Bilge water’s toughest problem: the emulsions that beat gravity — and the tech catching up at sea

International rules are unambiguous: vessels over 400 GT (gross tonnage) need to treat bilge water so the discharge contains no more than 15 ppm (parts per million) of oil, per MARPOL Annex I (swzmaritime.nl) (www.intechopen.com). Some waters, including the Baltic and the Great Lakes, go further with 5 ppm expectations (professionalmariner.com).

Bilge volumes are modest but persistent — typically 0.01–13 m³/day per ship (m³/day = cubic meters per day) — and they mix fuels, lube oils, and surfactants from cleaning agents (swzmaritime.nl) (professionalmariner.com) (swzmaritime.nl). Against that backdrop, satellite-detected slicks have ranked Indonesia’s waters among global “hotspots” for ship oil pollution, raising enforcement pressure (www.ft.com).

The MARPOL regime — adopted by Indonesia — effectively mandates onboard oily‑water separators (OWS, a bilge treatment unit) with alarms to prevent over‑15‑ppm discharge (swzmaritime.nl). But getting from emulsified bilge to clean effluent is where the technologies diverge sharply.

Conventional gravity separators

Traditional OWS are gravity-based “API” or lamella units (API = gravity separator design; lamella = inclined-plate sedimentation). They hold oily bilge long enough for lighter oil to float and be skimmed (www.intechopen.com) (www.intechopen.com). Many units add coalescing packplates made of oleophilic (oil‑attracting) polyethylene or fiberglass to help droplets merge and rise faster (www.intechopen.com).

Under calm conditions with distinct free oil, these systems can meet the 15 ppm spec. In the real world, bilge often contains detergents and surfactants that form stable emulsions, and ship motion agitates the mix — both undercut gravity separation (www.intechopen.com) (professionalmariner.com). One study found a plain gravity OWS removed only ~9% of an emulsion’s oil, versus ~80% removal when the same stream was polished by ultrafiltration (UF, a pressure‑driven membrane process) (nepis.epa.gov).

Bottom line on performance: these units are designed for ≤15 ppm output but often exceed that if emulsifiers are present; percent‑removal on emulsions is low (nepis.epa.gov). The payoff is simplicity, low capital cost, and a large installed base — gravity still dominates on cost grounds (dataintelo.com). They also run without consumable chemicals.

Limitations are structural: efficiency collapses with ship pitching/rolling in rough seas (professionalmariner.com). Long retention times and routine skimming/valve work add operational drag. In vendor catalogs, the primary phase of oil removal fits under waste-water physical separation.

Coalescing plate separators

Coalescing-plate OWS are an upgraded gravity design that packs the vessel with inclined, corrugated, oleophilic plates to capture droplets, coalesce them, and speed ascent (www.intechopen.com). Common media include polyethylene, fiberglass, or nylon (www.intechopen.com).

Performance improves markedly on fine droplets in non‑emulsified feeds — coalescing OWS can reduce oil to mid‑single‑digit ppm and more consistently approach 15 ppm; cascade coalescers lift percent‑removal vs a bare tank. Still, they rely on buoyancy, so strong emulsions and heavy agitation remain problematic (www.intechopen.com).

Advantages include small footprint and no chemical cost; many modern bilge systems use plate interceptors as the first stage to capture bulk free oil and macro‑droplets (www.intechopen.com). Market data still favor gravity/coalescing on cost‑effectiveness (dataintelo.com).

Limitations persist: plates saturate and need cleaning or replacement. Industry sources note coalescer or chemical floc systems can reject up to ~25% of treated volume as oily sludge/back‑flush waste (professionalmariner.com) (nepis.epa.gov). As gravity devices, they’re still vulnerable to ship motion and are often paired with downstream polishing (flotation, filtration, or chemical flocculation) to assure 15 ppm output.

Membrane filtration (UF/NF/RO)

Membrane OWS use porous barriers to physically retain oil and solids. Typical trains apply ultrafiltration (UF) or nanofiltration (NF), sometimes followed by reverse osmosis (RO) for final polishing. Because most oil droplets exceed UF pore size, removal efficiency is very high. Bench and pilot work reports UF permeate at 0.9–1.1 ppm oil (link.springer.com). UF+RO systems have produced essentially oil‑free water; in one study UF alone yielded <10 ppm and RO removed nearly all remaining oil (www.researchgate.net).

