7.2 Sand, DE, and Cartridge Filtration
Key Takeaways
- The three basic public-pool filter types on AFO-style study items are high-rate sand (granular media), diatomaceous earth (DE / precoat), and cartridge.
- Filters remove suspended solids. They do not strip dissolved TDS, and they do not remove most combined chlorine — chloramines are a chemistry, oxidation, and indoor-air problem, not a media swap.
- Typical 2024 MAHC design bands — model code, not unpublished AFO statute — include high-rate granular media often at no more than 15 GPM/ft² with adequate bed depth, precoat/DE often near 2 GPM/ft² (vacuum up to 2.5 GPM/ft² with body-feed), and surface cartridge design not exceeding 0.30 GPM/ft² (listings commonly show 0.375 GPM/ft²).
- Sand and DE are backwashed to an approved waste; cartridge elements are removed, cleaned, and replaced. Never backwash or dump spent media to a storm drain or waterway when that discharge is illegal.
- Record the clean start-up pressure differential; a rising split between influent and effluent is the dirty-filter cue. Follow the manufacturer listing and the AHJ for the backwash or clean trigger — the public 2025 AFO handbook does not print a national psi table.
Domain 2D on the 2025 AFO Exam Candidate Handbook is filters and filtration. Independent teaching for AFO candidates uses that public heading, NSF/ANSI 50 listings as the usual equipment standard, and 2024 MAHC figures as model-code / design typical numbers. They are not unpublished NRPA statutes. Your AHJ may adopt MAHC language, an older state table, or a manufacturer listing. When they disagree, the inspector's code wins.
Chapter 6 already told you that a dirty filter raises head and drops flow. This section owns the vessels and media. Chapter 8 will make you size filter area from GPM and loading rate. Here you must name the three types, know what they catch, and know how you clean them without sending waste to a creek.
The three basic types
Handbook-style study questions treat three families as the basic public-pool set:
- Sand (granular media) — a pressure tank (or gravity bed in older plants) filled with silica sand or multimedia. Water is driven through the bed. Particles lodge in the pores and on the surface. Cleaning is backwash: reverse flow that expands the bed and carries dirt to waste.
- Diatomaceous earth (DE), also called precoat — fabric-covered grids or septa inside a tank. You coat those grids with a slurry of DE powder. The DE cake is the filter. DE captures finer particles than typical high-rate sand. Cleaning is backwash (and often a recharge of new precoat). Vacuum DE and pressure DE are both precoat machines.
- Cartridge — pleated elements in a housing. Water passes through the fabric. There is no backwash to waste in the same way as sand or DE. You remove, clean, and eventually replace the elements. Commercial plants keep a spare set so one set can dry and the other can run.
Relative ranking if you forget a decimal: sand is built for the highest loading rate (most GPM per square foot of media face). DE is built for finer particle capture at a much lower loading rate. Cartridge sits at a still lower commercial loading rate and is serviced by cleaning/replacement, not a sight-glass backwash.
What filters remove — and what they do not
Filters remove suspended solids: turbidity, hair that beat the strainer, sunscreen globs, rust flakes, and many particles that make water look dull. Good filtration is why a legal residual still looks like swimming-pool water instead of dishwater.
Filters do not remove dissolved material. Total dissolved solids (TDS) — salt, leftover ions from treatment, sweat minerals that actually dissolved — pass the bed. Most combined chlorine (chloramines) is dissolved oxidizer demand, not a speck you can backwash out. Dumping DE or swapping cartridges will not replace breakpoint, UV, ozone, or indoor air handling. Chemistry chapters already taught combined chlorine; here the trap is believing the filter is a sanitizer.
Filtration helps disinfection by taking particles out of the way, and some precoat cakes are discussed in public-health literature for parasite-sized particles. That still does not make the filter a chlorine residual. Cloudy water can be a filter problem, a chemistry problem, or both. A clean sight glass after backwash with still-cloudy pool water is telling you to look at coagulants, pH, and oxidation — not to backwash again for sport.
Loading rates as typical design bands
Loading rate is GPM per square foot of filter area. Same 333 GPM design flow needs a large cartridge bank, a moderate DE grid area, or a smaller high-rate sand face — because the allowed GPM/ft² is so different. Chapter 8 will grind the arithmetic. Remember the bands and label them as MAHC / listing / AHJ typical, not as a secret AFO table.
High-rate sand / granular media. 2024 MAHC §4.7.2.2.3.1 tells high-rate granular media filters to operate at no more than 15 GPM per square foot when the bed is at least 24 inches deep as that section measures it. Shallower beds are held to a lower maximum (MAHC uses 12 GPM/ft² when bed depth is under 15 inches). Older design tables and some NSF listings still talk about a 15–20 GPM/ft² high-rate band. Teach 15 GPM/ft² as the current MAHC high-rate ceiling with an adequate bed, and 15–20 as the older/typical high-rate conversation — then follow the listing plate and the AHJ.
DE / precoat. 2024 MAHC §4.7.2.3.1 sets pressure precoat at not greater than 2 GPM/ft² of effective area. Vacuum precoat is not greater than 2 GPM/ft², or 2.5 GPM/ft² when used with continuous precoat media feed (body-feed). That is much lower loading than high-rate sand, which is why DE grids look huge for the same GPM.
