Why pulp and paper wastewater is hard to treat for COD
Pulp and paper mills consume 250–300 m³ of water per ton of paper produced, which makes the ETP one of the largest hydraulic loads on any industrial site (S3). That water leaves the mill carrying spent pulping liquor, bleach-plant filtrate, paper-machine white water, and recycled-fiber deinking effluent, each with a distinct COD profile. Kraft digester and bleach-plant streams dominate the COD load (79% and 85% reductions reported in primary clarification studies), while paper-machine white water carries mostly suspended fiber and filler (S3).
The defining chemistry problem is the BOD/COD ratio of 0.15–0.35, far below the 0.4–0.8 typical of food or municipal wastewater (S4). A low BOD/COD ratio means most of the COD is non-biodegradable: lignin fragments, chlorinated organics, and high-molecular-weight tannins. The dark brown color is the visible signature of those same lignin and tannin derivatives, and conventional activated sludge strips only 10–30% of kraft color in practice (S4). That color is not just cosmetic; it correlates with residual COD, AOX, and lignin that survive biological treatment and dictate whether the mill can meet its discharge permit.
Stream-by-stream the picture is mixed. Pulping liquor carries 60–90% of the dissolved COD and most of the color. Bleach-plant effluent contributes AOX, chlorophenols (90–99% removal in well-operated mills), and high-color filtrate. Paper-machine white water is mostly suspended fiber and fillers (CaCO₃, clay, TiO₂) with low dissolved COD. Recycled-fiber deinking effluent sits between these — moderate COD, moderate color, mostly biodegradable surfactants and inks. A single-stage biological plant cannot deal with all four streams at once, which is why the four-stage train exists.
How COD is removed: the four-stage treatment train
Every modern paper-mill ETP is built as a four-stage train because no single unit operation covers the full COD, color, and toxicity load. The stages run in the order the water sees them: primary, anaerobic, aerobic or membrane, and tertiary polishing.
Stage 1 — Primary clarification or DAF. Screening, fiber recovery, and either a clarifier or a dissolved air flotation system for pulp and paper primary treatment strip suspended solids — fibers, fillers, and the particulate fraction of COD. Typical suspended COD removal is 10–30% (S3, S4). DAF is favored for paper-mill white water because it floats low-density fiber, while clarifiers handle higher-density grit and filler.
Stage 2 — Anaerobic. UASB or EGSB reactors take the high-strength dissolved COD that survives primary treatment. Chinnaraj and Venkoba Rao (2006) reported 80–93% COD removal from agro-based pulp and paper effluent in a UASB (S3). The reactor's main value is twofold: it removes the bulk of biodegradable COD without aeration, and it produces biogas that offsets plant steam demand.
Stage 3 — Aerobic or MBR. Activated sludge, MBBR, or MBR membrane bioreactor for pulp and paper effluent polish the anaerobic effluent. AS delivers 60–95% COD (S3), MBBR 50–70% COD in standalone operation (S3), and MBR variants 72–99% COD including the submerged anaerobic MBR (83% COD) and fixed-bed MBR (92–99% COD) configurations (S3).
Stage 4 — Tertiary PAC/PAM. Polyaluminum chloride coagulation plus anionic polyacrylamide flocculation handles residual color, lignin, and any COD that survives biology. Doses run 100–400 mg/L PAC and 1–3 mg/L PAM at pH 5.5–7.0, delivering 30–60% additional COD and 70–90% color removal after biology (S4). The next section compares the four stages head-to-head.
Stage-by-stage performance: COD removal and operating parameters
The table below consolidates the numeric performance ranges and operating notes for each stage. Influent and effluent COD values are typical operating windows for kraft and recycled-fiber mills based on the studies in S3; cumulative removal assumes the stages are run in series with reasonable intermediate equalization.
