Scouring and bleaching wastewater can hit pH 10–13 and carry fibers, oils, and organics that overwhelm plants. A four‑step pretreatment—screening, equalization, pH neutralization, and coagulation/flocculation—cuts >90% solids and ~40–50% organics before the main WWTP.
Effluent characteristics and targets
Textile scouring (and bleaching) effluent is highly alkaline (pH often ~10–13) with heavy loads of suspended fibers, oils/waxes and organic matter. Studies of wool scouring wastewater report COD (chemical oxygen demand, a proxy for oxidizable organics) ~1,400–1,500 mg/L (www.mdpi.com) and significant lanolin/oil content; cotton scouring effluents likewise carry tens to thousands of mg/L COD/BOD (biochemical oxygen demand) along with fine fiber solids. Total suspended solids (TSS) in textile waste can range extremely wide—reported 15–8,000 mg/L (www.frontiersin.org)—reflecting fibers and additives.
All cotton finishing wastes “contain fine fibers” that can clog filters and sludge beds—EPA notes fibers can “seal sand or carbon beds, clog equipment, and absorb chemicals” and are unsightly in trickling filters (nepis.epa.gov). Untreated effluents often violate discharge standards; one Indonesian plant had COD ≈1,640 mg/L and color 4,277 Pt‑Co (platinum‑cobalt units for color) versus far lower regulatory limits (id.scribd.com). The pretreatment train must remove coarse solids (fibers), buffer flows/pH, neutralize alkalinity, and coagulate/flocculate solids and organic matter so discharge to the main WWTP meets pH≈6.5–8.5 with greatly reduced TSS/COD.
Headworks screening (fiber removal)
Initial screening removes large fibers and debris to protect pumps and downstream units. A coarse screen or drum filter (e.g., 1–5 mm mesh) at the headworks typically removes the majority of >1 mm solids (often >70–90% by weight of coarse material), capturing fibers and lint. That aligns with EPA guidance emphasizing fiber removal to avoid clogging (nepis.epa.gov).
Plants commonly install an automatic screen at this “wastewater physical separation” stage, a headworks step reflected in screens and primary systems designed for lint, fiber, and debris.
Equalization tank design parameters
An equalization tank (8–24 h retention) homogenizes flow and pollutant concentrations. Textile streams fluctuate by batch operations and vary in pH/strength; equalization “dampens organic load fluctuations” and equalizes pH so downstream stages see a near‑constant feed (www.informedchoicematrix.net). Designers often allocate ~8 h retention for textile effluent equalization (www.informedchoicematrix.net), though some plans use up to 24 h for maximum stability, with sufficient mixing/aeration to avoid anaerobic zones.
What equalization balances:
- Flow surges: smooths pulses so downstream pumps and reactors run steadily (www.informedchoicematrix.net) (www.informedchoicematrix.net).
- pH swings: buffers extremes (e.g., occasional acid/alkali spills), reducing acid/base dosing later (www.informedchoicematrix.net).
- Load spikes: dilutes toxic “slug” loads from batch rinses, protecting bioreactors.
A professionally engineered 8–12 h equalization tank might cost on the order of USD 90,000 in local markets (www.informedchoicematrix.net). To kit out mixing and aeration, many facilities source supporting equipment to maintain homogeneous conditions.
pH neutralization controls
Scouring effluent must be neutralized to near‑neutral pH before coagulation or biological treatment. Typically, strong acids (e.g., sulfuric or hydrochloric acid) are dosed under pH feedback control until reaching ≈6.5–8.0 (Indonesian regulations generally require 6–9, textilelearner.net). Adjusting to ~7 prevents alkaline shock in the WWTP, can precipitate some constituents (e.g., carbonate/hydroxide), and optimizes coagulation.
In practice, acid dose might be on the order of ~0.5–1 kg H₂SO₄ per m³ of effluent to lower pH from ~13 to ~7, though exact values depend on strength; precise dosing is determined by titration/alkalinity tests. Metering is typically handled by a dosing pump matched with available water and wastewater chemicals inventory.
Coagulation–flocculation and clarification
After neutralization, chemical coagulation/flocculation removes remaining suspended solids and destabilizes organics. Coagulants such as aluminum sulfate (alum), polyaluminum chloride, ferric chloride/sulfate are added under rapid mixing, followed by flocculant polymers to grow settleable flocs. This is optimized via jar tests (pH, dose, mixing time). Coagulant efficacy is strongly pH‑dependent—highest at acidic pH (www.frontiersin.org)—so doses are chosen to reach end‑of‑mix pH ~5–6 where Al/Fe salts are most active. Coagulant dose is often tens to hundreds of mg/L; one jar test used 0.2 g/L polymer with FeCl₃ to achieve the highest removal (pmc.ncbi.nlm.nih.gov).
On performance: turbidity dropped from ~202 to 6.4 NTU (>96% removal) in a pilot with FeCl₃ + polymer (pmc.ncbi.nlm.nih.gov); coagulation/flocculation halved TOC (from 528 to 265 mg/L, ~50%) under optimal dosing (pmc.ncbi.nlm.nih.gov). In dye‑containing effluent, 20–30 mg/L FeCl₃ or alum achieved ~40–46% COD removal, with ~81% color removal and ~42–46% COD reduction reported (www.researchgate.net).
Formulation and supply often center on PAC (polyaluminum chloride) and polymers—plants specify PAC grades and pair with appropriate flocculants to reach target removal. Clarified water discharges from a settling unit; many facilities specify a clarifier to achieve gravity separation. Oils/greases (including lanolin) are partly removed by co‑precipitating with metal hydroxides and adsorption onto flocs, which reduces grease/oil content and improves downstream treatment. The settled sludge (alum flocs plus captured solids) typically constitutes ~50–150 mg/L of removed solids, which must be dewatered.
Performance outcomes and WWTP interface
A properly designed pretreatment achieves dramatic load reductions: coarse fibers and grit removed at the screen; flow/pH stabilized in equalization; and >90% of remaining TSS removed by coagulation–flocculation. Bench and pilot results show turbidity/TSS often drop by >95% and measurable organics (COD, TOC) by ~40–50% prior to discharge—e.g., ~97% turbidity and ~50% TOC removal (pmc.ncbi.nlm.nih.gov) and ~81% color/45% COD removal with alum/FeCl₃ (www.researchgate.net). After clarification, effluent TSS is often <50 mg/L.
In practical terms, raw scouring effluent (e.g., COD ~1,500 mg/L, www.mdpi.com) could exit pretreatment at ~700–800 mg/L COD with very low TSS (<50 mg/L). These gains ease the load on the main WWTP; for instance, if Indonesian discharge limits call for BOD/COD ≪ 100 mg/L, the pretreatment cuts the load burden on the biological plant by a factor of ~2. Equalization and neutralization ensure the pH and flow conditions entering the WWTP are stable, avoiding process upsets (www.informedchoicematrix.net) (www.informedchoicematrix.net). That stability protects downstream biological processes such as an activated sludge stage.
Taken together, the scheme meets regulatory and plant needs: screening prevents fiber‑induced fouling (nepis.epa.gov); equalization buffers spikes (www.informedchoicematrix.net); neutralization fixes pH to ~7; and coagulation–flocculation slashes solids and partially reduces COD. All design decisions (tank size, chemical doses) should be based on measured flow rates and pollutant loads in the scouring effluent; the cited studies provide benchmarks and a data‑driven rationale for each unit operation (www.frontiersin.org) (www.frontiersin.org).
