Why Indigo Denim Wash Water Is a Special Case for DAF
Indigo is a vat dye: it is reduced to a soluble leuco form in a hot alkaline hydrosulfite (NaOH + Na₂S₂O₄) bath, adsorbed onto cotton, and then re-oxidized on the fiber back to an insoluble pigment. That means the dye that bleeds off the wash range is overwhelmingly suspended pigment fines in the 0.1–5 µm range plus dissolved surfactant micelles carrying loose color — not a true solution. A DAF sized from a generic industrial-wastewater template will underperform because those fine, negatively charged particles attach poorly to rising bubbles without conditioning, and the hot alkaline liquor depresses dissolved-air solubility.
Medium-scale denim mills generate 180–430 L of effluent per kg of fabric, and dye washing is consistently identified as the most water- and pollutant-intensive step in textile finishing (Yaseen and Scholz, 2019; Naqvi et al., 2020; Tanveer et al., 2021). The four interfering species the DAF has to manage are: anionic soaping surfactants (50–300 mg/L MBAS), sizing residues, indigo fines, and free alkalinity at pH 10–12. Together they suppress bubble–particle attachment and force a coagulation/flocculation stage ahead of flotation. The standard DAF operating levers still apply — separation time, chemical dose, saturation pressure, bubble size, and saturation time (Dabiri et al., 2018; Patel et al., 2020) — but the envelopes narrow for indigo, and the recycle ratio, polymer selection, and saturator pressure become the variables that decide whether the unit passes its performance test.
Characterizing the Feed: What to Sample Before You Size
Sizing a DAF on textbook defaults is the single most common reason denim-mill pretreatment trains under-deliver. Build the calculation on measured data instead. Pull flow-proportional 24-h composites from the wash range across at least three production campaigns covering dark, mid, and light shades; indigo shade changes shift both volume and color load by a factor of three or more.
The minimum analytical panel is flow (m³/h, with peak and diurnal average), pH, temperature, TSS, TDS, COD, color in ADMI units, methylene-blue active substances (MBAS) for surfactant, sulfide, and laser-diffraction particle size distribution. The expected envelope for an indigo wash range is summarized below; if your samples fall well outside these ranges, revisit the upstream water-reduction plan before you finalize equipment sizing.
| Parameter | Typical range (indigo wash) | Implication for DAF sizing |
|---|---|---|
| Flow | 180–430 L/kg fabric | Drives hydraulic loading; convert kg/day to m³/h first |
| pH | 10–12 | Requires neutralization or coagulant that performs at high pH |
| Temperature | 50–80 °C | Above 60 °C, air solubility drops; recycle ratio must rise 20–30% |
| TSS | 200–1,500 mg/L | Sets air-to-solids demand and saturator recycle |
| COD | 800–3,000 mg/L | DAF is a pretreatment; expect only 30–50% COD removal |
| Color | 2,000–10,000 ADMI | Color tracks suspended indigo; 50–70% removal is realistic |
| Surfactant (MBAS) | 50–300 mg/L | Anionic micelles compete for coagulant; dose on the high end |
| Particle size | 0.1–5 µm indigo fines | Fine fraction floats poorly without polymer aid |
Two feed-side constraints drive equipment choices more than any spreadsheet number. First, the wash range discharges hot: typical liquor leaves at 60–70 °C, and at 65 °C dissolved-air solubility is roughly 35% lower than at 25 °C, so the saturator must work harder. Second, color and TSS correlate strongly with shade — a mill running 80% dark indigo will load the DAF roughly twice as hard per cubic meter as the same mill running light shades. Capture that ratio in your sampling or your design will be seasonally wrong.
DAF Sizing Procedure: The 7 Calculation Steps
Run these seven steps in a single spreadsheet. The output is a defensible surface area, a saturator recycle flow, and a flocculation contact time that you can put into an RFQ without vendor hand-waving.
- Convert fabric throughput to design flow. Take daily fabric in kg/day, multiply by the specific water consumption in L/kg, and convert to m³/h. Apply a peaking factor of 1.3–1.5× to capture shade-change surges and wash-box dumps.
