Textile effluent color can vanish with coagulants at cents per cubic meter—or with advanced oxidation at dollars per cubic meter. The right choice hinges on dye class, pH windows, sludge, and how much color is left to polish.
In jar tests on real reactive‑dye wastewater, one team needed a staggering 6000 mg/L of alum (aluminum sulfate) at pH ~2.4 to reach roughly 90% color removal (www.researchgate.net). By contrast, a pre‑hydrolyzed coagulant—polyaluminum chloride (PAC)—hit 100% color removal at only 800–2000 mg/L, at pH ~4, in the same study (www.researchgate.net).
That is the coagulation/flocculation story in miniature: conventional metal salts are cheap per kilogram but hungry in dose and sludge, while polymeric coagulants and aids slash mass and still pull the color. When stubborn chromophores remain, plants pay up for advanced oxidation processes (AOPs)—Fenton chemistry or ozonation—to break the molecules apart.
Costs span orders of magnitude. A typical coagulation stage lands around $0.05–0.10 per m³ in chemicals; AOP steps range around $1–10 per m³, and one Fenton study implies ~\$10–20/m³ from an $8.6/kg COD‑removed figure (link.springer.com).
Conventional metal salt coagulation
Aluminum and iron salts (alum, FeCl₃, ferrous sulfate) hydrolyze to form metal hydroxides that adsorb and destabilize dye colloids—coagulation (charge neutralization) and flocculation (aggregation) in the classic sense. These processes typically require moderate‑to‑high dosages (hundreds to thousands of mg/L) and pH adjustment, producing large flocs and correspondingly high sludge volumes (c.coek.info).
In practice these inorganics are very cheap ($0.1–0.3/kg) but the chemical consumption per cubic meter tends to be large. Ferric/ferrous salts can be more effective at slightly higher pH (optimal ~7–9), but likewise generate bulky sludge; color removal often plateaus below 80–90% for stubborn chromophores unless extreme doses/pH are used (c.coek.info). Plants standardizing their chemistry often procure base coagulants via coagulant programs.
Pre‑hydrolyzed polymeric coagulants
Pre‑hydrolyzed products—PAC, polyferric sulfate (PFS), polyaluminum‑ferric chloride (PAFCl)—behave like their inorganic counterparts with better pH tolerance and denser flocs. PAC (“polyaluminum chloride”) often outperforms alum at lower dose: the same reactive‑dye effluent achieved 100% color removal at only 800–2000 mg/L PAC (pH ~4), while alum required 6000 mg/L for ~90% (www.researchgate.net). Ferrous sulfate can remove 99% color at ~4000 mg/L but only at high pH (www.researchgate.net).
These polymeric metal salts trade somewhat higher material cost (still on the order of $200–400/ton) for lower required dose and faster settling; Verma et al. note that PAC “shows better colour removal efficiency in a wider pH range (7–10)” and generates flocs that settle quickly (c.coek.info). Sourcing high‑purity PAC for industrial service is routine through programs like PAC supply.
Polyelectrolyte aids and charge effects
A small amount (often 1–5 mg/L) of a high‑molecular‑weight charged polymer—polyelectrolytes such as polyacrylamide, polyDADMAC, or natural polymers like chitosan—can dramatically improve coagulation performance. These aids bridge particles to form “massive flocs” that settle rapidly and reduce sludge volume (c.coek.info). Jar tests show that adding ~1 mg/L cationic polymer can significantly raise dye removal and speed decanting (c.coek.info).
Since many dyes in effluents are negatively charged (anionic), cationic polymeric flocculants are usually most effective; “cationic polymer is preferred over anionic and nonionic” types for textile dyes (c.coek.info). One industry report found >90% color removal and ~60% COD (chemical oxygen demand) removal using a few mg/L cationic polyacrylamide on a dye wastewater sample, with minimal sludge formation (www.sinofloc.com). In practice, polymeric/co‑polymeric coagulants allow similar or higher color removal at 10–100× lower mass dose than alum (c.coek.info) (c.coek.info). Plants concerned with precision feed often rely on accurate metering—an application for an industrial dosing pump—to hit those single‑digit mg/L setpoints.
Performance windows and sludge yield
For many reactive and direct (anionic) dyes, optimized coagulation with PAC or with a cationic polymer aid can routinely remove ≥85–95% of color (link.springer.com) (www.researchgate.net). Conventional alum often stalls below ~80–90% under comparable conditions. Cationic (basic) dyes are harder to capture unless matched with anionic coagulants. Color removal by coagulation is rapid—on the order of minutes—but can leave residual chromophores (the color‑bearing chemical groups) if not fully adsorbed.
