Bleach plants drive up to 85% of a kraft mill’s wastewater load and color. A three‑barrier design—anaerobic and aerobic biology, then advanced oxidation or membranes—now targets COD, color, and AOX while recovering energy.
In kraft mills, up to ~85% of the mill’s wastewater flow originates in the bleach plant (www.intechopen.com), and it carries a stubborn mix: lignin‑derived organics, intense color, and chlorinated organics (AOX, or adsorbable organic halides). Typical loads clock in around 10–35 kg BOD and 30–70 kg COD per ADt (air‑dry ton) of pulp (Table 5: www.intechopen.com), with one survey at ~10 kg BOD5 and ~35 kg COD per ton from bleaching plus ~1.2 kg AOX (www.intechopen.com).
The raw BOD5/COD ratio is low (~0.3–0.5), signaling poorly biodegradable lignin, and bleaching yields very high color from high‑molecular‑weight lignin fragments (www.intechopen.com). Indonesian discharge standards (Por. 51/1995) cap combined pulp‑and‑paper effluent at 150 mg/L BOD5 and 350 mg/L COD (≈25.5 kg BOD5 and 59.5 kg COD per ton) with pH 6–9 (id.scribd.com). There’s no AOX limit specified, but global best practice often targets ~1.5 kg/ton (cwejournal.org).
Process upgrades help but don’t erase the problem: better cooking plus oxygen delignification can cut vented COD loads roughly in half (www.intechopen.com). The sector remains a heavy polluter (ranked ~6th globally: www.mdpi.com), spurring advanced treatment and even zero‑discharge ambitions.
Bleach‑plant loads and discharge limits
A workable treatment train starts with basic hydraulics and equalization, but the mass balance is set by the bleach line itself: 10–35 kg BOD and 30–70 kg COD per ADt (Table 5: www.intechopen.com; example dataset: www.intechopen.com). The color load is dominated by high‑MW lignin fragments (www.intechopen.com), and low BOD5/COD (~0.3–0.5) limits easy biodegradation.
Headworks are straightforward: screens and oil/grit removal stabilize downstream biology, with systems such as screens and primary separation commonly used to control debris before equalization. Compliance points are clear: BOD5 ≤150 mg/L, COD ≤350 mg/L, pH 6–9 under Por. 51/1995 (id.scribd.com), while AOX control follows best‑practice targets around ~1.5 kg/ton (cwejournal.org).
Two‑stage biological degradation
The backbone is anaerobic followed by aerobic treatment. In the first stage, UASB (upflow anaerobic sludge blanket) or fixed‑bed reactors remove roughly half the incoming COD; one upflow packed‑bed study on textile pulp bleach wastewater achieved 52–55% COD removal at 12‑hour HRT (hydraulic retention time) (www.scielo.br). AOX drops as well via reductive dechlorination—~40–46% AOX removal under steady operation was reported (www.scielo.br).
Higher AOX removals (up to ~80–90%) generally need cosubstrates or severe conditions (www.scielo.br; www.researchgate.net). After anaerobic treatment (including ANAMMOX or UASB setups), effluent typically retains ~30–50% of original COD with a lower BOD fraction; reported BOD5/COD dropped from ≈0.35 to ≈0.2 (www.scielo.br). Energy is a dividend: biogas production is ≈0.3–0.5 m³ CH4 per kg COD removed.
Stage two is aerobic polishing. Activated sludge is used in ≈60–75% of pulp‑mill biotreatment plants (www.intechopen.com), and systems typically remove >80–90% of the remaining BOD/COD depending on retention time. An activated sludge/BOD reactor, followed by ozonation, has shown ~75–80% COD and color reductions (pmc.ncbi.nlm.nih.gov). Aerobic trains range from conventional basins to activated sludge with extended aeration or membrane bioreactors (MBR) using sludge ages of 8–15 days.
For design, a basin or MBR is sized to reach final COD/BOD targets (e.g., COD <250 mg/L, BOD5 <50 mg/L). Nutrient dosing (N, P) and pH control are added to optimize degradation, often via a nutrient program and metered feeds through a dosing pump. In total, the two‑stage biology removes ~90–95% of incoming BOD/COD—e.g., ~50% in anaerobic plus ~80% of the remainder in aerobic yields ~90+% overall (www.scielo.br; pmc.ncbi.nlm.nih.gov). Bulking and sludge handling remain active operational concerns, but the effluent usually meets numeric BOD/COD limits (id.scribd.com) with residual organics primarily refractory lignin fragments.
