Why Deinking Reject Water Is a Different MBBR Problem
Deinking reject water is a concentrated sub-stream with 3,000–15,000 mg/L COD, 500–3,000 mg/L TSS, and a sustained temperature of 40–55°C — roughly three to ten times the organic load of clarified white water from the same paper recycling mill. The stream carries detached ink, fines, fatty acid soaps, sodium silicate, residual sodium hydroxide, and surfactant residues from the deinking chemistry, and these compounds together define a biological-treatment problem that does not map to municipal MBBR assumptions or to laundry wastewater MBBR baselines (where Kaldnes K1 at 20% fill is the academic reference case per S1 academic study, Tanjungpura University, Kusuma et al.).
The 40–55°C operating window is a constraint, not a free benefit: biofilm acclimation is faster than at ambient temperature, but the same warm stream strips dissolved oxygen and accelerates surfactant foaming in the aeration tank. Sodium silicate and fatty acid soaps depress BOD/COD ratios into the 0.30–0.45 range typical of deinking reject — biodegradable, but with a recalcitrant fraction that caps achievable removal. Without dissolved air flotation (DAF) upstream, fibre and ink pass straight into the MBBR, where they bridge between carrier elements, smother biofilm, and cause carrier loss over the outlet sieve. The same DAF logic that applies to FOG and fibre in pulp & paper primary treatment applies here, which is why a Zhongsheng DAF system for deinking reject pre-treatment sits ahead of any MBBR sizing conversation — not after it.
Stream Characterisation Data You Need Before Sizing
Deinking reject sizing fails when the input numbers are guessed. Gather these parameters from 24-hour composite sampling to capture digester wash-out peaks before opening a sizing spreadsheet:
| Parameter | Typical range | Sizing implication |
|---|---|---|
| Design flow, m³/d | 20–1,000 | Apply peak factor 1.2–1.5 to size equalisation and aeration |
| COD, mg/L | 3,000–15,000 | Drives tank organic loading and aeration |
| BOD, mg/L | 1,000–6,000 | Drives volumetric BOD loading (g BOD/m²·d) |
| BOD/COD ratio | 0.30–0.45 | Defines the biodegradable fraction and achievable effluent COD |
| TSS, mg/L | 500–3,000 | Sets DAF pre-treatment loading and MBBR sieve aperture |
| Temperature, °C | 40–55 | Shortens achievable HRT; raises DO demand |
| pH | 7.5–10.5 | Alkaline excursions inhibit nitrification; needs on-line trim |
| Surfactant (MBAS), mg/L | 20–200 | Sets foaming headroom on aeration and HRT extension |
| Residual H₂O₂, mg/L | 10–100 | >50 mg/L must be catalytically quenched upstream of the MBBR |
A BOD/COD ratio of 0.30–0.45 in deinking reject tells you two things at once: the stream is treatable biologically, but a recalcitrant 55–70% fraction will pass through to the effluent. A target of 500 mg/L effluent COD for in-plant pre-treatment is realistic; pushing below 150 mg/L requires polishing — typically a downstream MBR or sand filter. Residual hydrogen peroxide above 50 mg/L oxidises biofilm on contact and must be catalytically quenched, or the MBBR will fail to establish a carrier culture in the first 4–6 weeks (Zhongsheng field data, 2025-09). Equalisation sizing sits at 8–24 hours of buffer flow to dampen surfactant and pH surges before the MBBR; for context on white water, which is the lower-strength companion stream, see the companion MBBR sizing guide for white water discharge.
The 7-Step MBBR Sizing Method for Deinking Reject Water
The sequence below is auditable in a single spreadsheet. Each step carries an explicit assumption so the math can be defended in a P&ID review or a regulator submission.
Step 1 — Design flow. Establish average design flow Q in m³/d from 24-hour composite data, convert to QH = Q/24 m³/h, and apply a peak factor of 1.2–1.5 to size equalisation and the aeration blower.
Step 2 — Target effluent and COD removal mass. Set the effluent target — 500 mg/L COD for in-plant pre-treatment discharge, or <150 mg/L with downstream polishing. Daily COD removal mass (kg/d) = Q × (CODin − CODout) / 1,000.
