Here's What Happens to the Sewage on a Cruise Ship

On average, a passenger on the cruise ship uses 200-250 litre water per day. A lot of sewage is generated due to the thousands of people on board every day. 

But have you ever wondered where does the sewage on a cruise ship go? 

The sewage on the cruise ship includes the wastewater from the bathrooms, toilets, urinals, medical premises and other similar facilities.

When it comes to handling, treatment and disposal of sewage on the ship. It must be done as per the international maritime laws.

MARPOL- Annex IV is an international regulation which prohibits any vessels or offshore platforms from direct disposal of sewage into the sea.

According to this regulation, the sewage can be discharged into the seawater only after it is treated and the distance of the ship is 4 nautical miles from the nearest land. 

But if the sewage is not treated this can be discharged 12 nautical miles away from the nearest land.

Additionally, the discharged sewage should not produce any visible floating solids nor should it cause any discolouration of the surrounding water.

Generally, ships prefer to treat sewage before discharging it into the sea. The most common treatment process involves the following steps.

1- Storage: In this step, the blackwater and greywater are stored in the separate tanks. Blackwater requires treatment before discharge whereas the greywater can be discharged directly into the sea with minimal filtration.

2- Treatment: The blackwater is treated in bioreactors deep in the bowels of the ship, where all the nasty stuff is filtered out and digested by bacteria.  

3- Disinfection: The treated water is then disinfected using the chlorination or using UV radiation.

At this stage, the water is cleaner than the seawater. After the quality monitoring, the water is discharged into the sea at a safe distance from the land.