Textile finishing effluent is loaded with recalcitrant dyes and surfactants. A data‑driven train combining coagulation–flocculation, biology, and either advanced oxidation or membranes is delivering reuse‑quality water — with case studies showing 90–99% removal ranges across key pollutants.
Textile finishing (dyeing/printing) effluents are highly polluted with soluble dyes, surfactants, and auxiliary chemicals. Typical figures are COD (chemical oxygen demand) = 300–800 mg/L, BOD (biochemical oxygen demand) = 100–400 mg/L, and intense color in the hundreds of Pt‑Co units (a color measurement), at pH 6–11 (iwaponline.com). By volume, such wastewater constitutes roughly 20% of global industrial effluent (iwaponline.com), and the literature is blunt: “color removal from textile wastewater by cheaper, environmentally friendly technologies is still a major challenge” (pubmed.ncbi.nlm.nih.gov).
To meet regulatory limits (for example, Indonesian baku mutu for BOD/COD and color), full‑scale plants consistently converge on a profile: pretreatment and equalization; chemical coagulation–flocculation to strip out most color and suspended solids; biological oxidation or a membrane bioreactor (MBR) for soluble organics; and an advanced polish — either an advanced oxidation process (AOP) or tight membranes — for the last dyes and toxic organics (iwaponline.com, pubmed.ncbi.nlm.nih.gov).
Membrane options — from UF (ultrafiltration) to NF (nanofiltration) and RO (reverse osmosis) — are increasingly configured as modular membrane systems for industrial and municipal water treatment, especially when water reuse is the target.
Influent profile and compliance targets
The load that matters in finishing wastewater blends soluble dyes, salts, surfactants, and auxiliaries. COD sits at 300–800 mg/L, BOD at 100–400 mg/L, color in the hundreds of Pt‑Co, and pH swings between 6–11 (iwaponline.com). With roughly 20% of global industrial effluent implicated (iwaponline.com), uncontrolled discharge brings ecological risk because dyes are stable and often toxic, and reviewers underline how persistent color is to remove at low cost (pubmed.ncbi.nlm.nih.gov).
Engineering responses typically start with bar screening and equalization to buffer flow and pH swings (not explicitly cited, but standard in design), then proceed to chemistry and membranes or AOPs to reach discharge or reuse.
Preconditioning and flow equalization
Front‑end physical treatment removes debris and evens out shock loads before chemistry. Continuous screening is common; in practice, plants deploy equipment akin to an automatic screen for debris removal >1 mm. Equalization is paired with pH adjustment and nutrient control as needed, and accurate chemical feed is supported by a metering device such as a dosing pump.
Chemical coagulation–flocculation design
Coagulation–flocculation is the tried‑and‑true front‑end step to remove colloidal dyes and TSS (total suspended solids). Aluminum salts (alum, polyaluminum chloride) or iron salts are dosed, often with polymeric aids, and pH is nudged toward neutral or slightly acidic to optimize metal hydroxide formation. In bench “jar tests,” dyes are precipitated via charge neutralization and sweep floc by Al(OH)₃ or Fe(OH)₃ aggregates (link.springer.com). Typical coagulant doses land in the tens to a few hundred mg/L.
Performance is robust when optimized. Color removal can reach 90–95% with alum; for example, Merzouk et al. achieved ~94% Reactive Red dye removal with just 40 mg/L alum (link.springer.com). Even low‑cost water‑treatment sludge, rich in Al/Fe hydroxides, hit ~55% color removal and ~35% COD reduction on a real textile effluent in batch tests (link.springer.com). Color removal generally improves at acidic pH (~3–5) when Al/Fe species precipitate effectively (link.springer.com, link.springer.com).
Suspended solids removal typically exceeds 80–90% of turbidity/TSS, although COD removal is only partial at 10–50% because many organics remain soluble; in the water‑treatment‑residuals study above, coagulation yielded ~35% COD reduction (link.springer.com). Plants capture flocs in settlers or flotation. A clarifier such as a conventional clarifier handles the solids, while some facilities use dissolved‑air flotation.
Reagent choice spans alum to polyaluminum chloride; in many plants, operators specify a PAC such as a polyaluminum coagulant, then tune with polymeric aids from a catalog of flocculants. The step produces a metal‑hydroxide sludge with sizable volume (often 2–5% of wastewater by volume) that must be thickened/dewatered, and Al/Fe residuals must be managed with pH adjustment to avoid high Al, etc. Despite the handling burden, design targets commonly assume ≥90% TSS and ≥60–90% color removal via coagulants (link.springer.com, link.springer.com).
Advanced oxidation processes (AOPs) for recalcitrants
Even after coagulation and biological steps, recalcitrant dyes and organics can persist. Advanced oxidation — Fenton/photo‑Fenton, ozone, UV/H₂O₂, or photocatalysis — generates highly reactive radicals that break down complex molecules. Pilot studies show >90% degradation of residual dyes and COD; a photo‑Fenton system (Fe²⁺ + H₂O₂ + UV/sunlight) recorded ≈75–93% removal of complex dyes within 1–2 hours (iwaponline.com, iwaponline.com).
In one test, photo‑Fenton achieved 75.3% COD removal — driving final COD to ≈12 mg/L from ~49 mg/L — in 90 minutes using sunlight (iwaponline.com); another study found >93% decolorization of a dye mixture under photo‑Fenton, and combining AOP with a brief adsorption step produced ≈98% removal of color and organics in about 60 minutes (iwaponline.com, iwaponline.com).
