Inside the dirty secret of scrubber washwater — and the tech cleaning it up

Marine exhaust gas scrubbers don’t just strip SOₓ from ship plumes. They create a second pollution problem: washwater that’s acidic, turbid and laced with oil residues, heavy metals and PAHs (polycyclic aromatic hydrocarbons — hydrophobic combustion byproducts) (nepis.epa.gov) (enveurope.springeropen.com). On a 15 MW engine, open‑loop effluent flow is typically 200–500 L/s, with elevated Zn, V, Cu, Ni and characteristic PAHs such as phenanthrene and naphthalene (enveurope.springeropen.com) (enveurope.springeropen.com).

IMO/MARPOL Annex VI sets the bar: discharge pH ≥ 6.5 measured 4 m from the ship with ≤ +2 units change (liqtech.com) (enveurope.springeropen.com), turbidity ≤ 25 NTU (Nephelometric Turbidity Units — a measure of clarity) above intake, and PAHs ≤ 50 μg/L (micrograms per liter) as phenanthrene‑equivalents at 45 m³/MWh (megawatt‑hours) (liqtech.com) (enveurope.springeropen.com). No IMO metal limits are given.

Indonesia, a MARPOL signatory, enforces strict marine pollution controls (PP 22/2021 repealing PP 19/1999) that effectively prohibit unauthorized discharges; in practice, scrubber effluent in Indonesian waters must meet at least IMO limits and solid residues (sludge) must go to port reception — not dumped or incinerated onboard (peraturan.bpk.go.id) (nepis.epa.gov) (enveurope.springeropen.com).

Coagulation–flocculation design basis

The core step aggregates fine soot, oil droplets, metal hydroxides and other colloids into settleable flocs via chemical coagulation–flocculation (nepis.epa.gov) (patents.google.com). Ferric chloride, alum or polyaluminum chloride are dosed to neutralize charge and precipitate dissolved metals, often with pH adjustment (e.g., NaOH). Typical dosage is 0.01–0.1% v/v of flow with short mixing/hydrolysis times of about 30 seconds (patents.google.com) (patents.google.com).

Cationic flocculant polymers such as polyacrylamides are then added to bridge particles and accelerate settling. Bench studies show well‑designed coagulation with DAF (dissolved‑air flotation — microbubbles that lift flocs) consistently removes about 90–95% turbidity, with DAF at roughly ~95% versus sedimentation at ~90% (www.researchgate.net) (www.researchgate.net). In practice, scrubber unit trials (e.g., Wartsila) using coagulation+flocculation, oil‑skimming and flotation/GAC (granular activated carbon) reported very high cleaning of bleed‑off water, with pollutant‑loaded flocs collected for disposal (nepis.epa.gov) (nepis.epa.gov).

Key data: designs target >90% suspended solids/turbidity removal at this stage. Coagulant choice and pH are tuned to precipitate metals — Fe/Al coagulants at neutral pH remove ≥90% of Ni, Cu, Zn in industrial waters (nepis.epa.gov) (patents.google.com). PAHs, being hydrophobic, largely partition onto suspended solids and are removed with the flocs; any residual dissolved/colloidal PAH goes to polishing.

For dosing control and reliability in tight engine‑room spaces, operators typically deploy an accurate chemical dosing pump with coagulants such as polyaluminum chloride and polymer flocculants.

Clarification by plate settling or DAF

After coag‑floc, separation is by gravity or flotation. A lamella clarifier (inclined plates for compact settling) or DAF with skimming is common. DAF is particularly effective for oily/colloidal loads and can float >90% of flocculated solids (www.researchgate.net). A DAF stage after coagulation can consistently reach turbidity <5 NTU — well below the 25 NTU limit — by removing small flocs and oil droplets (www.researchgate.net).

In practice, some shipboard scrubber systems add a multicyclone pre‑filter that removes >90% of particles larger than 30–40 μm (nepis.epa.gov) before a sludge settling tank concentrates solids for periodic draw‑off. The clarified effluent then moves to polishing and can already meet the turbidity requirement.

Key data: typical designs remove ≥95% of suspended solids in the clarifier/DAF. US EPA trials found bleed‑off treatment (coag+DAF+skimming) produced sludge at roughly ~0.1–0.4 kg per MWh of engine power (nepis.epa.gov). Open‑loop systems generate ≈0.6% of fuel weight as solids (nepis.epa.gov). A 15 MW vessel at 0.07 m³/MWh bleed‑off (freshwater scrubber) might produce a few hundred kg of wet sludge per day.

For compact footprints, operators often specify a plate‑pack unit such as a lamella settler, while oily loads favor a DAF system. Conventional gravity designs still have a place — a clarifier sized for the detention time removes suspended solids effectively.

