Hydroxyl‑radical oxidation and pressure‑driven membranes are turning recalcitrant dyehouse effluent into reuse‑quality water, with pilots reporting 90–100% color removal and up to 96% COD cuts. The catch is membrane fouling — and the pretreatment it demands.
For decades, reactive dyes and stubborn organics in textile effluent have outlasted conventional treatment. Now, two technologies are changing the math: advanced oxidation processes (AOPs) that generate hydroxyl radicals (·OH, oxidation potential ≈2.8 V) to break molecules apart, and pressure‑driven membranes that physically retain whatever’s left. Together, they’re delivering dye removal that frequently hits 90–100% and total organic load reductions that beat strict discharge and reuse targets (link.springer.com).
In one integrated Fenton + membrane process, color fell 98% after Fenton alone and reached complete elimination after reverse osmosis (RO); COD and TOC dropped 96% and 95%, respectively, producing water “suitable for reuse” (pubmed.ncbi.nlm.nih.gov). Such performance far exceeds most regulatory limits (e.g., Indonesia’s textile effluent COD limit is on the order of tens of mg/L) and is increasingly showing up at pilot and full scale.
Hydroxyl‑radical oxidation (AOPs)
AOPs rely on in‑situ generation of ·OH that non‑selectively oxidizes stable dye molecules and other recalcitrant organics (link.springer.com). In a combined ozone/Fenton trial optimized via response–surface methods, O₃/Fe²⁺/H₂O₂ at 70 mg/L O₃, 1.748 g/L Fe²⁺, and pH 3 achieved ~89% color removal and ~82.5% COD removal on real dyeing effluent (link.springer.com).
Classical Fenton chemistry (H₂O₂ + Fe²⁺ at pH ~3) alone removed ~64% COD in 10 minutes; adding granular activated carbon (GAC) boosted COD removal to ~83–88%, with color (measured at 436–620 nm) nearly completely eliminated under those conditions (link.springer.com). Plants typically meter reagents with precision — a role served in practice by dedicated dosing pumps — and often add adsorption media such as activated carbon to consolidate gains.
Oxygen‑based AOPs also improve biodegradability (by cutting molecules into lower molecular‑weight byproducts), easing downstream treatment. Energy footprints vary widely: recent analyses put Fenton processes around ~0.98 kWh·m⁻³ per log reduction, ozone alone near ~3.3 kWh·m⁻³, and UV photocatalysis about ~90 kWh·m⁻³ (www.mdpi.com). Many studies conclude that enhanced Fenton — often photo‑Fenton or activated‑carbon‑assisted Fenton — gives the best balance of efficacy and cost (www.mdpi.com; link.springer.com).
Nanofiltration and RO performance
Pressure‑driven membranes are the polish. Nanofiltration (NF; typical molecular‑weight cut‑off ~100–300 Da) shows ≈86–98% dye rejection, including up to 97–98% removal of reactive dye under pH 3 using Hydranautics NF membranes in lab trials (www.mdpi.com; www.mdpi.com). NF removes larger organics (MW >200 Da) and multivalent ions, whereas RO is needed to capture small acids and salts. In practice, “NF will remove most dyes and high‑MW organics, while RO metabolizes small organics and salts (often >95% of remaining COD)” (pubmed.ncbi.nlm.nih.gov; www.mdpi.com).
Membranes can therefore polish AOP‑treated effluent to near zero‑color and very low organics. The integrated Fenton/MF/RO system cited above produced reuse‑suitable water, with 96% COD removal post‑RO and all color eliminated (pubmed.ncbi.nlm.nih.gov). Even without AOP, NF or RO alone can yield high‑quality water: recycled dye baths via NF can recover 85–90% of water and essentially 100% of dyes (www.mdpi.com; www.mdpi.com), and RO of pretreated effluent reduces conductivity by >85% (lowering Cl⁻, SO₄²⁻, etc.), making it potable or reusable (pubmed.ncbi.nlm.nih.gov).
Given textile effluent COD often ranges 500–5000 mg/L (www.mdpi.com), plants sequence AOPs and solids removal before membranes. Typical trains send clarified flow through microfiltration/ultrafiltration, then NF/RO for discharge or reuse; NF permeate (clean water) is typically 70–90% of feed, with concentrate recycled or further treated (www.mdpi.com). In deployment, that maps to installed nanofiltration units and full RO/NF/UF systems sized for industrial duty.
