A multi‑stage design that starts with screening and clarification, leans on anaerobic and aerobic biology, and finishes with membranes and carbon is now delivering EU‑grade effluent — including BOD around 25 mg/L and near‑zero color — from notoriously tough pulp‑and‑paper wastewater.
Raw pulp‑and‑paper effluent is a brutal mix: thick with fibers and debris, sky‑high in organics, and deeply colored. Typical recycled‑paper mill wastewater clocks in at chemical oxygen demand (COD, an oxidant‑based measure of total organics) around 3,300–6,300 mg/L and five‑day biochemical oxygen demand (BOD₅, the biodegradable fraction) near 1,650–4,000 mg/L (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).
The fix is not a single machine but a train: screen and settle first, let microbes do the heavy lifting next, then strip out color and refractory organics with advanced polishing. Done right, primary and secondary steps alone commonly deliver 85–98% BOD₅ removal and 60–85% COD removal (intechopen.com), with tertiary membranes and adsorption pushing effluent quality to “free of color and organic compounds” and into reuse territory (mdpi.com).
Primary screening and clarification design
Front‑end solids control starts with screening and grit removal (bar racks, de‑sanding), then primary clarification (sedimentation). In practice, primary clarifiers remove on the order of 80–90% of suspended solids, but little BOD (mdpi.com) (intechopen.com). One study summed it up: “...80% suspended solids removal on average, but little BOD removal” (mdpi.com).
Upstream equipment spans coarse screens to compact pre‑treatment modules; mills typically combine front‑end screens with integrated systems such as waste‑water physical separation for debris and grit control.
Clarifier design values are long by primary standards — two‑stage settlers with roughly 2–4 hours of retention — to capture fibers and fines. Chemically assisted settling (coagulants that bridge particles and flocculants that strengthen flocs) materially improves performance: trials with alum, lime, or MgSO₄ report ~90% turbidity and total suspended solids (TSS) removal with notable BOD/COD cuts (mdpi.com). For example, after gravity settling, dosing ~3400 mg/L MgSO₄ achieved ~89–90% reductions in turbidity, COD, TSS, and color, while 1.4 g/L lime delivered ~94% turbidity and 86% COD removal (mdpi.com).
Settling tanks themselves are purpose‑built; mills commonly deploy a clarifier to remove suspended solids within the multi‑hour detention window cited above.
Coagulant addition is precise by necessity. Plants meter alum or polyaluminum chloride in the flocculation zone using a dosing pump, then reinforce particle aggregation with tailored aids (e.g., “coagulants” and “flocculants” in the literature) to boost clarifier efficiency by 30–50% (mdpi.com). In practice, the two‑stage primary clarifier with flocculation removes ~70–90% of settleable solids and trims ~20–30% of soluble organics (mdpi.com) (mdpi.com).
Despite that, the clarifier overflow can still carry ~70–80% of the influent BOD and COD, making a biological stage non‑negotiable.
Primary effluent quality and loading
The sheer strength of raw effluent sets the tone: COD ~3,300–6,300 mg/L and BOD₅ ~1,650–4,000 mg/L in recycled‑paper wastewater (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Even with 80–90% TSS removal ahead of it, the biological stage must handle hundreds to thousands of mg/L of residual COD.
Anaerobic digestion for high‑strength loads
When influent BOD tops 2,000–3,000 mg/L, mills frequently add an anaerobic step first (e.g., UASB, “upflow anaerobic sludge blanket,” or EGSB, “expanded granular sludge bed”). Studies show 70–85% COD removal for pulp/paper effluent under mesophilic/thermophilic operation, with Bakraoui et al. reporting 80.8% COD removal (and 90% TSS removal) at an organic loading of 7.3 g/L·d in a UASB — quantified elsewhere as 7.27 g COD/L·d (pmc.ncbi.nlm.nih.gov).
