The Disc Filter Mechanism in One Paragraph
A disc filter works by passing wastewater from the inside of stacked, vertically mounted filter discs to the outside through a fine polyester mesh (typically 10–60 µm). Suspended solids are retained on the inner mesh surface and build a cake layer that progressively improves capture. When pressure differential reaches a setpoint, a spray bar backwashes the mesh using filtered effluent — without interrupting filtration. A reference Hydrotech HF2220 unit with 18 µm mesh achieved 89.7% particle removal and reduced effluent suspended solids to 3–8 mg/L (per MDPI 2019, Billund WWTP study).
Inside the Disc: How Solids Are Actually Captured
Disc filters capture solids through a two-stage mechanism that product pages rarely decompose into operating physics.
Stage 1 — direct interception. Particles larger than the mesh aperture are sieved on the inner disc surface; this is the "rated" capture and corresponds to the nominal pore size printed on the data sheet. For an 18 µm mesh, particles above 18 µm are stopped at the woven filaments.
Stage 2 — cake filtration. As solids accumulate, the deposit itself becomes a progressively finer filter, capturing sub-mesh particles that would otherwise pass through. The MDPI 2019 Billund study explicitly attributes part of its 75.6% mass removal to this cake-mediated capture, noting that the cake "progressively improves" retention beyond what the bare mesh alone would achieve. More dirt produces better filtration — until head loss forces a backwash cycle.
The MDPI authors observed an unexpectedly large number of MP particles whose size exceeded the pore size of the disc filter, which they interpreted as evidence of cake-mediated capture combined with some bypass. This finding is a useful empirical anchor for any engineer defending a mesh specification: field performance routinely exceeds the rated pore size because of the cake layer, but it is not absolute.
Submergence is the second physical lever. On a HUBER RoDisc, up to 65% of disc area is below the waterline at any instant (per HUBER product documentation). This defines the effective filter area available at any moment and is the denominator in any hydraulic loading calculation. Discs rotate only during backwash on HUBER designs, which means the cake builds continuously on a stationary mesh until the ΔP setpoint triggers the cleaning cycle.
The Backwash Cycle: ΔP Trigger, Spray Bar, No Fresh Water
Disc filters self-clean without operator intervention, but the control logic is more specific than standard commercial pages indicate.
As cake builds, head loss across the mesh rises. Because the HUBER RoDisc is a gravity-flow system with a fixed overflow weir, the upstream water level in the tank climbs in proportion to the pressure differential. When water level reaches the setpoint — typically corresponding to a ΔP in the 30–50 kPa range for tertiary polishing duty — the controller initiates a backwash event (HUBER product documentation).
On a HUBER-style unit, the shaft begins slow rotation and a spray nozzle bar fires filtrate at the mesh from the outside, dislodging the cake. The wash water plus dislodged solids fall into a collection trough and discharge axially from the machine. On continuous-rotation designs, only a 5–15% segment of disc area is offline at any moment, and the segment is cleaned for 30–120 seconds before returning to service. Filtration does not stop during backwash — the upstream process sees no flow interruption, and the only throughput effect is a 5–10% reduction in available filter area during the cleaning window.
The wash medium is filtrate, not fresh water — this is the water-reuse advantage that drives the 95–98% recovery figure commonly quoted for tertiary polishing. Periodic chemical cleaning (CIP) is required to remove biofouling or inorganic scale that backwash alone cannot dislodge; the typical interval is monthly to quarterly for tertiary polishing duty, and shorter for industrial wastewaters with high scaling potential.
Where a Disc Filter Sits in the Treatment Train
A disc filter is a polishing device, not a primary clarifier, and assumes the upstream process train has already performed the bulk of solids, organic, and oil/grease removal.
The canonical municipal train is: primary sedimentation → activated sludge → secondary clarification → disc filter → UV or RO. At the Billund WWTP studied by MDPI in 2019, secondary clarifier effluent carried approximately 20 mg/L SS; the disc filter reduced this to 3–8 mg/L before discharge. The disc filter sits between the biology and the disinfection/membrane barrier because both downstream steps are sensitive to particulate fouling: UV transmittance drops sharply above ~10 mg/L SS, and RO membranes fail rapidly when the silt density index (SDI₁₅) exceeds 3–5.