Sludge yield is typically ~2–5% of treated volume (as settled cake) for inorganic coagulants, whereas organic polymers produce significantly less solids (c.coek.info). The key trade‑offs are chemical cost versus sludge cost: inorganic salts are cheap per kg, whereas polymeric aids cost a few dollars per kg, but their very low dosing usually yields lower total cost where sludge disposal is costly (c.coek.info). Facilities boosting settling area often add modular internals; a tube settler is a common capacity upgrade in clarification trains.
Fenton’s reagent oxidation (AOP)
Fenton chemistry—Fe²⁺ + H₂O₂—generates hydroxyl radicals (•OH), highly reactive species that cleave dye molecules. Typical operation is at low pH (~3), with molar H₂O₂:COD ratios around ~1:1. In one pilot on real textile wastewater (COD ~1600 mg/L), a Fenton step (H₂O₂ ≈2000 mg/L plus Fe²⁺) removed ~70–73% of both COD and color in 1 hour (link.springer.com). pH and radical quenchers (e.g., excess colorants) can limit efficiency.
Energy demand is moderate (mixing only), but chemical use is high—approximately 2 g H₂O₂ per liter plus iron salts—driving operating cost. The same study estimated chemical Fenton (CFenton) at about $8.6 USD per kg COD removed, implying ~\$10–20 per m³ of raw effluent (link.springer.com). Fenton’s sludge is mostly iron hydroxide flocs (≈<10% of the initial COD as solids), much less than coagulant–sludge volume (bluewaterlab.co). Because of cost, it is often used as a polishing step after biological or coagulation stages; studies report that Fenton can quickly break down persistent reactive dyes (azo, anthraquinone, complex metal‑organic), substantially improving effluent biodegradability (bluewaterlab.co).
Ozonation parameters and costs
Ozone (O₃) bubbled through effluent—often at elevated pH to slow ozone decomposition—directly attacks unsaturated chromophores. Bench tests on reactive dyes achieved 83–96% decolorization within 45–90 minutes, with anthraquinone and copper–complex (copper formazan) dyes fully decolorized in that study (journals.sagepub.com). COD is partially oxidized, but full mineralization often requires high doses.
There is no sludge; residuals are largely O₂ and CO₂ plus low‑level organics/aldehydes. The drawbacks are capital intensity and energy: reported power consumption sits on the order of 1–4 kWh/m³ for moderate dye concentrations. While full cost per m³ depends on electricity, one study estimated a UV/H₂O₂ (a comparable AOP) step at about $0.3–0.8 per m³ (bluewaterlab.co). Ozone alone is usually more expensive due to power—water treatment AOP texts often cite tens of dollars per ton of ozone produced. Operators considering UV‑based oxidation typically look at reactor hardware akin to UV systems as part of that train.
Costs, hybrids, and equipment choices
In summary, coagulation/flocculation—especially with optimized polymeric coagulants—can achieve high color removal (often >90%) for many dyes at low capital and moderate consumable cost; the trade‑off is chemical/sludge handling (link.springer.com) (www.researchgate.net). A typical coagulation stage might cost on the order of $0.05–0.10 per m³ in chemicals (alum/PAC), whereas Fenton or ozone treatments can cost on the order of $1–10 per m³ depending on conditions, with ~\$10–20/m³ implied by $8.6/kg COD removed (link.springer.com).
Many textile effluent treatment plants therefore use a hybrid approach: low‑cost coagulation for bulk color/COD, followed by a targeted AOP “polish” to meet strict color limits. That choice is often reinforced by the availability of optimized chemistries such as PAC (e.g., industrial‑grade PAC) and customizable pre‑hydrolyzed blends (e.g., PAC/ACH families), plus modular clarification hardware like a compact lamella settler.
Dye class sensitivity and regional adoption
Effectiveness varies by dye class: anionic reactive dyes respond well to cationic coagulants and also to AOP, whereas cationic (basic) or dispersed dyes may need stronger AOP or tailored coagulants (c.coek.info). Indonesian and Southeast Asian plants face the same trade‑offs; many report outdated ETPs unable to remove deep color without upgrades, and new investments often involve polymeric coagulant aids or small ozone/UV units (bluewaterlab.co).
As color targets tighten, the most durable play has been methodical optimization: jar‑test the coagulation chemistry first, trim dose with a cationic polyelectrolyte from a calibrated flocculant program, and reserve radicals—Fenton or ozone—for the recalcitrant residue.
