Color‑rich wastewater from finishing and coating can push COD into the thousands and test regulatory limits. A staged design—coagulation/flocculation, advanced oxidation, and membranes—now sets the compliance and reuse playbook.
Textile finishing and coating throw off a potent mix: suspended solids, surfactants, waxes, stubborn dyes, and trace metals. Untreated, chemical oxygen demand (COD, a proxy for oxidizable organic load) can run in the hundreds‑to‑thousands mg/L, with intense coloration [mdpi.com]. Indonesia’s standards are tight—BOD ≤60 mg/L, COD ≤150 mg/L, and TSS ≤50 mg/L for textile mills (see BSR guideline excerpt, additional excerpt, additional excerpt). Non‑compliance under Law No. 32/2009 can trigger fines up to IDR 15 billion (~USD 950,000), permit loss, or criminal charges [has-environmental.com].
That enforcement backdrop—and corporate “zero discharge” goals—has pushed plants toward robust, multi‑stage treatment that marries physical separation, chemical clarification, and advanced polishing.
Effluent composition and treatability
Typical finishing/coating effluent (with batik wastewater as a close analog) contains suspended and dissolved solids, high BOD/COD, dyes, surfactants, waxes, and trace metals from pigments such as Cr, Cu, and Ni [mdpi.com]. A low BOD/COD ratio (≈0.1–0.2) is common, signaling slow biological degradation due to complex aromatics and resins [mdpi.com].
For planning, designers assume COD >1000 mg/L, TSS >200 mg/L, and residual color (no numerical color limit is specified in the cited regulation). Early headworks typically start with debris control; packaged wastewater physical separation systems handle screens and primary removal, and continuous debris control can be implemented via an automatic screen.
Coagulation/flocculation for color and solids
Purpose: Coagulation/flocculation (C/F) neutralizes colloidal charges so insoluble dyes and particulates aggregate and settle. Inorganics like aluminum sulfate (alum), ferric chloride/sulfate, polyaluminum chloride (PAC), and polyferric sulfate (PFS) are standard, with cationic polymer aids (e.g., polyDADMAC) as needed [mdpi.com]. Optimal dosing is typically at pH ≈5–7, with 100–500 mg/L common pending jar tests; natural coagulants like chitosan are under study but see modest industrial use.
In practice, C/F can remove a large fraction of solids and much of the color. Reported outcomes include COD removal up to ≈83% and color removal up to ≈82% on textile wastewater [mdpi.com]. More broadly, turbidity and TSS reductions of 80–95% are typical, while COD removal is often 30–60% because soluble organics remain [mdpi.com].
Operation notes: rapid mix then gentle flocculation, followed by settling and sludge handling. Dosing is nonlinear and overdosing can leave residual metals; jar tests are essential. Plants frequently meter aluminum‑ or iron‑based coagulants such as PAC with an accurate dosing pump, and add polymer flocculants to build robust flocs.
Outcome: after C/F, effluent turbidity can fall below 5 NTU with TSS <100 mg/L and dramatic color reduction (~70–90%) [mdpi.com]. Settling is typically handled in a clarifier prior to downstream processes.
Advanced oxidation for recalcitrant dyes
Advanced Oxidation Processes (AOPs) generate hydroxyl radicals (•OH) to break down chromophores and hard‑to‑oxidize organics. Fenton’s reagent (Fe²⁺ + H₂O₂ at pH ~3) is a workhorse; studies report roughly 50–80% COD and color removal depending on dose and contact time. One test on real dyehouse wastewater achieved ~70.6% COD and 72.9% color removal in 60 minutes using electrochemical Fenton; the chemical Fenton cost was ~$8.6 per kg COD removed, about half that of the electro‑Fenton variant [pmc.ncbi.nlm.nih.gov].
Photo‑Fenton or UV/H₂O₂ can accelerate oxidation. When optimized, Fenton‑based treatments have achieved >90% COD and color removal in lab tests, with one hybrid Fenton/UV run reporting ~93.7% COD and 89.5% color removal [researchgate.net]. UV irradiation can be delivered through inline ultraviolet systems for controlled exposure.
Ozonation (O₃) targets dye chromophores and organics and is especially effective on color: one pilot fully decolorized Reactive Black 5 in 5 minutes at ~25 mg/min ozone with ~75% COD removal [researchgate.net]. Typical practice doses ozone in the tens of mg/L; COD removal usually lags color (often 60–80% during the same contact). Ozone does not add solids, though bromate formation is a consideration where bromide is present.
Design notes: AOP reactors slot after any biological treatment and before final polishing; pH and residence time are critical (low pH for Fenton, higher for ozone). Radicals can leave intermediates; a short biological step or activated carbon polish sometimes follows. Energy and chemical costs (H₂O₂, electricity for UV or ozone) are material. Outcome: combined systems typically exceed 90% color removal, often to “visibly clear,” and drive COD down substantially—post‑AOP COD may often fall below 200 mg/L (depending on intensity)—far below the 150 mg/L limit if followed by mild polishing [researchgate.net] [pmc.ncbi.nlm.nih.gov].
