Bleach-plant wastewater carries high COD/BOD, lignin derivatives, and chlorinated AOX—plus stubborn color—at volumes that can hit 50–80 m³ per tonne of pulp and ~70–80 tonnes per tonne of paper. Meeting Indonesia’s 150 mg/L BOD₅ and 350 mg/L COD caps demands a multi-stage plan that marries anaerobic and aerobic biology with AOPs or membranes.
Bleach-plant wastewater is a concentrated cocktail of organics with high COD (chemical oxygen demand) and BOD (biochemical oxygen demand), lignin derivatives, chlorinated compounds measured as AOX (adsorbable organic halides), and dark color. Some mills generate on the order of 50–80 m³ of wastewater per tonne of pulp, creating ~70–80 tonnes of effluent per tonne of paper produced (mdpi.com). Typical untreated Kraft bleach effluent can have COD on the order of hundreds to thousands of mg/L and AOX in the tens of mg/L. Indonesian regulations (Minister of Environment Decree No.51/1995) set maximum pulp mill effluent limits of BOD₅ ≈150 mg/L and COD ≈350 mg/L (fliphtml5.com) (often expressed per ton of pulp), and raw bleach effluent far exceeds these limits, necessitating multi-stage treatment. Mills worldwide are transitioning to chlorine‑free bleaching to lower AOX formation.
Regulatory benchmarks in Indonesia cap BOD₅ at ~150 mg/L and COD at 350 mg/L for pulp mills (fliphtml5.com). No specific AOX or color limits are given, but best practices target AOX removal >80% and color removal to near ASTM 20 (clear discharge) for regulatory compliance. Indonesian pulp facilities must also meet national water quality standards (e.g., PP71/2014), which usually enforce BOD/COD and TSS standards similar to international pulp & paper effluent guidelines.
Anaerobic stage: UASB/EGSB performance
Anaerobic digestion (e.g., UASB/EGSB) removes bulk organics and produces biogas. Bench‑scale tests treating peroxide‑bleached Kraft effluent report COD removals of ~45–55% (scielo.br), AOX removals of ~40–45% (scielo.br), and substantial biogas generation. In Chaparro et al. (2011), two chlorine‑bleach effluents treated anaerobically achieved ~50% COD and ~42% AOX removal (scielo.br). A byproduct was darker color, since biodegradation of lignin can produce colored intermediates (scielo.br). Design notes cite full‑scale operation at 25–35 °C with retention times of several days, relatively low operating costs (energy‑neutral via biogas), and modest sludge production.
For engineered anaerobic lines, facilities often specify complete digestion packages; category equipment such as anaerobic and aerobic digestion systems is commonly paired with process monitoring to stabilize load swings before downstream polishing.
Aerobic polishing: activated sludge and MBR
An aerobic step is required to consume residual organics and nitrify ammonia (if present). Aeration (e.g., extended activated‑sludge or SBR) can achieve very high COD removal if given sufficient time. Aerobic biodegradation tests on Eucalyptus Kraft bleaching effluent (with partial chlorine substitution) showed overall COD removal of 82–98% after ~2 weeks of treatment; after 13 days in acclimated aeration tanks, TOC was reduced 80–91% and COD by 82–98% for both chlorine‑bleached and peroxide‑bleached effluents (scielo.br). Final COD could be as low as ~10–30 mg/L for chlorine‑free effluent vs 100–180 mg/L for chlorine‑bleached effluent (scielo.br).
In well‑operated aerobic stages, >90% of biodegradable COD can be removed (scielo.br). Aerobic MLSS (mixed liquor suspended solids) levels are typically 3,000–5,000 mg/L (or rely on MBR with higher MLSS). Nitrifying activated sludge or trickling filters reduce residual BOD/COD and eliminate ammonia. Implementations range from conventional activated‑sludge basins to SBR trains, and high‑MLSS options such as membrane bioreactors for tighter polishing.
Integrated biology: equalization to clarifier
A common sequence is equalization and neutralization → anaerobic reactor → aeration tank → clarifier. The anaerobic stage cuts the bulk of COD (while generating methane), and the aerobic stage “polishes” to regulatory BOD/COD levels. One sequential study combining UASB with ~72 h aerobic polishing could meet discharge COD limits when followed by further treatment (scielo.br). Even after anaerobic + aerobic, color remains high, requiring chemical polishing (scielo.br).
Equalization buffers pH swings (chlorinated stages are often acidic) ahead of solids separation. Secondary solids removal via a clarifier typically yields ~5–20 mg/L suspended solids before tertiary treatments.
