Why indigo denim wash water is uniquely hard on an MBR
Indigo denim finishing consumes roughly 80–150 L of process water per kilogram of fabric — the 150 L/kg figure for textile manufacturing is documented in a 2025 Bangladesh field study covering 14 mills (PMC, 2025-04). That same stream carries three fouling vectors that hit a submerged PVDF MBR simultaneously: insoluble indigo particles in the 1–50 µm colloidal range, settleable pumice fines from stone-wash steps, and non-ionic/anionic surfactants that foam under aeration. Raw denim wash water typically runs at COD 800–2,500 mg/L, TSS 200–1,500 mg/L, pH 9–11, and a BOD:COD ratio below 0.2 because indigo and its auxiliaries are largely non-biodegradable (Zhongsheng field data, 2026).
The reason this matters for membranes, not just for general biology, is that each vector attacks a different failure mode. Indigo deposits foul PVDF surfaces as a thin blue film that compresses under permeate suction. Pumice grit scours the membrane skin during aeration scour cycles, accelerating irreversible damage. Surfactants lower surface tension, drive foaming in the aeration tank, and lift solids into the MBR cassette. Sending this raw stream to a submerged PVDF MBR system without pretreatment collapses TMP within days, not weeks. Pretreatment, in this article, is membrane protection — sized against the fouling envelope the MBR cannot survive.
The six-stage pretreatment train for denim wash water
The treatment train below is the minimum sequence a denim mill can hand to a draftsman and drop straight onto a P&ID. Each stage is tied to one fouling vector and has a numeric target the operator can trend.
- Coarse screening. A GX rotary bar screen with 3–6 mm openings captures lint, thread, and fabric scraps at the headworks. A self-cleaning brush discharge keeps the bar surface clear at peak flow; overflow protection is set at the upstream junction box.
- Grit / vortex removal. A vortex or detritor chamber sized for peak flow targets 95% removal of particles above 200 µm — the pumice and sand fraction that destroys pump impellers and aerator bearings. Detention here is short (1–3 min) but the cutoff is sharp.
- Equalization. An 8–24 h HRT basin with diffused aeration dampens the diurnal swing. Design target: pH 9–11 collapsed to less than ±0.5 pH units, COD variation dampened to less than ±20% before downstream biology sees it.
- pH correction. A PLC-controlled chemical dosing skid injects CO₂ or sulfuric acid to land pH at 6.5–8.0. For denim streams, sulfuric acid is common because it also suppresses carbonate scaling downstream.
- Coagulation / flocculation + DAF. Polyaluminum chloride (PAC) dosed at 50–200 mg/L plus 1–5 mg/L anionic polymer is flash-mixed then sent to a ZSQ DAF system with a 20–40% recycle. This stage targets the colloidal indigo — typically 40–60% COD and 90%+ TSS reduction.
- Fine filtration / media polish. A multi-media polish filter with sand, anthracite, and garnet brings the effluent to under 50 µm — the last guard before the MBR cassette.
| Stage | Unit operation | Target parameter | Vector removed |
|---|---|---|---|
| 1 | Rotary bar screen 3–6 mm | <5% headloss at peak flow | Lint, thread, fabric scraps |
| 2 | Vortex / detritor | 95% removal >200 µm | Pumice, sand |
| 3 | Equalization basin | 8–24 h HRT, ±0.5 pH | Flow and load shock |
| 4 | Acid dosing skid | pH 6.5–8.0 | Alkalinity, scale precursors |
| 5 | PAC + DAF | 40–60% COD, >90% TSS | Colloidal indigo, surfactants |
| 6 | Multi-media polish | Particle size <50 µm | Floc carryover |
Target water quality entering the MBR
Every numeric inlet below is the threshold the MBR must see to hit the published performance envelope of 91% COD, 99.4% TSS, and 80% color removal at HRT 1.3 days reported for a textile MBBR-MBR system (UPC, 2021). Violate any of them and the membrane pays for it.
| Parameter | Target to MBR | Consequence if exceeded |
|---|---|---|
| TSS | <100 mg/L | TMP rise 3–5× faster, weekly CIP |
| COD | 300–800 mg/L | F/M upset, foam events |
| BOD | 100–300 mg/L | Starved or shocked biomass |
| pH | 6.5–8.0 | Nitrification collapse, scale |
| Oil/grease | <25 mg/L | Irreversible pore plugging |
| Temperature | <40 °C | Membrane integrity loss |
| Particle size | <50 µm | Surface deposition, blind spots |
The hard rule from operating data: TSS and oil are the two parameters that most reliably break an MBR's CIP interval. Holding them in range is what lets a denim mill stretch clean-in-place from weekly to monthly (Zhongsheng field data, 2026). The submerged PVDF MBR system is sized against these inlets, not against raw denim wash water.