| Stage | Typical influent COD (mg/L) | Typical effluent COD (mg/L) | COD removal | Key equipment | Energy / footprint note |
|---|---|---|---|---|---|
| Primary — DAF or clarifier | 2,000–8,000 (raw) | 1,500–6,000 | 10–30% (suspended COD) | DAF cell, primary clarifier, fiber-recovery screen | Low energy; small footprint for DAF; recovers saleable fiber |
| Anaerobic — UASB / EGSB | 3,000–8,000 | 300–1,000 | 80–93% (S3, Chinnaraj and Venkoba Rao 2006) | UASB or EGSB reactor, gas holder, sludge bed | No aeration energy; produces biogas that offsets steam; tall reactors, moderate footprint |
| Aerobic — AS (CAS) | 300–1,000 (post-anaerobic) | 30–200 | 60–95% COD; 95–99% BOD; 97% TSS (S3) | Aeration tank, clarifier, RAS loop | Aeration is the dominant energy draw; large footprint; sensitive to bulking |
| Aerobic — MBBR | 500–2,000 | 150–800 | 50–70% standalone; up to 98% toxicity reduction at 25 kg COD/m³·d (S3) | Moving-bed biofilm reactor, screen, clarifier | No sludge recycle; smaller tankage than CAS; biofilm-driven |
| Aerobic/MBR — MBR variants | 300–2,000 | 10–150 | 72–99% across configurations; FBMBR 92–99% (S3) | Membrane tank with PVDF MF/UF at 0.1–1 μm pore size | ~60% smaller footprint than CAS; higher CAPEX; membrane fouling control required |
| Tertiary — PAC + PAM | 30–200 (post-biology) | 10–80 | 30–60% additional COD; 70–90% color (S4) | Flash mixer, floc tank, settling tank or filter, PLC-controlled dosing skid | Chemical cost dominates; modest footprint; sludge volume to handle |
Two patterns are worth flagging. First, the biggest single-step COD drop happens in the anaerobic stage, not the aerobic. Second, MBR variants close the gap to <50 mg/L effluent COD that AS alone cannot reach, which is why the MBR route has become the default for mills with strict color and AOX limits. The table also makes the energy trade-off visible: anaerobic saves aeration energy at the cost of a taller reactor, and MBR saves footprint at the cost of membrane replacement and cleaning.
Choosing between activated sludge, MBBR and MBR for pulp and paper effluent
The biological step is the highest-leverage decision in the train because it sets the floor under which tertiary chemistry has to operate. The three viable options have very different cost and footprint profiles.
Conventional activated sludge (CAS). Capital cost is the lowest of the three, and 60–95% COD removal is achievable across the published studies (S3). The downside is footprint: an aeration tank sized for a typical post-anaerobic BOD load plus a secondary clarifier with RAS/PRAS pumping takes significant real estate, and the process is sensitive to bulking, foaming, and toxic shocks from bleach-plant spikes. CAS is the right pick when the mill has land, moderate discharge limits (e.g., 100–250 mg/L COD), and a stable influent.
Moving-bed biofilm reactor (MBBR). A 1994 pilot removed 98% toxicity and 70% COD at an organic load of 25 kg COD/m³·d (S3); a 1997 newsprint pilot hit 65–75% COD and 85–95% BOD at 4–5 h HRT (S3). Full-scale combined-effluent MBBR, however, achieved only 50% SCOD and 21.53% COD when fed a blended pulp, powerhouse, and chemical-recovery stream (Das and Naga 2011, in S3). That gap between pilot and full-scale matters: MBBR alone is unlikely to meet strict limits without polishing chemistry.
Membrane bioreactor (MBR). Configurations include submerged anaerobic MBR (83% COD), fixed-bed MBR (92–99% COD), and MBR coupled with advanced oxidation (90%+ COD with ozonation polishing) per S3. PVDF membranes at 0.1–1 μm pore size retain nearly all biomass and most colloids, giving a clean effluent suitable for reuse. Footprint is roughly 60% smaller than CAS, but membrane CAPEX, aeration for scouring, and chemical cleaning (CIP) all add OPEX.
Decision rule: if the discharge limit is <50 mg/L COD, if color and AOX are regulated, or if the mill is footprint-constrained, specify MBR. If land is available and the limit is 100–250 mg/L COD, MBBR or CAS with tertiary PAC will deliver the required removal at lower total cost. Either way, the downstream PLC-controlled PAC and PAM dosing skid is what closes the residual color and COD gap after the biology.
Tertiary coagulation: PAC dose, pH and polymer selection
Tertiary coagulation is where the last 30–60% of COD and 70–90% of residual color actually come out, and it is the step that turns dark brown biologically treated effluent into a light-straw discharge that meets strict standards (S4). The two parameters that matter most are pH and PAC basicity.
High-basicity PAC (60–75% basicity as Al₂O₃) carries more high-charge polymeric aluminum species, which improves charge neutralization of negatively charged lignin colloids (S4). The optimum pH window is 5.5–7.0, with many mills hitting best results at pH 6.0–6.5 (S4). Outside that window, color removal drops sharply because the aluminum species shift toward lower-charge forms that floc poorly.
The table below maps color-removal targets to PAC dose, expected COD reduction, and the supporting PAM dose. Use it as a starting point for jar tests, then verify against the mill's actual effluent.
| Target color removal | PAC dose (mg/L) | Expected additional COD removal | PAM dose (mg/L) | Operating pH |
|---|---|---|---|---|
| Moderate (50–70%) | 100–200 | 30–40% | 1–2 | 6.0–7.0 |
| High (70–85%) | 200–300 | 40–55% | 2–3 | 5.5–6.5 |
| Strict (80–95%) | 250–400 | 50–60% | 2–3 | 5.5–6.0 |
Anionic polyacrylamide at 1–3 mg/L is paired with the PAC to build larger, faster-settling flocs and to reduce residual aluminum in the final effluent (S4). Cationic PAM is generally avoided in this application because it adds to the dissolved solids load and can hurt downstream reuse. Dosing must be PLC-controlled with online pH and streaming-current monitoring — manual dosing drifts within hours and the color removal collapses with it. Settling typically happens in a dedicated high-rate sedimentation tank or a sand filter, depending on the residual TSS target.
