Oily Water Separator (15ppm)

critical IMO Required 28 models total

A 15ppm Oily Water Separator treats engine room bilge water so that any discharge overboard contains no more than 15 parts per million of oil, the MARPOL Annex I limit, and is paired with an oil content monitor that automatically stops discharge if that limit is exceeded.

Read more — Oily Water Separator (15ppm) explained ▾

What defines a 15ppm Oily Water Separator

An Oily Water Separator (OWS) processes bilge water collected from the engine room bilge wells, separating free and emulsified oil from water before any discharge overboard. The 15ppm designation refers to the discharge standard it must meet under MARPOL Annex I: treated effluent may contain no more than 15 parts of oil per million parts of water. The separator itself only does part of the job — it is always fitted with a downstream Oil Content Meter/Monitor (OCM) that continuously samples the outlet and automatically closes a three-way valve to divert flow back to the bilge holding tank the moment the reading approaches the limit. Without a working, calibrated OCM, an OWS has no legal discharge path at all.

15ppm oily water separator process — Flow diagram of a bilge oily water separator showing bilge water passing through a coarse separation stage and a coalescer stage, separated oil rising to a sludge tank, and clean water passing an oil content monitor and three-way valve that either discharges it overboard or recirculates it when off specification.

Main components

Separation stages

Most designs use two or three stages: a first gravity or coalescing stage that removes the bulk of free oil, followed by a fine coalescing or membrane/absorption stage that reduces emulsified oil to meet the 15ppm limit. Some systems add a final polishing filter cartridge.

Oil Content Monitor (OCM)

A separate, IMO-type-approved instrument sampling the treated outlet continuously, alarming and automatically closing the overboard valve on any reading approaching 15ppm, and logging every discharge event.

Three-way automatic valve

Directs treated water either overboard or back to a bilge holding tank, actuated automatically by the OCM and also operable manually for testing.

Sludge and oil collection

Separated oil is drained to a sludge or waste oil tank for later incineration or shore disposal; this stream, and the volumes logged, are cross-checked against the Oil Record Book.

Selection and sizing

Capacity is set by expected bilge water generation rate for the engine room size and machinery type, expressed in cubic metres per hour, with common sizes from around 0.25 to 5 m3/h for merchant vessels. Selection also depends on the range of oil types and emulsifying agents expected — heavy fuel residues, lubricating oil, detergents used in tank cleaning — since some separator technologies handle stable emulsions poorly. Material selection for wetted parts needs to tolerate the corrosive, contaminated nature of bilge water over years of intermittent running.

Regulations and class

MARPOL Annex I and IMO Resolution MEPC.107(49) set the type-approval standard for 15ppm bilge separators and their oil content monitors; a unit must carry an IMO type approval certificate referencing this resolution to be fitted. The OCM must be calibrated to a recognised standard and both instruments are checked at every Port State Control inspection likely to review the engine room, since bilge water discharge is one of the most heavily enforced pollution offences in shipping. Class surveys verify the automatic stopping device is operative and that Oil Record Book entries for every discharge, transfer or internal transfer of bilge water match the OWS operating log.

Requirement	Reference
Discharge limit	15 ppm oil content, MARPOL Annex I
Type approval standard	IMO Resolution MEPC.107(49)
Automatic stopping device	Mandatory, tied to OCM reading
Record keeping	Oil Record Book Part I, checked against OWS log

Typical faults

Coalescing media fouling — heavy sludge or detergent residue blinds the media, reducing separation efficiency and forcing frequent OCM trips.
OCM sensor drift or fouling — a dirty sample cell gives false readings, either nuisance-tripping the discharge valve or, more seriously, under-reading actual oil content.
Three-way valve seizure — a valve stuck in the overboard position defeats the automatic stopping function entirely and is treated as a serious deficiency.
Bypass piping — illegal or undocumented piping around the OWS, historically the cause of the largest MARPOL prosecutions, remains one of the first things a Port State Control officer looks for.

What to look for in a supplier

Current IMO type approval certificate for both the separator and the OCM, matching the specific model to be installed.
Demonstrated performance on the emulsion types actually generated onboard, not only on clean test oil.
Availability of spare coalescing media and OCM sample cells through a global service network, since these are consumables with limited life.
Clear commissioning documentation package for class approval and Port State Control file.

Never let anyone touch the OWS piping without an entry in the Oil Record Book to match: an unexplained valve position or an undocumented pipe run is the single fastest way to turn a minor equipment fault into a detention.

