What a Disc Filter Does in a Wastewater Train
A disc filter consists of a stack of round polyester filter meshes mounted on a horizontal or vertical shaft inside a closed tank, with wastewater flowing inside-to-outside through the mesh (per HUBER RoDisc and Hydrotech HF2220 product descriptions). Positioned between secondary clarification or dissolved air flotation (DAF) and the downstream barrier—typically UV disinfection or a membrane system—its function is to remove residual suspended solids left by upstream processes. HUBER markets the RoDisc specifically to protect UV and membrane systems from particulate fouling by reducing suspended solids load (S4).
Field performance data supports this application. A 2019 MDPI study on a Hydrotech HF2220 reference unit at a Swedish municipal plant—13 discs, 18 µm mesh, 1200 m³/h capacity—recorded 89.7% particle removal by count and 75.6% by mass, with effluent suspended solids (SS) of 3–8 mg/L (S3). This effluent range makes disc filters the preferred choice for tertiary polishing, pre-RO polishing, and microplastic capture: tight enough on solids to prevent RO membrane silt density index (SDI) excursions, mechanically simple for unattended operation, and compact enough for retrofits in basins with limited hydraulic head (S2, S4).
The Six Design Parameters That Drive Sizing
Six key variables determine whether a disc filter specification is feasible: mesh pore size, number of discs, submergence fraction, hydraulic loading rate per m² of submerged disc area, backwash trigger and pressure, and reject water volume. Errors in any of these parameters can result in premature blinding, effluent target failure, or excessive throughput loss to drain.
Mesh pore size selection directly impacts performance. Standard polyester meshes range from 10–60 µm. HUBER's finest standard mesh is 10 µm (S4); the MDPI study used 18 µm, while literature also reports 15 µm for retaining particles larger than 300 µm (S3). Finer apertures reduce effluent SS but increase blinding frequency and backwash demand.
Parameter 2 — Number of discs per unit. The Hydrotech HF2220 reference unit uses 13 discs (S3); a single HUBER RoDisc shaft can carry up to 35 discs (S4). Disc count serves as the primary capacity adjustment for a given disc diameter.
Parameter 3 — Submerged disc fraction. Up to 65% of disc surface is submerged during operation on the HUBER RoDisc (S4); the WesTech SuperDisc uses a partially submerged drum configuration that varies with rotor position (S2). Submergence determines the effective filter area available at any moment.
Parameter 4 — Hydraulic loading rate per m² of effective submerged disc area. The calculation is: design flow ÷ (number of discs × disc area × submergence factor). The Hydrotech HF2220 achieves 1200 m³/h across 13 discs at 18 µm (S3), serving as the published benchmark for sizing. Where influent TSS is high, a multi-media filter for upstream TSS reduction is often paired with the disc filter to maintain hydraulic loading within specifications.
Parameter 5 — Backwash trigger and pressure differential. Both HUBER and WesTech designs initiate backwash when the upstream water level reaches a predefined maximum ΔP. HUBER uses spray nozzle bars with filtrate as the wash water at high pressure (S4); WesTech employs a high-pressure backwash spray while the rotor continues turning (S2).
Parameter 6 — Reject water volume and recovery. Reject consists of filtrate used for backwash plus captured solids. Recovery for tertiary polishing typically exceeds 95%; the rejection fraction is 2–5% of throughput, calculated as (Q_rej / Q_in) × 100.
| Parameter | Typical Range / Value | Source |
|---|---|---|
| Mesh pore size | 10–60 µm (finest 10 µm) | S4 |
| Discs per shaft | 13 (Hydrotech HF2220); up to 35 (HUBER RoDisc) | S3, S4 |
| Submerged disc fraction | Up to 65% | S4 |
| Capacity (reference) | 1200 m³/h at 13 discs, 18 µm | S3 |
| Backwash fluid | Filtrate (not fresh water) | S2, S4 |
| Effluent SS | 3–8 mg/L at 18 µm | S3 |
Mesh Size Selection Logic: 10 µm vs 18 µm vs 40 µm
Mesh aperture most directly influences both capital and operating costs, and engineers often over-specify it. Select pore size based on the actual effluent target, not the lowest number on the OEM price list.