On efficiency, Nelson‑grade UF has shown >80–90% oil removal on bilge water, far surpassing gravity (nepis.epa.gov). Filtrate as low as ~1 ppm is achievable (link.springer.com), and high‑pressure membranes can drive oil to <1 ppm (www.researchgate.net). Membranes capture emulsified and dispersed oil — what gravity misses.

Advantages include “consistently high” oil removal, compact footprint, and minimal oily residue — often only a concentrate/backwash stream amounting to a few percent of feed — with no separation chemicals required (nepis.epa.gov) (nepis.epa.gov). They work underway and are unaffected by ship motion; ceramic and antifouling coatings have improved robustness in recent years. Many offshore vessels use membrane polishers after a primary OWS.

Limitations revolve around cost and fouling. Oil, salts, and biofilms raise pressure and force cleaning downtime, lifting operating cost above gravity systems (nepis.epa.gov) (nepis.epa.gov). In practice, membranes sit as a secondary stage after pretreatment. Operators often select UF modules akin to ultrafiltration. For polishing, RO options such as sea water RO are sometimes added. Nanofiltration categories like nano filtration also appear in bilge trains.

Chemical emulsion breakers

Demulsifiers (surfactants or inorganic coagulants such as PAC — polyaluminum chloride) are dosed ahead of separation to neutralize stabilizers and aggregate fine droplets into flocs for easier removal (nepis.epa.gov). Onboard, these chemicals are typically injected into a mixing (floc) tank upstream of the separator at ~100–500 ppm (about 0.1–0.5 L per m³ of water), with flow‑controlled dosing (rxsolgroup.com).

Demulsifier treatment is often part of a multistage OWS: free oil is removed mechanically first, then flocculation targets the remaining emulsified phase (rxsolgroup.com). Properly formulated programs can drive oil well below MARPOL limits; PAC‑based “bilge flocculants” claim typical discharges under 3 ppm (omanchem.com).

The trade‑off is sludge: chemical flocculation can generate 5–25% of treated volume as oily paste that must be landed or incinerated; reagents add recurring cost and require dose control (nepis.epa.gov) (professionalmariner.com). Dosing on ships is commonly handled by a dosing pump. PAC‑based programs align with PAC coagulants.

Comparative considerations and trends

Engineering trade‑offs are stark. Gravity OWS (with or without coalescers) are low‑cost and low‑complexity but frequently limit‑discharge unless aided by polishing or chemicals. Membranes are high‑performance — think millisecond throughput with effluent ≪5 ppm — but high‑cost and fouling‑prone (nepis.epa.gov) (nepis.epa.gov). Chemical demulsifiers sit in the middle: moderate retrofit cost and strong gains (to ~3 ppm), at the expense of 5–25% sludge (omanchem.com) (nepis.epa.gov).

Evidence reflects the mix‑and‑match reality. An EPA review noted that adding UF or similar polishing to gravity OWS can lift oil removal from single digits to ~80% (nepis.epa.gov). Market analyses say gravity still holds the largest share on simplicity, while “more stringent‑clean” options (centrifuges, membranes) are gaining under tighter rules (dataintelo.com) (dataintelo.com).

In Asia‑Pacific — including Indonesia — rapid fleet growth and stricter enforcement are driving demand for advanced systems (dataintelo.com). In practice, operators layer solutions: gravity/coalescing first, then membrane polishing or chemical dosing to guarantee the alarm‑gated 15 ppm compliance (swzmaritime.nl). If bilge is heavily emulsified or targets trend toward 5 ppm, a UF module or routine demulsifier dosing may be warranted despite cost; conversely, small ships with light oil load can often rely on coalescing gravity units in calm conditions.

Source notes and references

Key references include IntechOpen’s 2020 separation review (www.intechopen.com) (www.intechopen.com), an EPA technical report on membrane and chemical performance (nepis.epa.gov) (nepis.epa.gov) and field studies of UF/RO (www.researchgate.net) (link.springer.com). Regulatory context and pollution hotspots are covered by SWZ Maritime, Professional Mariner, and the Financial Times (swzmaritime.nl) (professionalmariner.com) (www.ft.com). Manufacturer and market data support sludge fractions and adoption trends (nepis.epa.gov) (omanchem.com) (dataintelo.com).