Pulp and paper mills discharge up to 50–200 m³ of wastewater per ton of product — a dark, high‑COD stream that’s tough to treat. A staged plant design—clarify, biologically remove, then polish with membranes or carbon—now consistently drives color to near‑zero and COD into the tens of mg/L.
Pulp and paper manufacturing produces very high‑strength effluent—typically 50–200 m³ of wastewater per ton of product (mdpi.com). It carries extremely high organic loads (COD often 1,000–3,000 mg/L) from lignins, resin acids, and process chemicals, plus suspended fibers and wood fines (mdpi.com) (pmc.ncbi.nlm.nih.gov).
One mill study logged raw wastewater at COD ≈2,820 mg/L, BOD₅ ≈975 mg/L, TSS ≈784 mg/L, and color ≈6,660 Pt‑Co units (Pt‑Co is a color scale) (mdpi.com). The stream often has low biodegradability (BOD/COD <0.4) and high AOX (adsorbable organic halides) and color from lignin residues. Indonesian discharge standards are stringent: a 1995 Ministry of Environment decree caps COD at ≤350 mg/L, TSS ≤150 mg/L, BOD₅ ≤150 mg/L, and pH 6–9 (123dok.com). The path to compliance is a multi‑stage train: primary clarification, robust biological treatment, and advanced polishing (sciencedirect.com).
Primary clarification and solids removal
Raw effluent first meets coarse screening and grit removal—coarse screens (≈3–5 mm) strip out wood chips, fibers, and grit to protect downstream units and recover fibers (mdpi.com). Where mills want continuous debris removal, an automatic screen sits at the headworks.
Settling follows. Primary clarifiers (sedimentation tanks) sized at roughly 4–6 hours of detention are typical; a clarifier in this range commonly removes ~30–50% of TSS and ~10–20% of COD, easing the biological load (mdpi.com). In one test, a 4‑hour settle dropped TSS by ~30% (784→550 mg/L) and COD by ~15% (2,820→2,410 mg/L), with similar turbidity gains (mdpi.com).
For footprint‑constrained plants, a lamella settler can reduce clarifier footprint by up to 80% compared with conventional basins. If the mill elects to use coagulant/flocculant aids to boost gravity settling, coagulants and flocculants are dosed—though the paper notes that highly aggressive primary coag‑floc (e.g., alum) can remove up to ~90% TSS and ~85% COD, a move that functionally merges into tertiary treatment and increases sludge (mdpi.com). Chemical feed is typically controlled via a dosing pump for accuracy.
Anaerobic digestion for high‑load organics
Given the high organic strength, a two‑stage biological system starts with anaerobic digestion. Anaerobic reactors such as UASB (upflow anaerobic sludge blanket) remove concentrated soluble organics and produce biogas; bench work on recycled‑paper wastewater shows ≈80% COD removal under mesophilic conditions (pmc.ncbi.nlm.nih.gov). A 70 L UASB at ~15 h HRT (hydraulic retention time) treating ~7.2 g COD/L·d achieved 80.6–80.8% COD removal, with methane yields around ≈0.07–0.09 m³ CH₄ per g COD removed (pmc.ncbi.nlm.nih.gov).
In design, mills often size anaerobic reactors for outlet COD near ~500 mg/L from ~2,500 mg/L influent (≈80% removal) at 12–24 h HRT, depending on temperature and loading (pmc.ncbi.nlm.nih.gov). System vendors supply complete anaerobic digestion trains for pulp effluents, with low sludge yields compared to aerobic basins.
Aerobic polishing and secondary clarification
Post‑anaerobic, an aerobic stage (e.g., activated sludge) removes the remaining BOD/COD. With MLSS (mixed liquor suspended solids) around ~3–5 g/L, DO (dissolved oxygen) >2 mg/L, and 8–12 h aeration, conventional activated sludge can deliver >85–90% removal on the residual organics, often pushing total COD to the tens of mg/L (sciencedirect.com). Vendors package activated sludge systems with secondary clarifiers to drive effluent SS toward ≈20 mg/L and to nitrify ammonia where required.
Some mills opt for a membrane bioreactor (MBR), which combines biology with ultrafiltration to produce reuse‑quality effluent; a membrane bioreactor also simplifies solids separation. Literature consistently reports higher overall removal and lower energy/greenhouse gas impacts when anaerobic and aerobic stages are combined than when either is used alone (sciencedirect.com).
Color and refractory organics: tertiary options
Even after biological treatment, color from lignin fragments and some chlorinated organics (AOX) persist. Coagulation/flocculation post‑secondary can be highly effective: alum around ~1.2 g/L at pH ~6 removed ~92% COD, ~98% TSS, and ~96% color in lab tests (mdpi.com). Post‑coagulation filters, such as sand or cloth, can remove >80% of remaining TSS but leave dissolved COD largely untouched (mdpi.com); a sand filter is a common choice.
Activated carbon (adsorption) is a strong finisher for residual color and COD. In one trial, powdered activated carbon at 5 g/L for ~4 h drove COD down to 26.6 mg/L and color to 30.6 Pt‑Co units—essentially erasing visible color (mdpi.com). Lower doses (1–3 g/L) achieved 80–95% of that effect, while GAC systems often report ~70–90% color removal and >50% COD removal depending on contact time; mills typically deploy activated carbon systems after solids polishing. The study cautions that 5 g/L is a high dose in full‑scale design.
