Textile dyeing and finishing generate strongly colored, high‑salinity wastewater at staggering volumes — and regulators are tightening the screws. A multistage, centralized plant that couples primary chemistry with robust biology and membrane polishing is emerging as the compliance workhorse.
Textile wet processing is a water‑intensive, high‑pollution enterprise. In India alone, dyehouses are tied to ~425×10^6 gallons/day of water use (www.sciencedirect.com), while a single pair of jeans can take ~500 gallons (~1,900 L) to make (www.sciencedirect.com). Globally, these processes account for roughly 17–20% of industrial effluent by volume (www.sciencedirect.com).
The wastewater is strongly colored, high in pH and salinity, and loaded with BOD (biochemical oxygen demand), COD (chemical oxygen demand), suspended solids, salts and recalcitrant organics such as heavy metals and unbound dyes (www.sciencedirect.com) (webapps.ilo.org). Untreated streams can run COD well above 1,000 mg/L and color beyond regulatory limits. In parts of Indonesia, uncontrolled discharge has caused “severe water pollution,” with the textile sector among the world’s highest for dye‑related burdens (webapps.ilo.org).
Standards are unforgiving: typical river discharge targets are <50 mg/L BOD, <100 mg/L COD, and “no visible color,” and in Indonesia national rules (“Permen LHK”) and international guidelines (IFC EHS) effectively mandate ≥90% removal of organics and color before discharge. The only practical answer is a multi‑barrier design with quantified performance and reuse options.
Influent profile and standards
A robust plant design starts from the effluent reality above and builds to hit those BOD, COD, and color limits. The concept: primary conditioning to strip solids and chromophores (color‑forming molecules), a biological core to mineralize organics, and advanced polishing to close the compliance gap.
Headworks and equalization
Front‑end screening protects downstream equipment; mills commonly adopt continuous debris removal at the headworks with an automatic screen. Equalization then evens out the batchy nature of dyehouses: an EQ tank sized for ~4–6 hours of flow, with aeration or mechanical mixing, stabilizes pH/flow swings, prevents anaerobic zones, and even recovers heat to feed a uniform load to the next step (www.informedchoicematrix.net).
pH is adjusted to ~6.5–8.5 before biology, typically via controlled chemical addition using a dedicated dosing pump and basic mixing ancillaries that many plants group under wastewater ancillaries.
Coagulation, flocculation, clarification
The color problem is tackled early via coagulation/flocculation. Common coagulants include ferric chloride, alum (Al₂(SO₄)₃), or polyaluminum chloride (PAC; often abbreviated “PAC” in coagulation literature), typically dosed at 100–1,000 mg/L. Jar tests show the upside: 6,000 mg/L alum at pH~2.4 achieved ~90% color removal for a reactive dye, while PAC at 800 mg/L and pH~4 achieved near 100% dye removal (www.researchgate.net). In practice, PAC often outperforms alum by lowering the required dose.
Rapid mix and flocculation contact times are typically ~15–30 minutes and ~20–40 minutes, respectively, with optional polymer aids sourced as flocculants to strengthen aggregates. Downstream gravity units such as a clarifier or dissolved‑air flotation can remove >80–95% of suspended solids and a large fraction of color. In routine operation, TSS removals in this stage run 80–90%, while COD drops ~30–50% mainly by pulling out entrained solids.
Where oils or fine solids persist, plants add a DAF as a compact clarification step. Coagulation produces metal hydroxide sludge that must be thickened and dewatered; a typical yield is ~10–20 m³ of sludge per 1,000 m³ treated per day.
Note: the acronym “PAC” also refers to powdered activated carbon in adsorption stages later; this article distinguishes polyaluminum chloride (coagulant PAC) from powdered activated carbon (adsorbent PAC) to avoid confusion.
Biological treatment configurations
Secondary treatment mineralizes dissolved organics. Conventional activated sludge or biofilm systems are tuned for ~85–95% BOD removal, with typical effluent BOD <20–30 mg/L when the food‑to‑microorganism ratio (F/M) and sludge age (solids retention time; SRT) are held in range. A moving bed bioreactor (MBBR) or a membrane bioreactor (MBR) can deliver similar endpoints with different footprints and solids management strategies.
In one pilot on real textile effluent, an aerobic MBR achieved ~91% COD removal — down to ~170 mg/L COD — at 1.3 day HRT (hydraulic retention time), and removed >90% total nitrogen and phosphorus (www.mdpi.com). Plants often choose an integrated MBR to capture solids tightly; aerobic systems alone, however, tend to struggle with residual color.
Anaerobic staging and energy recovery
To boost biodegradability and harvest energy, mills add anaerobic steps — UASB/EGSB reactors (upflow anaerobic sludge blanket/expanded granular sludge bed) — ahead of, or coupled with, aerobic polishing. Anaerobic digestion can remove ~60–80% of COD and yields biogas at ~0.3 m³ CH₄ per kg COD removed, with actual removal dependent on reactor design and co‑substrates (pmc.ncbi.nlm.nih.gov). A hybrid moving‑bed biofilm + MBR system treating raw dye effluent realized ~93% COD removal (pmc.ncbi.nlm.nih.gov), although color removal remains modest without further polishing.