Advanced oxidation trade‑offs
Biology leaves recalcitrants—especially color and AOX—on the table. Advanced oxidation processes (AOPs) such as ozone (O3), Fenton reagent (H2O2/Fe2+), photo‑Fenton (UV/H2O2/Fe), or UV/H2O2 leverage radicals to mineralize organics. Bench data show fast COD destruction but partial color removal: ozone treatment achieved ≈88–93% COD removal but only ~41–47% color removal under optimum conditions (contact ~9–12 minutes) (www.mdpi.com; www.mdpi.com).
Photo‑Fenton reached ~90–94% COD removal but ~40% color removal (www.mdpi.com). Real‑case bleaching waste treated by Fenton/UV saw AOX cut by up to ~85–95% under strong conditions (www.researchgate.net). Trade‑offs matter: Fenton runs at pH 2–3 with high H2O2, raising cost; ozone is more neutral but less effective on AOX and can generate bromate/chlorate by‑products if halides are present. UV‑based steps are typically delivered in compact ultraviolet systems, with oxidants metered by a dosing pump.
In practice, AOP follows biology. An AS/UASB‑treated kraft effluent processed by ozone at pH≥8 delivered ~75% COD and ~77–80% color removal (pmc.ncbi.nlm.nih.gov), illustrating how post‑biological oxidation breaks stubborn color molecules while trimming residual COD.
Membrane polishing and reuse
Membranes offer a physical barrier for high‑MW organics and chromophores. Ceramic or polymeric UF (ultrafiltration) followed by NF (nanofiltration) has been piloted on bleaching effluent: a two‑stage UF→NF train removed ~40% of remaining COD and ~66% of lignin (a color proxy) in a German study (pmc.ncbi.nlm.nih.gov). Under optimized conditions, UF+NF achieved up to ~73% lignin removal with ~35–45% COD removal (pmc.ncbi.nlm.nih.gov).
With an NF cutoff near ~1 kDa, only very small molecules pass, so color is largely retained. In practice, UF or MBR effluent feeds NF; the retentate is recirculated or further treated (e.g., incinerated or digested). Deployed as polishing, membranes can reduce AOX when organics are large or adsorb to the matrix, and remove suspended‑bound fractions. The trade‑off is capital and concentrate disposal, but the payoff is very clear, low‑AOX effluent often <10 mg/L AOX and <50 mg/L COD, suitable for reuse. Typical units include UF pretreatment and NF polishing, assembled from modular membrane systems depending on load and footprint.
Integrated train and outcomes
A robust layout looks like this: primary screening/grit removal → neutralization/equalization → anaerobic reactor (HRT ~8–12 h) → aerobic activated sludge or MBR (SRT 10+ d) → AOP (ozone or UV/H2O2) → NF membrane. Upstream debris control is typically handled with primary screening modules before flow equalization. Biology alone can cut COD from ~3000–5000 mg/L down to a few hundred mg/L (www.scielo.br; pmc.ncbi.nlm.nih.gov), after which AOP oxidizes 80–90% of the remaining organics (www.mdpi.com) and NF removes residual turbidity/color moieties.
Overall, the multi‑barrier train achieves >90–95% COD removal with substantial color and AOX reduction. End‑of‑pipe targets of <100 mg/L BOD5 and <300 mg/L COD meet the 1995 Indonesian limits (id.scribd.com), with visible color removal by ~70–90%. For example, combined AS+ozone treatment has reported ~77% color removal (pmc.ncbi.nlm.nih.gov).
AOX control is equally material: integrated trains aim below ~1–2 kg/ton pulp or a few µg/L, with studies (e.g., Zaki et al., Ribeiro et al.) showing AOX drops on the order of ~80–90% with AOP (www.researchgate.net). Modern mills in South America and Asia are closing water loops; European BAT‑level outcomes report final COD loads of ~5–20 kg/ton pulp (www.intechopen.com; www.intechopen.com).
The economics pencil out via energy capture and risk reduction: biogas co‑generation covers much of plant energy, while incremental AOP power and membrane fouling costs trade against compliance and water reuse value. The result is effluent within typical discharge limits (e.g., COD <300 mg/L, AOX low) and alignment with zero‑liquid‑discharge goals of leading producers (pmc.ncbi.nlm.nih.gov; www.researchgate.net).
