Reactive dyeing loads wastewater with 30–60 g/L of salts, pushing salinity to levels that stall biology and force membranes into the game. The emerging template blends salt‑tolerant microbes with RO/NF and, in some cases, full zero liquid discharge.
High salt content in dyehouse effluent isn’t a bug; it’s the process. Large doses of NaCl/Na₂SO₄ (30–60 g/L) used to fix reactive dyes leave wastewater with 1–10 g/L total dissolved solids (TDS)—roughly 0.1–1% salinity (Elsevier). Real‑world sampling has found 12.6 g/kg (~1.26%) salt, about one‑third of seawater salinity (IWA Publishing).
Overlay that with very high color and organics—COD in the hundreds of mg/L and color up to 652 Pt‑Co mg/L (IWA Publishing)—plus surfactants and sulfates, and the treatment challenge comes into focus. Indonesian regulations (PermenLH 5/2014 and amendments) set strict limits on COD, BOD, color, heavy metals, and more, but do not specify salt levels; in practice, managing salinity is necessary both to keep downstream biology stable (high‑salt discharges can fail general effluent tests) and to enable water reuse.
Reactive dyeing salt loads and impacts
Biology is the first casualty when salinity rises. Above ~5–8 g/L, most sludge bacteria are stressed and treatment performance drops (Elsevier). In one lab study, COD removal fell from ~80.7% at low salt to 59.4% at 5,000 mg/L, then to only 14.9% at 10,000 mg/L (Elsevier). Dye removal also dipped (~60% at 5,000 mg/L TDS) and collapsed near 10,000 mg/L (Elsevier, Elsevier).
Those figures set the boundaries: conventional activated sludge must be adapted or it risks missing BOD/COD targets at textile dyehouse salinities.
Salt‑tolerant biological operation
Two levers change the outcome: organisms and operations. Halophilic/halotolerant microbes and acclimation can extend performance. For example, adding Halobacter halobium to activated sludge enabled effective COD removal in 1–5% NaCl wastewater (ResearchGate). A Salinivibrio strain decolorized high‑strength dyes in saline media (up to 3 g/L dye) with >80% efficacy (PMC). Mixed consortia, via gradual acclimation or bioaugmentation, can tolerate up to ~5 g/L—and in special cases up to 10 g/L—though performance declines steeply as salinity approaches 1% (~10,000 mg/L) (Elsevier).
On configuration, sequencing batch reactors (SBRs) are commonly chosen for saline effluents and are well‑suited to variable loads (Elsevier). An SBR treating real dye effluent (~1–1.3% salinity) achieved ~70% color removal (IWA Publishing), but COD and sulfate removal required an external carbon source—molasses—to reach 87% sulfate reduction versus 23% untreated (IWA Publishing). High salinity can push sludge to rely on cosubstrate (ethanol/methanol/molasses) for stability (IWA Publishing).
Operations typically sequence anaerobic–aerobic steps (anaerobic breaks azo bonds; aerobic degrades amines). Plants aiming at this flexibility often select a sequencing batch reactor and pair it with biological digestion support. For stable dosing of external carbon, a metered dosing pump is standard practice. Performance summary: with acclimation and selection, biological reactors can handle moderate salinity (up to ≈5 g/L) and deliver ~60% COD/dye removal at 5,000 mg/L TDS, but at ~10,000 mg/L (~1%) performance collapses to <20% removal (Elsevier, Elsevier). Biology does not remove salts; it is a load‑reduction stage before physical desalination.
Membrane desalination for salt removal
Reverse osmosis (RO) is the primary industrial method to strip dissolved salts; typical salt rejection is >95–99%, especially with ultrafiltration (UF) pretreatment (MDPI). In practice, RO follows sedimentation, UF, granular activated carbon (GAC), and similar barriers to control fouling. An integrated textile case combined ozone + nanofiltration (NF) + RO + ion‑exchange to treat RO brine and achieved 77% water recovery and 66% NaCl recovery (ResearchGate); converting 115,000 m³/yr of effluent to reused water and 680 t NaCl saved ~$176k in water and $37k in salt, enabling a zero liquid discharge objective (ResearchGate).
RO’s downside is brine: 5–10% of influent volume, carrying all the salts and residual dyes/organics. This reject often reaches ~50,000 mg/L TDS (EP‑BD), demanding further treatment. Plants typically protect membranes with pretreatment like a clarifier and GAC—commercially, activated carbon—and upstream solids control. UF and NF are standard steps: an industrial UF–NF installation in Italy’s Prato textile district removed >98% hardness (PMC), producing permeate with hardness <9°f (less than 160 mg/L as CaCO₃) and Cl⁻ <500 mg/L (PMC); bench tests of spiral‑wound NF showed ~96–98% hardness and ~79–86% chloride removal from brackish water (PMC), with typical NF/RO rejection rates of ~85–98% for hardness and chlorides reported (PMC, PMC).
