Pulp and paper effluent is brown because of lignin and other wood‑derived organics. Three technologies—coagulation/flocculation, activated carbon adsorption, and membrane filtration—tackle that color with very different performance profiles and costs.
The trademark brown hue in pulp and paper mill wastewater comes from lignin, tannins, and chlorinated organics shed during pulping and bleaching. In real plants, untreated effluent can show UV254 absorbance on the order of 1–2 m−1 and color in the hundreds of APHA (American Public Health Association) units—driven mostly by soluble high‑molecular‑weight aromatics and smaller chlorinated phenolics or dyes if bleaching or recycling is involved (MDPI) (ResearchGate).
As water reuse rises on boardroom agendas, mills in strict or water‑scarce regions—particularly parts of Asia‑Pacific—are moving toward advanced tertiary treatment and even zero liquid discharge (ZLD), which puts color removal squarely in scope (Verified Market Research) (MDPI).
Which tool fits best? It depends on the fraction of high‑MW versus low‑MW colorants, the outlet target (discharge vs. reuse), and total cost. Here’s how coagulation/flocculation, activated carbon, and membranes stack up on pulp mill color.
Coagulation/flocculation mechanisms and results
Metal coagulants such as alum (Al₂(SO₄)₃), ferric chloride, ferrous sulfate, lime, and magnesium sulfate hydrolyze to create “sweep flocs” that neutralize charges and enmesh organics. In practice, coagulation zeroes in on colloidal and high‑MW organics—especially lignin and humic substances—that drive apparent color (MDPI) (MDPI).
Effectiveness hinges on pH and dose. In one mill study, alum at 1200 mg/L (optimum at pH 6) removed 96% of color (initial 519 units), while magnesium sulfate (MgSO₄) required ~3400 mg/L to reach ~89% color removal (MDPI) (MDPI). In another trial, polyaluminum chloride (PACl) paired with polymer at pH 9 cut ~58% of UV254 absorbance (a surrogate for color) (ResearchGate). Optimized doses often deliver 80–95% apparent color removal, particularly when lignin dominates (MDPI).
Compound sensitivity matters. Coagulation excels on negatively charged, high‑MW lignin; reviews report up to ~90% lignin removal with alum and ~100% using oxo‑titanium sulfate plus alum (PubMed). Smaller, soluble colorants—mono‑/di‑chlorophenols or dyes—are less responsive, often requiring polymers or follow‑on steps. In recycled‑paper effluent simulations, coagulation without flocculant aids removed only ~50–60% UV254 absorbance (ResearchGate).
There is a sludge trade‑off. Typical mill doses range from 200 to >1000 mg/L; at 1200 mg/L alum (with pH adjustment), significant sludge forms, while MgSO₄ needs ~3400 mg/L. Sludge production is of order 100–200 mL per liter of Sludge Volume Index (MDPI). Dewatering and disposal add cost.
On cost, coagulation is low CAPEX, moderate OPEX (chemicals plus sludge). One 6000 m³/d scheme—settling + lime/alum coagulation + carbon polishing—ran about $2.09 per m³; a similar alum+AC hybrid was ~$2.31 per m³. Tertiary activated carbon polishing alone commonly lands at $1.10–$3.30 per m³ (MDPI) (MDPI). Chemical cost for coagulant/pH agent is modest (often < $0.50/m³ for requires), but sludge handling inflates the total.
Operationally, mills dose metal salts and polymers with precision—often via a dosing pump—and settle flocs in a clarifier. PACl (polyaluminum chloride) is a common choice for color, and suppliers position it for industrial use (polyaluminum chloride). Ancillary aids—commercial coagulants and flocculants—are selected based on pH and organics.
Activated carbon adsorption as polishing
Activated carbon (AC) adsorbs dissolved organics onto a porous surface, targeting relatively small and hydrophobic species—including phenolic oligomers and chlorinated aromatics—that can bypass coagulation (MDPI) (Springer). In practice, AC is a tertiary polish after biological or chemical treatment.
Reported color cuts are striking. Granular AC at 5 g/L achieved 99.5% color removal (dropping from ~255 to ~30 units) after 4 hours; another study using plastic‑derived AC (1.25 g per 100 mL, 7 h contact) removed 96.5% of color in actual paper mill wastewater (MDPI) (Springer). Even at shorter times, AC often removes >80% of color: one test saw 83% drop in 2 h, 96% in 4 h (MDPI).
AC’s affinity is broad. It picks up residual lignin fragments and chlorinated aromatics; very hydrophilic, low‑MW acids are less retained. As a polish, AC can take effluent to near colorless—often <5–10 APHA units (MDPI).
Dose and contact time drive performance. Typical dosing falls between 0.5–5 kg/m³; one continuous contactor held 5 kg/m³ with ~0.5 kg/m³ makeup to sustain inventory (MDPI). Columns need periodic backwash or replacement as they saturate.
Costs are higher than coagulation. Tertiary AC polishing typically runs $1.10–$3.30 per m³; using low‑cost or waste‑derived carbons can reduce material cost, with studies showing 94–96% color removal on such media (MDPI) (Springer). AC itself is the main expense; energy use is modest. Spent AC requires regeneration or disposal.
As a unit process choice, mills often deploy a fixed‑bed or contactor of activated carbon after clarification or coagulation, sometimes preceded by simple prefilters to keep fines from clogging media—cartridge housings are typical here (cartridge filter).
Membrane filtration (UF/NF/RO) performance and power
Membranes separate by size and charge. Ultrafiltration (UF, typical molecular weight cut‑off ~1–100 kDa) retains suspended solids, colloids, and larger organics; nanofiltration (NF, ~200–1000 Da) and reverse osmosis (RO, <100 Da) block smaller dissolved organics and salts. In pulp streams, UF often precedes NF/RO to remove >99% of TSS and turbidity (ResearchGate).