The biogas dividend is material: on the order of 50–60 L methane per kg COD removed (pmc.ncbi.nlm.nih.gov), cutting net aeration energy later in the train. Commercial offerings span complete wastewater anaerobic and aerobic digestion systems to match mill load and footprint.
Where anaerobic is bypassed, the full organic burden shifts to the aerobic stage; extended‑aeration or high‑rate reactors can be sized accordingly.
Aerobic treatment and nitrification
Activated sludge (suspended‑growth mixed liquor) or biofilm systems finish the bulk organic removal. Typical activated sludge runs at MLSS (mixed liquor suspended solids) ~2,000–6,000 mg/L, hydraulic residence time (HRT) around 4–8 hours, and sludge age 5–15 days (intechopen.com). Modern plants report combined primary + secondary performance of 85–98% BOD₅ removal and 60–85% COD removal (intechopen.com).
Aeration basins can be deployed as classic activated‑sludge systems or as moving‑bed biofilm reactors with carriers. Nitrification/denitrification is achievable where needed: nitrogen removals of 20–50% and phosphorus 40–85% are cited in industry reviews (intechopen.com).
Suspended‑biofilm designs such as MBBR increase surface area for microbes, while hybrid approaches like membrane bioreactors (MBR) couple biological treatment with ultrafiltration to produce reuse‑grade effluent.
Tertiary membranes, adsorption, and oxidation
After biological treatment and secondary clarification, effluent frequently retains 50–200 mg/L of COD and stubborn color from lignin and chlorinated organics. Tight discharge limits drive tertiary steps. Coagulation/flocculation can be extended with alum, iron salts, or PACl to curb residual organics, but advanced adsorption or membranes often do the heavy lift (mdpi.com).
Granular or powdered activated carbon adsorbs lignin and chlorophenols. In combined tests, coagulation followed by GAC columns delivered ~99% color removal and ~90% reduction of phenolic compounds (mdpi.com).
Advanced oxidation processes (AOPs) — ozonation, Fenton’s reagent, UV/H₂O₂, or electrochemical oxidation — can mineralize >90% of residual COD and color, with solar‑assisted Fenton reported to remove “more than 90% of COD and total polyphenols” and an electrochemical oxidation pilot achieving ~84% COD and 96% color removal (mdpi.com). These steps are energy‑intensive; application is an economics call (mdpi.com).
Membranes are proven polishers. Post‑biological ultrafiltration or microfiltration removes residual suspended solids and delivers a clean feed to tighter membranes; it also yields a bioreactor locked sludge. Studies show UF permeates from pulp‑and‑paper wastewater are high‑quality and often reusable in process (mdpi.com).
Downstream, nanofiltration and reverse osmosis strip dissolved organics and color; field cases report NF permeate “free of color and organic compounds” from secondary‑treated mill effluent (mdpi.com). An economic analysis concluded UF/NF is likely the best option for stringent pulp effluent, enabling water reuse and “zero‑liquid‑discharge” (ZLD) with major indirect benefits (reduced freshwater use, lower emissions), albeit at higher CAPEX (mdpi.com).
Final polishing, disinfection, and targets
For finishing, mills deploy ultrafine filtration or slow sand filters, then disinfect (UV or chlorination) to meet microbial limits; remaining turbidity is typically driven to near zero upstream. Discharge pH is adjusted to neutral (generally 6–9). Disinfection can be chemical‑free using UV systems for 99.99% pathogen reduction.
The final specification is tight but feasible with the train above: BOD <25–50 mg/L, COD <125 mg/L, TSS <30 mg/L, and virtually no visible color, with EU BAT guidance specifically noting “BOD is expected to be around 25 mg/L” after full treatment (eur-lex.europa.eu) (mdpi.com).
Measured performance and case data
Step‑by‑step efficiencies track the engineering: primary clarification at 80–90% TSS and ~20–30% COD/BOD removal; anaerobic digestion at 70–85% COD removal; aerobic bioreactors at ≥90% of remaining BOD₅ and ~60–85% of remaining COD (pmc.ncbi.nlm.nih.gov) (intechopen.com); advanced polishing removing >90% of color and refractory organics (mdpi.com) (mdpi.com). Combining flocculation and treatment yielded overall COD cuts above 90% in lab work (e.g., [50†L125‑L129] showed 84% COD removal in a single coag flocculation step after primary).
