Pulp & Paper Water Treatment

Pulp mill wastewater typically has COD of 1,500–5,000 mg/L in combined effluent, with peaks of 10,000–20,000 mg/L from black liquor condensate spills or upset conditions. BOD/COD ratios of 0.3–0.5 indicate a significant refractory organic fraction that resists biological treatment — requiring either extended aeration (HRT 24–48 hours) or advanced oxidation for color and AOX removal.

ECF (Elemental Chlorine Free) bleaching using ClO₂ has dramatically reduced AOX generation compared to elemental chlorine, but AOX levels of 0.5–1.5 kg/tonne of pulp are still typical. Regulatory limits in the EU (0.25 kg AOX/tonne in BAT-AEL) and North America drive continuous optimization of bleaching chemical dosing — where even small improvements in ClO₂ dose accuracy translate to measurable AOX reduction.

Kraft mill effluent is intensely colored due to lignin degradation products, with true color values of 1,000–5,000 Pt-Co units. Color is aesthetically unacceptable in receiving waters and blocks light penetration needed for aquatic ecosystems. Color removal requires either biological treatment (which removes 30–50% of color) supplemented by chemical coagulation (60–80% additional removal) or advanced oxidation (ozone, O₃/H₂O₂) for near-complete decolorization.

Pulp mill chemical environments include: ClO₂ solution at 8–10 g/L (corrosive, explosive gas release risk), hot NaOH at 10–50% and 80–95°C in the causticizing circuit, H₂SO₄ at 93–98% for ClO₂ generation, methanol as a reducing agent, and lime slurry at 15–30% solids. Each requires specific pump metallurgy, diaphragm materials, and valve configurations.

Chlorine dioxide is the primary bleaching agent in ECF pulp mills. It is generated on-site by reacting sodium chlorate (NaClO₃) with a reducing agent (methanol CH₃OH or H₂SO₄/HCl) in a generator. The resulting ClO₂ solution (typically 8–10 g/L) must be injected precisely into the bleaching sequence — typically in D₀ (first ClO₂ stage), D₁, and D₂ stages — at dosing rates of 15–40 kg ClO₂/tonne of pulp depending on target brightness and kappa number.

TCF (Totally Chlorine Free) and enhanced ECF sequences use H₂O₂ as a bleaching and brightening agent. Typical dose: 5–30 kg H₂O₂/tonne of pulp at 3–5% concentration, applied in the EP (Extraction with Peroxide) or P (Peroxide) stages at 60–80°C and pH 10.5–11.5. H₂O₂ is a strong oxidizer that decomposes catalytically on contact with Fe, Mn, Cu — requiring passivated or PTFE-lined pump components.

Paper machine white water contains fine fibers (0.1–5% consistency), fillers, sizing agents, and dissolved organics. Saveall systems (DAF or sedimentation) recover valuable fiber and filler while producing clarified water for reuse. Effective fiber recovery requires optimized coagulant dosing (PAC at 50–200 mg/L or alum at 100–400 mg/L) and retention aid polymer injection.

Kraft mills operate high-pressure recovery boilers (60–100 bar) and power boilers (40–80 bar) that demand ultra-pure feedwater. Internal treatment includes phosphate (Na₃PO₄) or coordinated phosphate-pH control, oxygen scavenger (sodium sulfite Na₂SO₃ for low-pressure boilers, DEHA/carbohydrazide for high-pressure), and filming amine for condensate pH control.

Aerated stabilization basins (ASBs) and activated sludge systems treating kraft mill effluent require: alkalinity supplementation (NaOH or lime) to maintain nitrification pH above 7.0, nutrient dosing (phosphoric acid H₃PO₄, urea CO(NH₂)₂) for BOD:N:P ratio of 100:5:1, and antifoam injection to control surface foam from resin acids and fatty acids.

Residual color after biological treatment (typically 500–2,000 Pt-Co) can be removed by chemical coagulation (ferric chloride at 100–300 mg/L achieves 60–80% color reduction) or advanced oxidation (ozone at 0.5–1.5 g O₃/g COD, or O₃/H₂O₂ at molar ratio 1:0.5). 

Pulp mill sludge is a mix of primary fiber sludge and biological excess sludge. Combined sludge requires polymer conditioning (cationic PAM at 3–8 kg/dry tonne) before dewatering on belt presses, screw presses, or centrifuges.