MBR operating envelope after proper pretreatment
With the inlet targets above met, the MBR can be operated inside a stable window. The envelope below is what an automation engineer programs into the SCADA trending screens.
| Parameter | Operating window | Note |
|---|---|---|
| Membrane pore size | 0.1 µm (PVDF) | Submerged flat-sheet or hollow-fiber |
| MLSS | 8,000–12,000 mg/L | Higher end tolerated with flat-sheet |
| F/M | 0.05–0.15 kg BOD/kg MLSS·d | Indigo stream runs low |
| HRT | 18–32 h | 1.3 d reference at 91% COD (UPC, 2021) |
| SRT | 30–60 d | Stabilizes nitrification |
| Aeration scour | 0.3–0.6 m³ air/m²·h, 10 s on / 30 s off | Keeps DO >2 mg/L |
| TMP ceiling | 25–35 kPa | Trigger CIP at ceiling |
| CIP interval | 30–90 days | NaOCl + citric acid sequence |
Flat-sheet versus hollow-fiber is a real choice on a denim stream. Flat-sheet modules tolerate higher TSS excursions and are mechanically easier to wipe down during CIP — useful when the upstream equalization is not perfectly damped. Hollow-fiber packs more area per cassette but fouls faster on particulate upsets. For a 500 m³/d line with denim's load variability, a DF series flat-sheet MBR module is the conservative pick (Zhongsheng field data, 2026). For a side-by-side view of how this differs from a print-paste wash stream, the MBR configuration for print paste wash guide is a useful comparator.
Sludge and chemical handling on the pretreatment side
Solids are the side of the train engineers under-design until startup. Screenings and grit from stages 1 and 2 land in a dumpster at roughly 0.3–0.8 kg per 1,000 L of wash water — landfilling or incineration is typical (Zhongsheng field data, 2026).
DAF float from stage 5 and any chemical sludge from pH correction come off the train at 2–5% dry solids. They are routed to a plate-and-frame filter press for dewatering to a cake above 22% DS; the pressate returns to the headworks. The plate-and-frame filter press is sized on peak DAF float, not average, or it will bottleneck on Monday-morning sludge.
Spent CIP chemicals from the MBR — alkaline NaOCl spent and acidified citric acid rinse — must be pH-neutralized and held out of the MBR. Sending them back to the bioreactor is a guaranteed biomass toxicity shock. PAC is delivered as a 10% solution; polymer is delivered as an emulsion. Both need a day tank and the PLC-controlled chemical dosing skid needs calibration at least quarterly, with pump curves verified against the running flow.
2026 specification snapshot for a 500 m³/day denim-wash MBR train
One concrete build-up an engineering manager can benchmark vendor quotes against, sized for a 500 m³/day post-stone-wash and post-desize combined stream:
- Pre-equalization tank: 200 m³ (≈10 h HRT at average, 4 h at peak)
- Rotary screen: 3 mm aperture, 50 m³/h peak, with brush discharge
- DAF unit: 75 m³/h hydraulic, 25% recycle, 40–60% COD removal design point
- Chemical dosing skid: 3 pumps — PAC, polymer, sulfuric acid — on shared PLC
- Multi-media polish: 50 m³/h, backwash on DP, effluent to <50 µm
- Submerged MBR cassette: ~600 m² PVDF membrane area, flat-sheet, with 0.3–0.6 m³ air/m²·h scour blower
Estimated electrical load is 60–90 kW, dominated by the DAF recycle pump and MBR permeate/air scour. Quoted CAPEX band sits at 280,000–450,000 USD equipment-only in 2026, depending on automation level and material of construction (SS304 vs SS316 in the DAF and dosing skid) (Zhongsheng field data, 2026). For DAF-specific sizing, the DAF configuration for indigo denim wash water guide walks through recycle ratios and contact-zone geometry. For membrane-side engineering, the MBR working principle and engineering specs reference covers aeration, SRT, and TMP in more detail.
Frequently Asked Questions
What pretreatment does indigo denim wash water need before MBR?
Six stages in series: coarse screening (3–6 mm), grit/vortex removal, equalization (8–24 h), pH correction to 6.5–8.0, coagulation/DAF (PAC 50–200 mg/L, polymer 1–5 mg/L), and multi-media polish to under 50 µm. This sequence is required to bring TSS below 100 mg/L and oil/grease below 25 mg/L at the MBR inlet (Zhongsheng field data, 2026).
What is the typical BOD:COD ratio for denim wash water, and why does it matter for an MBR?
Denim wash water typically runs at a BOD:COD ratio below 0.2 because indigo and most dye auxiliaries are recalcitrant. A low ratio means the MBR's F/M is dominated by slowly degradable COD; MLSS must be held at 8,000–12,000 mg/L with SRT 30–60 d to keep biomass active and avoid foaming (UPC, 2021).
How often will the MBR need chemical cleaning on a denim stream?
With the six-stage train above, CIP runs every 30–90 days at a TMP ceiling of 25–35 kPa, using a NaOCl soak (typically 500–1,000 mg/L free chlorine) followed by a citric acid rinse. If pretreatment is bypassed or under-sized, CIP collapses to weekly, which is the standard failure signature for this stream (Zhongsheng field data, 2026).
