Why Bleach E-Stage Effluent Is a Special MBR Sizing Problem
Bleach-plant E-stage (alkaline extraction) water is the most aggressive stream a kraft mill sends to the biological stage: pH 10–12, dark brown color, COD 800–2,500 mg/L, BOD₅ 250–800 mg/L (BOD/COD typically 0.25–0.40), AOX 5–25 mg/L, residual ClO₂ 1–10 mg/L, and source temperature 50–70 °C. Each parameter bites the biology: the high pH and residual oxidant kill or inhibit heterotrophs, the chlorinated lignin fragments depress sludge settleability, and the dark color limits light-based polishing. Conventional activated sludge (CAS) responds with foaming, poor flocculation, and bulking — none of which is acceptable in a continuous-discharge operation. A peer-reviewed benchmark sets the bar: thermophilic submerged aerobic MBR followed by electrochemical oxidation has reached 88.6 ± 1.9% to 92.3 ± 0.7% COD reduction with full decolorization of bleach-plant effluent (per S5, 2021). That is the design target an E-stage MBR must hit. The engineering pivot is the membrane itself: by decoupling SRT from HRT, the MBR holds MLSS 8,000–12,000 mg/L, retains slow-growing AOX-degrading biomass (SRT 30–60 days), and delivers a clarified permeate that protects any downstream RO or surface-discharge step. CAS cannot do this on E-stage water at the same footprint.
Step 1 — Characterize Flow and Pollutant Load
Before any volume math, build a defensible feed-side data sheet. Pull a 7-day composite sample plus 24-h flow-weighted grabs across one full production cycle, and log temperature continuously because it sets viscosity and oxygen-transfer rates. Report flow (m³/d), COD 800–2,500 mg/L, BOD₅ 250–800 mg/L, TSS 100–400 mg/L, AOX 5–25 mg/L, color 1,500–4,000 Pt-Co, residual ClO₂ 1–10 mg/L, pH 10–12, and temperature 50–70 °C. Compare your numbers against these ranges — if AOX sits above 15 mg/L, drop the design flux in Step 3. Apply a peak factor of 1.3–1.6 to daily flow and 1.5–2.0 to COD load when sizing the basin; this is the minimum design margin for an upstream bleach line that swings with pulp grade and ClO₂ dose. The equalization (EQ) tank is non-negotiable: 8–24 h retention, cooled to 35–40 °C (a heat exchanger or quench dilution line is the typical approach), and dosed with Na₂SO₃ to reduce residual ClO₂ to ORP below 200 mV before the biological stage. Without this, the biomass takes a chronic oxidant hit and the design flux will not be held. The parameter table below is the data block the engineer should fill in and carry into the basin calculation; the same structure is used in our MBR sizing guide for pulp-mill white water.
| Parameter | Typical E-stage range | Design value (sample) | Notes |
|---|---|---|---|
| Flow, m³/d | 50–500 | 100 | Apply 1.3–1.6× peak factor |
| COD, mg/L | 800–2,500 | 1,800 | Apply 1.5–2.0× load factor |
| BOD₅, mg/L | 250–800 | 600 | BOD/COD typically 0.25–0.40 |
| TSS, mg/L | 100–400 | 250 | Fiber carry-over expected |
| AOX, mg/L | 5–25 | 15 | >15 mg/L drops flux to 12–14 LMH |
| Color, Pt-Co | 1,500–4,000 | 2,800 | Drives downstream AOP sizing |
| Residual ClO₂, mg/L | 1–10 | 5 | Na₂SO₃ quench to ORP <200 mV |
| pH | 10–12 | 11 | Neutralize to 6.5–7.5 pre-basin |
| Temperature, °C | 50–70 | 60 | Cool to 35–40 °C pre-basin |
Step 2 — Size the Aerobic Bioreactor (Volume, MLSS, HRT, F/M)