Step 3 — Carrier fill. Select 40–60% carrier fill. Deinking reject trends higher than the 20% K1 baseline documented in the S1 academic study because the recalcitrant fraction needs more biofilm area to hit a defensible effluent. 50% is a robust baseline; above 60% the aeration pattern starts to short-circuit and carrier bridging becomes a maintenance burden (Zhongsheng field data, 2025-11).
Step 4 — HRT. Set HRT to 8–18 hours. The upper end (14–18 h) applies to surfactant shock loads and cooler operation; the lower end (8–10 h) is defensible at 50–55°C with stable influent. The S2 academic review (IJSR, Calderón 2012, Spain) reports 5–15 hour HRT for MBBR on lower-strength influents; deinking reject should sit at the upper end of that envelope or above.
Step 5 — Reactor volume and organic loading check. Reactor volume V (m³) = QH × HRT. Sanity-check with tank organic loading 5–15 g BOD/m²·d, calculated as (BODin × Q / 1,000) / (V × specific surface area). For a 500 m²/m³ HDPE carrier, this check will catch volumetric errors before fabrication.
Step 6 — Aeration. Size aeration at 0.3–0.6 Nm³ air per kg COD removed, then add 20% headroom for surfactant-induced foaming. At 8,000 mg/L COD and the 0.3 Nm³/kg baseline, a 100 m³/d reject stream draws roughly 200–240 Nm³/h of process air.
Step 7 — Carrier specification. Specify HDPE carrier media with 500–1,200 m²/m³ specific surface area, density close to 0.95 g/cm³ for fluidisation in a 3–5 m deep tank, and a sieve aperture of 8–12 mm to retain carriers without fouling. For polishing downstream of the MBBR when reuse is the target, an MBR integrated system can be sized to the same envelope; see the MBR sizing context below.
Worked Examples: 50 m³/d vs 500 m³/d Reject Stream
| Parameter | Example 1 — small mill | Example 2 — large mill |
|---|---|---|
| Design flow Q, m³/d | 50 | 500 |
| Influent COD, mg/L | 8,000 | 6,000 |
| Influent BOD, mg/L | 3,200 (BOD/COD 0.40) | 2,100 (BOD/COD 0.35) |
| Target effluent COD, mg/L | 500 | 500 |
| Carrier fill, % | 50 | 60 |
| HRT, h | 12 | 14 |
| Reactor volume V, m³ | ~25 | ~290 (two parallel trains of ~145 m³) |
| COD removed, kg/d | 375 | 2,750 |
| Aeration, Nm³ air/min | ~1.2 | ~7.5 |
| Diffuser layout | Single tank, two-stage aerobic zone | Two parallel trains, fine-bubble disc diffusers |
The 60% fill and 14 h HRT in Example 2 are not arbitrary — at 500 m³/d the surfactant load is high enough that longer residence time buffers shock, and the higher carrier surface is justified by the lower BOD/COD ratio of 0.35. A 20% K1 academic fill (per S1) would undersize the biofilm area for the 6,000 mg/L COD load and force the tank organic loading above the 15 g BOD/m²·d ceiling. The 5–15 hour HRT documented in the S2 Spain study (Calderón 2012) is the lower-strength baseline; deinking reject sits at the upper end because surfactant kinetics, not just COD, drive the residence-time requirement.
MBBR vs MBR vs SBR for Deinking Reject Water
| Criterion | MBBR | MBR | SBR |
|---|---|---|---|
| Footprint, m² per m³/d treated | 0.4–0.8 | 0.3–0.6 | 0.6–1.0 |
| Effluent TSS, mg/L | 30–80 | <1 | 20–50 |
| Effluent COD achievable, mg/L | 300–500 | <150 with downstream polishing | 250–450 |
| Surfactant tolerance | Good with 20% aeration headroom | Good; membrane fouling risk at >150 mg/L MBAS | Moderate; foaming in aeration phase |
| Sludge yield, kg/kg COD removed | 0.20–0.30 | 0.25–0.35 | 0.25–0.35 |
| CAPEX driver | Reactor volume + carriers | Reactor + membrane modules | Reactor + decanters + controls |
| OPEX driver | Blower power, carrier top-up | Membrane cleaning, blower, permeate pump | Blower, decanters, batch controls |
| Best-fit reject flow range, m³/d | 50–500 | 10–2,000 (per MBR module envelope) | <30 |
MBBR is the right balance for 50–500 m³/d reject streams with a 500 mg/L COD discharge target and operator-light operations. MBR is preferred when the next stage is water reuse — a PVDF membrane at <1 μm matches the 32–135 m³/d DF module envelope and produces an effluent suitable for in-mill dilution water or shower water. SBR only wins for very small flows (<30 m³/d) where batch simplicity outweighs MBBR's continuous-flow advantage. When the MBBR is the right primary stage but reuse is the downstream target, the MBR membrane bioreactor module lines up directly behind it for polishing; cross-reference the MBR sizing guide for white water reuse for the polishing-side calculation, and the DAF sizing guide for white water streams for the upstream fibre/ink removal step.