Media filtration and membrane polishing

To meet strict PAH and metal targets, tertiary filtration follows. Multimedia sand/carbon beds or cartridge filters remove fines and adsorb organics; GAC (granular activated carbon) is a go‑to for dissolved PAHs, pushing levels to μg/L. Ultrafiltration (UF) membranes are reported to remove ~91–92% of medium‑weight PAHs such as phenanthrene, fluoranthene and pyrene (www.mdpi.com), with reverse osmosis (RO) or nanofiltration (NF) achieving even higher organics rejection — though at higher cost. In practice, some shipboard systems send centrifuge supernatant through a membrane unit to assure turbidity/PAH/pH limits are met (patents.google.com) (patents.google.com).

Key data: bench and pilot work shows membrane or adsorption polishing can cut PAHs by >80–90%. A membrane bioreactor (MBR — biological treatment integrated with a membrane barrier) study removed 66–85% of 16 priority PAHs (www.mdpi.com). Designs target final effluent turbidity ≪25 NTU and PAH ≤ 50 μg/L phenanthrene‑equivalents (ideally <10 μg/L); many scrubber systems using coagulation+DAF+GAC meet the IMO threshold. Metals (now mostly precipitated or filtered) are expected < IMOWQS (no UN regs), and European inputs remain below chronic criteria after treatment (nepis.epa.gov).

Typical trains combine a multimedia bed — often a sand media filter to knock down fines — followed by an adsorber such as activated carbon for dissolved organics. Where cartridges are preferred for footprint or maintenance reasons, a cartridge filter in a corrosion‑resistant stainless housing is common. For higher assurance on colloids and emulsions, UF modules such as ultrafiltration systems or integrated membrane systems are deployed.

Sludge dewatering and port disposal

Coag‑floc and filter backwash yield sludge at 95–98% moisture that must be dewatered. Centrifuges typically concentrate to ~20% dry solids; high‑pressure filter presses reach >30%. A Japanese scrubber treatment patent describes a centrifuge followed by membrane filtration, sending the concentrated cake to a tank (patents.google.com) (patents.google.com).

After dewatering, the cake still contains heavy metals, PAHs and spent coagulant. It is treated as hazardous B3 waste and must go ashore; guidelines require residues not be discharged to sea nor incinerated onboard (nepis.epa.gov). Wartsila trial data show sludge from a fresh‑water scrubber bleed‑off stored for shore disposal (nepis.epa.gov).

Key data: global scrubber use — 3,000 ships (2019 data) — implies thousands of tons of sludge annually if untreated (enveurope.springeropen.com). Dewatering can reduce volume 5×–10×. Example: a 15 MW ship generating ~0.3 kg sludge/MWh for 24 h (~360 kg/day wet) yields about ~50 kg/day of filter cake after press. In Indonesia, such waste must go to licensed hazardous landfills or incinerators per PP 22/2021 and Permen LH (sludge with Pb, Ni, PAHs is a B3 waste) (peraturan.bpk.go.id).

Performance summary and fleet context

In summary, a robust scrubber washwater plant combines (a) coagulation/flocculation (Fe/Al salts + polymer), (b) clarification (DAF or settling), and (c) polishing filtration (sand/membrane + activated carbon) to meet IMO limits on turbidity, pH, metals and PAHs (enveurope.springeropen.com) (www.researchgate.net). Each stage has known performance: >95% SS removal (turbidity <5 NTU) (www.researchgate.net), >80–90% PAH removal via GAC/membrane (www.mdpi.com), and metal/hydrocarbon capture in >95% of sludge. Final effluent can meet the IMO 25 NTU and 50 μg/L PAH standards (enveurope.springeropen.com). Sludge (~0.1–0.6% of fuel weight) is thickened (centrifuge/press) and shipped to onshore disposal (nepis.epa.gov) (nepis.epa.gov).

Actual trials report coag/DAF + oil skimming + GAC removed essentially all PM/PAH, with discharge pH ~7.6 and nitrates <10 mg/L (nepis.epa.gov). Scrubber use has been rising rapidly — 3,000 units installed by 2019 (enveurope.springeropen.com) — so reliable removal to the tight discharge criteria (pH ≥ 6.5, turbidity ≤ 25 NTU, PAH ≤ 50 μg/L) is a design non‑negotiable. Meeting them yields measurable outcomes (e.g., >90% TSS and metal removal, PAH to low μg/L) consistent with IMO and Indonesian expectations (enveurope.springeropen.com) (nepis.epa.gov).

Sources and referenced guidance

Authoritative studies and guidelines include the US EPA scrubber review (nepis.epa.gov) (nepis.epa.gov); IMO/EPA regulations (liqtech.com) (enveurope.springeropen.com); academic reviews (enveurope.springeropen.com) (www.mdpi.com); industry reports (enveurope.springeropen.com) (nepis.epa.gov); and Indonesian regulatory texts (peraturan.bpk.go.id). All pollutant removal figures and design parameters above are drawn from these sources.