Where oil, grease, or high TSS is present in the feed, a DAF system for upstream FOG and TSS reduction is typically specified ahead of the disc filter to prevent premature mesh blinding and reject-volume spikes from FOG breakthrough. HUBER specifies two non-negotiable pre-treatment requirements: a 3 mm rotary bar screen for pre-screening and zero oil/grease in the feed. Failure on either condition causes premature blinding, spray bar fouling, and inability to meet effluent targets regardless of how the disc filter itself is sized.
Downstream beneficiaries are clear. UV disinfection needs low SS to transmit UV effectively (typically < 10 mg/L). RO membranes need low SDI, which disc filters at 40–60 µm reliably support. The MDPI 2019 influent/effluent numbers are the cleanest proof point for the train: 89.7% particle count removal, 75.6% mass removal, and effluent SS of 3–8 mg/L from a 20 mg/L feed.
| Process Node | Typical Influent SS | Typical Effluent SS | Function |
|---|---|---|---|
| Primary sedimentation | 200–400 mg/L | 100–200 mg/L | Bulk settleable solids |
| Activated sludge + secondary clarifier | 100–200 mg/L | ~20 mg/L | Biological oxidation + clarification |
| DAF (if FOG present) | 100–500 mg/L | 20–50 mg/L | Oil/grease + floatable solids |
| Disc filter (18 µm reference) | ~20 mg/L | 3–8 mg/L | Tertiary polishing, microplastic capture (per MDPI 2019) |
| UV / RO | 3–8 mg/L | Disinfected or desalted | Disinfection or membrane barrier |
Mesh Pore Size: 10 vs 18 vs 40–60 µm and When to Specify Each
Mesh aperture is the single specification that most directly drives both capital cost and operating cost.
10 µm is the performance ceiling. HUBER offers it as the finest standard mesh. It delivers the lowest effluent SS and the highest TSS removal, but it blinds fastest, demands the most frequent backwash, and produces the highest reject volume and spray-bar pump duty. Specify 10 µm only when the downstream process (typically a reuse discharge limit or an RO system treating high-fouling feedwater) genuinely cannot tolerate higher SS.
15–18 µm is the standard for tertiary reuse and microplastic capture. The MDPI 2019 study used 18 µm and recorded 89.7% particle count removal, 75.6% mass removal, and 3–8 mg/L effluent SS. This aperture is the default for municipal tertiary polishing where the goal is reliable SS reduction without the backwash penalty of 10 µm. Plants experiencing downstream membrane fouling often find 18 µm solves the problem without the reject-volume cost of 10 µm.
40–60 µm is the pre-RO polishing standard where the objective is SDI reduction and gross solids removal, not absolute TSS. The coarser mesh extends backwash intervals and reduces reject volume, which is why it dominates seawater and brackish RO pretreatment trains. The trade-off rule: every halving of pore size roughly doubles backwash load. Present this to clients as reject volume (m³/day) rather than as a percentage for clearer impact.
| Mesh Aperture | Typical Use Case | Effluent SS (Reference) | Reject Volume | Backwash Frequency |
|---|---|---|---|---|
| 10 µm | Strict reuse limits, high-fouling RO feed | < 3 mg/L | Highest (baseline × ~2) | Highest |
| 15–18 µm | Tertiary reuse, microplastic capture | 3–8 mg/L (per MDPI 2019) | Moderate (baseline) | Moderate |
| 40–60 µm | Pre-RO SDI reduction, seawater RO pretreatment | 5–15 mg/L | Lowest | Lowest |
Sizing a Disc Filter: Discs, Submergence, and Hydraulic Loading
The sizing math is straightforward once the reference benchmark is set.
The Hydrotech HF2220 reference unit handles 1200 m³/h across 13 discs at 18 µm with 65% submergence (per MDPI 2019). For a design flow of 800 m³/h at the same 18 µm aperture, scaling linearly gives approximately 9 discs (800/1200 × 13 = 8.7, rounded up to 9). This disc count is the starting-point estimate that the OEM must then confirm against their specific disc diameter, spacing, and submergence geometry.