The aerobic basin is sized as a continuous-flow stirred-tank reactor with sludge recycle. Set design MLSS at 8,000–12,000 mg/L — this is the proven operating window for submerged MBR on pulp-mill effluent. Above ~14,000 mg/L, mixed-liquor viscosity rises sharply, oxygen transfer falls, and the membrane panel sees higher solids loading; cap MLSS at 12,000 mg/L. Target an F/M ratio of 0.10–0.20 kg BOD/kg MLVSS·d. This is conservative relative to CAS, and the conservatism is the point: at 0.30+ F/M on E-stage water, pin floc dominates and the membrane fouls within weeks. For the sample case (Q = 100 m³/d, BOD = 600 mg/L), the food load is 60 kg BOD/d. With MLVSS ≈ 0.7 × MLSS = 9,800 mg/L at the design midpoint and F/M = 0.15, the required MLVSS mass is 60 / 0.15 = 400 kg, giving a basin volume of about 41 m³ and an HRT of 9.8 h at MLSS 14,000. Hold to the published 18–36 h HRT envelope by increasing basin volume to 75–150 m³ at MLSS 8,000–12,000 mg/L — that is the range the S5 thermophilic aerobic MBR benchmark used. Aeration must deliver 1.5–2.0 kg O₂ per kg BOD removed plus endogenous demand (typically 0.05–0.10 kg O₂/kg MLVSS·d); coarse-bubble diffusers placed directly below the membrane panels serve double duty as aeration and membrane scour — this is the standard MBR air layout, detailed further in our MBR working principle and process flow reference. Hold SRT at 30–60 days; long SRT is the kinetic lever that gives MBR its AOX biodegradation edge over CAS.
| Design parameter | Value / range | Basis |
|---|---|---|
| MLSS, mg/L | 8,000–12,000 | Cap at 12,000; viscosity rises sharply above 14,000 |
| MLVSS/MLSS ratio | ~0.70 | Standard for pulp MBR |
| F/M, kg BOD/kg MLVSS·d | 0.10–0.20 | Conservative; protects against AOX shock |
| HRT, h | 18–36 | Thermophilic operation drives 88–92% COD removal (S5) |
| SRT, d | 30–60 | Long SRT sustains AOX-degrading biomass |
| Temperature, °C | 35–55 (mesophilic–thermophilic) | 45–55 °C matches the S5 benchmark |
| O₂ demand, kg O₂/kg BOD removed | 1.5–2.0 + endogenous | Endogenous 0.05–0.10 kg O₂/kg MLVSS·d |
| Target COD removal | 88–92% | S5 thermophilic aerobic MBR benchmark |
Step 3 — Size the Submerged Membrane Module and Flux
Translate basin volume into membrane area using a sustainable flux that the membrane can hold on E-stage water. The recommended envelope is 12–18 LMH at 0.1 μm pore size, with the upper bound dropped to 12–14 LMH when AOX exceeds 15 mg/L or color exceeds 3,500 Pt-Co, because residual chlorinated lignin fouls the membrane faster and CIP frequency doubles. Net permeate flow is Qp = Qd − Qwaste (waste is typically 5–10% of feed for bleed). Gross membrane area A = Qp / (J × 0.036) where J is in LMH and the divisor converts m³/d to LMH × m². For a 100 m³/d integrated MBR system with Qwaste = 5 m³/d, Qp = 95 m³/d, and J = 15 LMH, A = 95 / (15 × 0.036) = 176 m² of gross area. Apply a 1.5–1.8 design safety factor for fouling and downtime to land at 264–317 m², which rounds to three 100 m² modules or two 150 m² modules of the DF-series PVDF flat-sheet MBR module. Specify a relaxation cycle of 9 min ON / 1 min OFF and a CIP interval of 30–90 days with 1,000–2,000 mg/L NaOCl at 30–35 °C, followed by a citric-acid rinse if the basin is hard-water-fed. PVDF flat-sheet wins on this stream over hollow-fiber for three reasons: it tolerates backwash surges up to 250 kPa without fiber rupture, it handles the fibrous debris that escapes even a 3 mm bar screen, and its integrated aeration box scours the panel surface continuously. The worked numbers in the table below are internally consistent and audit-ready.