Advanced oxidation processes (AOPs)—ozone, ozone/UV, or Fenton’s reagent—also attack colored lignins. Ozonation has achieved ~84% COD and ~96% color removal, and Fenton + UV has mineralized >90% of COD and polyphenols in published work, albeit with higher energy and operating costs (mdpi.com).
Membranes offer a clean final barrier. Ultrafiltration (UF) strips colloids to produce a color‑free permeate; UF followed by nanofiltration (NF) or reverse osmosis (RO) removes >95–99% of remaining organics and color, yielding water near drinking quality (mdpi.com). As pretreatment or polish, ultrafiltration systems are common; to reach the tightest internal reuse targets, mills add nanofiltration or RO. Full‑train suppliers package UF/NF/RO systems with established membrane brands such as Toray or Filmtec. For disinfection in reuse applications, UV provides a 99.99% pathogen kill rate without chemicals—an approach consistent with ultraviolet systems.
In a refinement loop, RO rejects can be returned to earlier stages or treated via AOP before discharge. When fully implemented, membrane polishing can drive final COD/TSS to <20 mg/L with an essentially colorless effluent (mdpi.com) (mdpi.com).
Illustrative performance through the train
Starting point: Raw influent around COD ~2,500 mg/L; TSS ~800 mg/L; color ~6,500 Pt‑Co units.
Primary clarification (≈4 h detention): ~30% TSS removal to ~500 mg/L and ~15% COD removal to ~2,100 mg/L; color falls ~20% to ~5,200 (all from pilot data) (mdpi.com).
Anaerobic digestion (~18 h HRT): ~80% COD removal on what remains (2,100→420 mg/L), some TSS/sludge removal, strong methane production; color largely unchanged because lignin fragments are not anaerobically removed (pmc.ncbi.nlm.nih.gov).
Aerobic reactor (12–24 h): ~90% additional removal on the residual COD/BOD (420→42 mg/L) with secondary clarification yielding effluent ≈20 mg/L SS; some nitrification occurs; modest color drop may accompany oxidation (sciencedirect.com).
Tertiary filtration: cloth or sand filters reduce particulates further, bringing SS <10 mg/L; where cloth media are used without chemicals, pilots showed up to 81% TSS removal but only ~10% COD removal (mdpi.com).
Coagulation + carbon: coagulant dosing and 4 h granular/powder carbon contact (often 1–2 g/L in full‑scale designs) cut COD a further ~90–95% and drive color toward zero; bench data with 5 g/L PAC drove COD to ~26.6 mg/L and color to 30.6 Pt‑Co units (mdpi.com).
Optional membrane polishing: UF/RO brings COD well below 20 mg/L and TSS to near‑zero with a clear effluent; UF permeate has been fully reused in several paper processes in studies (mdpi.com) (mdpi.com). Suppliers frequently support such add‑ons with ancillary equipment.
Outcome: The full train can reliably meet or exceed Indonesia’s standards. Conservative expectations are final COD on the order of 10–50 mg/L, BOD₅ <20 mg/L, TSS <10 mg/L, and color <50 Pt‑Co (often <10), well below COD 350 mg/L and TSS 150 mg/L limits (123dok.com).
Reuse potential, cost, and energy balance
With UF/RO polishing, treated water can be reused in paper machines or cooling—studies note UF permeate can be fully reused and that UF projects costing about ~$19M can pay back in ~5 years if reclaimed water is sold at €1/m³ (mdpi.com). For mills prioritizing RO in brackish ranges, brackish‑water RO units are routinely integrated after UF.
Anaerobic digestion contributes to the energy balance through biogas: reported methane yields are ≈0.07–0.09 m³ CH₄ per g COD removed in bench digestion of paper mill wastewater (pmc.ncbi.nlm.nih.gov); typical design guidance also cites 0.3–0.5 m³/kg COD. Where process changes or maintenance plans require short‑term capacity, plants often lean on wastewater ancillaries to maintain performance.
Filtration and sedimentation are relatively low‑cost polishing steps: cloth filters (no chemicals) achieved 81% TSS removal but only ~10% COD removal in pilots, while adding carbon or coagulants boosts COD removal at added material cost (mdpi.com). Choosing between adsorption and membranes often hinges on reuse value as much as on discharge limits; suppliers bundle membrane systems that can be justified by internal water reuse.
Water reuse is increasingly common at the sector level: one study notes Finnish mills cut freshwater use by ≥75% through internal recycling initiatives (mdpi.com).
Data‑backed removal and compliance
Across the literature and pilots, multi‑stage treatment achieves “COD reduction ~99%, color removal ~99%, TSS removal ~98%,” provided stages are tuned for the specific stream (mdpi.com) (mdpi.com). Post‑process disinfection and pH correction finalize the effluent for discharge or reuse; UV remains a low‑OPEX option aligned with UV disinfection.
Sources and citations
Figures and design parameters are drawn directly from industry and peer‑reviewed sources: overviews and performance bounds (sciencedirect.com), pulp and paper effluent characterization and treatability (mdpi.com), primary settling performance data (mdpi.com), UASB COD removal and methane yield in paper mill wastewater (pmc.ncbi.nlm.nih.gov), coagulation/adsorption color and COD removal (mdpi.com), tertiary options including AOPs and filtration economics (mdpi.com) (mdpi.com), Indonesian regulatory limits via KD MethodsNY (Indonesian Ministry regs) (123dok.com), and reuse case data including UF cost and payback (mdpi.com).