Pre-Treatment and Rejection-Mode Troubleshooting
Four failure modes account for the majority of MBBR underperformance on deinking reject. Each is diagnosable from a single operating parameter.
DAF pre-treatment failure. When DAF surface loading drifts above its 4–300 m³/h capacity envelope (per the ZSQ DAF range referenced above), fibre and ink pass into the MBBR. The diagnostic is MBBR sieve pressure-drop rising 20–30% week-on-week; the corrective action is to verify DAF surface hydraulic loading at 15–25 m³/m²·h and recycle-ratio at 20–30% before touching the MBBR.
Aeration upset. Dissolved oxygen below 2 mg/L collapses COD removal and triggers filamentous bulking. The check is a portable DO probe at the outlet end of the carrier tank; the corrective action is diffuser descaling and blower balancing to restore 0.3–0.6 Nm³ air per kg COD removed, with the 20% surfactant foaming headroom applied on top.
pH excursion. Deinking chemistry is alkaline (pH 7.5–10.5), and excursions above 9.0 inhibit nitrification and stress heterotrophs. The fix is on-line pH trim, typically with an automatic chemical dosing system for pH and peroxide trim tied to a PID loop on the equalisation tank outlet.
Cold-weather operation. Below 25°C, biofilm kinetics roughly halve for every 10°C drop, and the deinking stream loses its thermal advantage. Mitigate with tank insulation and a 2–4 hour HRT extension; check that the equalisation tank is sized to the upper end of its 8–24 hour envelope to retain heat through the buffer volume.
Frequently Asked Questions
What carrier fill percentage should an MBBR use for deinking reject water?
40–60% carrier fill, with 50% as a robust baseline. Deinking reject trends higher than the 20% K1 fill documented in the S1 academic study (Kusuma et al., Tanjungpura University) because the recalcitrant COD fraction needs more biofilm area to reach a 500 mg/L effluent. Above 60%, aeration pattern degrades and carrier bridging becomes a maintenance burden (Zhongsheng field data, 2025-11).
What HRT should I size the MBBR for on deinking reject water?
8–18 hours, with 12–14 hours as a defensible baseline for most mills. The 5–15 hour range reported in the S2 academic review (Calderón 2012, Spain) is the lower-strength baseline; deinking reject sits at the upper end because surfactant kinetics, not just COD, drive the residence-time requirement. Hotter streams (50–55°C) allow the shorter end of the envelope; surfactant shock loads push toward 18 hours.
Is DAF pre-treatment required before an MBBR on deinking reject water?
Yes. With influent TSS of 500–3,000 mg/L, fibre and ink will bridge between carrier elements, smother biofilm, and cause carrier loss over the outlet sieve within 2–4 weeks of operation. DAF with 15–25 m³/m²·h surface hydraulic loading and 20–30% recycle ratio is the standard upstream step; the same logic applies to the DAF sizing guide for white water streams.
What effluent COD can an MBBR reliably hit on deinking reject water?
500 mg/L COD for in-plant pre-treatment discharge is a robust single-pass target, given the 0.30–0.45 BOD/COD ratio of typical reject. Pushing below 150 mg/L for water reuse requires polishing — typically an MBR stage downstream of the MBBR, sized to the 10–2,000 m³/d envelope described in the MBR sizing context above.
How do I size aeration for an MBBR on deinking reject water?
0.3–0.6 Nm³ of process air per kg COD removed, plus 20% headroom for surfactant-induced foaming. For an 8,000 mg/L COD stream at 100 m³/d, that is roughly 200–240 Nm³/h of process air delivered through fine-bubble disc diffusers sized to maintain dissolved oxygen above 2 mg/L at the outlet end of the carrier tank.
