Garnet filter media for multimedia water filters
Garnet is the heaviest of the common filter media. At a specific gravity of 4.0, GRM Garnet sits at the bottom of a multimedia bed as the finest polishing layer, and its coarse grades can replace silica gravel in the support layers. It works in filters of every size, from a home water tank or pool filter to a city treatment plant.
Weight decides the job.
A media filter is cleaned by backwashing: water is pumped up through the bed, lifting and loosening it. When the flow stops, the media settle again, and the heaviest grains settle first.
Under water a GRM grain weighs about 1.8 times as much as a sand grain of the same size: its specific gravity minus the water it displaces is 3.0, against 1.65 for sand. So garnet returns to the bottom after every backwash even though its grains are the finest in the bed, and the layers stay in the right order.2,6
Sand only
After backwashing, the finest sand settles on top, so solids are caught in the top few centimetres and the filter blocks early.6
Multimedia with garnet
Solids travel deeper into the bed, so it stores more dirt, runs longer between backwashes and still finishes on a fine layer.2,6 In an ASCE study, beds of anthracite, sand and garnet ran with nearly half the head loss of a conventional sand filter, with no loss of filtrate quality.3
GRM safety data sheet; filter garnet1; silica sand14; anthracite minimum2.
Coarse to fine. Light to heavy.
A multimedia filter stacks three media in one bed. Water meets the coarsest, lightest grains first and the finest, heaviest grains last, so the whole depth of the bed does useful work. Garnet is the layer that makes the bottom of that sequence possible.
Layer proportions for municipal and industrial beds.1 Media sizes2; GRM effective sizes calculated from the GRM size chart.15 Follow the filter designer’s layer depths.
Four jobs for garnet in a bed
Fine polishing layer. The finest, heaviest layer at the bottom of the media. GRM 50#, or 80# where the design calls for extra-fine media.
Transition layer. Steps the bed down from the support to the fine media. GRM 14/40.
Support underbed. Holds the bed and shields the distributor. GRM 8/12.
Coarse support. Replaces support gravel for an all-garnet bed. GRM 4/8 and 2/4.
A fine garnet layer can filter down to about 10–20 µm.6 Multimedia filters are designed for 17–24 m/h, against about 5 m/h for rapid sand filters.2
From the home to the city.
Wherever water passes through a backwashed granular filter, garnet can do one of those four jobs. The same few GRM grades cover every market below.
| Market | Typical filter | Where garnet goes | GRM grades | What it adds |
|---|---|---|---|---|
| Home and farm water | Backwashing tank filters for sediment, iron and manganese | Underbed at the distributor; fine layer | 50# · 8/12 · 4/8 | Stays under heavy iron-removal media; keeps media out of the valve; finer water from one tank1,11,13 |
| Swimming pools and spas | Sand filters on the circulation loop | Fine layer below the sand; support at the laterals | 50# · 8/12 | Moves sand-filter capture toward cartridge-filter levels while keeping the simple backwash6,7 |
| Municipal drinking water | Rapid gravity filters, dual and tri-media | Fine bottom layer; all-garnet support | 50# · 80# · 8/12 · 4/8 · 2/4 | 3.5–5× the design rate of rapid sand; about 50% less head loss2,3 |
| Wastewater and reuse | Tertiary multimedia filters | Fine polishing layer | 50# | Deeper solids capture and longer runs for reuse-quality effluent6 |
| Desalination and RO | Pressure multimedia filters before cartridges and membranes | Fine polishing layer; all-garnet support | 50# · 8/12 · 4/8 · 2/4 | Removes most particles down to 5 µm or less, protecting the membranes9 |
| Cooling towers | Side-stream media filters | Fine layer in higher-rate beds | 50# | Cleaner heat exchangers and less biocide; more water per m² of filter2,10 |
| Process water and boilers | Multimedia pretreatment ahead of ion exchange and RO | Fine polishing layer; support | 50# · 8/12 · 4/8 | Inert, insoluble and low in chloride, so it adds little to water bound for demineralising12 |
| Oil and gas | Seawater injection and sulphate-removal feed filters | Fine polishing layer; all-garnet support | 50# · 80# · 8/12 · 4/8 · 2/4 | Anthracite–sand–garnet beds raise uptime on sulphate-removal packages9 |
Clearer pool water. More water from the same filters.
