Filter sand: the benchmark
Graded silica sand is the medium every other medium is compared against: dense, cheap, chemically inert, and understood for a century. Specified by effective size and uniformity coefficient, a fine filter sand bed delivers roughly 20 – 30 µm filtration at sensible velocities. Its weaknesses are operational: sand beds channel as flow wears preferential paths, they can compact and form mudballs where coagulant or biological growth binds grains, and a biofilm establishes readily on the grain surface — which is why sand filters slowly lose performance between media changes even when backwashed on schedule.
Glass media — but which glass?
"Glass filter media" covers two products that share a raw material and little else.
Generic crushed glass is recycled bottle glass, crushed and graded. It is a fair sand substitute: the grains are smoother and slightly angular, it channels somewhat less than sand, backwashes a little easier at similar density, and uses recycled material. Quality varies meaningfully between suppliers — grading consistency and dust content are the things to check. Performance is best thought of as sand, slightly improved.
Activated Filter Media (AFM) is Dryden Aqua's engineered product, and the word activated is the difference: the glass is processed to increase the grain's surface area and given a negative surface charge. The claims that follow are the manufacturer's, and they are worth reading as such rather than as independent test results. Dryden Aqua states that the charged, high-area surface adsorbs fine and charged particles that plain grains only capture mechanically, taking filtration finer than sand — the company publishes figures down toward 4 – 5 µm in standard operation, finer with coagulant assistance — and that the surface resists biofilm, so the bed does not develop the channelling, mudballing and bacterial carryover that ages a sand bed. Treat those as the supplier's performance figures for its own product, ask for the test conditions behind them, and ask which approvals the grade holds for the duty you have in mind, with the certifying body, the standard and the scope named. Neither DWT nor this article certifies AFM for potable use.
The honest framing: generic glass competes with sand on price and sustainability; AFM competes with sand on performance and lifetime, at a genuine premium. Where the filtrate quality matters — potable polishing, pre-RO duty, pool and reuse water — the case for the premium rests on the manufacturer's published performance, so ask for the grade's data sheet and its test basis and judge the figures against what your duty actually needs. DWT stocks AFM; ask for the grade guide.
Anthracite: the team player
Anthracite is crushed hard coal — chemically inert, and at roughly half the density of sand it is used at larger grain sizes. Alone it is a coarse, fast filter; its real role is the dual-media bed: a layer of coarse anthracite above fine sand. Because anthracite is light and coarse while sand is dense and fine, backwashing automatically restratifies the layers in the right order. The coarse top layer captures the bulk solids in depth, the fine sand beneath polishes, and the bed as a whole holds two to three times the solids of sand alone before the same pressure drop — longer runs, less backwash water, better tolerance of solids spikes. The cost is care in design: matched grain sizes and controlled backwash rates, or the layers mix.
Choosing
| Duty | Sensible medium |
|---|---|
| General sediment duty, budget-led | Sand, or generic glass |
| Potable, pre-RO, reuse, pool — quality-led | AFM |
| High solids loads, long filter runs | Dual media (anthracite over sand) |
| Existing sand filter underperforming | Check velocity first; then consider AFM re-bed |
Rule of thumb: sand when good-enough is good enough; AFM when the filtrate quality or the bed's long-term stability is the point — and treat "glass media" on a quote with suspicion until you know whether it means activated or merely crushed.