Process fundamentals matter: Fenton/photo‑Fenton tends to run best around pH≈3; UV/H₂O₂ is effective on azo bonds but demands power; and catalytic variants (e.g., catalytic ozonation, electro‑Fenton) can improve rates. Typical operating figures include H₂O₂ in the tens–hundreds mg/L, reaction times of 30–90 minutes, and energy of 0.5–2.0 kWh/m³ for power‑intensive UV processes (iwaponline.com). Plants often pair UV equipment such as an ultraviolet system with oxidants, then follow with adsorption media like activated carbon to capture any intermediates that form.
Membrane filtration and reuse options
Membranes physically remove fine solids and many dissolved organics. UF (0.01–0.1 µm) reliably knocks down turbidity/TSS by >90%, but its color rejection is limited; in one pilot, UF delivered >90% turbidity reduction on a light‑shade effluent yet only ~7–35% color removal (www.scielo.org.za). Many plants deploy UF as a pretreatment, in the mold of an ultrafiltration module, to protect tighter membranes.
NF and RO push further. NF at ~8–15 bar can reject 80–95% of COD/color; a polyamide NF90 membrane cut feed COD from ~486 mg/L to ~24 mg/L (≈95% removal), with color rejection >90% across tests (www.scielo.org.za, www.scielo.org.za). A looser NF (SR90) produced acceptable reuse quality only on the light‑shade sample. RO, typically run at 15–25 bar, can reach up to 99% COD reduction, with permeate TDS/salinity dropping accordingly and enabling reuse (www.scielo.org.za).
Designers must wrestle with fouling: dye adsorption and pore blocking can drive flux down by >25–60% in a few hours (www.scielo.org.za), making coagulation pretreatment essential. Periodic chemical cleanings are required; in practice, teams standardize a regime supported by specialized membrane cleaners. Tight designs lean on nanofiltration to meet reuse standards or opt for a brackish‑water RO unit if a higher cut on COD and salts is needed.
Membrane bioreactors and biological oxidation
An MBR (membrane bioreactor) combines biological degradation with membrane separation, retaining biomass while it oxidizes soluble organics. Textile MBR studies report COD removal around 90–95% with virtually complete solids capture; a pilot logged 91% COD removal (effluent ≈170 mg/L) and 99.6% TSS removal (www.mdpi.com, www.mdpi.com). Color removal is moderate (~50–70%), so MBR effluent often still needs polishing. Key design factors include a long sludge age (months), low hydraulic retention time (~1–2 days), and UF membranes near 0.08 µm (www.mdpi.com).
Where space is tight or variability is high, packaged membrane bioreactors are used; in other cases, conventional activated sludge or a moving bed bioreactor (MBBR) sits upstream of membrane or AOP polishing.
Integrated train and expected performance
A comprehensive treatment train for finishing effluent stacks four steps: (1) preconditioning — bar screens/grit and an equalization basin to buffer flow and pH swings (not explicitly cited, but standard in design); (2) coagulation/flocculation — rapid mix, flocculation basins, and a clarifier or dissolved‑air flotation using alum or FeCl₃, designed for ~90% TSS capture and substantial color removal (link.springer.com, link.springer.com); (3) a biological reactor — an aerobic MBR or activated sludge/MBBR to degrade soluble organics; for instance, a 24–48 h SRT MBR can reach ~90% COD removal with near‑100% solids retention (www.mdpi.com, www.mdpi.com); and (4) polishing — either AOP (e.g., Fenton/ozonation) to oxidize remaining chromophores (iwaponline.com, iwaponline.com) or nanofiltration to finish off color/COD (www.scielo.org.za, www.scielo.org.za), with UV/hypochlorite disinfection sometimes used to break color.
Expected outcomes with this cascade include BOD <20–50 mg/L, COD <100 mg/L, TSS <50 mg/L, and color “non‑detect” (clear water). Case studies show feasibility: coagulation plus photo‑Fenton/decolorizing delivers >97% dye removal (iwaponline.com), and NF‑treated water can be directly recycled to dyeing (www.scielo.org.za, www.scielo.org.za). Modern systems in Indonesia and worldwide increasingly adopt MBRs and AOPs; one industrial MBR upgrade in Indonesia boosted capacity 2.5× while meeting direct‑discharge standards (including color) using coagulation + MBR + chlorine polish (supremewatertech.com).
For membrane‑centric polishing and potential ZLD (zero‑liquid‑discharge) via brine recovery, teams lean on modular membrane systems. Energy for high‑pressure operation is a line‑item — think 10–25 bar pumps — but the payoff is reuse‑quality permeate and, with RO, up to 99% COD reduction (www.scielo.org.za).
Design verification and optimization
Data‑backed design underpins performance: jar‑test curves size coagulant dose; pilot assays define biological kinetics and membrane flux; and lab tests (or RSM models) optimize AOP reagent levels (link.springer.com, iwaponline.com). High removal rates are achievable but incur cost and tradeoffs; this approach ensures compliance with Indonesian effluent rules while recovering as much water and energy as feasible.
Sources: Peer‑reviewed studies of textile effluent treatment (link.springer.com, link.springer.com, iwaponline.com, www.scielo.org.za, www.mdpi.com, iwaponline.com), regulatory guides, and industry reports have been synthesized to produce these design guidelines.