Membrane polishing and reuse
Membrane filtration provides the physical barrier to mop up residual solids, dyes, salts, and low‑molecular organics. Microfiltration/ultrafiltration (MF/UF; MF pore size ~0.1–1 μm, UF ~0.01–0.1 μm) clear colloids and high‑MW solutes and are often used as pretreatment. UF has removed >90% turbidity and TSS in studies, but only 7–35% of dissolved dyes (color) [scielo.org.za]. For this duty, plants deploy skid‑mounted ultrafiltration systems ahead of tighter barriers.
Nanofiltration (NF) targets dyes and divalent salts, typically achieving 60–90+% removal of residual color and COD; an NF90 membrane rejected >90% of dye in a reactive‑dye effluent [scielo.org.za], with COD rejection around 76–83% on lighter loads [scielo.org.za]. Plants often standardize around packaged nano‑filtration for decolorization and partial organics removal.
Reverse osmosis (RO) is the tightest barrier (<0.001 μm equivalent), removing virtually all dissolved salts, metals, dyes, and organics; >95–99% removal of turbidity, organics, and most salts is reported, with ~99% COD removal uniquely achieved by RO in comparative tests [scielo.org.za] [watertreatmentmagazine.com]. For industrial reuse targets, engineers specify brackish‑water RO trains sized to TDS and recovery.
Sequence and operations: An integrated train commonly runs UF (or MF) → biological A/O (optional) → C/F → AOP → NF → (RO). In some designs, NF can partly replace AOP for color if dose is increased. Membrane fouling is real—one textile NF trial saw 20–60% flux decline within hours and required chemical cleanings that restored ~76–82% flux [scielo.org.za] [scielo.org.za]. Plants manage this with membrane cleaners and dose antiscalants to control mineral deposition; NF typically runs at ~10–20 bar, RO at ~15–60 bar.
Outcome: sequential membranes can dramatically polish effluent. In one pilot, NF drove conductivity down to <100 μS/cm from ~8,300 μS/cm and cut total organic carbon by ~65–85% [scielo.org.za]. Many systems meet textile wash‑reuse criteria after NF; RO is added when deionized‑grade reuse is the goal [scielo.org.za] [watertreatmentmagazine.com].
Integrated train for Indonesian mills
A comprehensive plant layout pairs screening and sedimentation with chemical clarification, oxidation, and membranes. One example: screening and sediment removal up front, then C/F in a flash tank with alum or ferric at ~200–400 mg/L and pH ~6, followed by a settling step in a clarifier. If BOD warrants, an optional biological stage can be inserted using packaged wastewater biological digestion modules.
Polishing proceeds with an AOP reactor—Fenton at pH 3–4 or an ozone contactor—and a membrane stack (UF/MF + NF/RO). This multi‑stage approach can deliver ~85–95% overall COD reduction, BOD near‑zero, TSS <10 mg/L, and effectively eliminate color [mdpi.com] [scielo.org.za]. Coagulation alone has removed ~82% of color in reported cases [mdpi.com]; membranes plus oxidants have driven COD reductions above 90% in studies [researchgate.net] [scielo.org.za].
Results reported from textile plants show C/F + AOP + NF/RO cutting COD below 100 mg/L and producing water suitable for partial reuse (e.g., cooling or rinsing) [scielo.org.za] [watertreatmentmagazine.com]. That comfortably fits within the cited Indonesian BOD/COD/TSS limits of 60/150/50 mg/L (sources: BSR excerpt, additional, additional).
Operations and optimization checkpoints
Jar‑test coagulant choice and pH, oxidant dose and residence time in AOP, and membrane transmembrane pressure and flux are the key knobs. Periodic backwash and chemical cleaning are required on membranes; chemical cleans restored ~76–82% flux in one NF trial after a 20–60% decline within hours [scielo.org.za] [scielo.org.za]. Plants often standardize on integrated membrane systems so pretreatment and cleaning protocols are harmonized.
The stakes are clear: meeting standards prevents penalties up to IDR 15B (~US$950k) and aligns with sustainability commitments [has-environmental.com]. The staged approach—screens and sedimentation, chemical C/F, AOP, and membrane polishing—has repeatedly produced colorless effluent with BOD near‑zero and TSS <10 mg/L in reported designs, and it’s now the reference path for finishing/coating wastewater [mdpi.com].
Selected sources: Zakaria et al. (2023) on composition and coagulation performance [mdpi.com] [mdpi.com]; Eslami et al. (2013) on Fenton removals and cost [pmc.ncbi.nlm.nih.gov]; Ima W. N. et al. (2021) on ozonation of Reactive Black 5 [researchgate.net]; Chollom et al. (2015) on UF/NF/RO performance and fouling [scielo.org.za] [scielo.org.za] [scielo.org.za] [scielo.org.za] [scielo.org.za] [scielo.org.za]; Behl (2025) on RO capabilities [watertreatmentmagazine.com].
