What an MBR Process Flow Diagram Actually Shows
An MBR process flow diagram is a labeled, stream-connected drawing of a membrane bioreactor train, used by process engineers to communicate how wastewater moves through biological and physical unit operations before discharge or reuse. MBR is an activated-sludge system with a submerged membrane filtration step inside or directly downstream of the aeration tank, so the diagram must show both the liquid and the gas streams that conventional activated-sludge (CAS) P&IDs handle separately. Because MLSS and SRT are intentionally higher than CAS — typically 8,000–12,000 mg/L and 20–40 days respectively — the aeration basin can be physically smaller, but the diagram must call out every piece of equipment that makes that elevated biomass workable (per CED S2).
Every MBR process flow diagram covers the same seven canonical unit operations: coarse screening, grit removal, fine screening (2–3 mm), anoxic basin (for nitrogen trains), aerobic basin, membrane tank with submerged cassette, and permeate/disinfection polish. Two stream families must appear on the P&ID: liquid streams (influent, permeate, recycle, WAS) and gas streams (process air for oxygen demand, scouring air for membrane fouling control). Drawing only the liquid train is the most common reason an MBR P&ID is rejected at 30% review. The U.S. EPA factsheet (EPA S1) uses the Traverse City, Michigan facility block diagram as the canonical reference, and it is the layout most reviewers expect to see mirrored on the issue package.
Variant 1 — BOD Removal + Nitrification MBR Flow Diagram
The BOD-removal-only variant is the simplest defensible MBR train and is the right starting P&ID for ammonia-limited discharges where total nitrogen is not regulated. The single liquid train reads: influent → coarse screen → grit chamber → 2–3 mm fine screen → aerobic aeration tank (with submerged PVDF flat-sheet MBR membrane cassette (DF series) immersed in the mixed liquor) → permeate suction pump → CIP tank tie-in → disinfection. The air train has two distinct duties and must be drawn as two separate headers: a dedicated scouring blower feeds the integrated aeration box under each flat-sheet cassette, and a separate process blower supplies biological oxygen demand to the aerobic basin. The two blowers are not interchangeable — combining them on a single header is a recurring review comment.
The sludge stream is drawn off the recycle line at 1–1.2 wt% TS and routed to a plate and frame filter press for WAS dewatering or a lamella thickener (per EPA S1). The waste-activated-sludge concentration is a real specification, not a guideline; below 1 wt% the press throughput collapses, above 1.2 wt% the sludge piping plugs. Typical operating envelope for this variant: MLSS 8,000–12,000 mg/L, SRT 20–40 d, HRT 4–8 h, design flux 10–25 LMH — these are typical engineering ranges that an MBR P&ID reviewer will look for in the design basis. The Traverse City reference numbers put hard data on the envelope: BOD 280 → <2 mg/L, TSS 248 → <1 mg/L, Ammonia-N 27.9 → <0.08 mg/L, TP 6.9 → 0.7 mg/L (EPA S1).
Variant 2 — Pre-Anoxic Denitrification MBR Flow Diagram
Adding a pre-anoxic basin upstream of the aerobic basin converts the BOD-removal train into a total-nitrogen removal system without redrawing the rest of the process. In this variant, the nitrate-rich sludge return is pumped from the membrane tank back to the anoxic basin — not the aerobic basin — at a recycle ratio of roughly 3–5× permeate flow (per CED S2, Figure 18). The anoxic basin carries a mechanical mixer only; no diffused air, no blower tap-off. Because influent BOD is consumed anoxically in the front of the train, the downstream process-blower duty drops, and the scouring blower duty is unchanged.
Two operational payoffs follow from this layout. First, denitrification recovers roughly 50% of the alkalinity consumed by nitrification, which directly reduces the caustic dose on the alkalinity-feed line — a line that the BOD-only variant requires and this variant can shrink. Second, total nitrogen in the permeate drops to single-digit mg/L without a separate denitrification filter downstream. The pre-anoxic HRT is typically 1–2 h, which is a real space allocation on the general arrangement drawing, not a footnote. The BOD-removal/nitrification block of this variant is sized from the CED S2 worked calculation reproduced in the next section, so the membrane area, tank volume, and scouring air numbers are consistent between the two P&ID variants.