For textile effluents in the brackish range, mills deploy ultrafiltration as pretreatment, nano‑filtration for divalents and dyes, and RO sized for up to ~10,000 mg/L TDS feeds via brackish‑water RO—with ion removal polishing where needed via ion‑exchange. Electrodialysis (ED) can also desalinate by moving ions through membranes; RO and ED are cited as the only practical desalination methods for textile effluent (MDPI), but ED alone is rarely used at full scale in this sector. Because membranes foul, plants pair pretreatment with periodic membrane cleaning and dose antiscalants to manage scaling; a brine management step is always needed.
Zero liquid discharge architectures
Zero liquid discharge (ZLD) means no liquid effluent—only solids like sludge or salt crystals leave the site. Practically, ZLD stacks membranes with thermal concentration. A typical train: coarse filtration/clarification → UF/GAC → RO → (optionally ED) → thermal concentration. RO brine is routed to evaporators/crystallizers—multi‑effect units or mechanical vapor recompression—or to solar evaporation ponds if land is available. Electrodialysis or membrane distillation can serve as intermediate concentration steps before thermal evaporation.
The payoffs are compelling. One study reported ~77% of influent volume and ~66% of NaCl recovered for reuse (ResearchGate). In Indonesia, industry seminars report water‑recycling systems achieving >80% reuse of plant water (JPNN). A brine‑recovery scheme projected 115,000 m³ water and 680 t salt per year reused, saving ~$213,000 annually (ResearchGate).
The hurdles are equally real. A typical ZLD textile plant uses ~35% more energy (and carbon emissions) than a conventional plant, largely due to thermal brine treatment (ResearchGate). RO rejects (~10% of volume) can reach ~50,000 mg/L TDS, making disposal energy‑hungry (EP‑BD). In India, regulators require ZLD for dyeing units >25 m³/day (Fibre2Fashion), but industry cautions that without cheap energy or careful design, ZLD can be economically burdensome.
Given these trade‑offs, many plants pursue hybrids: stepwise salt or water recovery (e.g., partial rinse reuse, cascade RO) to shrink loads before any full ZLD. In Indonesia, vendors are exploring systems that recycle >80% of textile effluent internally (JPNN). Robust deployments put salt‑tolerant bioreactors before desalination and then stage membranes and evaporation. Plants often anchor such trains with integrated membrane systems and adapt pretreatment to the waste matrix.
Design and decision factors
Three rules of thumb emerge from recent data. First, membranes are the only practical way to remove the bulk of dissolved salts (RO/ED), targeting 95–99% removal (MDPI). Second, salt‑tolerant biology can still cut organics and color by ~50–80% at moderate salinity, enabling cleaner feeds to membranes (IWA Publishing, Elsevier). Third, integrated systems report water reuse rates of 75–90+% (ResearchGate, JPNN)—but with ~2–3× higher energy/carbon demand for ZLD versus conventional treatment (ResearchGate).
That makes piloting essential: run SBRs at actual salinity to quantify COD/color removal; test UF/NF/RO rejection and fouling; map brine disposal or recovery and the costs of partial versus full ZLD. Example outcomes already cited—RO/NF/ion‑exchange achieving 77% water and 66% salt recovery, and a 115,000 m³ water plus 680 t salt reuse case saving ~$213,000 annually—help bound expectations (ResearchGate). To support pilot trains, operators commonly include a clarifier upstream and pretreat RO with UF to reduce fouling risk.
The bottom line from recent studies and industry reports is consistent: some level of salt and water recovery—if not full ZLD—is becoming necessary for textile plants to meet rising regulations and sustainability goals. Full salt recovery can save hundreds of thousands of USD per year in water purchases (ResearchGate), offset by the high capital and energy of thermal brine treatment.
Sources and methods
This guide draws on peer‑reviewed studies, industry reports, and regulations, including treatment trials with high‑salt textile effluent (IWA Publishing, Elsevier), membrane reuse case studies (ResearchGate, PMC), and ZLD assessments (ResearchGate, ResearchGate). These sources underpin the performance figures and outcomes cited. For plants building out pretreatment to stabilize membranes, options include GAC units and integrated membrane systems tailored to textile loads.
