What an IFAS Process Flow Diagram Represents
An IFAS (Integrated Fixed-Film Activated Sludge) process flow diagram depicts a hybrid bioreactor in which free-floating plastic carrier media are suspended inside an aerated activated-sludge basin, combining biofilm growth on the media with suspended-growth biomass in the mixed liquor. The flow path runs: influent → screening/primary clarification → IFAS aeration tank (with retained carriers and an aeration grid) → secondary clarifier with a return activated sludge (RAS) loop → disinfection → discharge or reuse, with waste activated sludge (WAS) sent to sludge handling. The visual shorthand used in IFAS drawings is distinctive: the aeration tank symbol is split between a "mixed liquor" zone and a "media-filled zone," with the same reactor vessel serving both biomass populations simultaneously.
IFAS is one of three commonly diagrammed hybrid or moving-bed configurations alongside the moving bed biofilm reactor (MBBR) and pure biofilm processes, as illustrated in comparative process flow diagrams such as the ResearchGate figure 26670059 which places MBBR and IFAS side by side. The principal engineering benefit the diagram conveys is higher treatment capacity in the same tank volume — typically a 50–100% increase in organic loading without expanding civil footprint, because biofilm on carriers provides active biomass that does not require the same settling properties as suspended MLSS (Taylor & Francis, Water and Wastewater Engineering, Chapter 35, 2026).
Stream-by-Stream Walk Through the IFAS Flow Train
A complete IFAS train follows seven streams of water and sludge from raw influent to final effluent, each with a defined piece of equipment and a design parameter.
Step 1 — Influent and screening. Raw wastewater enters the headworks through a rotary mechanical bar screen, typically 6 mm aperture in modern IFAS plants, to remove rags, plastics, and fibrous material that would otherwise foul the carrier retention screens downstream. Headworks screening is the first line of defense for the carriers themselves; even small rag accumulations on a retention screen can plug and cause mixed-liquor overflows within hours.
Step 2 — Primary clarification. An optional primary clarifier or grit chamber removes settleable solids and reduces loading on the IFAS basin. In municipal plants, a high-rate lamella clarifier for primary sedimentation can reduce TSS by 50–70% and BOD by 25–35% before the water reaches the IFAS reactor, which lowers the required aeration volume.
Step 3 — IFAS aeration tank(s). The basin is usually configured as a multi-zone train — often a pre-anoxic selector followed by 2–4 aerobic cells in series. Carrier retention screens (perforated plates with 6–10 mm slots) are placed at the outlet of each aerobic cell to keep the 10–25 mm PE media inside the basin while letting mixed liquor flow forward to the clarifier.
Step 4 — Aeration grid and DO control. Coarse-bubble or membrane diffusers maintain a 2–4 mg/L dissolved-oxygen setpoint at the carrier surface. Aeration serves a dual role: oxygen delivery for BOD removal and nitrification, plus the mixing energy required to keep carriers in fluidization (typically 15–25 W/m³ of basin volume).
Step 5 — Secondary clarification. Mixed liquor flows to a secondary clarifier where MLSS is separated from the clarified effluent. A lamella clarifier for secondary clarification is frequently used for high-rate IFAS mixed liquor because the high MLSS (3,000–5,000 mg/L) plus biofilm sloughing pushes surface overflow rates higher than conventional designs tolerate.
Step 6 — RAS and WAS streams. The clarifier underflow splits into two streams: RAS returns settled biomass to the head of the IFAS basin at 50–100% of forward flow, while WAS is wasted to sludge handling. WAS from IFAS typically dewaters well on a plate and frame filter press, reaching 22–28% dry solids because the biofilm fraction improves sludge dewaterability relative to straight CAS.
Step 7 — Disinfection and discharge/reuse. Clarified effluent passes through chlorination, UV, or a chlorine dioxide generator before reuse or surface discharge, sized to deliver a CT of 1.5–3.0 mg·min/L for fecal coliform compliance at 200 CFU/100 mL typical reuse thresholds.
Key Design Parameters to Annotate on the Diagram
A complete IFAS process flow diagram should carry numerical callouts on each unit operation, not just flow arrows. The table below consolidates the parameters an engineer needs to see annotated on the drawing.
| Parameter | Typical IFAS Range | Why It Appears on the Diagram |
|---|---|---|
| Carrier fill fraction (FILL) | 20–40% of aerated volume | Defines the media zone geometry inside the aeration tank symbol |
| Media specific surface area | 500–800 m²/m³ for standard PE carriers | Drives biofilm surface loading and nitrification rate |
| MLSS in IFAS tank | 3,000–5,000 mg/L | Sets clarifier underflow design and RAS return rate |
| HRT (aerobic zone) | 4–8 h for secondary treatment | Determines basin volume for a given design flow |
| F/M ratio | 0.05–0.15 kg BOD/kg MLVSS·d | Lower than CAS because biofilm adds effective biomass |
| SRT (suspended) | 10–30 days | Biofilm allows operation at lower SRT without losing nitrification |
| DO setpoint at carrier surface | 2–4 mg/L | Online DO probes control blower output and mixing intensity |
| Carrier retention screen aperture | 6–10 mm slots | Retains 10–25 mm media while passing mixed liquor |
| Surface overflow rate (clarifier) | ≤ 1.0 m/h at 4,000 mg/L MLSS | Retrofit constraint when upgrading an existing CAS clarifier |
The total active biomass in an IFAS tank is the sum of suspended MLSS plus attached biomass on the carriers, typically 3,000–5,000 mg/L suspended plus 4,000–8,000 mg/L attached per carrier dry weight (Zhongsheng field data, 2026). This attached biomass explains why IFAS outperforms CAS at the same MLSS.
