growth

Micron Rating Guide:

Rating	What It Catches
40+ microns	Visible debris, leaves, large particles
20-40 microns	Sand, dirt, visible particles (typical sand filter)
10-20 microns	Fine particles, some algae spores
4-5 microns	Very fine particles, most algae (AFM without flocculant)
1 micron	Bacteria, ultra-fine particles (AFM with APF/ZPM)

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⚙️ Besgo Valve System

The Calplas AFM-520 is designed for use with the Swiss-engineered Besgo backwash valve system, which replaces traditional 6-way multiport valves with a more efficient, automated solution.

Besgo Valve Positions:

Position	Function
Filter	Normal filtration mode – water flows through media to pool
Backwash	Reversed flow cleans media, water to waste

Besgo System Advantages:

• ✅ Automatic switching between filter and backwash modes
• ✅ No need to turn pump off during valve changeover
• ✅ Fail-safe design – returns to filter mode on power failure
• ✅ Lower pressure loss than traditional multiport valves
• ✅ Silent operation with no pressure hammer
• ✅ Valve operates in less than 2 seconds
• ✅ Wall-mounted design saves floor space
• ✅ Control panel available for scheduled automatic backwashing

⚠️ Note: Besgo valve and coupling set sold separately. 3-5 bar mains water pressure or compressed air required for valve actuation.

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🎯 Ideal Applications

Application	Pool Size	Suitability
🏠 Residential above-ground pools	Up to 48,000 L	✅ Ideal
🏠 Residential in-ground pools	Up to 48,000 L	✅ Excellent
🧖 Large swim spas	Up to 20,000 L	✅ Well suited
🏨 Small holiday let pools	Up to 36,000 L	✅ Good with AFM media
🏢 Light commercial	Up to 30,000 L	⚠️ Consider AFM-640 for higher bather loads

Best Suited For:

• Pool owners seeking superior water clarity with AFM media
• Installations requiring automated backwash capability
• Projects where long filter media lifespan is important
• Pools where space allows for side-mounted valve systems

❓ Frequently Asked Questions

Q: What size pool will this filter handle? A: The Calplas AFM-520 is suitable for pools up to 48,000 litres with a flow rate of 6 m³/h, providing an 8-hour turnover. For optimal 6-hour turnover, it’s ideal for pools up to 36,000 litres.

Q: How often do I need to backwash? A: Backwash when the pressure gauge reads 0.5 bar above your clean baseline reading. This is typically every 1-2 weeks depending on pool usage and debris load.

Q: What filter media should I use? A: The filter is optimised for AFM (Activated Filter Media) with a capacity of 11 bags (231 kg total). Traditional silica sand can also be used – 10 bags (250 kg total).

Q: How long does filter media last? A: AFM media lasts 10-15+ years without replacement. Traditional sand should be replaced every 5-7 years.

Q: What pump do I need? A: Match with a pump delivering 4-6 m³/h for normal filtration. Ensure the pump can deliver 8-10 m³/h for effective backwashing. Do not exceed 10 m³/h maximum.

Q: Do I need a multiport valve? A: The AFM-520 is designed for use with Besgo backwash valves rather than traditional multiport valves. The Besgo valve and coupling set must be purchased separately.

Q: What warranty is included? A: Calplas provides a 5-year warranty on the filter tank. Contact Parts4Tubs for full warranty terms and conditions.

Q: Can I use glass media instead of AFM? A: Yes, standard glass filter media will work, though AFM (Activated Filter Media) is specifically recommended as the filter is designed to optimise AFM performance with its special diffuser system.

Q: Is this filter suitable for commercial use? A: The AFM-520 is primarily designed for residential pools. For light commercial applications, ensure the pool volume and bather load match the filter’s capacity. Consider the larger AFM-640 or AFM-720 for busier commercial pools.

Q: What’s the difference between this and the FA series? A: The AFM series has a 900mm filter bed height compared to 350-450mm in the FA series, providing superior filtration depth and water contact time for better water quality.

📥 Downloads & Resources

📄 Technical Datasheet (PDF) – Complete specifications and dimensions

🏆 Why Choose the Calplas AFM-520

✅ Premium European Manufacturing – Hand-crafted in Spain by Calplas, manufacturing filters since 1970

✅ Superior Filtration Depth – 900mm filter bed height provides exceptional water contact time and filtration efficiency

✅ AFM-Optimised Design – Special diffuser prevents channelling and ensures even media distribution

✅ Built to Last – GRP construction with steel reinforcement, UV-resistant gelcoat finish

✅ Easy Maintenance – Transparent lid and large sight glass make monitoring simple

✅ Automation Ready – Designed for Besgo automatic backwash valves for hands-free operation

✅ Certified Quality – Manufactured to EN 13121-3 European pressure vessel standards

✅ 5-Year Warranty – Manufacturer-backed warranty demonstrates build quality confidence

🤝 Why Buy from Parts4Tubs

✅ UK-based company with knowledgeable local support

✅ Fast UK delivery on in-stock items

✅ Expert advice from pool and spa specialists

✅ Competitive pricing on premium filtration equipment

✅ Full warranty support – we handle warranty claims directly

✅ Easy returns within 30 days if not completely satisfied

✅ Compatible accessories available – AFM media, Besgo valves, spare parts

✅ Technical guidance – sizing advice and installation support available

Need Help? Contact our team for expert advice on filter sizing, pump matching, or installation requirements.

✨ Summary

The Calplas AFM-520 is a premium semi-high bed filter engineered specifically for use with AFM activated glass media, delivering exceptional water clarity for residential pools up to 48,000 litres. With its 900mm filter bed depth, special AFM diffuser, and robust GRP construction backed by a 5-year warranty, this Spanish-manufactured filter represents a professional-grade solution for pool owners seeking superior filtration performance. Designed for compatibility with Besgo automatic backwash systems, the AFM-520 offers both outstanding water quality and convenient automated operation. Available now from Parts4Tubs with expert sizing advice and UK-wide delivery.