Swimming pools and spas
A pool sand filter is a backwashed media filter in miniature. A fine GRM layer under the sand moves it toward cartridge-level capture, and it is still cleaned by a simple backwash. Pool sand is usually replaced every three to five years;8 garnet’s hardness resists wear in backwash.1
How to add it: GRM 8/12 around the laterals, a layer of GRM 50#, then the filter maker’s pool sand on top, to the filter maker’s depths.
Municipal drinking water and wastewater
Most treatment works filter water after coagulation and settling, and filtration is the last physical barrier before disinfection. Garnet’s place is the fine bottom layer of a tri-media bed of anthracite, sand and garnet.
Multimedia filters are designed for 3.5–5 times the throughput per square metre of rapid sand filters.2 Upgrading existing sand filters can add plant capacity without building new filter boxes; regulators often ask for extra monitoring to show water quality holds at the higher rate.2
Metres per hour is cubic metres of water per square metre of filter per hour. Converted from gal/ft²/min.2
Multimedia filters of anthracite, sand and garnet remove most particles down to 5 µm or less before the water reaches the cartridges and membranes, so fewer cartridge changes and less membrane fouling.9
Side-stream media filters keep heat exchangers cleaner and cut biocide demand.10 A multimedia bed with garnet runs at a higher rate, so it treats more water per square metre of filter.2
Multimedia filters are the first stage ahead of ion exchange, RO and boiler make-up.12 GRM Garnet is insoluble and inert, with 10–15 ppm chlorides, so it adds very little to water about to be demineralised.
An all-garnet support bed. Rarely offered.
Every media filter sits on graded support layers that hold the media above the underdrain and spread the backwash evenly. Those layers are usually silica gravel, because published filter-garnet grades stop at about 2.5 mm.1,11 GRM crushes hard rock, so its 4/8 and 2/4 grades take garnet up to 8 mm and beyond, and the whole support bed can be garnet.
Lighter gravel sits under heavy garnet. In backwash the two can mix, risking water bypassing the bed and an uneven support.11
One dense material from the fine layer to the underdrain. Under water, garnet weighs about 1.8 times as much as silica gravel of the same size, so backwash is less able to lift it. AWWA B100 lists high-density gravel as a filter material alongside ordinary gravel.4
All ten grades, every layer of the bed.
Filter designers specify media by effective size (ES, the d10: the size 10% of the grains pass) and uniformity coefficient (UC, d60 ÷ d10; closer to 1 means more even grains). Every GRM grade can be supplied as filter media; the highlighted grades are the usual choices for a layered bed.
| Grade | ES (d10) | UC | Role in the bed |
|---|---|---|---|
| 120# | 0.11 mm | 1.45 | Extra-fine polishing where the design calls for it |
| 80# | 0.16 mm | 1.43 | Extra-fine polishing layer |
| 50# | 0.26 mm | 1.38 | Fine polishing layer: the standard multimedia garnet |
| 20/60 | 0.23 mm | 1.98 | Fine-to-medium layer with a broader grading |
| 14/40 Specialty Filter | 0.65 mm | 1.82 | Transition layer between the support and the fine media |
| 36# | 0.38 mm | 2.63 | Medium layer with a broad grading |
| 8/80 Alpha Blast | 0.25 mm | 3.40 | Very broad grading; by the filter designer’s specification |
| 8/12 | 1.73 mm | 1.17 | Support underbed and distributor cover |
| 4/8 | 2.58 mm | 1.55 | Coarse support in place of fine gravel · rarely offered |
| 2/4 | 5.30 mm | 1.45 | Coarsest support over the underdrain, in place of coarse gravel · rarely offered |
SG 4.0–4.1, so very few grains wash up into the sand.