Advanced oxidation: ozone, Fenton, photo‑Fenton
Biology struggles with recalcitrant color and AOX. Advanced oxidation processes (AOPs) mineralize these compounds. Ozonation is effective on organic load but only partially removes color: optimized ozonation of raw pulp wastewater (9 min contact, pH 5) removed ~88.5% COD but only ~41.2% of color (mdpi.com). When applied to biologically‑treated effluent, ozone removed ~95.9% COD and ~46.4% color (mdpi.com). Ozone does not directly target AOX.
Fenton and Photo‑Fenton (H₂O₂/Fe²⁺ with/without UV) attack halogenated organics via hydroxyl radicals. Under acidic conditions (pH 5), the Fenton process achieved ~88% TOC removal, ~85% color removal and ~89% AOX removal within 30 min; Photo‑Fenton (UV plus Fenton) reached ~85% TOC, ~82% color and ~93% AOX in 5 min (inis.iaea.org). In other trials, raw effluent treated by Fenton reached 86% COD removal and ~26% color removal (mdpi.com), and Photo‑Fenton reached ~90% COD with ~40–41% color removal (mdpi.com).
Hydrogen peroxide alone can oxidize some chromophores: pure H₂O₂ contact gave ~85% COD and 28–39% color removal (mdpi.com), with limited AOX impact unless combined with UV/Fe. Design notes for Fenton include significant H₂O₂ demand, iron sludge generation, and pH adjustment; Photo‑Fenton’s UV demand adds energy cost but often yields higher AOX degradation. Facilities implementing Photo‑Fenton often pair UV sources with reagent management; UV systems in the class of ultraviolet disinfection units and precise chemical metering via a dosing pump can support this stage.
Studies consistently find that an AOP post‑treatment can “finish off” the color/AOX that survives biology. One review notes biological treatment is cost‑effective but leaves residual color (mdpi.commdpi.com). Using ozone or Fenton after aerobic polishing can push AOX removal above 90% and achieve near‑total color reduction (inis.iaea.org) (pubmed.ncbi.nlm.nih.gov). In summary, AOP can achieve very high COD and AOX removals (often >85–90%) under optimal conditions (mdpi.com) (inis.iaea.org), though typical color removals per pass are more modest (20–50% in one step; mdpi.com), often requiring multiple passes or combinations (e.g., ozone + UV). AOPs are best viewed as tertiary polishing after biology.
Membrane filtration: UF, NF, RO
Membranes can physically reject dissolved organics and colorants for tertiary polishing. Nanofiltration (NF) membranes (e.g., NF270) have been tested on pulp wastewater. Under suitable (neutral pH, high‑shear) conditions, NF removed >80% of organic carbon and virtually all color; in cross‑flow fibrous tests, the best NF membranes “almost completely eliminated the color” and removed ~80% of COD and ions (pubmed.ncbi.nlm.nih.gov). NF reduces residual color and lowers AOX (organics) but often leaves monovalent salts. Category solutions like nano‑filtration slot here when some mineral passage is acceptable.
Reverse osmosis (RO) yields the highest purity at higher energy cost. In one pilot, cellulose‑acetate RO (40 bar, pH=10) on biologically‑ and Fenton‑treated effluent achieved ~82% COD removal and 96% color removal (pubmed.ncbi.nlm.nih.gov). When coupled with upstream NF (to soften/buffer feed), RO removed >99% of hardness and major ions (pubmed.ncbi.nlm.nih.gov), yielding permeate that was essentially demineralized and colorless. RO requires rigorous pre‑treatment to prevent fouling (e.g., MF/UF for particulates, bio‑loss, antiscalants), and produces a concentrated brine (10–50% of feed) that must be handled or recycled. In practice, ultrafiltration often precedes RO, and scaling control uses formulations in the class of membrane antiscalants. For high‑recovery reuse, facilities deploy brackish‑water RO or integrated membrane systems designed for industrial effluents.
UF alone will not remove color effectively (molecules <10 kDa pass), but it protects downstream membranes. Overall, membranes can push effluent purity to meet reuse or ZLD (zero liquid discharge) goals. For business/security decisions: NF/RO investment is high but can recover water and reduce discharge. Performance data suggest >90% COD removal and essentially total color removal are achievable (pubmed.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov) if appropriately designed.
Sequencing and design parameters
Integrated process flow: equalization/neutralization collects bleach‑plant drains in an equalization basin to buffer pH swings and load peaks (chlorinated stages are often acidic). The anaerobic reactor then operates at ~30–35 °C and HRT ~3–5 days, with ~40–60% COD removal and nascent methane; biogas may cover part of the operating expense. Aerobic polishing via an aeration tank or SBR can reduce BOD₅/COD to <50 mg/L with sufficient residence (e.g., an SBR 12–24 h cycle or oxidative pond), with 3–5 days HRT or higher MLSS for full polishing (scielo.br).