State‑of‑the‑art installations report final effluent with BOD often <20–30 mg/L, COD ~50–100 mg/L, and color (e.g., ADMI, a color index) near zero after tertiary treatment (mdpi.com) (eur-lex.europa.eu). One pilot logged 96% color removal and 84% COD removal with ozone added to NF concentrate; another achieved >99% color removal via chemical/GAC polishing (mdpi.com) (mdpi.com).
Regulatory benchmarks and context
In Indonesia (as elsewhere), discharged pulp‑and‑paper effluent faces strict limits; older national thresholds targeted BOD ~150 mg/L and COD ~350 mg/L for pulp mills. Modern practice pushes far lower. EU BAT conclusions for integrated paper mills (recycled fiber) cite 0.4–1.4 kg COD per tonne product — implying effluent COD on the order of 50–100 mg/L for typical flows — and state “BOD is expected to be around 25 mg/L” after full treatment (eur-lex.europa.eu) (eur-lex.europa.eu).
Energy and water impacts
The multi‑stage layout does more than comply. Anaerobic digestion offsets energy via biogas, while membranes enable reuse, cutting freshwater intake and discharge volumes (mdpi.com). Combined primary and secondary treatment alone typically removes ~85–90% of total solids (mdpi.com) (intechopen.com), while advanced steps peel off nearly all remaining fractions.
In sum, the design yields robust desulphurization of pulp mill effluent with measurable outcomes — notably >95% reduction in BOD/SS loads and near‑complete color elimination (pmc.ncbi.nlm.nih.gov) (mdpi.com) — supporting both environmental goals and regulatory compliance.
Equipment and chemical notes
Front‑end solids management uses screens and primary settlers; many plants deploy automatic screens before sedimentation. Coagulation is typically handled with alum or PAC (polyaluminum chloride), available in industrial grades such as PAC, dosed precisely upstream of flocculation.
Biological stages align with standard offerings — e.g., activated‑sludge aeration for high‑rate carbon removal or biofilm carriers in MBBR systems where footprint or load variability demands.
Tertiary membranes are widely configured as integrated UF/NF/RO systems, with UF used as pretreatment to protect tighter membranes and NF removing dissolved color and organics to meet stringent targets.
Sources and references
Design parameters and removal rates throughout this report are drawn from recent reviews and case studies, including Mainardis et al. on tertiary AOP/adsorption and reuse benefits (mdpi.com) (mdpi.com); Mehmood et al. on primary coagulation performance (mdpi.com) (mdpi.com); Cabrera on pulp‑mill biological treatment (intechopen.com) (intechopen.com); Bakraoui et al. on UASB COD removal and OLRs (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov); and the European Commission’s BAT conclusions for pulp, paper, and board (eur-lex.europa.eu).
References: M. Mainardis et al. (2022), “Sustainable alternatives for tertiary treatment of pulp and paper wastewater,” Sustainability 14(10), 6047 (mdpi.com) (mdpi.com); K. Mehmood et al. (2019), “Treatment of pulp and paper industrial effluent using physicochemical process for recycling,” Water 11(11), 2393 (mdpi.com) (mdpi.com); M.N. Cabrera (2017), “Pulp Mill Wastewater: Characteristics and Treatment,” In Biological Wastewater Treatment and Resource Recovery (IntechOpen) (intechopen.com) (intechopen.com); M. Bakraoui et al. (2019), “Biogas production from recycled paper mill wastewater by UASB digester,” Biotechnol. Reports 25, e00402 (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov); European Commission (2014), Commission Implementing Decision 2014/687/EU, BAT conclusions for pulp, paper and board (eur-lex.europa.eu).