Cartridge. 2024 MAHC §4.7.2.4.1 sets the design filtration rate for surface-type cartridge filters at not more than 0.30 GPM/ft². The MAHC annex notes that elements are often listed at 0.375 GPM/ft², but design should stay more conservative because cartridges do not recover 100% of flux after fouling. If an exam stem says "commercial cartridge loading," 0.375 appears in many listings; if the stem is MAHC design, 0.30 is the code number. Either way, cartridge loading is an order of magnitude below sand.
These numbers are model-code and listing typical. They are not an unpublished AFO statute. If a question only wants ranking: sand highest loading, DE finest common cake, cartridge no equivalent backwash-to-waste ritual.
Backwash, clean, replace
Sand. Isolate as the valve nest is labeled, then backwash. 2024 MAHC §5.7.2.2.2 wants a backwash rate of at least 15 GPM/ft² unless the manufacturer or NSF/ANSI 50 listing forbids that rate and approves another. Watch the sight glass. Continue until the leaving water is clear (MAHC §5.7.2.2.3). Return the valves to filter. The first minutes back on line can be ugly; some plants waste that slug. Do not backwash as a way to "raise chlorine." You are throwing away heated, treated water and, if you do it wrong, sending a dirt slug into the pool.
DE. Backwash to strip the dirty cake, then re-precoat. Mix DE powder into a slurry (water plus media — never dump dry powder into a running suction line as a party trick) and introduce it so the grids take an even coat. 2024 MAHC §4.7.2.3.2 wants closed-loop precoating so media and debris are not returned to the pool during the coat. Grid integrity is the DE-specific disaster: a torn septum dumps DE into the basin — cloudy water, possible bather irritation, and a filter that is no longer filtering. Inspect grids when the tank is open. Do not run a DE filter without a proper precoat "to save powder."
Cartridge. Shut down, relieve pressure, remove elements. Rinse per the manufacturer (hose, soak, or both). Do not high-pressure-blast pleats into rags unless the maker says that tool is allowed. Swap in the spare set. Replace elements that are torn, crushed, or permanently blinded. This is not a backwash-to-waste cycle; the dirt you wash off still has to go to an approved drain, not the lawn.
Never backwash to a waterway illegally
Spent backwash water is dirty, chlorinated, and sometimes full of DE. 2024 MAHC §4.7.2.3.3.2 says discharged precoat media shall be handled under applicable local, state, territorial, federal, and tribal laws. Storm sewers, creeks, and lakes are not a free dumpster. Many plants must send backwash to sanitary sewer or an approved holding tank. DE slurry in a waterway is both a pollution offense and a picture you do not want on the evening news. If the AHJ or sewer authority limits discharge, that limit is part of filtration, not an optional courtesy.
Pressure differential as the dirty-filter cue
A healthy pressure filter has two stories: influent (before the media) and effluent (after). The differential is the split. On start-up with clean media, record both gauges (or the single gauge with the manufacturer's valve nest). As the bed or cartridge loads, influent rises and/or effluent falls — the split widens. That widening is your cue to backwash or clean.
2024 MAHC §5.7.2.2.4 tells operators to backwash at a differential pressure increase over the initial clean-filter reading. The public MAHC text does not replace your nameplate; many manufacturers still print a cue on the order of 8–10 psi above clean. Treat that as a listing/manufacturer typical, not an unpublished AFO psi. If you never recorded the clean differential, you have no baseline — so log it after every media change and after a proper backwash.
A high single influent gauge with a throttled return valve is not the same story as a wide differential across a dirty sand bed. Read two points, or you will backwash a clean filter because someone closed a valve.
| Feature | High-rate sand | DE / precoat | Cartridge |
|---|---|---|---|
| What does the filtering | Sand or multimedia bed | DE cake on grids/septa | Pleated fabric elements |
| Typical loading (label as MAHC/listing) | Highest; MAHC high-rate often ≤15 GPM/ft² with deep bed; older talk 15–20 | Much lower; MAHC often 2 GPM/ft² (vacuum 2.5 with body-feed) | Lowest; MAHC design ≤0.30 GPM/ft²; listings often 0.375 |
| Particle story | Good commercial turbidity control | Finest common cake among the three | Fine when clean; blinds and must be washed/replaced |
| Cleaning | Backwash to waste until sight glass clears | Backwash, then slurry re-precoat; inspect grids | Remove, clean, replace; keep a spare set |
| Waste caution | Chlorinated backwash to approved drain | Backwash and spent DE are regulated waste — not a waterway | Wash-water still goes to an approved drain |
| Signature failure | Channeling, mudballs, too-shallow bed at high rate | Torn grid dumping DE into the pool | Torn or permanently blinded elements |
Name the three types. Know suspended versus dissolved. Quote MAHC loading bands as model code. Backwash sand and DE; wash or replace cartridges. Watch differential, not a single vanity gauge. Keep waste legal. That is filtration hardware for an aquatic facility operator.
The three basic filter types on AFO-style study items are which of the following?
A sand filter is freshly backwashed and the sight glass ran clear, but combined chlorine is still high and TDS has been climbing all summer. What should the operator conclude about the filter?
Where must sand or DE backwash and spent DE slurry go?
What is the practical dirty-filter cue on a pressure filter?