The dumping of oil and oily water into the sea has been banned for decades.

However, it is probably still practiced today.

This is confirmed by a large-scale investigation by Deutsche Welle (DW) with the investigative network Lighthouse Reports.

Accordingly, thousands of cargo and container ships "systematically circumvented this ban to save time and money - with devastating consequences for the environment".

According to the report, the oil is drained "under cover of darkness," DW quotes a young marine engineer who has witnessed the crime several times.

The name of the German seaman and that of his tanker are not mentioned to protect the man.

The report suggests that as the movement of goods by sea increases – 90 percent of all goods are shipped – the problem is also growing.

More than 55,000 cargo and container ships are in use worldwide, without their work "half of humanity would starve and the other half would freeze to death".

Environmental organization SkyTruth estimates the amount of oily water discharged into the oceans in this way could be more than 200,000 cubic meters annually, the report said, citing satellite imagery.

"That's about five times more than the 1989 oil spill when the Exxon Valdez ran aground in Alaska's Prince William Sound."

Report: Disposal in the port costs time and money

In the practice of illegal disposal on the high seas, also known as "bilge dumps", "the bilge water would be transferred to another tank, usually the waste water tank, using a small, portable pump".

The bilge is the lowest space on a ship that is directly above the ship's planking or above the keel.

This is where the leakage water that has penetrated the ship's hull and, in the case of more modern ships, also condensation water - especially that from the air conditioning system - collects.

"You can assemble this pump in five minutes and then disassemble it and hide it in five minutes," one mariner is quoted as saying.

The international regulations that have been in force since the 1970s state that bilge water must be treated before it is discharged into the sea.

This is done with the help of a so-called oil separator, which separates the oil from the water.

It is mandatory for large ships.

Bilge water that is pumped into the sea after treatment may only contain a very small residual amount of oil.

The remaining oil sludge has to be stored on board and later disposed of in port.

The quantities must also be recorded in an oil diary.

According to the report, however, this oil diary can be “easily manipulated”.

According to the research, many ships bypass the prescribed treatment and pump the bilge water unfiltered into the sea, including the toxic metals lead and cadmium that are contained in the bilge water.

“There are no fees for disposal in the port, and the shipowner does not lose valuable hours transporting the goods.

That increases profit margins," the report said.

Five whistleblowers to DW and its media partners have confirmed the serious allegations made.