Solid‑liquid separation uses a secondary clarifier (or membranes) to remove biomass, targeting ~5–20 mg/L suspended solids. AOP or chemical polishing follows: an ozone contactor (~5–15 mg O₃/L, 5–10 min) or a Fenton reactor (H₂O₂ + FeSO₄ at pH ~5) to degrade residual color/AOX. A post‑treatment adsorption step (e.g., activated carbon) can capture oxidation byproducts. Where very high purity or reuse is required, a membrane tertiary—NF or RO—completes the train; as noted, an RO stage achieved ~96% color removal with significant COD polishing in a case study (pubmed.ncbi.nlm.nih.gov).
Performance targets and monitoring
Illustrative mass balance: each tonne of pulp might initially produce 100 m³ effluent with COD ~3000 mg/L. A combined anaerobic + aerobic treatment can drop COD from ~3000 to <300 mg/L (90% removal) (scielo.br) (scielo.br), then to <50 mg/L (<98% total removal) after AOP/membrane. Typical raw AOX (5–50 mg/L) might be cut to ~25% of original by anaerobic, ~10% by subsequent aerobic, and to <10% by AOP; Photo‑Fenton processes can remove ~90% of AOX (inis.iaea.org). Color measured in ADMI units or UV absorbance often remains 60–80% after biology (scielo.br) but can be reduced by 90–100% via AOP or NF/RO; a practical target is >95% decolorization.
Biomass yield benchmarks: anaerobic sludge yield ~0.02–0.1 kg VSS/kg COD; aerobic ~0.5 kg VSS/kg COD, with sludge handling to be factored. In‑situ monitoring (online COD, UV254, AOX) can guide booster oxidant doses or membrane cleaning—work typically supported by chemical metering packages such as a dosing pump. An example outcome combining activated sludge + ozone + RO attained final effluent BOD <20 mg/L, COD <50 mg/L, color removal ~96% (pubmed.ncbi.nlm.nih.gov).
Trade‑offs and implementation choices
Biological reactors are cost‑effective for bulk organics (low energy) but leave recalcitrant color/AOX (mdpi.com). AOPs are more expensive (reagent/energy) but remove stubborn pollutants; adding ozone or Fenton after a biodelegated stage drove AOX removals from ~40% (bio only) to ~90% (inis.iaea.org) (mdpi.com). Business decision: use anaerobic/aerobic for 80–90% removal, then quantify if AOP is needed to meet final targets (often yes for strict color/AOX).
Membrane polishing demands capital and strict pretreatment but yields reuse‑grade or near‑zero discharge outcomes. One RO stage reduced COD by 82% and color by 96% in a case study (pubmed.ncbi.nlm.nih.gov). NF removed >80% organics and practically all color without pretreatment under good flux (pubmed.ncbi.nlm.nih.gov). Membranes create ~5–30% volume of concentrated brine. For large mills, ZLD schemes could recycle this brine or discharge to evaporation ponds. NF fits when some minerals in permeate are acceptable; RO for full desalination. Where AOP creates byproducts, polishing with activated carbon and maintaining wastewater ancillaries helps protect membrane uptime.
Indonesian mills are increasingly incorporating AOP (ozone) and membrane (UF/RO) for water reuse and effluent standards. A pilot combined UF‑RO system at a bleached pulp mill logged ~82% COD and 96% color rejection (pubmed.ncbi.nlm.nih.gov). There is interest in novel media (e.g., activated carbon pre/post‑treatment) and fungal treatment for lignin. Compliance anchors include the 1995 effluent standards (Keputusan MENLH No.51/1995; BOD ≤150 mg/L, COD ≤350 mg/L: fliphtml5.com) and newer regulations (e.g., PP 22/2021, PP 82/2021) emphasizing Best Available Technology (BAT) for hazardous substances like AOX, even if not explicitly listed. Producers are adopting BAT‑equivalent processes—abandoning elemental chlorine bleaching and integrating ozonation or RO—to comply and reduce environmental impact.
From a technical/financial standpoint, design choices depend on plant scale and reuse goals. A large pulp mill might justify full anaerobic + aerobic + ozone + RO (with biogas energy recovery), whereas a smaller mill might use aerobic + Fenton only. Trends in the region (driven by water scarcity) include closed‑loop water recycling; in such cases, investment in membrane separation is warranted. Environmental engineers should pilot‑test configurations on actual effluent—local wood species (Acacia, Eucalyptus) and process conditions can alter treatability—and reference up‑to‑date guidance alongside the studies cited here: scielo.br; scielo.br; mdpi.com; inis.iaea.org; pubmed.ncbi.nlm.nih.gov; pubmed.ncbi.nlm.nih.gov.