The general hydraulic loading formula is: design flow ÷ (number of discs × disc area × submergence factor). On a HUBER RoDisc, submergence is fixed at up to 65%; on a partially submerged drum design, submergence varies with rotor position and must be integrated over the rotation cycle. A HUBER RoDisc shaft can carry up to 35 discs, so a 9-disc duty fits comfortably within a single-shaft envelope and leaves room for future capacity expansion.
Reject water is the second sizing output. With recovery in the 95–98% range, reject accounts for 2–5% of throughput. At 800 m³/h continuous duty, 2% reject equals 16 m³/h (384 m³/day); 5% reject equals 40 m³/h (960 m³/day). If the upstream process includes oil-bearing waste, expect the higher end of this range. Where reject handling matters — for example, when routing reject back to head-of-works creates a solids recycle loop — a filter press selection for the reject stream is often the downstream decision.
OEM confirmation is mandatory before procurement. Disc diameter, spacing, and submergence fraction vary between HUBER and other vendors, and a 9-disc count on one platform may deliver different effective area than 9 discs on another. For higher flow rates that exceed a single shaft's disc capacity, parallel trains are specified rather than oversized shafts, because maintenance access and redundancy both favor modular units.
Frequently Asked Questions
What mesh size should I specify for pre-RO disc filtration?
For pre-RO polishing where the goal is SDI reduction rather than absolute TSS removal, 40–60 µm is the standard choice because it extends backwash intervals and reduces reject volume. The MDPI 2019 study used 18 µm for tertiary reuse and microplastic capture and achieved 3–8 mg/L effluent SS; finer meshes like 10 µm are reserved for reuse limits or RO feeds with high fouling potential where the reject-water cost is justified.
How much reject water does a disc filter produce?
Recovery for tertiary polishing typically exceeds 95%, so reject accounts for 2–5% of throughput. At 800 m³/h design flow, that is 16–40 m³/h, or 384–960 m³/day at continuous duty. Every halving of mesh pore size roughly doubles backwash load, so 10 µm mesh produces the upper end of this range while 40–60
Frequently Asked Questions
How does a disc filter work in wastewater treatment?
A disc filter utilizes a series of vertically mounted, circular filter discs covered in fine filter media, typically polyester or stainless steel mesh. As influent wastewater flows into the center drum and passes outward through the media, suspended solids are captured on the internal surfaces. As the solids build up, the head loss increases, triggering a rotation of the discs and the activation of backwash spray nozzles that remove the accumulated cake, which is then diverted to a waste trough.
What mesh size should I use on a disc filter for tertiary polishing?
For tertiary polishing applications, the industry standard mesh size typically ranges from 5 to 20 microns. Utilizing a 10-micron nominal aperture is common for achieving high-quality effluent standards in municipal wastewater treatment, ensuring the effective capture of fine organic matter and suspended particles while maintaining hydraulic throughput.
How often does a disc filter backwash and how much water does it use?
Backwash frequency is determined by the differential pressure across the filter media, typically activating when a head loss of 150 to 300 mm is reached. Under normal operating conditions, backwash water consumption generally accounts for 0.5% to 3% of the total plant flow, depending on the influent solids loading and the specific mesh porosity selected.
Can a disc filter replace a secondary clarifier?
While disc filters are highly efficient at removing suspended solids, they are designed primarily as tertiary filtration or solids separation units and cannot replace the biological processing functions of a secondary clarifier. However, when paired with membrane bioreactors or integrated fixed-film activated sludge (IFAS) systems, disc filters can significantly reduce the footprint requirements of downstream clarification processes by handling higher solids concentrations.
What suspended solids level can a disc filter achieve?
In well-optimized tertiary treatment systems, a disc filter can consistently reduce Total Suspended Solids (TSS) concentrations to less than 5 mg/L, with many high-performance units achieving effluent TSS levels of 1 to 2 mg/L. Performance is highly dependent on the influent particle size distribution and the consistency of the upstream biological process stability.