Dyehouses routinely discharge wastewater with 1–10 g/L salt — and sometimes 6% NaCl. Conventional biology chokes; membrane trains and zero liquid discharge (ZLD) promise recovery, but at a cost.
Textile dyeing and finishing effluents aren’t just colored — they’re salt-laden. Published data put total dissolved solids (TDS, a measure of dissolved salts and minerals) in typical textile effluent at 1–10 g/L (redalyc.org). Lin et al. report dye baths up to 6% NaCl (60 g/L) or 5.6% Na₂SO₄ (mdpi.com), a hypersaline hit by municipal standards. In a pilot case in China, membrane separation was flagged as necessary because diluted effluent still carried ~6 wt% NaCl (mdpi.com).
One complication: salinity itself is rarely regulated in Indonesian effluent standards — rules focus on organic and toxic loads — yet mills must still manage salt-loaded wastewater in practice (researchgate.net).
Salinity levels and sources (dyehouse TDS)
Reactive dyeing of cellulosic fabrics adds NaCl or Na₂SO₄ to neutralize fiber charge — typically 20–100 g of salt per kg fabric — driving wastewater salinity to the 1–10 g/L range, and at times to percent-level values (redalyc.org; mdpi.com). One industry was estimated to discharge ~0.2 million tonnes of salt per year (redalyc.org).
Pretreatment to remove solids and fats becomes table stakes before any advanced step; primary systems such as a clarifier or packaged wastewater physical separation reduce grit and load ahead of biology or membranes.
Biological treatment under salt stress (SBR and activated sludge)
High salt impairs conventional biology. In sequencing-batch reactors (SBRs, time-sequenced activated sludge), COD removal (chemical oxygen demand, a proxy for oxidizable organics) fell by ~20% once influent TDS reached 3,000 mg/L; at 6,000 mg/L, COD removal dropped further (to ~75% of baseline) and phosphate uptake collapsed (researchgate.net). Mirbolooki et al. (2017) found that with 1 g/L TDS and moderate dye, COD removal was ~81%; at 5 g/L it fell to ~59%; and at 10 g/L it plummeted to ~15% — with biomass wash-out above 5 g/L (redalyc.org).
Nitrification sometimes endures — >97% even at 6,000 mg/L in one SBR study — but phosphate removal failed above ~1.5 g/L (researchgate.net), and other studies show 0.5–5% salt cuts nitrifier kinetics by >20% (redalyc.org). The mechanisms are well known: osmotic stress on microbes and impaired oxygen transfer. In practice, traditional aerobic systems perform well only up to ~1–3 g/L TDS and degrade rapidly beyond that (researchgate.net; redalyc.org). Facilities running batch-mode biology can benchmark these limits against a sequence batch reactor.
Salt-tolerant bioprocess options (halophiles, AGS, MBR/MBBR)
When full desalination is infeasible, halotolerant or halophilic (salt-loving) consortia can carry the organic load. One mixed culture from Tibetan salt-fields fully decolorized 600 mg/L of azo dyes (Congo Red, Direct Black G, Amaranth, Methyl Red, Methyl Orange) in 8–24 hours under anoxic conditions while tolerating 0–80 g/L NaCl — and decolorization remained rapid even at 60+ g/L (pmc.ncbi.nlm.nih.gov).
Aerobic granular sludge (AGS) seeded with halophiles maintained stable 0.8–1 mm granules and high EPS production (extracellular polymeric substances, the biofilm glue) at 40–85 g/L salt, whereas conventional sludge granules collapsed beyond ~20–40 g/L. At 85 g/L NaCl the halophile reactor still produced EPS and held ~100 mg Na⁺/g biomass (vs. 125 mg/g for ordinary sludge), indicating better osmotic regulation (liebertpub.com).
Biofilm systems such as moving bed bioreactors (MBBR) and membrane bioreactors (MBR, biology paired with ultrafiltration) can deliver good COD removal up to a few g/L salt, though large-scale data for saline MBRs in textiles remain scarce (researchgate.net; liebertpub.com). The takeaway: halophilic reactors can handle ~50–80 g/L NaCl, whereas conventional sludge must stay under ~3–5 g/L (liebertpub.com; pmc.ncbi.nlm.nih.gov).
Membrane desalination trains (UF/NF/RO/ED)
To remove salts outright, reverse osmosis (RO) and related membranes do the heavy lifting. One hybrid achieved 99.95% salt removal after ultrafiltration (UF, a fine physical sieve typically pretreating RO): an ultrafiltration step at 5,000 Da MWCO first concentrated dyes (from ~998 to 7,953 mg/L) and let nearly all NaCl/Na₂SO₄ pass (>99.42%), then high-pressure RO or bipolar-membrane electrodialysis (BMED) removed the salts; the result was essentially complete desalination with dye recovery (pubs.acs.org). In continuous full-scale practice, RO can typically achieve 90–98% salt rejection with recovery ratios up to 70–80%.
