Oily Water Separators: 

In the engine room, tucked away among valves, gauges, and pipelines, lies a piece of equipment that plays a quiet but vital role in protecting our oceans: the Oily Water Separator (OWS).

It doesn’t get much attention—until it stops working, or worse, gets misused.

This article breaks down the basics of OWS, how it works, where things often go wrong, and how small daily habits can make a big difference. Whether you’re a young engineer or a seasoned officer, it’s a reminder of why compliance with MARPOL isn’t just a box to tick—it’s a reflection of our professionalism at sea.

What Is the OWS and Why Does It Matter?

An OWS is designed to treat bilge water—the oily mixture that collects in the bottom of the engine room. It removes oil so that only treated water, containing less than 15 parts per million (ppm) of oil, is discharged into the sea.

Under MARPOL Annex I, ships over 400 GT must have this system installed. For ships over 10,000 GT, an Oil Content Meter (OCM) and automatic stopping devices are also required.

Why is the 15 ppm limit so critical? Because anything more than that isn’t just pollution—it’s a regulatory violation. And in today’s maritime industry, those violations can result in heavy fines, detentions, and serious damage to professional reputations.

Sampling Point: A Small Detail That Matters Big

One of the most common issues flagged during inspections is the sampling point arrangement.

According to MEPC.107(49), the sample point should be installed in a vertical section of the discharge pipe, as close as possible to the OWS outlet. This ensures a true, representative sample of what’s actually being discharged.

Why vertical? Because oil floats. In a horizontal pipe, oil can cling to the upper section, leading to false “clean” readings.

Many Port State Control (PSC) officers, particularly in regions like Paris MoU, are trained to spot incorrect sampling lines. A poorly placed sample point has led to many detentions.

Tip: During your rounds, check your sampling point. Is it vertical? If not, raise the issue. Fix it before it becomes a deficiency.

Understanding the Oil Content Meter (OCM)

The OCM—commonly referred to as the 15 ppm bilge alarm—monitors the quality of water discharged from the OWS. If the oil content exceeds 15 ppm, the system automatically redirects the discharge back to the bilge or OWS.

Modern OCMs (especially those compliant with MEPC.107(49)) come with data logging features. These store operational history and allow inspectors to cross-check with entries in the Oil Record Book (ORB).

Important: If an OCM is replaced, the stored data must remain available for at least 18 months.

Common Violations: Tricks That Can Cost You Everything

Unfortunately, some operators still attempt to bypass the system. These practices are illegal—and inspectors are increasingly trained to detect them.

 1. Using Flushing Water to Fake a Reading

Closing the sample valve and opening the flushing line can fool the OCM into reading clean water—allowing overboard discharge even when the bilge water is oily. However, most modern OCMs now raise alarms if flushing water is detected during discharge.

2. Permanently Bypassing the Sample

Some try to keep the sample valve closed to force a permanent clean reading. But if an inspector checks the discharge pipe and finds oil residue, it’s an immediate red flag.

3. “Magic Pipes”

These are illegal modifications—hidden discharge lines, freshly painted flanges, or unnecessary blanking plates. They’re tampering, and if discovered, they can lead to detention, dismissal, or even prosecution.

What Good Practice Looks Like

Instead of shortcuts, let’s focus on daily actions that protect both the environment and your career:

Monitor OCM readings regularly. Don’t wait for alarms.

Clean the coalescer unit. A clean separator works more efficiently.

Log operations accurately and on time. Your ORB is a legal document.

Test the auto-stop function monthly. Make sure it works as designed.

Mentor junior engineers. Teach them the correct way from Day 1.

Compliance Is a Culture, Not a Checklist

The real challenge isn’t the equipment—it’s the mindset.

When someone says, “It’s fine, no one will notice,” or “Let’s fix it later,” that’s where trouble begins.

Being professional at sea means doing the right thing even when no one is watching. Especially then.

Because how you treat your OWS reflects how seriously you take your role—not just as an engineer or officer, but as a custodian of the sea.