Design flexibility matters: module sizing, volume allowance for shock loads, and the ability to switch aerobes/anaerobes. For peaking flows, scaling aeration basins and sludge recycle to MLSS (mixed liquor suspended solids) of 3–4 g/L and F/M ~0.1–0.2 helps sustain >90% BOD removal under variable loads.
Membrane filtration polishing
Advanced membranes are the closer for strict discharge and reuse. Ultrafiltration (UF) — including submerged modules — strips remaining suspended and colloidal matter; many plants integrate UF as an MBR barrier. Nanofiltration (NF) can then remove ~76–90% of COD and >90% of color, with salt rejection of 65–90% (higher for divalent ions), often producing permeate suitable for dye bath reuse (www.scielo.org.za) (www.scielo.org.za). Reverse osmosis (RO) can push COD removal up to ~99% with near‑complete decolorization (www.scielo.org.za).
For secondary effluent polishing, mills pair NF with a modular brackish‑water RO train to meet reuse or very tight discharge limits. Suppliers increasingly package UF/NF/RO as integrated membrane systems to simplify control and footprint. Membrane trains concentrate salts and inorganics, so a brine handling or reclamation plan is required.
Activated carbon adsorption
Activated carbon is the backstop for chromophores and residual organics. Powdered activated carbon (also abbreviated “PAC” in adsorption practice) is typically dosed at 100–300 mg/L in contact tanks, and one EPA study found 100 mg/L PAC only yielded ~20–30% extra COD removal (nepis.epa.gov). As a result, PAC is commonly used as a polishing adjunct after membranes or oxidation, delivered via a dedicated activated carbon system.
Granular activated carbon (GAC) filters can treat at ~5–20 m/h but require frequent replacement or regeneration when loaded with dyes. A practical design rule is ~0.5–2 kg PAC per kg of COD targeted for adsorption, with breakthrough monitoring to schedule changeouts.
Performance envelope and targets
Across the train, combined performance targets are robust: ~95% TSS removal, ~90–95% COD/BOD removal, and >90% color removal. In aerobic service, a representative range is COD 80–90%, BOD 90–95%, TSS ~85–95%, and color 30–70%. An MBR can concentrate biomass and outperform conventional activated sludge: one study reported 99.6% TSS removal vs 66% for conventional sludge, with ~67% color removal vs ~32% in CAS (www.mdpi.com).
For anaerobic UASB/EGSB systems, COD removal typically runs ~60–80%, with biogas yields of ~0.25–0.35 m³ CH₄ per kg COD removed, though color removal is modest (pmc.ncbi.nlm.nih.gov). A hybrid MBBR–MBR plant reported >90% COD removal and 85–99% color removal with remaining COD ≈30 mg/L, enabling on‑site reuse or strict discharge (pmc.ncbi.nlm.nih.gov).
Operational data points
Pilot data underscore the gap between configurations. On ~940 mg/L COD influent, aerobic CAS (HRT 2 days) versus MBR (HRT 1.3 days) produced MBR effluent COD ~170 mg/L (91% removal) versus CAS ~300 mg/L [40†L119-L127]; TSS in CMBR effluent was ~20 mg/L versus 316 mg/L from CAS (www.mdpi.com). A combined MBBR–MBR treating >150 m³/d delivered final effluent BOD ~10–20 mg/L and color <50 true‑color units (pmc.ncbi.nlm.nih.gov).
Design parameters and flexibility
Key sizing anchors include: EQ volume for ~4–6 hours; rapid mix ~15–30 minutes and flocculation ~20–40 minutes; biological HRT of ~1–3 days with SRT (sludge age) of ~15–30 days or equivalent biofilm fill. Plants routinely tune MLSS to 3–4 g/L and F/M ~0.1–0.2 to ride out shock loads while maintaining >90% BOD removal. Where compactness is critical, dissolved air flotation can complement gravity units, and membrane barriers can be integrated as part of the biological stage.
Compliance, reuse, and options
Putting it all together, a centralized textile ETP (effluent treatment plant) built along these lines can meet BOD <50 mg/L, COD <100–200 mg/L, TSS <50 mg/L, ammoniacal‑N <5 mg/L, and “no visible color” criteria common to Indonesian and global standards. In referenced designs, an MBR stage alone delivered ≈91% COD removal (e.g., from ~1,900 to ~170 mg/L; www.mdpi.com), while an MBBR–MBR hybrid hit ≈93% COD removal with >99% TSS removal (pmc.ncbi.nlm.nih.gov). Primary coagulation can remove ≈60–90% of color, and biological plus membrane polishing can reach >85–99% color removal (www.mdpi.com) (pmc.ncbi.nlm.nih.gov).
For regions facing water stress, NF/RO‑based reuse or even ZLD (zero liquid discharge) schemes — evaporating RO brine — are increasingly considered. Where reuse is the goal, many plants deploy an NF step ahead of RO using a modular nano‑filtration rack, then close with RO; others package the entire tertiary barrier using integrated membrane systems. Oxidation steps (ozone or UV/H₂O₂) are also options to degrade residual dyes, albeit with higher energy and chemical consumption. Cost analyses for similar projects have shown positive net benefits — IRR ≈18% — driven by reduced effluent fees and onsite water reuse (pmc.ncbi.nlm.nih.gov).