Pretreatment is non-negotiable because RO is sensitive to fouling by organics and color; flotation, coagulation, and UF/NF (nanofiltration, a tighter membrane that targets multivalent ions and larger organics) are standard. Mills often combine a DAF unit and coagulants upstream of ultrafiltration. Where hardness or larger organics complicate RO, nano‑filtration can trim the load.
Hardware scale and energy are significant. A Chinese reuse plant (50,250 m³/d) needed 5,280 RO membranes ($500 each) and 1,224 UF membranes ($2,167 each), implying ~$1.41M in membrane inventory and ~$1.41M/year replacement costs (mdpi.com). Typical RO energy use at 25–40 bar is on the order of 3–6 kWh/m³. For feeds in the 1–10 g/L TDS band, brackish-water skids such as brackish-water RO address the core range, while percent-level salinity pushes designs toward higher-pressure trains, including sea-water RO configurations.
Concentrate management remains the crux. At 70–80% recovery, 20–30% of inflow becomes brine at 5–10× the feed salt concentration. Hybrid NF/RO trains often reach ~75–80% recovery (researchgate.net), after which oxidation, electrodialysis (ED), thermal evaporation, or crystallization handle the remainder (researchgate.net). Electrodialysis stacks — especially BMED — can split salts into acids and bases; after dye removal via TUF (tight ultrafiltration, a fine-pore UF), Lin et al. converted Na₂SO₄ into H₂SO₄ and NaOH while generating ultrapure water, turning “waste” salt into chemicals and approaching ZLD (pubs.acs.org). System integrators typically package such steps within modular membrane systems.
ZLD strategies and economics (closed-loop reuse)
ZLD (zero liquid discharge) means no wastewater is discharged — all water is recovered or evaporated, and only salts/solids leave the gate. A typical ZLD train: robust pretreatment (flocculation/sedimentation), biological/equalization for BOD/COD (biological oxygen demand/chemical oxygen demand), a membrane cascade (MF/UF → RO/NF) to reclaim ~70–90% of water, and brine concentration via thermal evaporators or ED/crystallizers to handle the last 10–30%; the end result is nearly 100% water reuse. The textile and desalination sectors are singled out as ZLD-successful in reviews (sciencedirect.com).
The advantages are clear: no effluent discharge; freshwater draw cut dramatically (often >90%); potential recovery of dyestuffs (via UF), precipitated salts, or acids/bases (via BMED). The challenges are equally clear: energy and capital. Jahan et al. (2022) note that OPEX and CAPEX are far greater than traditional methods, with many ZLD schemes proven at pilot or small scale (mdpi.com).
Practical examples in Asia include a Chinese design at 75,000 m³/d (UF/RO followed by crystallizers) targeting >95% water recovery, and an Indian ZLD system (50 KLD lab sale) claiming full reuse. With careful integration — e.g., absorbent heat exchangers for brine evaporation or hybrid ED — net water recovery can approach 98–99%. One TUF–BMED pilot turned a concentrated 8% Na₂SO₄ stream into pure water and H₂SO₄/NaOH, i.e., a near-ZLD outcome (pubs.acs.org).
Across designs, reported outcomes cluster around >90% water reuse, >95% salt recovery, and very low liquid waste; some membrane-ZLD designs target 100% closed loop (excluding small blowdowns). A review notes textile companies “can achieve an almost full recovery and reuse of wastewater using RO and other membrane techniques,” though system energy may double compared to conventional treatment (sciencedirect.com).
Design implications and the bottom line
Typical textile salinity — 1–10 g/L — is one to two orders of magnitude higher than domestic wastewater, and conventional plants suffer. Salt stresses slow bacterial metabolism, reduce sludge activity, and can collapse granular flocs. Halotolerant biology can shoulder much of the organic load up to ~5–10 g/L salt (redalyc.org; liebertpub.com), while membranes can strip salt entirely and produce reuse-quality water — with a concentrated brine to manage (pubs.acs.org; sciencedirect.com). The ultimate play, ZLD, closes the loop via combinations of these technologies. Data from research and field pilots show water recoveries above 90–95% are achievable, at the expense of higher energy and power draw (sciencedirect.com; mdpi.com).
Sources and notes
Peer‑reviewed studies, industry case reports and reviews underpin these figures. Examples include an SBR study quantifying COD removal vs. TDS (researchgate.net); a granular reactor paper demonstrating organism survival at 85 g/L salt (liebertpub.com); advanced membrane research documenting 99.95% desalination of textile dye water (pubs.acs.org); global reviews of textile ZLD emphasizing high recovery but energy/cost barriers (sciencedirect.com; mdpi.com). For regulatory context in Indonesia, see (researchgate.net); for salinity loads and dye-bath salt additions, see (redalyc.org; mdpi.com). A large Chinese plant’s membrane counts and costs are documented here (mdpi.com). Hybrid NF/RO concentrate handling options, including oxidation, are reviewed here (researchgate.net).