2000h

Service Interval

5 yr

Class Survey

15 yr

Typical Lifetime

Typical Manufacturers

RWO Alfa Laval DVZ Compass

Inspector Mode Show All 28 total

21 manufacturers · 28 models

Alfa Laval

PureBilge 2.5

2.5 m3/h · 1.5 kW · Bilge water (oily) · membrane oily water separator

Common Failures & Inspection Points

Membrane module fouling
CIP system malfunction
Oil content sensor failure

Service: Membrane technology; requires regular chemical cleaning cycles.

Spare Parts: Ersatzteile über Alfa Laval oder zugelassene Vertriebspartner. Lead time: 2-6 Wochen.

Strengths

Achieves ≤15 ppm oil discharge, complying with MARPOL Annex I
Compact footprint suitable for space‑constrained installations
Low power consumption compared with traditional gravity separators
Automated Clean‑In‑Place (CIP) cycle reduces manual maintenance
Integrated oil‑content sensor provides real‑time compliance monitoring

Weaknesses

Membrane fouling requires regular chemical cleaning cycles
Sensitive to high solids or particulate loads; may need upstream filtration
CIP system complexity can lead to downtime if malfunctioning
Higher upfront cost than simple gravity OWS units
Limited flow capacity (≈2.5 m³/h) may be insufficient for large‑scale bilge streams

Typical Vessels: Coastal tankerContainer shipGeneral cargo vesselOffshore supply vesselFishing vessel

Certifications: IMO Type ApprovalDNV

Decision Guide: Choose if you need a high‑efficiency, space‑saving OWS that meets the 15 ppm oil limit and offers automated cleaning. Avoid on vessels with very high bilge water flow rates or heavy solid loads where membrane fouling would be excessive.

Use Cases: Commonly installed on new builds and retrofits of coastal tankers, container ships, offshore supply vessels, and other merchant ships that must comply with MARPOL Annex I while conserving deck space and power. Ideal for operations where continuous monitoring and automated cleaning are valued.

Datasheet

PureBilge 2515

Application

Bilge water treatment, 15ppm oily water separator

Common Failures & Inspection Points

Area: Sampling point integrity and representativeness
Check: Verify sampling point is in vertical section of effluent piping near OWS outlet, with adequate pressure (≥0.2 bar) and flow rate (≥3 l/min). Check piping is sealed, inspect for tampering or bypass valves. Verify sample line has no isolation points.
Area: 15ppm bilge alarm accuracy and response time
Check: Test alarm calibration within ±5 ppm accuracy. Verify response time ≤5 seconds for alarm activation. Test automatic overboard discharge valve closure within ≤20 seconds total. Check alarm has audible/visual indication. Verify alarm stores operational data (date, time, status) for minimum 18 months.
Area: Coalescer and filter element condition and replacement
Check: Inspect coalescer for blockage, degradation, or sediment accumulation. Check filter elements for fouling or clogging (service life typically 1-2 years). Verify replacement schedule compliance. Inspect internal seals and gaskets for integrity. Check heater elements (if equipped) for corrosion or scaling.
Area: Oil content monitor (OCM) calibration and functionality
Check: Verify OCM calibration by manufacturer or authorized agent within 5-year intervals. Test with clean water supply to measuring cell. Confirm sensor precision for oil content regulation. Check OCM display/recording function. Test bypass activation when oil content <threshold. Confirm cell replacement after 5 years of service.
Area: Separator tank condition, sludge, and sediment accumulation
Check: Inspect separation chambers for cleanliness, oil residue, and water level sensors. Check for sludge or emulsion buildup at tank bottom. Verify automatic sludge/oil discharge valve operation. Test level sensors and transfer solenoid valves. Confirm oil recovery system functioning and recovered oil properly disposed.

Typ-universelle Inspektionspunkte fuer Oily Water Separator (15ppm) (verifiziert, 2026-06).

Oily Water Separator (15ppm)

What defines a 15ppm Oily Water Separator

Main components

Separation stages

Oil Content Monitor (OCM)

Three-way automatic valve

Sludge and oil collection

Selection and sizing

Regulations and class

Typical faults

What to look for in a supplier

Typical Manufacturers

21 manufacturers · 28 models

Alfa Laval

RWO Water Technologies

Wärtsilä

Victor Marine

ACIR Marine

Chongqing Gohi Marine Equipment

Deyuan Marine

DVZ

DVZ-Services

Facet Filtration

GEA

Hanyoung Engineering

Hi-Sea Marine

Imarinex

Llalco

Marine Technical Engineering (MTE)

Marinfloc

RWO

Simplex Americas

Taiko Kikai

Zava Marine