- Select hydraulic loading rate (HLR). For indigo, use 5–10 m/h; generic industrial DAF often runs 10–20 m/h, but fine indigo particles and high surfactant loading force a more conservative envelope. Pick 7–8 m/h unless you have jar-test data supporting the upper end.
- Compute flotation surface area. A = Q / HLR, with Q in m³/h and A in m². Round up to the next standard cell size, and prefer two cells in parallel for redundancy on a single-stream wash range.
- Set air-to-solids ratio and saturator pressure. A/S = 0.03–0.06 kg air per kg TSS, with 0.04 as the design center. Saturator pressure 5–6 bar (gauge) gives dissolved-air concentrations of roughly 60–75 mg/L at 25 °C, falling to 40–55 mg/L at 65 °C.
- Compute saturator recycle flow. Recycle % = (A/S × TSS) / (C_air − C_out), where C_air is dissolved air at saturation and C_out is the residual dissolved air in the clarified stream. For indigo this typically lands at 25–35% of design flow.
- Size the saturator. Retention 45–90 s at 5–6 bar; specify nozzles or porous diffusers that produce 5–10 µm bubbles, the size window where attachment efficiency on 1–5 µm particles peaks.
- Size the flocculation stage and skim rate. Flocculation HRT 10–20 min at velocity gradient G = 50–80 s⁻¹ ahead of the DAF; skim velocity 0.5–1.0 m/min across the surface to move float to the beach.
Two guardrails belong on the spreadsheet next to the formulas. First, never let recycle exceed 40% of design flow — above that, recycle short-circuits the contact zone and washes floc out of the cell. Second, if the saturator pressure is held at 6 bar but the influent is hotter than 60 °C, the operator must either drop pressure (which loses A/S) or cool the feed to 40–50 °C; cooling is almost always the cheaper option at the scale of a wash range.
Worked Example: 50,000 kg/day Denim Mill
Inputs: 50,000 kg fabric/day, specific water consumption 250 L/kg, peaking factor 1.5×. Average flow = 50,000 × 0.250 = 12,500 m³/day ≈ 520 m³/h; design flow = 520 × 1.5 ≈ 780 m³/h. Influent TSS 800 mg/L, temperature 65 °C, pH 11.
Sizing: HLR 8 m/h → required flotation area = 780 / 8 = 97.5 m² → round to two 50 m² cells in parallel (100 m² total) for redundancy. Saturator at 6 bar, A/S = 0.04 → recycle ≈ 25–30% of design flow, i.e. roughly 200 m³/h pumped through the saturation tank. Conditioning: 150 mg/L polyaluminum chloride (PAC) plus 2 mg/L anionic polyacrylamide, with 15 min flocculation HRT.
Expected performance at these design numbers, for an indigo feed of this envelope:
| Parameter | Influent | DAF effluent | Removal |
|---|---|---|---|
| TSS (mg/L) | 800 | 120–240 | 70–85% |
| Color (ADMI) | 2,000–10,000 | 600–5,000 | 50–70% |
| COD (mg/L) | 800–3,000 | 400–2,100 | 30–50% |
| Surfactant (MBAS, mg/L) | 50–300 | 20–120 | 40–60% |
Treat these numbers as a pretreatment envelope, not a polish. A DAF on indigo wash water reliably removes suspended fines, color bound to those fines, and a chunk of the surfactant; a downstream biological or membrane step handles the dissolved color and COD that the DAF cannot touch. On the equipment side, the unit at the heart of this train is the ZSQ series dissolved air flotation system, and the coagulant/polymer feed should be a PLC-controlled coagulant and polymer dosing skid sized for the 150 mg/L PAC and 2 mg/L polyacrylamide doses above with a 2× turndown envelope for shade changes.
Chemistry and Process Configuration for Indigo
Charge chemistry drives the result. At pH 10–12, indigo fines and anionic surfactant micelles both carry a net negative surface charge; a cationic coagulant (PAC at 100–300 mg/L, or alum at 150–400 mg/L) neutralizes that charge and bridges the fines into microflocs. Follow with a low-dose anionic or non-ionic polyacrylamide at 0.5–3 mg/L for floc strength. Avoid cationic polyacrylamides on this feed — at the doses needed to bridge indigo, they restabilize the colloid and turn the clarifier grey rather than clearing it.
