When cruise lines advertise their ships, they usually parade their most exotic travel destinations, their award-winning gourmet restaurants, their luxury cabins and lounge areas, as well as the non-stop entertainment they offer on board. We are rarely shown any photos of the plumbing system, the garbage storage and all the other service facilities that ensure that all that glamour on board stays glamorous.

Have you ever looked at one of those giant cruise ships that accommodate thousands of people and wondered, how on earth does this ship carry water for all of these people? What happens to all the disgusting sewage water? Where does all the stinky garbage go? As always, The Shipyard is here to answer.

All This Water and Not a Drop to Drink

The biggest cruise ship in the world, Royal Caribbean’s Symphony of the Seas, has a capacity of 6,680 passengers, each of whom uses on average between 200 and 250 liters of water per day. For a ship this size, this is a total of 1.5 million liters of water per day, for the passengers alone. There is no way in the world a ship could store so much water on board for the duration of the journey. So where does the water for the showers, sinks, cooking, etc. come from?

Cruise ships turn sea water to drinking water. Special pumps in the hull suck the raw water in and transfer it to the ship’s desalination plant. The plants are complex systems composed of water storage tanks, pipelines, pressure tanks and pumps. Two main methods are used to create fresh water: reverse osmosis and evaporation.

In the reverse osmosis process, a pump pressurizes the seawater and forces its molecules through a semi-permeable membrane. The result is called pure water permeate. Most dissolved salts, organic compounds, bacteria and suspended solids are larger than the mesh of the membrane and are unable to pass through.

In the alternative purification process, the H2O is evaporated from the seawater in specially designed evaporators, such as the Wärtsilä Serck Como multi-stage flash (MSF) evaporators in the picture below. Heat from the engine’s cooling water (75-95 ° C) is typically used as a source of heat for the evaporation process. The resulting condensate is then processed depending on the intended use.

After the water is desalinated through either evaporation or reverse osmosis, it is further mineralized and disinfected before it can be used as drinking water. Several methods for water disinfection are used on cruise ships including chlorination, ozonation, silver-ion treatment, UV radiation, filtration and heating up.

The resulting fresh water is stored in tanks, which are usually found in the hull of the cruise ships. With the aid of special pumps, the water finally reaches the end user in the bathrooms, laundry, the kitchen, etc. through kilometers of pipes that run through the ship. During the construction of the tanks, particular care is taken to protect the water from contamination. The plants achieve a quality of water that meets the drinking water standard on land.

Going Down the Drain – What Happens to Sewage Water?

Modern day cruise ships can carry several thousands of people who use the showers, toilets, pools and spa areas every day and all this results in astounding amounts of sewage material. Additional waste water is generated by the laundry, kitchen and other servicing facilities on board. Throughout the years, many cruise lines have been accused of dumping their wastewater directly into the ocean, but maritime regulations have changed in the recent past and all modern cruise ships are obliged to have their own wastewater treatment plant on board. To understand how it functions, let’s look at the four main types of wastewater on ships and how each type is processed.

Black Water

Black water is the first and most ghastly category. It refers to highly polluted waste water, containing fecal solids from the toilet system, bacteria, as well as traces of prescribed medications and hormones. To go from gruesome to golden, black water is treated in several steps – mechanically, biologically, as well as chemically. The first step is the mechanical treatment of the water, whereby coarse impurities (particles with a diameter of less than one millimeter) are mechanically filtered out. In the second step, the water goes through biological purification, where microorganisms decompose the organic matter. In the third step, the water runs through filters with extremely fine sifts (0.2 microns) that sort out all microorganisms. The filter membranes are advanced enough, so that no further disinfection of the water is necessary. The final step is nitrogen and phosphorus reduction. After all this treatment, black water may be disposed of overboard, if the ship is more than 12 nautical miles from land.

Grey Water

Grey water refers to most of the sewage on board, for example, from showering, bathing, washing hands, the laundry and the kitchen. Grey water is not as heavily polluted as black water, but still contains considerable amounts of pollutants, such as residues of personal care products, detergents or pharmaceutical products. On most ships it is therefore collected and treated together with the black water. According to current regulations, grey water may be dumped into the ocean 4 nautical miles from land.

Bilge Water

Bilge water may form in the lower part of the ship’s hull, the so-called bilge, for example due to condensate. Cruise ships have their own bilge water treatment system, which separates the gathered water from oil residues. The treatment of bilge water is extremely important, as even a small drop of oil can poison between 600 and 1,000 liters of water. The residues are separated from the water either mechanically, or chemically, and then transferred to special tanks. The separated oil residues are either burned on board or delivered ashore.

The wastewater treatment plants on ships create by-products which are dehydrated and dried in a centrifuge. In order to prevent the reproduction of germs and to save space on board, the by-products are burned in an incinerator after they are dried. Once the ship docks, the ashes are disposed of with the remaining waste.

Ballast Water

Ballast water is taken in or expelled by seagoing vessels in order to increase the stability of the ship. The water is stored in appropriate ballast tanks and a quantity of up to 4000 cubic meters (141,258 cubic feet) is needed depending on the size of the ship.

When seawater is sucked into the ballast tanks, small marine organisms, such as plankton, also get sucked in. When the ship sails to another location and expels its ballast tanks, these microorganisms are released into a foreign ecosystem, where they can spread rapidly and disrupt the balance of the existing ecosystem. To avoid this, ships are now obliged to filter and disinfect their ballast water through UV electrolysis or chemical treatment.

Where Has All the Garbage Gone?

16 tons of food waste are produced on average each week on a medium-sized cruise ship.

Since storage space on board is limited, a portion of the food leftovers is thrown overboard at the end of each day and ends up as fish food. This practice continues because food waste regulations are not as strict as those for sewage, for example. Despite the fact that food waste is theoretically not toxic to the environment, large quantities of it lead to the overfertilization of the oceans and fast algae growth, which in turn leads to a decrease in oxygen levels in the ocean. According to the International Maritime Organization (IMO) regulations, shredded food garbage may be released three nautical miles away from land. In certain special zones, the regulation states at least twelve nautical miles away from land. These regulations are often criticized for being outdated, as the passenger capacity on cruise ships, and therefore the quantities of garbage produced, has drastically grown in the past decade.

Cardboard, metals and plastics are collected and pressed together by hydraulic garbage compressors. Glass is crushed into small pieces by onboard crushing machines and then filled into large bags, each weighing a whole ton when full. Non-recyclable garbage is burned in incinerators and turned to ashes.

In the ports, the shipping companies then pass on the bundled waste packages to local disposal companies. The Marine Pollution Code of the IMO, states that no vessel or offshore platform may dump ashes from incinerators, oil from the kitchen or plastics into the sea, no matter the distance from land.

Part 2 Coming Soon…

In next week’s post, you can find out how the laundry rooms, the housekeeping, and the kitchens operate aboard a cruise ship.

The Shipyard