On real effluent, a ceramic UF (30 kDa) at acidic pH achieved 99% turbidity removal and 81% UV254 reduction (ResearchGate). UF alone removed ~15% of lignin in a black liquor study, but NF membranes in that work rejected >92% TOC—indicating near‑complete removal of dissolved organics when the MWCO is tight enough (MDPI) (MDPI). In pilots combining UF then NF, permeate was near‑clear with TOC down ~90% (MDPI). In short, UF removes most colloidal color, NF/RO removes the rest; achievable color removal with UF+NF is typically 95–99%, often taking apparent color to near zero.
Head‑to‑head, membrane filtration has outperformed coagulation on UV254 removal in specific trials: ~80% versus ~60% in the Simonič study (ResearchGate)—and in that work, UF (acidic feed) removed 81% of UV254 while the best coagulation scheme achieved ~58% (note the higher pH and absence of a carbon step) (ResearchGate).
Operations are energy‑intensive relative to coagulation/adsorption. UF modules typically run at 1–3 bar, with fluxes around 75–100 L/m²·h at 1–2 bar. NF/RO operate roughly 10–40 bar, with fluxes ~20–30 L/m²·h at 20–35 bar (ResearchGate) (MDPI) (MDPI). UF cleaning by backwash can restore 88–91% flux, implying mostly reversible fouling—as [28] notes. Energy use scales with pressure: UF on the order of 0.5–1 kWh/m³; NF/RO several kWh/m³. A black liquor NF case estimated ~13 kWh/m³ if treating all liquor with evaporation versus only 0.6 kWh/m³ for a partial NF scenario (MDPI). By contrast, coagulation or AC require negligible electricity beyond mixers.
Pre‑treatment is mandatory to control fouling. UF often follows primary clarification or coagulation; NF/RO are typically final polishing steps, sometimes preceded by activated carbon or ion‑exchange for scaling ions (ResearchGate). In practice, mills spec an upstream UF skid as pretreatment and a brackish‑service RO train for polishing (ultrafiltration; brackish water RO), and deploy NF when hardness or divalent ions and color are both targets (nanofiltration). Where scaling control is critical, a resin bed is common (ion exchange).
While quantitative cost data are sparse, tertiary NF/RO is often cited around $1.10–$3.30 per m³—on par with AC polishing in some contexts (MDPI). CAPEX and power, plus membrane replacement and cleaning chemicals, dominate OPEX. Membranes produce no metal‑laden sludge but do create a concentrate; in pulp service the brine is often burned or evaporated.
One consequence noted in the literature: UF permeate often already meets discharge laws on pH, BOD, and turbidity (ResearchGate)—it’s the color fraction that NF or AC then polishes. For plants building out tertiary trains, packaged membrane systems simplify integration with existing clarifiers and filters.
Performance by compound type
Lignin and other high‑MW color bodies: all three options remove these well; coagulation plus AC is standard. Specialized coagulants such as oxo‑titanium sulfate can push lignin removal toward 90–100% (PubMed).
Chlorinated phenolics and dyes: AC and NF/RO outperform coagulation, since these small aromatics are less charged and often remain soluble after flocculation but adsorb strongly or are physically rejected (Springer).
Refractory organics (AOX): AC or advanced oxidation (not covered here) are typical; coagulation has limited effect on fully dissolved chlorine species.
Cost and operations summary
Coagulation/flocculation: 80–95% bulk color removal under optimized conditions; chemicals are often < $0.50/m³ for requires, but sludge handling is a key liability (MDPI) (PubMed).
Activated carbon: 90–99% total color removal typical as a polishing step; $1.10–$3.30 per m³; media cost and regeneration dominate (MDPI) (MDPI) (Springer).
Membranes (UF/NF/RO): ~80–99% across color fractions with UF removing colloids and NF/RO removing dissolved color; highest CAPEX and energy, plus cleaning and replacement (ResearchGate) (MDPI).
Adoption trends and hybrid playbooks
Tight discharge limits and reuse targets are pushing hybrid designs that blend coagulation with a membrane or adsorption polish. One case from Pakistan used primary clarification + alum coagulation + activated carbon to meet reuse standards, with greater than 90% reductions enabling recycling to the process (MDPI). Literature likewise points to coagulation + AC (or UF) + NF/RO as a reliable integrated path (MDPI) (ResearchGate).
Spending in pulp/paper water reuse is expected to grow briskly; one market report forecasts ~15% annual growth from 2024 to 2031 in Asia‑Pacific and other regions (Verified Market Research). Facilities typically treat bulk flows with lower‑cost coagulation and deploy AC or membranes selectively on blowdown or recycle streams to hit stringent color endpoints.
Bottom line for mills
Chemical coagulation is a cost‑effective workhorse for coarse color removal but produces metal‑laden sludge and may leave a residual tint. Activated carbon is a powerful polish, often removing >95% of remaining color, albeit at higher operating cost. Membrane filtration achieves the highest purity—removing essentially all color bodies—but demands significant capital, energy, and disciplined operation. Many mills adopt a hybrid: coagulate, then adsorb or membrane‑polish, matching the train to influent composition, target quality, and total cost.
References
Key studies and reports: MDPI (performance and costs), MDPI (cost breakdowns), ResearchGate (UF vs. coagulation), Springer (activated carbon from plastic waste), MDPI and MDPI (UF/NF/RO on black liquor), PubMed (coagulation–flocculation review), and Verified Market Research (market trends).