Position the DAF in the train after pH/temperature adjustment and before biological or RO polishing. The integrated configuration of pH neutralization → DAF → biological → RO is the established default for indigo-bearing textile streams (Mofrad et al., 2020; Jadhav and Jadhav, 2021; Verma et al., 2021), and the DAF's job is to protect the downstream biology and membranes from indigo-fine fouling. On the back end, the float is typically 2–4% dry solids and dewateres to 18–25% DS on a plate-and-frame filter press or a screw press; budget that into the sludge-handling line so the DAF does not become the bottleneck. For comparison with two adjacent duty cases, see the worked examples for DAF sizing for slaughterhouse blood water and DAF sizing for brewery spent-yeast water; the chemistry differs, but the surface-area and recycle logic transfer directly.
Commissioning, Common Pitfalls, and Performance Verification
Three failure modes sink most denim DAF installations. Address them in the commissioning plan and the unit will pass its performance test on the first 72-h run.
| Pitfall | Symptom | Fix |
|---|---|---|
| Operating DAF at 65–70 °C | Air comes out of solution in the cell, floc blanket boils, A/S collapses | Cool influent to 40–50 °C with a plate heat exchanger, or accept a 20–30% recycle penalty and oversize the saturator |
| Skipping or under-dosing flocculation | Grey haze in the effluent, no float mat, color removal < 30% | Tune G to 50–80 s⁻¹, hold 10–20 min HRT, and verify polymer dose with a jar test on the actual hot feed |
| Recycle above 40% | Floc washes out of the contact zone, surface boils, solids re-enter the effluent | Cap recycle at 25–35%; if A/S still falls short, raise saturator pressure from 5 to 6 bar before adding more recycle |
Verification protocol: run jar tests across a PAC range of 50–300 mg/L and a polyacrylamide range of 0.5–5 mg/L on the actual composite sample, then confirm on a bench DAF or modified jar at the design A/S. For plant acceptance, run a 72-h continuous trial with online ADMI color and TSS on the DAF inlet and outlet; document shade mix, temperature, and recycle flow over the trial so the performance numbers are traceable. The same monitoring approach used for DAF sizing for paper-mill white water transfers directly: stable online measurement is the only way to catch a coagulant under-dose or a saturator pressure drift before they show up in the effluent.
Frequently Asked Questions
What hydraulic loading rate should I use to size a DAF for indigo denim wash water?
Use 5–10 m/h for indigo, with 7–8 m/h as the typical design center. That is roughly half the 10–20 m/h envelope used for generic oily-water DAF, because 0.1–5 µm indigo fines and anionic surfactant micelles attach to rising bubbles more slowly than the larger oil droplets those generic curves were built around. Below 5 m/h the cell becomes uneconomically large; above 10 m/h the effluent turns grey and color removal collapses.
Which coagulant and polymer doses work for a DAF on indigo effluent?
Dose 100–300 mg/L of a cationic coagulant — polyaluminum chloride at 150 mg/L is a common center point, with alum at 150–400 mg/L as the alternative — followed by 0.5–3 mg/L of anionic or non-ionic polyacrylamide. Cationic polyacrylamides over-dose on this feed and restabilize the colloid, so avoid them. Always confirm the dose with a jar test on a composite from the actual wash range, because shade mix alone can shift the optimum by a factor of two.
What removal efficiency can a DAF realistically hit on indigo wash water?
A properly sized and conditioned DAF on indigo wash water delivers 70–85% TSS removal, 50–70% color (ADMI) removal, 30–50% COD removal, and 40–60% surfactant (MBAS) removal on a feed of 800 mg/L TSS, 2,000–10,000 ADMI color, and pH 10–12. Treat it as a pretreatment step: the DAF protects downstream biological and RO units, it does not replace them. The ZSQ series dissolved air flotation system is one equipment platform commonly specified into this duty envelope.
