About 20 million people go on cruises every year. If you are part of this statistic, chances are you have experienced at least a passing moment when you wondered what really happens to your poop when you flush the toilet.
Does the waste go straight into the ocean, or is it stored somewhere and then released later on? Exactly how do cruise ships get rid of human waste?
How Do Cruise Ships Get Rid Of Human Waste?
An average cruise ship with a passenger and crew capacity of 3,000 produces approximately 21,000 gallons of sewage daily. In the not-so-distant past, this sewage
was simply thrown overboard through “storm valves” connected to the sides of the ship.
Today, however, there are really strict international maritime laws that cruise ships must abide by due to growing concerns over marine pollution.
For starters, the International Convention for the Prevention of Pollution from Ships requires ships to be at least 5km or 3 nautical miles from land before dumping treated sewage.
How cruise ships deal with sewage is not that different from what happens in our home sewage systems.
When you flash the loo, the wastewater generally goes to a wastewater treatment system aboard the ship, where it is passed through multiple treatment and disinfection processes.
These treatment units are required to meet strict International Maritime Organization standards, and they have to be installed and used according to the manufacturer’s keen instructions.
In a functional treatment system, the sewage should ideally come out the other side as drinkable water before it is pumped back into the ocean.
A typical biological treatment plant works by decomposing raw sewage. This process usually involves injecting fresh air into the sewage chamber,
which helps the aerobic bacteria working on decomposition to thrive. In the absence of air, it’s the anaerobic bacteria that thrive instead,
which are known to produce toxic gasses with negative health implications. Also, if anaerobic bacteria are used to decompose the sewage,
a dark black liquid that is not acceptable for discharging is formed. So the goal is to maintain the flow of fresh air in the treatment plant.
There are usually three main chambers in a ship’s biological sewage plant:
1. Aeration chamber
Raw sewage is ground into small particles and then fed into this chamber. The sewage is broken down into small particles to increase the surface area and speed up the decomposition process.
Inorganic sewage, water, and carbon dioxide are produced in this stage.
Air is typically forced into the air chamber via the diffuser. Even the pressure with which air is pushed into the air chamber contributes to the quality of the decomposition.
High pressure prevents the air and sewage from mixing properly, so most of the air escapes without making any impact. It is therefore extremely important to maintain controlled pressure inside the treatment plant.
2. Settling tank
From the aeration chamber, the mixture of sludge and liquid is transferred into the settling tank, where the clear liquid stays on the top while the sludge settles at the bottom.
The sludge should not be kept in the settling tank as it can trigger the growth of anaerobic bacteria. As such, it is normally recycled and reused in the breakdown of sewage.
3. Chlorination and collection
The clear liquid from the settling tank is overflown here and then disinfected using chlorine to get rid of e-Coli bacteria.
The treated liquid is kept for at least an hour more to fight off any remaining e-Coli. Some plants also use ultraviolet radiation for extreme disinfection.
The resultant liquid is then discharged overboard or into the holding tank for future disposal, depending on where the ship is at that particular time.
If it is near the coastline or in a restricted zone, the liquid is preserved in the holding tank until it can be safely discharged into the ocean.
According to new regulations, passenger ships are prohibited from discharging sewage in a Special Area unless the ship has an approved sewage treatment plant onboard that meets the phosphorus and nitrogen removal standard.
The Problem with Waste Disposal on Cruise Ships
All cruise ships produce the following types of waste:
- Polluted discharge containing lubricants, fuel, and chemical additives
- Air pollution from the engines
- Solid wastes (including glass, cans, food waste, cardboard, wood, paper, plastic, etc.)
- Hazardous wastes (including batteries, fluorescent light bulbs, print shop wastes, solvents, paint waste, photo processing wastes, and perchloroethylene from dry cleaning)
- Oily bilge water
- Black water or sewage
- Grey water from galleys, laundries, shower, and sinks
In addition to harming the environment with these waste products, the cruise ships themselves can also have a negative impact on surrounding ecosystems purely due to their enormous size.
Designed like floating cities, these vessels are extremely sophisticated to navigate and can be difficult to steer away from sea creatures at a short notice.
As a result, cruise ships are responsible for injuring and killing large numbers of marine life.
The weird part is that cruise ships are getting even larger with every new vessel, so ports now have to increase the depth and frequency of dredge projects,
which means literally digging deeper channels that can accommodate larger, deeper ships. This naturally comes at the cost of destroying local habitats,
but even without dredging, the mere passage of these gigantic cruise ships can stir up sediment and compromise bottom-dwelling life.
Like with sewage/black water, the discharge of oil/oily water into U.S. waters and adjoining shorelines is prohibited unless it has been processed in an oil-water separator
and the oil concentration reduced to 15 parts per million (ppm). Even then, the liquid should be discharged at least 12 miles offshore.
Ships usually have an “Oil Record Book” for recording every time they discharge oil into the ocean.
When it comes to garbage disposal, cruise ships are not supposed to dump this waste within a certain distance from the shore, typically from 3-25 miles.
Any type of plastic disposal is prohibited everywhere at sea, and ships usually have to maintain a Garbage Record Book for keeping up with all garbage discharges.
FAQs on Ships’ Waste
Does sewage treatment on a cruise ship smell?
This is a pretty obvious question for people who have never seen a ship’s onboard sewage treatment plant in operation. The general answer is yes, any biological sewage treatment plant smells. However, the smell is usually contained within the chambers to ensure the safety and comfort of the operating crew. If you open the manhole above the screen filter, you will be treated to a pungent odor reminiscent of rotten eggs.
Do sewage treatment plants work like septic tanks?
Both septic tanks and sewage treatment plants seem to work in a similar fashion, but there are notable differences. Sewage treatment plants treat sewage using a dedicated system with chambers, while a septic tank is a closed chamber installed deep into the soil where sewage decomposes on its own. In one, decomposition takes place using controlled air flow, while in the other the process happens naturally whether aerobically or anaerobically.
How do cruise ships get rid of human waste? Generally, they pass the sewage through biological sewage treatment plants aboard the ship before discharging it into the ocean.
Sewage disposal should always be done far away from land as the sewage may still contain harmful substances like viruses, pathogens, bacteria, and even metals.
These substances can kill marine animals and contaminate seafood if released near coasts, leading to various health complications.
Read: Boat lift mechanics