7.5–8.0 Mohs resists attrition in backwash, for a long service life.1
Insoluble, inert, and only 10–15 ppm chlorides to leach into the water.
Every garnet layer from one source, with one chemistry and one set of test data.
Garnet filter media: common questions.
Why is garnet used as the bottom layer in a multimedia filter?
Garnet is the heaviest common filter media, with a specific gravity of about 4.0. After each backwash the heaviest grains settle first, so garnet returns to the bottom even though its grains are the finest in the bed. That keeps the order coarse and light at the top, fine and heavy at the bottom.
What size garnet do I need for a multimedia filter?
The fine polishing layer usually has an effective size in the 0.18–0.60 mm range. GRM 50# (effective size 0.26 mm, uniformity coefficient 1.38) is the standard fine garnet; 80# is finer. 14/40 forms the transition layer, 8/12 the support underbed, and 4/8 and 2/4 replace support gravel. Follow the filter designer’s specification.
What ratio of anthracite, sand and garnet should I use?
Municipal and industrial beds are commonly about 55% anthracite, 30% silica sand and 15% garnet by depth, with garnet support layers below. The filter designer sets the exact depths.
Can garnet replace gravel as a support layer?
Yes. AWWA B100 lists high-density gravel alongside ordinary gravel. GRM 4/8 and 2/4 take garnet up to 8 mm and beyond, so the whole support bed can be garnet. Under water it weighs about 1.8 times as much as silica gravel of the same size, so backwash is less able to lift it into the layers above.
Can I use garnet in a swimming-pool sand filter?
Yes. Put GRM 8/12 around the laterals, then a layer of GRM 50#, then the filter maker’s pool sand on top, to the filter maker’s depths. A fine garnet layer captures particles down to about 10–20 µm, against 20–40 µm for sand alone. Check the filter maker’s media guidance first.
Is garnet filter media safe for drinking water? Is it certified?
GRM Garnet is insoluble and inert, with 10–15 ppm chlorides. Drinking-water projects usually require certification to EN 12910 in Europe or AWWA B100 in North America; ask us for the test data available for your project.
How often does garnet filter media need replacing?
Garnet is hard, 7.5–8.0 Mohs, and resists wear in backwash, so it usually outlasts the lighter media above it. Operators top up or replace media when inspection or filter performance shows it is needed.
Specifications and further reading.
Other uses of GRM Garnet
Sources
- Clack Corporation. Garnet filter media specification.
- Engelhardt, Hach (2012). Granular media filtration for water treatment.
- ASCE (1968). Filtration through a trimedia filter. Journal of the Sanitary Engineering Division 94(3).
- AWWA B100-25. Granular filter material.
- EN 12910:2012. Products used for treatment of water intended for human consumption: garnet.
- Carbotecnia. Garnet deep-bed filter media.
- Pool Research. Sand vs DE vs cartridge filters.
- Atomic Filters. How to choose swimming pool sand for filters.
- VWS Westgarth. Multi-media filtration.
- Water Technology (2019). Side-stream media filtration in cooling towers.
- Tohkemy. Filter garnet and support garnet.
- Ion Exchange. What is a multimedia filter.
- SoftPro. Gravel underbedding in softeners and filters.
- Graco. How to choose the right abrasive (silica sand specific gravity).
- GRM Garnet (2026). Product data sheet and size chart.
GRM Garnet values are typical and do not constitute a specification. Effective size and uniformity coefficient are calculated from the GRM size chart; confirm by laboratory test before specifying. Drinking-water projects usually require certification to EN 129105 or AWWA B100.4 Third-party figures describe garnet in general, not GRM Garnet, unless stated.