Water for a cruise ship is a constant balancing act! Between what the ship can produce, what those on board use, and making sure that the vessel’s stability (its ability to remain completely upright, and sail safely through any weather) is maintained. A mid-sized ship can use 25-30 tonnes of fresh water every hour, over 600 tonnes per day – more than half a million litres! Once used, all this water needs to be properly treated and disposed of.
Unlike other forms of waste, there aren’t many (if any) port facilities in the world capable of processing waste water in these quantities, so ships themselves employ the latest technology to do so. Much of this technology has been developed specifically for cruise ships, to better allow them to achieve this critical balance. Waste water is separated into grey- and black-water, with black water from toilets, and grey water from just about everywhere else. They are separated because, unlike the water in many cities, they are treated very differently.
Grey water is a very simple process – it is simply collected, and discharged into the ocean when permitted. Like food waste, each country may set its own limitations, and each company can impose further restrictions. An example of the limitations for grey water is 4 Nautical Miles from land (around 7 Kilometres). Grey water represents the clear majority of the waste water discharged from a ship, and as such any chemical used such as with shampoos, body washes or laundry detergents are heavily diluted before being discharged, and of course further diluted as the ship sails through the ocean. The chemicals used for cleaning on board are selected to be friendly to the ship’s waste treatment systems, as well as the environment.
Black water gets more complicated, and this is where the technology comes to power! The toilet systems on a ship use a vacuum collection method (like the suction seen on an aeroplane toilet system) which greatly reduces the amount of water needed to flush. Instead of the 4-5 litres found on many conventional flush toilets, this can be as low as 500 millilitres of water. This immediately reduces the amount of waste water created.
Once collected, the waste is processed to break up any solids – this is simply a mechanical process. The resulting liquid is then added to a ‘bioreactor’ chamber. Not to be confused with something out of a superhero movie, this chamber contains a bacterial culture that feeds on the waste, rapidly growing and creating clumps in the chamber. This bacterial culture is the key to the entire process, but it is sensitive to chemicals and some products such as dairy. Another reason cruise ships are so selective of the chemicals used on board! As the culture finishes its work digesting the waste, they are filtered out in several stages, resulting in ‘permeate’, a clear, near-drinkable waste product, and ‘biomass’ which is the clumped together bacteria.
The permeate is treated by UV light, ensuring any remaining bacteria etc. are killed, and passes through a sampling process before being discharged. If the bacterial culture is thriving, the permeate meets or exceeds the standards required for drinking water in many parts of the world! But be assured, rather than piping it back to the ship, it is discharged into the ocean. Because it is so clean, many countries allow this to be pumped out 24/7, even in port. To ensure we continue to meet their standards, we send samples ashore for lab testing. The results are graded ‘A’, ‘B’ or ‘C’, with each grade having a set of discharge limitations.
The biomass is another story – the bacteria effectively concentrate the solids from the black water, so much greater limitations are placed on its discharge. Like food waste, this is generally more than 12 Nautical Miles from land. Because the bacterial culture is sensitive to environmental changes, it simply cannot survive in the harsh salt water in the ocean – they become another source of food for the fish! The treatment systems are so efficient that only 25-30 tonnes are generally produced each day – only 5% of the total water usage.
As well as a distance from land, there are speed requirements that the ship needs to meet while discharging waste. Ships need to be doing at least 6 knots (around 10 kilometres per hour), to ensure that any waste water doesn’t pool in one place but is spread out, diluting the waste. Where the itinerary of the ship provides limited opportunities for them to go outside 12 Nautical Miles, cruise ships can be seen doing big loops in the ocean, providing enough time to empty their tanks. These are affectionately known as ‘poop-loops’! Even while sailing these patterns, the ship still needs to exceed both distance and speed requirements.
A cruise ship treats its waste water to a very high standard when compared to the systems commonly used by city treatment plants around the world, which serve tens or hundreds of thousands of people. City treatment stations may only discharge into the ocean a few kilometres from shore, some even into local waterways. The treatment of waste water by cruise ships is something to be envied by municipalities worldwide, and continued development of more effective treatment systems will ensure that our impact on the ocean environment we rely upon is reduced as much as possible.
How can you help reduce waste water on your cruise vacation?
- Don’t flush any waste except ship-provided toilet paper. Other items may cause blockages to the toilet vacuum system, or the treatment plant.
- Conserve water where possible. Using less water reduces the amount of waste water produced.
- As chemicals and products used on board are environmentally friendly (and treatment system friendly!), try to use ship-provided shampoo and body wash.
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