Design Parameters That Must Appear on Every MBR P&ID
A reviewer reads the parameter table before the diagram. The table below is the minimum set of values that an MBR P&ID has to carry in the title block or in a tied design-basis sheet. Influent/effluent targets, MLSS, SRT, HRT, flux, and scouring air ratio are all non-negotiable; missing any one of them is grounds for rejection at 30% review.
| Parameter | Value or Range | Source / Note |
|---|---|---|
| Influent flow (Qo) | 7,571 m³/d (2 MGD) — example basis | CED S2 Example 1 |
| Peak flow cap | 1.5–2× average design flow | EPA S1 — equalization required above this |
| Influent BOD / TSS / NH₃-N / TP | 280 / 248 / 27.9 / 6.9 mg/L (example) | EPA S1, Traverse City |
| Effluent BOD / TSS / NH₃-N / TP | <2 / <1 / <0.08 / 0.7 mg/L (example) | EPA S1, Traverse City |
| MLSS | 8,000–12,000 mg/L | Typical MBR range |
| SRT | 20–40 d | Typical MBR range |
| HRT (aerobic) | 4–8 h | Typical MBR range |
| Design flux (J) | 10–25 LMH (membrane-vendor specific) | Obtain from module datasheet |
| Membrane pore size | 0.1 µm (PVDF flat-sheet or hollow-fiber) | CED S2 / HydroPure DF series |
| Fine-screen cut-off | 2–3 mm, immediately upstream of membrane | EPA S1 — all MBR manufacturers require this |
| Required membrane area, Am = Qo / J | 26,288 m² (7,571 m³/d basis); 282,956 ft² (2 MGD basis) | CED S2 Example 1 |
| Membrane module volume, Vm | 219 m³ (7,571 m³/d); 7,736 ft³ (2 MGD) | CED S2 Example 1 |
| Scouring air flow = SADM × Am / 60 | 131 m³/min (7,571 m³/d); 4,642 cfm (2 MGD) | CED S2 Example 1 |
| Sludge recycle ratio (pre-anoxic variant) | 3–5× permeate flow | CED S2 Figure 18 |
| Pre-anoxic HRT | 1–2 h | Typical engineering range |
Two equations govern the membrane block of every MBR P&ID and should appear in the calculation note attached to the drawing: Am = Qo / J (membrane area in m², Qo in L/h, J in LMH) and Scouring Air = SADM × Am / 60 (m³/min, with SADM in m³ air/h per m² membrane). The CED S2 worked example is reproduced directly: at Qo = 7,571 m³/d, the calculation returns Am = 26,288 m², Vm = 219 m³, and scouring air = 131 m³/min; at 2 MGD it returns Am = 282,956 ft², Vm = 7,736 ft³, and scouring air = 4,642 cfm. Design flux is module-vendor specific — do not publish a single number on the P&ID; obtain J from the selected membrane module datasheet and tie it to the BOM.
Component-Level Equipment List for the MBR Train
Every block on the P&ID has to resolve to a procurable equipment item. The 2026-component mapping below lets a process engineer copy a complete equipment list straight into the BOM without going back through vendor catalogs.
- Fine screen: GX series rotary mechanical bar screen, 2–3 mm aperture, stainless rake teeth, dual overload protection. Sits immediately upstream of the membrane tank per EPA S1 — this is not optional.
- Membrane cassette: PVDF flat-sheet, 0.1 µm pore, 80–225 m² per unit, integrated aeration box, individually replaceable elements, stainless frame. The flat-sheet geometry gives 10–20× lower energy consumption than external cross-flow systems (HydroPure DF series catalog). Hollow-fiber is acceptable but is harder to clean on a per-element basis.
- Scouring blower: dedicated blower sized to SADM × Am / 60. For the CED S2 7,571 m³/d basis that is 131 m³/min; do not oversize for the process-air duty.
- Process blower: separate unit sized to biological oxygen demand in the aerobic basin. Combined scouring + process headers are a recurring review rejection.