IFAS vs MBBR vs Conventional Activated Sludge: Reading the Diagrams Side by Side
Reading the three process flow diagrams next to each other is the fastest way to spot the structural differences an engineer must communicate to a client. The table below consolidates the visual cues and treatment envelopes.
| Feature | IFAS | MBBR | Conventional AS (CAS) |
|---|---|---|---|
| Biomass form | Biofilm on carriers + suspended MLSS | Biofilm on carriers only | Suspended MLSS only |
| RAS loop on diagram | Yes (return line from clarifier) | No | Yes |
| Carrier retention screen on diagram | Yes, at IFAS tank outlet | Yes, at reactor outlet | No |
| Carrier fill annotation | 20–40% of aerated volume | 20–67% of reactor volume | Not applicable |
| Typical MLSS | 3,000–5,000 mg/L | <500 mg/L (no sludge inventory) | 2,000–4,000 mg/L |
| SRT range | 10–30 days | No sludge wastage in pure MBBR mode | 5–15 days |
| Typical TN effluent | <10 mg/L with pre-anoxic zone | 5–15 mg/L with downstream denitrification | <10 mg/L with separate anoxic basin |
IFAS is typically specified when the MLSS capacity of an existing CAS basin is exhausted but the civil footprint cannot grow, or when a nitrification upgrade is needed at lower SRT than CAS requires. For very low effluent nitrogen targets where an existing IFAS retrofit still falls short, the MABR working principle guide describes a counter-diffusion biofilm technology that can be added downstream without expanding basin volume.
Sizing Notes for the Aeration Basin and Carrier Inventory
Three calculations translate the diagram annotations into a buildable design.
Aeration volume. V ≈ Q × HRT. For Q = 10,000 m³/d and HRT = 6 h, V ≈ 2,500 m³ in the aerobic IFAS zone. Add a pre-anoxic selector of 20–30% of this volume for partial denitrification if TN is in the design basis.
Carrier volume and count. V_carrier ≈ 0.30 × V at 30% fill → ~750 m³ of media. For standard PE carriers at 500–800 m²/m³ specific surface area, the basin carries 375,000–600,000 m² of biofilm surface area, the parameter that controls nitrification rate (Zhongsheng field data, 2026).
Air demand and clarifier check. Assume 1.8–2.5 kg O₂/kg BOD removed for a combined nitrification-denitrification IFAS train, then size the blower room that appears on the diagram from that figure. For the secondary clarifier, hold the surface overflow rate ≤ 1.0 m/h at 4,000 mg/L MLSS — this is the most common retrofit constraint when an existing CAS clarifier is being reused for IFAS mixed liquor.
For reuse-quality effluent where the secondary clarifier cannot meet the target, the clarifier can be replaced with an MBR membrane bioreactor while keeping the IFAS aeration tank and carrier inventory unchanged — a configuration that delivers sub-5 mg/L TSS and is increasingly specified for industrial water reuse.
Frequently Asked Questions
What is the difference between IFAS and MBBR?
IFAS retains suspended MLSS and a RAS loop with carrier media inside the aeration tank; MBBR uses carrier media as the sole biomass with no RAS stream and no clarifier-driven sludge inventory. On a flow diagram, IFAS shows an extra return line from the secondary clarifier that MBBR does not have.
What is the typical carrier fill percentage in IFAS?
Carrier fill is typically 20–40% of the aerated volume for standard polyethylene (PE) carriers, with most municipal IFAS designs operating in the 25–35% range to balance biofilm surface area against mixing energy requirements.
Why is a screen used at the IFAS outlet?
A perforated retention screen with 6–10 mm slots keeps the free-floating 10–25 mm media inside the basin while letting mixed liquor flow forward to the secondary clarifier. Without this screen, carriers would wash out with the effluent in minutes.
When should an engineer choose IFAS over MBBR?
Specify IFAS when an existing CAS infrastructure must be upgraded for higher organic load or nitrification without adding new basins, and when the site can accommodate a clarifier plus RAS pumping. MBBR is the better fit for footprint-constrained greenfield plants or when no sludge inventory is desired.
Can IFAS be combined with MBR?
Yes. For reuse-quality effluent, the secondary clarifier can be replaced with an MBR membrane step (an MBR membrane bioreactor) while keeping the IFAS aeration tank, the carrier inventory, and the existing blower room unchanged. This hybrid IFAS-MBR configuration is common in
