13.1 Tertiary Filtration Equipment: Granular Media, Disc & Cloth Filters
Key Takeaways
- A granular media filter is built in layers — filter media, support gravel, and an underdrain — with backwash troughs above the bed, and each layer has a distinct function that fails in a distinct way.
- Dual-media filters place lighter, coarser anthracite above heavier, finer sand so that flow encounters coarse-to-fine grading, which stores solids through the depth of the bed instead of plugging the surface.
- Effective size and uniformity coefficient describe a filter media, with effective size being the sieve opening passing 10 percent of the media by weight and uniformity coefficient being the ratio of the 60 percent size to the 10 percent size.
- Cloth disc filters mount media on partially submerged rotating discs and clean themselves with suction shoes, using far less backwash water and floor space than granular media filters.
- Air scour and surface wash break up the compacted surface layer that water alone cannot dislodge, and without them a granular bed develops mudballs and cracks.
13.1 Tertiary Filtration Equipment: Granular Media, Disc & Cloth Filters
Exam Focus: "Tertiary treatment equipment — Filtration; Media filtration (e.g., sand, anthracite, disc filter)" is listed in the Equipment content area, and the matching process appears again in Treatment Process Evaluation and Adjustment. Tertiary filtration is the step that takes a secondary effluent from roughly 15 to 30 mg/L TSS down to single digits.
1. Why Plants Filter
Secondary clarification cannot remove the fine, low-density particles that escape a biological floc. Tertiary filtration exists to:
- Meet effluent limits tighter than conventional secondary standards — for example permits requiring effluent TSS well below 30 mg/L.
- Remove the particles that shield pathogens from UV disinfection and consume chlorine, so downstream disinfection actually works.
- Remove particulate phosphorus after chemical precipitation.
- Produce water suitable for reuse — plant service water, irrigation, or industrial supply.
2. Granular Media Filter Construction
From the bottom up:
| Layer | Function | Failure Mode |
|---|---|---|
| Underdrain | Collects filtered water and distributes backwash water evenly across the entire bed | Broken or plugged laterals or nozzles cause uneven backwash, which leaves dirty zones and can boil media out in others |
| Support gravel | Graded layers holding fine media above the underdrain | Gravel upset — violent or uneven backwash disturbs the graded layers and lets media migrate into the underdrain |
| Filter media | Does the actual filtration | Media loss over the troughs, mudballs, surface cracking, compaction |
| Freeboard / backwash troughs | Carry spent backwash water away above the expanded bed | Troughs set too low let media wash out during backwash |
Media Properties
- Effective size (ES) is the sieve opening that passes 10 percent of the media by weight. Smaller effective size means finer filtration and faster headloss development.
- Uniformity coefficient (UC) is the ratio of the 60 percent passing size to the 10 percent passing size. A UC near 1.0 means very uniform grains; a high UC means a wide range of sizes that pack tightly and plug at the surface.
- Specific gravity determines where a medium settles after backwash: anthracite is lighter (roughly 1.4 to 1.7) than silica sand (roughly 2.6), and garnet or ilmenite is heavier still (roughly 4.0 to 4.5).
Single, Dual, and Multi-Media
A single-media sand filter does most of its work at the very top of the bed. Solids plug that thin surface layer, headloss climbs quickly, and run times are short.
A dual-media filter solves this by exploiting specific gravity. After backwash, the lighter, coarser anthracite settles on top of the heavier, finer sand. Flow therefore travels from coarse to fine, so large particles are captured deep in the anthracite while progressively finer particles are captured below. The result is in-depth filtration: far more solids storage capacity, longer runs, and lower headloss for the same water quality. A multi-media (mixed-media) filter adds a garnet layer at the bottom for still finer polishing.
Exam point. The reason anthracite goes on top is not that it filters better — it is that anthracite's lower specific gravity makes it re-stratify above the sand every time the bed is backwashed and re-settles. Grading is achieved by density, automatically.
3. Filter Configurations
| Configuration | How It Works | Characteristics |
|---|---|---|
| Conventional gravity filter | Water flows down through a fixed bed; the filter is taken offline periodically for backwash | Simple, robust, well understood; needs backwash pumps or a clearwell and a place to send spent backwash |
| Traveling bridge filter | A moving bridge backwashes one narrow cell at a time while the rest of the filter stays in service | Continuous service, shallow bed, low backwash volume per event |
| Continuous backwash upflow sand filter | Water flows upward through a downward-moving sand bed while an airlift continuously washes sand at the center and returns it clean to the top | Never taken offline; no backwash pumps or valves; steady small reject stream |
| Cloth disc filter | Cloth media on partially submerged discs rotating slowly on a center shaft; solids collect on the outside of the cloth and are removed by suction shoes as the discs turn | Very small footprint, very low backwash volume, low headloss, no media to lose; increasingly the default retrofit choice |
| Pressure filter | Granular media in a closed pressure vessel | Used where downstream pressure is needed; harder to observe the bed |
4. Backwash and Cleaning Systems
Water alone cannot dislodge the compacted, biologically sticky surface layer that forms on a wastewater filter. Auxiliary scour is essential:
- Air scour injects air beneath or into the bed, and the rising bubbles abrade the grains against one another to break the deposits loose. Air scour is normally applied first, or simultaneously at reduced water rate, then water alone completes the wash.
- Surface wash uses fixed or rotating high-pressure nozzles just above the settled bed to attack the surface crust.
- Backwash water is supplied from a dedicated backwash pump, a clearwell, or an elevated tank.
Without effective auxiliary scour, a granular bed develops:
- Mudballs — accreted lumps of media, biological solids, and grease that grow, sink into the bed, and remove filtering volume.
- Surface cracking and shrinkage — the bed pulls away from the walls, and water short-circuits down the crack, bypassing the media entirely.
- Media cementing — a hardened layer that no backwash will restore, requiring media replacement.
Backwash troughs must sit high enough above the settled bed to accommodate expansion during backwash. Media appearing in the spent backwash, or a measurably falling bed depth over time, means the backwash rate is too high or the troughs are too low.
In a dual-media filter, why is the coarser anthracite layer placed above the finer sand layer rather than below it?
An operator inspecting a drained granular media filter finds hardened lumps of media, biological solids, and grease throughout the bed, along with cracks where the media has pulled away from the walls. What deficiency does this indicate?
A small plant needs to add tertiary filtration but has very limited floor space and no capacity to handle a large backwash return stream. Which filtration technology best fits these constraints?