- Permeate pump: suction-side centrifugal or peristaltic, duty/standby arrangement. Sized for 1.5–2× average flow, not peak instantaneous flow.
- Sludge handling: lamella clarifier or plate and frame filter press for WAS dewatering, fed from the WAS line at 1–1.2 wt% TS (per EPA S1).
- Disinfection polish: chlorine dioxide disinfection generator (ZS series), sized to permeate flow. ClO₂ is preferred over Cl₂ for MBR permeate because it does not react with residual ammonia to form chloramines.
- RO polish (reuse projects): industrial RO unit up to 95% recovery downstream of the MBR permeate for water-reuse or ZLD polishing. See the related fertilizer wastewater MBR solution guide for a worked reuse-train example.
For full MBR train packaging, the integrated MBR wastewater treatment system bundles the cassette rack, blowers, permeate pump, and control panel under one BOM line. A pre-anoxic retrofit typically uses the same package with an added anoxic mixer and recycle pump.
Common MBR Flow-Diagram Mistakes and How to Avoid Them
Five P&ID errors account for the majority of MBR drawing rejections at 30% and 60% review.
- Skipping the 1–3 mm fine screen. Every MBR manufacturer requires it immediately upstream of the cassettes (EPA S1). Without it, hair, fibers, and ragging foul the membrane in weeks, not months.
- No CIP tie-in or isolate valves around each membrane cassette. Chemical cleaning is a maintenance fact of life; the CIP tank connection, the per-cassette isolation valves, and the per-cassette permeate drain all have to be on the P&ID, otherwise a future cleaning event requires cutting pipe.
- Permeate pump sized for peak flow. EPA S1 caps peak design flow at 1.5–2× average. Sizing the pump for peak instantaneous flow drives capital cost up and operating energy wasted; flow equalization is the right answer above the cap.
- Missing or unlabeled WAS line. The waste-activated-sludge line at 1–1.2 wt% TS to sludge handling is a real stream on the P&ID. Omitting it causes MLSS creep and eventually flux loss on the membranes.
- Scouring air and process air on the same header. They are two different blowers on two different duties. The scouring blower runs continuously at SADM × Am / 60 to keep foulants off the membrane; the process blower modulates to oxygen demand. Combining them means the membrane cannot be scoured during low-DO periods.
If the project is a new build rather than a retrofit, the flat-sheet MBR membrane cost and ROI guide for 2026 ties the cassette selection back to a defensible 20-year lifecycle cost, which is the next thing a procurement reviewer will ask for after the P&ID is approved.
Frequently Asked Questions
What does an MBR process flow diagram include that a conventional activated-sludge P&ID does not?
An MBR P&ID adds the submerged membrane cassette, a dedicated scouring-air blower, a permeate suction pump on the suction side, and a CIP tank tie-in. The biological section is otherwise the same as CAS, but the recycle, WAS, and air headers are drawn separately for the membrane duty and the process-air duty.
What is the typical membrane area and scouring air requirement for a 2 MGD MBR?
Per the CED S2 worked example, a 2 MGD basis returns Am = 282,956 ft² of membrane area, Vm = 7,736 ft³ of module volume, and a required scouring air flow of 4,642 cfm. In metric, the equivalent 7,571 m³/d basis gives 26,288 m², 219 m³, and 131 m³/min.
Can an MBR be designed for total nitrogen removal without a separate denitrification filter?
Yes. A pre-anoxic denitrification MBR places an anoxic basin upstream of the aerobic basin and recycles nitrate-rich mixed liquor from the membrane tank back to the anoxic basin at 3–5× permeate flow. Denitrification in the anoxic zone recovers roughly 50% of the alkalinity consumed by nitrification, reducing caustic dose on the alkalinity feed line.
What pore size and screen cut-off should be specified on an MBR P&ID?
Specify 0.1 µm membrane pore size (PVDF flat-sheet or hollow-fiber) and a 2–3 mm fine screen immediately upstream of the membrane tank. Both numbers are manufacturer requirements, not preferences, and both must appear on the P&ID equipment tags.