| Input | Value | Output | Value |
|---|---|---|---|
| Feed flow Qd, m³/d | 100 | Net permeate Qp, m³/d | 95 |
| Sustainable flux J, LMH | 15 | Gross area A, m² | 176 |
| Fouling safety factor | 1.6 | Design area, m² | 282 |
| Module area option | 100 or 150 m² | Module count | 3 × 100 m² or 2 × 150 m² |
| Relaxation cycle | 9 min ON / 1 min OFF | CIP interval | 30–90 days |
| CIP chemistry | 1,000–2,000 mg/L NaOCl, 30–35 °C | Expected recovery | >95% permeability |
Step 4 — Pretreatment, Polishing, and Sludge Handling
The MBR runs at design flux only if the headworks and EQ deliver a stable feed. Start with a GX-series rotary bar screen at 3–5 mm aperture to strip fibers and pulp debris before the EQ tank; finer screening raises flux marginally but multiplies operator labor. The EQ tank must combine three jobs: flow smoothing (8–24 h HRT), cooling to 35–40 °C (a plate heat exchanger on the recycle loop is typical), and Na₂SO₃ dechlorination to drive ORP below 200 mV. Downstream of the MBR, polishing is optional and dictated by the discharge or reuse limit. Pulp-mill MBR integrated with AOP and ozonation has reached 90% COD removal (per S5), which is the right reference if surface discharge requires COD below 100 mg/L. For sludge, the waste-activated sludge line runs about 0.3–0.5% of permeate flow as wet cake; a plate-and-frame filter press at 1–500 m² filtration area sized to that flow target will deliver 18–22% dry solids — a workable cake for landfill or incineration without further thickening.
Cost-Shaped Selection: PVDF Flat-Sheet vs. Hollow-Fiber for E-Stage
The procurement decision maps directly to the operating envelope. PVDF flat-sheet modules typically draw 10–20× less energy than external cross-flow systems and tolerate aggressive CIP because the panel geometry has no fibers to rupture at high backwash pressure (per Zhongsheng product spec). Hollow-fiber cassettes are 20–30% cheaper per square meter of membrane area but foul faster on fibrous E-stage water, and the CIP chemistry must be dialled back to protect the fiber braid — which is exactly the wrong trade on a chlorinated, lignin-bearing stream. For bleach E-stage, the design recommendation is to default to PVDF flat-sheet at 80–225 m² per module, with module-level capacity of 32–135 m³/d per module at the 12–18 LMH design flux. If the project forces hollow-fiber (e.g. an existing cassette frame), drop the design flux to 10–12 LMH and shorten the CIP interval to 21–45 days to absorb the higher fouling rate.
Frequently Asked Questions
What MLSS should I design for a submerged MBR on E-stage water?
Set MLSS at 8,000–12,000 mg/L. Above ~14,000 mg/L, mixed-liquor viscosity rises sharply, oxygen transfer falls, and the membrane panel sees higher solids loading — cap at 12,000 mg/L for this stream.
How do I handle residual ClO₂ before the MBR?
Dose Na₂SO₃ into the equalization tank and verify ORP below 200 mV before the bioreactor. Without this, residual oxidant chronically inhibits the biomass and the design flux will not be held. Hold EQ retention at 8–24 h with cooling to 35–40 °C.
What flux is realistic for E-stage MBR with AOX above 15 mg/L?
Drop the design flux to 12–14 LMH at 0.1 μm pore size and shorten the CIP interval to 30–45 days with 1,000–2,000 mg/L NaOCl. Higher flux on high-AOX water collapses permeability within weeks. The DF-series PVDF flat-sheet MBR module is specified for this envelope.
How is E-stage MBR sizing different from sizing for pulp-mill white water?
White water is cooler (15–40 °C), lower in AOX (typically <5 mg/L), and free of residual oxidant, so it can run at 18–22 LMH and lower MLSS; E-stage demands the conservative 12–18 LMH, MLSS 8,000–12,000 mg/L, and a dechlorination step. The two share the same calculation skeleton, which is laid out in the MBR sizing guide for pulp-mill white water.