10 µm. The performance ceiling. HUBER offers it as the finest standard mesh (S4). It delivers the lowest effluent SS and highest TSS removal but blinds fastest, requiring the most frequent backwash—resulting in the highest reject volume and pump duty on the spray bar. Specify 10 µm only when the downstream process (typically RO or a reuse discharge limit) cannot tolerate higher SS.
15–18 µm. The standard choice for tertiary reuse and microplastic capture. The MDPI 2019 study used 18 µm and achieved 89.7% particle removal by count and 75.6% by mass, with effluent SS of 3–8 mg/L (S3). This aperture is the default for municipal tertiary polishing where the goal is reliable SS reduction without the backwash penalty of 10 µm. For plants experiencing downstream membrane fouling, the ultrafiltration troubleshooting field guide explains how upstream mesh selection affects TMP rise rates.
40–60 µm. The standard for pre-RO polishing where the objective is SDI reduction and gross solids removal, not absolute TSS. The coarser mesh extends intervals between backwashes and reduces reject volume, making it dominant in seawater and brackish RO pretreatment trains. The trade-off rule: every halving of pore size roughly doubles backwash load, so mesh selection must align with the actual effluent target rather than being over-specified.
HUBER RoDisc vs WesTech SuperDisc: Parameter Comparison
While both vendors offer disc filters, their designs optimize for different site constraints. The following table compares the two leading designs to allow direct evaluation without reverse-engineering datasheets.
| Parameter | HUBER RoDisc | WesTech SuperDisc |
|---|---|---|
| Disc configuration | Up to 35 vertical discs on a horizontal shaft | Rotor drum with stacked discs |
| Finest mesh | 10 µm | Polyester media, defined pore size |
| Submergence | Up to 65% of disc surface | Partially submerged drum, varies with rotor position |
| Drive / rotation | Slow rotation during backwash only | Continuous rotation, filters during backwash |
| Backwash water source | Filtrate (no fresh water) | Filtrate via high-pressure spray |
| Tank integration | Modular, suited to new build and retrofit | Integral unit, optional freestanding stainless tank |
| Footprint signal | High capacity in small tank via high disc count | Minimal hydraulic head — retrofit into existing basins |
| Sources | S4 | S2 |
The design differences reflect distinct priorities. HUBER maximizes disc count on a single shaft, allowing a small tank to deliver high throughput—making the 35-disc RoDisc ideal for new tertiary trains with limited footprint (S4). WesTech prioritizes retrofit compatibility in existing filter basins with constrained hydraulic head, where its integral frame, low drive head, and continuous-rotation backwash minimize civil modifications (S2). The choice typically depends on whether the project is greenfield (HUBER) or brownfield (WesTech).
Worked Sizing Example: 800 m³/h Tertiary Duty
For a design flow of 800 m³/h and a target effluent SS of ≤ 5 mg/L, 18 µm mesh serves as the starting aperture—consistent with the MDPI 2019 reference unit (S3). The Hydrotech HF2220 reference handles 1200 m³/h with 13 discs (S3), so an 800 m³/h duty scales linearly to approximately 9 discs (800/1200 × 13 = 8.7, rounded up to 9).
This disc count provides an initial estimate, but OEM confirmation is required because disc diameter, spacing, and submergence vary by design. A HUBER RoDisc at 65% submergence will deliver a different effective area per disc than a WesTech SuperDisc drum.
Reject water volume is the next consideration. With recovery in the 95–98% range, reject accounts for 2–5% of throughput. At 800 m³/h, 2% reject equals 16 m³/h (384 m³/day at continuous duty); 5% reject equals 40 m³/h (960 m³/day). Present this to clients as a daily volume rather than a percentage for clearer impact. The trade-off between finer mesh and reject volume is direct: reducing pore size from 18 µm to 10 µm roughly doubles backwash frequency, meaning the 384–960 m³/day range represents a minimum. If the upstream process includes oil-bearing waste, a DAF system for upstream oil and grease removal is typically specified ahead of the disc filter to prevent reject volume spikes from FOG breakthrough.
