What a Disc Filter Process Flow Diagram Actually Shows
A disc filter process flow diagram is a four-stream schematic: influent entering the disc stack from the inside, filtrate discharging from the outer housing, a backwash supply tapped from the filtrate launder, and a backwash reject stream carrying dislodged cake to sludge handling. Every credible disc filter PFD — for tertiary reuse, pre-RO polishing, or industrial polishing — must show those four streams, the boundary tags with upstream secondary clarification (or DAF) and downstream UV or RO, and the control loop that initiates the backwash cycle. Without those four streams and the ΔP setpoint logic, the diagram is not a PFD; it is a stylized illustration.
The physical equipment is a vertical array of grooved plastic discs clad in polyester or stainless steel mesh (10–60 µm aperture), mounted on a central hollow shaft inside a pressurized or gravity tank. Influent enters the center of the stack and is forced outward through the mesh; suspended solids are captured on the inner surface and build a cake layer that progressively improves capture. On a HUBER RoDisc — the geometry most municipal engineers transcribe first — up to 65% of disc area is below the waterline at any instant, and the unit operates as a gravity-flow system with a fixed overflow weir that defines the upstream water level driving the backwash control loop. The canonical train position is primary sedimentation → activated sludge → secondary clarifier → disc filter → UV or RO, with a 3 mm rotary bar screen upstream of the disc filter to keep fibrous debris off the mesh.
Stream-by-Stream Walkthrough: Influent to Filtrate
Secondary clarifier overflow enters the disc filter through a flow-distribution weir into the inlet drum. On HUBER RoDisc designs, the inlet drum is sized so the fixed overflow weir sets a stable upstream water level — that level is the physical variable the controller watches, because it rises in proportion to head loss across the mesh as the cake thickens. From the inlet drum, water passes into the center of the disc stack and is forced outward through the mesh under the differential head. Per the MDPI 2019 Billund WWTP study, an 18 µm mesh captured 89.7% of particles and reduced influent SS from 20 mg/L to 3–8 mg/L, with 75.6% mass removal attributed in part to cake-mediated capture beyond the bare mesh rating. Filtrate collects in the outer housing and discharges by gravity over the outlet weir toward the next unit operation. Downstream SS targets are <10 mg/L for UV transmittance and SDI₁₅ <3–5 for RO feedwater.
The instrumentation block on the PFD has four canonical tags for this stream group. FIT-301 is the influent flow transmitter, usually a magnetic flow meter on the inlet launder. LIT-301 is the level transmitter in the inlet drum — it is the primary backwash initiator because, on a gravity unit, water level is the proxy for ΔP. PDI-301 is the differential pressure transmitter across the disc stack, redundant with LIT-301 and used for the 30–50 kPa setpoint logic. FIT-302 is the filtrate flow transmitter on the outlet launder, used to back-calculate recovery.
| Tag | Service | Location on PFD | Typical Range / Setpoint |
|---|---|---|---|
| FIT-301 | Influent flow | Inlet launder to drum | Full scale at design flow (e.g. 0–800 m³/h) |
| LIT-301 | Upstream water level | Inlet drum | ΔP equivalent 30–50 kPa triggers backwash |
| PDI-301 | Differential pressure | Across disc stack | 30–50 kPa setpoint, tertiary duty |
| FIT-302 | Filtrate flow | Outlet weir to next stage | 95–98% of FIT-301 at steady state |
The Backwash Loop and Reject-Handling Branch
The backwash loop is a side branch that draws from the filtrate launder, not from raw water. When LIT-301 reaches the level corresponding to a ΔP of 30–50 kPa (HUBER product documentation), the controller starts disc rotation and opens the spray bar — filtration does not stop. A backwash pump — sized for the spray nozzle bar flow at 3–5 bar nozzle pressure — pulls filtrate from the outlet launder and fires it through particulate-resistant spray nozzles at the mesh from the outside. The dislodged cake plus spray water falls into an internal collection trough and discharges axially from the machine. On continuous-rotation designs, only 5–15% of the disc area is offline at any moment, and the cleaning window lasts 30–120 s before the segment returns to service. The upstream process therefore sees no flow interruption; on the PFD this is annotated as a partial-capacity event, not a shutdown line.
The reject destination is a routing decision the engineer makes on the diagram. Cake plus spray water can be sent to a sludge dewatering device — a filter press for the backwash reject stream is the common choice when the upstream biology already stabilizes the solids — or it can be returned to head-of-works for re-treatment. Quantify the reject stream as 2–5% of disc-filter throughput, equivalent to 0.5–3% of total plant flow per commercial references. At 800 m³/h design flow, 2% reject is 16 m³/h (384 m³/day); 5% reject is 40 m³/h (960 m³/day). If the upstream feed carries oil or grease, expect the upper end of this range and route the reject through a dedicated handling train rather than back to the head of the works, where a recycle loop will compound the FOG load.