Fouling mechanisms and flux loss
The major challenge is fouling. Organic dyes, colloids, and salts rapidly foul NF/RO elements; multivalent ions such as Ca²⁺, Mg²⁺, and SO₄²⁻ form scalants. Van der Bruggen et al. note that in textile wastewater “fouling is assumed to be caused by (ad)sorption of organic compounds, which has a large influence” on flux, and high salt reduces driving pressure via osmotic backpressure (www.lenntech.com).
In practice, membranes operated on raw or just‑biologically‑treated textile effluent can see rapid flux decline — >60% loss in hours if unchecked (www.mdpi.com; www.lenntech.com). Unmitigated fouling would otherwise reduce NF flux by ~50–65% in a short period (www.mdpi.com).
Pretreatment trains and anti‑scaling controls
Robust pretreatment is non‑negotiable. Coagulation–flocculation using alum, FeCl₃, or lime is standard; studies show coagulation removes high‑MW, hydrophobic organics (dominant foulants), dramatically improving UF/NF flux (www.researchgate.net). Plants often dose coagulants via controlled systems; in practice, it’s common to specify coagulant programs matched to dye load and pH.
Activated media can add another barrier: beyond Fenton’s ~64% COD removal in 10 minutes, adding 2 g/L activated carbon boosted COD removal to ~83–88% while color at 436–620 nm was nearly completely eliminated (link.springer.com). That aligns with widespread use of activated‑carbon filters to adsorb residual color and organics before membranes.
Physical filtration is equally important. Sand or multimedia filters capture suspended solids, typically front‑ending microfiltration/ultrafiltration; a recent pilot found that a sand filter + 5 kDa UF sequence gave the best pretreatment for dyehouse effluent, cutting turbidity >40% and significantly lowering TOC before NF/RO, which translated into much lower membrane fouling (www.mdpi.com; www.mdpi.com). In practice, that maps to granular sand/media filters ahead of ultrafiltration skids, and finally to NF/RO.
Additional measures include pH adjustment (to keep calcium salts soluble) and anti‑scalant dosing where needed (www.mdpi.com). Plants commonly deploy targeted membrane anti‑scalants alongside pretreatment; effective programs have been reported to extend membrane life by years and cut operating cost by 20–30% in reuse systems, where fouling control remains the dominant operational cost driver.
Adoption, reuse, and energy context
The pace of adoption is high: over 200 pilot/full‑scale textile plants have been reported with AOP or NF components globally. Removal efficiencies often exceed 90%, and many textile mills achieve >70% water reuse. One combined scheme coupling NF and electrochemical oxidation has recycled up to 100% permeate water and nearly all dye, economically saving ~75% of salt costs (www.mdpi.com). Energy‑wise, Fenton’s ~0.98 kWh·m⁻³ per log reduction compares favorably to ozone alone (~3.3 kWh·m⁻³) and UV photocatalysis (~90 kWh·m⁻³), explaining why enhanced Fenton variants often lead on cost–performance (www.mdpi.com; link.springer.com).
Across these setups, the membrane block spans NF and RO configured for reuse targets — a fit for modular RO/NF packages in dyehouses — while pretreatment leans on coagulation, granular media, adsorption, and UF to keep fouling in check.
Source notes and references
Numeric examples include Fenton‑based treatment removing 80–90% of COD and nearly all color from reactive‑dye wastewater, with a pilot showing COD/TOC reductions of 96–95% and color down to non‑detectable levels via combined Fenton oxidation + MF/RO (link.springer.com; pubmed.ncbi.nlm.nih.gov). This level of performance far exceeds most regulatory limits (e.g., Indonesia Permen LHK 16/2019) and underscores how AOP + NF/RO trains can meet strict standards.
Sources: All claims above are drawn from recent peer‑reviewed and industry studies (pubmed.ncbi.nlm.nih.gov; link.springer.com; link.springer.com; www.mdpi.com; www.lenntech.com; www.researchgate.net; www.mdpi.com; www.mdpi.com), along with regulatory guidelines (Indonesia Permen LHK 16/2019) on textile effluent limits and global technical reports on dyehouse treatment.