Pre-Treatment, Backwash, and Operating Constraints
A disc filter functions as a polishing device, not a primary treatment unit—it assumes upstream processes have already performed the bulk of solids removal. HUBER specifies two non-negotiable pre-treatment requirements: pre-screening with a 3 mm perforated plate and absence of oil or grease in the wastewater (S4). Failure to meet either condition results in premature mesh blinding, spray bar fouling, and inability to meet effluent targets regardless of sizing.
A 3 mm rotary bar screen for disc filter pre-screening is the standard upstream device. For oil and grease removal, DAF is the typical solution; the engineering basis is detailed in the DAF engineering and selection guide.
Both HUBER and WesTech designs use filtrate—not fresh water—for backwash (S2, S4), which should be framed as a water-reuse advantage. The filtration process continues during backwash, eliminating dead time. Periodic chemical cleaning is required to remove biofouling or inorganic scale from the mesh (S4); CIP frequency depends on influent water chemistry but typically ranges from monthly to quarterly for tertiary polishing. The other key operating constraint is ΔP: when upstream water level reaches the predefined maximum, backwash triggers automatically, and the spray bar operates until ΔP returns to baseline.
Frequently Asked Questions
What mesh size should I specify for pre-RO polishing?
40–60 µm. This range provides the SDI reduction RO membranes require without the backwash penalties associated with 10–18 µm meshes
Frequently Asked Questions
What mesh size should I use for a disc filter ahead of RO?
A disc filter used as pretreatment for reverse osmosis (RO) systems typically requires a mesh size between 50 and 200 microns, depending on the feedwater quality and RO membrane specifications. For most municipal and industrial applications, a 100-micron mesh is standard to protect RO membranes from fouling while balancing flow efficiency.
If the feedwater contains high levels of fine particulates (e.g., silt or colloidal matter), a finer mesh (50–80 microns) may be necessary. However, finer meshes increase pressure drop and backwash frequency, so pilot testing is recommended per ASTM D4189 or ISO 13946 standards.
How many discs are needed to treat 1000 m³/h with a disc filter?
The number of discs required for a 1000 m³/h flow rate depends on the disc filter model and its hydraulic loading capacity. Most industrial disc filters handle 10–25 m³/h per disc, with high-efficiency models reaching up to 30 m³/h per disc under optimal conditions.
For 1000 m³/h, a minimum of 40–100 discs would be needed, assuming a conservative loading rate of 10–25 m³/h per disc. Factors such as suspended solids concentration, mesh size, and backwash frequency may require additional discs to maintain performance. Consult manufacturer specifications (e.g., ISO 16889 for filtration efficiency) for precise sizing.
What pre-screening is required before a disc filter?
Pre-screening is essential to prevent large debris from damaging or clogging disc filters. A coarse screen with openings between 1–3 mm (1000–3000 microns) is typically installed upstream to remove leaves, plastics, and other macro-contaminants. For high-turbidity water, a hydrocyclone or settling basin may be used to reduce heavy sediment load.
In wastewater applications, a bar screen (6–20 mm spacing) or rotary drum screen (0.5–2 mm) is often employed. Pre-screening requirements are outlined in standards such as ISO 16889 (multi-pass test) and AWWA B100 for granular media filtration.
Can a disc filter keep running during backwash?
Yes, most modern disc filters operate in a continuous mode, allowing filtration to continue during backwash cycles. The system uses a segmented design where only a portion of the discs (typically 5–15%) is backwashed at a time, ensuring uninterrupted flow. Backwash duration usually lasts 30–120 seconds per segment.
Flow rates may experience a minor reduction (5–10%) during backwash due to diverted water for cleaning. Systems are designed to comply with ISO 16889 for hydraulic efficiency, ensuring minimal disruption to downstream processes like RO or cooling towers.
What suspended solids level can a disc filter achieve?
A well-designed disc filter can reduce suspended solids to 5–20 mg/L, depending on feedwater quality and mesh size. With a 100-micron mesh, effluent turbidity is typically below 10 NTU, while finer meshes (50 microns) can achieve 2–5 NTU under optimal conditions.
For comparison, ISO 16889 specifies a minimum filtration efficiency of 90% for particles larger than the nominal rating. Performance may degrade if feedwater exceeds 100 mg/L suspended solids; in such cases, a multi-stage filtration system (e.g., disc filter + ultrafiltration) is recommended.