Sizing the Disc Filter on the Flow Diagram
General hydraulic loading formula: design flow ÷ (number of discs × disc area × submergence factor). On a HUBER RoDisc, submergence is fixed at up to 65%, so the equation collapses to a disc-count calculation against effective area per disc. Worked example anchored to the MDPI 2019 Hydrotech HF2220 reference: 1200 m³/h across 13 discs at 18 µm and 65% submergence. For an 800 m³/h design at the same 18 µm aperture, scale linearly: 800/1200 × 13 = 8.7, rounded up to 9 discs. A HUBER RoDisc shaft carries up to 35 discs, so 9 discs fits comfortably within a single-shaft envelope and leaves headroom for future capacity expansion without a parallel train.
OEM confirmation is mandatory before procurement. Disc diameter, spacing, and submergence fraction vary between HUBER and other platforms, and a 9-disc count on one geometry may deliver a 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. Reject volume is the second sizing output the PFD must show: at 95–98% recovery, expect 2–5% of throughput as reject; the sizing table below captures the key numbers an engineer will pencil onto the diagram.
| Parameter | Value | Source / Note |
|---|---|---|
| Reference unit | Hydrotech HF2220, 13 discs, 18 µm, 65% submergence | MDPI 2019 Billund study |
| Reference throughput | 1200 m³/h | MDPI 2019 Billund study |
| Design flow (worked example) | 800 m³/h | Engineer's design point |
| Calculated disc count | 800/1200 × 13 = 8.7 → 9 discs | Linear scale, OEM confirmation required |
| Single-shaft capacity (HUBER RoDisc) | Up to 35 discs | Manufacturer documentation |
| Recovery | 95–98% | Tertiary polishing duty |
| Reject rate | 2–5% of throughput (16–40 m³/h at 800 m³/h) | 0.5–3% of total plant flow |
Three Disc Filter PFD Variants: Reuse, Pre-RO, and Industrial
Variant A — municipal reuse / microplastic capture. Secondary clarifier → 18 µm disc filter → UV. This is the Billund reference train: 89.7% particle count removal, 3–8 mg/L effluent SS, and 95–98% recovery. The dominant failure mode is rapid blinding when 10 µm mesh is specified without an effluent-SS justification; stay at 15–18 µm unless downstream limits demand finer.
Variant B — pre-RO polishing. Secondary clarifier (or DAF) → 40–60 µm disc filter → cartridge filter → RO. The coarser mesh extends backwash intervals and reduces reject volume, which is why it dominates seawater and brackish RO pretreatment trains. The dominant failure mode is SDI creep when upstream biology is unstable; specify a redundant LIT/PDI on the disc filter outlet and alarm at SDI₁₅ > 5.
Variant C — industrial with FOG/TSS. DAF upstream for oil/grease removal → 3 mm bar screen → disc filter (typically 18 µm) → RO or reuse. HUBER specifies zero oil/grease to the disc filter, so a DAF pre-treatment for FOG and TSS reduction ahead of the disc filter is non-optional on this duty. The dominant failure mode is FOG breakthrough when the DAF is undersized; verify the DAF's oil removal guarantee on the same flow and temperature envelope, and protect the downstream RO membrane protected by the disc filter with an SDI₁₅ interlock before the high-pressure pump starts.
| Variant | Pre-treatment | Mesh | Next Stage | Dominant Failure Mode |
|---|---|---|---|---|
| A — Reuse / microplastic | Secondary clarifier | 18 µm | UV disinfection | Rapid blinding at 10 µm |
| B — Pre-RO polishing | Secondary clarifier or DAF | 40–60 µm | Cartridge + RO | SDI creep from unstable upstream biology |
| C — Industrial FOG/TSS | DAF + 3 mm bar screen | 18 µm | RO or reuse | FOG breakthrough if DAF undersized |
Engineers can integrate advanced oxidation downstream of the disc filter by inserting an AOP skid between the filter and the final disinfection step. Engineers weighing electrocoagulation as an alternative upstream of the disc filter for refractory COD or metals should review a separate electrocoagulation flow diagram as an alternative upstream of the disc filter; for a downstream AOP train, the matching AOP process flow diagram for advanced polishing after the disc filter covers the boundary tags and oxidant dosing loop.
Frequently Asked Questions
What mesh aperture should be specified on a disc filter PFD for tertiary reuse?
15–18 µm is the standard for municipal tertiary reuse and microplastic capture; the MDPI 2019 Billund study used 18 µm and recorded 89.7% particle count removal, 75.6% mass removal, and 3–8 mg/L effluent SS. Specify 10 µm only when a downstream reuse limit or high-fouling RO feed genuinely requires it, because every halving of pore size roughly doubles backwash load.
What ΔP setpoint initiates the backwash cycle on a disc filter PFD?
30–50 kPa across the disc stack for tertiary polishing duty, per HUBER product documentation. On a gravity-flow RoDisc, the controller watches LIT-301 (upstream water level) because it rises in proportion to ΔP; a redundant PDI-301 across the stack provides the direct measurement and the alarm.
How much reject does a disc filter generate, and where should it be routed?
Reject accounts for 2–5% of disc-filter throughput (0.5–3
