Hotel Service Unit 18 Q5

A single-stage static oily water separator (OWS) designed to meet MARPOL Annex I requirements is a relatively simple but effective system for separating oil from bilge water or other oily wastewater onboard ships.

It operates primarily based on the principle of gravity separation, allowing oil and water to separate naturally due to their density difference.

1. Static Oil Separators – Gazebo SpA

Construction and Components:

• Inlet: The oily water mixture enters the OWS through an inlet pipe, often equipped with a strainer to remove coarse debris.
• Separation Chamber: This is the main body of the OWS, typically a cylindrical or rectangular tank. It provides a quiescent (calm) environment for gravity separation to occur.
• Baffles or Plates: The separation chamber may contain baffles or plates that create a longer flow path and increase the surface area for oil to rise and separate from the water.
• Oil Collection Chamber: Located at the top of the separation chamber, this area collects the separated oil, which is less dense than water and floats to the surface.   1. Why does oil spread over the surface of water but water does not spread over the surface of oil? – BYJU’S byjus.com
• Skimmer or Overflow: A skimmer or overflow system removes the collected oil from the oil collection chamber, transferring it to a sludge tank or oily waste holding tank for further processing or disposal.
• Clean Water Outlet: The treated water, with reduced oil content, exits the OWS through an outlet pipe located near the bottom of the separation chamber.
• Oil Content Monitor (OCM): An OCM is installed on the clean water outlet to continuously monitor the oil content in the discharged water. It ensures compliance with the MARPOL Annex I limit of 15 ppm (parts per million) oil content.
• Automatic Stopping Device: The system incorporates an automatic stopping device that halts the discharge if the OCM detects an oil content exceeding 15 ppm.

Operation:

1. Oily Water Entry: Oily water is pumped into the OWS through the inlet pipe.
2. Gravity Separation: The mixture enters the separation chamber, where oil droplets, being less dense than water, rise to the surface due to gravity. Baffles or plates help enhance this separation process.   1. density | STEM Newcastle blogs.ncl.ac.uk2. Baffle Plate Configuration to Enhance Separation in Horizontal Primary Separators www.researchgate.net
3. Oil Collection: The separated oil accumulates in the oil collection chamber at the top of the tank.   1. Static Oil Separators – Gazebo SpA www.gazebo.it
4. Oil Removal: The skimmer or overflow system removes the collected oil and transfers it to a separate tank.
5. Clean Water Discharge: The treated water, now with reduced oil content, is discharged overboard through the outlet pipe, provided it meets the 15 ppm limit monitored by the OCM.
6. Automatic Stop: If the OCM detects an oil content exceeding 15 ppm, the automatic stopping device halts the discharge, preventing pollution.

Limitations of Single-Stage Static OWS:

• Limited Efficiency: While effective for simple oil-water mixtures, it may struggle to separate stable emulsions or remove very fine oil droplets.
• Sensitivity to Flow Rate: Exceeding the designed flow rate can reduce separation efficiency and lead to non-compliant discharges.
• Requires Sufficient Settling Time: The separation process relies on gravity, so adequate residence time in the chamber is crucial for effective oil removal.   1. Oily Water Separator — Working Principle and Construction | by MarinersPoint | Medium medium.com

Despite these limitations, single-stage static OWS remains a common and cost-effective solution for many vessels, especially smaller ones or those with less complex bilge water compositions. However, for larger vessels or those dealing with more challenging oily mixtures, multi-stage or enhanced OWS systems that incorporate additional separation technologies like coalescers or centrifuges may be required to meet MARPOL requirements.Sources and related content