2026 selection framework: matching the train to your mill
The four numeric profiles below cover the configurations a procurement team will see most often in 2026 vendor bids. Each assumes primary DAF, and the difference is what sits between DAF and tertiary chemistry.
Kraft or sulfite virgin-pulp mill with strict color and AOX limits. Primary DAF → UASB or EGSB → MBR → PAC/PAM tertiary. Expected overall COD removal 95–99% with effluent COD typically <50 mg/L and color in the low Pt-Co range. Highest CAPEX and OPEX, but the only configuration that consistently hits the strictest EPA 40 CFR Part 430 BPT and BAT limits for bleached kraft mills and the EU Industrial Emissions Directive 2010/75/EU benchmarks for color and AOX.
Recycled-fiber (RCF) or deinking mill with moderate limits. Primary DAF → AS or MBBR → PAC polishing. Expected overall COD removal 80–90% with effluent COD in the 100–200 mg/L range. Lower CAPEX, lower chemical usage, and the simpler regulatory pathway. This is the right choice when influent COD and color are already moderate because the lignin has been removed in the upstream pulping step (S4).
Unbleached or mechanical-pulp mill with land available. Primary DAF → aerated stabilization basin (ASB) → PAC polish. ASBs in cited studies achieved 67% COD and 90% BOD under nitrogen supplementation, and 84–88% COD in more recent trials (S3). Lowest CAPEX of any option, but footprint is large and performance is temperature-sensitive.
The trade-off is consistent: each added stage raises CAPEX and OPEX but lowers residual COD and color. The framework above helps locate where the bottleneck actually is. For mills weighing DAF against a primary clarifier at the head of the train, the DAF vs clarifier comparison for paper-mill primary treatment walks through the decision. For a broader view of polishing options beyond PAC, the tertiary wastewater treatment selection guide covers sand filtration, activated carbon, and AOP alternatives.
Frequently Asked Questions
What overall COD removal can a four-stage pulp and paper ETP achieve?
A four-stage train (primary DAF or clarifier, anaerobic UASB or EGSB, aerobic or MBR, and tertiary PAC/PAM) routinely achieves 95–99% overall COD reduction, with final effluent typically below 50 mg/L COD for bleached kraft mills operating under EPA 40 CFR Part 430 or the EU Industrial Emissions Directive 2010/75/EU. The anaerobic stage contributes the largest single-step drop (80–93% COD), and tertiary PAC coagulation adds another 30–60% on the residual (S3, S4).
What does a BOD/COD ratio of 0.15–0.35 imply for biological treatment?
A BOD/COD ratio of 0.15–0.35 means 65–85% of the influent COD is non-biodegradable, primarily lignin fragments, chlorinated organics, and high-molecular-weight tannins (S4). This forces a hybrid train: anaerobic and aerobic biology handle the biodegradable fraction, while the non-biodegradable fraction must be removed by primary clarification, membrane separation, or tertiary coagulation. A low BOD/COD ratio is also why conventional activated sludge alone rarely gets below 100–200 mg/L effluent COD on kraft effluent.
What PAC dose and COD removal should I expect from tertiary coagulation?
PAC doses of 100–400 mg/L at pH 5.5–7.0 deliver 30–60% additional COD removal and 70–90% color removal after biological treatment, with anionic PAM at 1–3 mg/L supporting floc formation (S4). Moderate color targets (50–70%) typically need 100–200 mg/L PAC, while strict targets (80%+) require 250–400 mg/L PAC. High-basicity PAC (60–75% basicity) is preferred because of its higher proportion of polymeric aluminum species.
How much COD does a UASB reactor remove in pulp and paper applications?
UASB and EGSB reactors achieve 80–93% COD removal on agro-based and kraft pulp and paper effluent, taking influent COD of 3,000–8,000 mg/L down to 300–1,000 mg/L (Chinnaraj and Venkoba Rao 2006, in S3). The energy advantage is the main reason anaerobic is placed before aerobic: UASB needs no aeration and produces biogas that can offset mill steam demand, while aerobic aeration is typically the largest single OPEX line in a paper-mill ETP.
Which regulations govern pulp and paper effluent discharge?
US mills operate under EPA 40 CFR Part 430 (Pulp, Paper and Paperboard Effluent Guidelines), which sets BPT, BAT, and BCT limits for conventional pollutants and adds specialized standards for bleaching streams. EU mills operate under the Industrial Emissions Directive 2010/75/EU, which sets BAT-AEL ranges for COD, color, AOX, and TSS. Both frameworks drive the move toward four-stage trains with MBR or enhanced biological aeration combined with tertiary coagulation for bleached kraft mills.