The reality is that raw sewage (known as black water) is dumped at sea more than 3 or 4 miles from land after it has been macerated (ground into small bits) and disinfected and held in a tank for a while for in theory the bugs to be killed. In some areas, sewage cannot be discharged at sea due the proximity to environmentally sensitive areas e.g. coral reefs. Sewage can also be discharged at ports but some of the smaller ones don’t have the facilities to handle the volume. Discharge at sea is obviously cheaper. The sewage is discharged at sea and dispersed whilst the ship is moving over a long distance e.g. 8 miles, and done at night in the early hours of the morning.
The most recently built ships have more efficient treatment which can treat to near drinking water quality but the above minimal treatment is what the US federal laws require. However the majority of ships were built some years ago.
There have been major fines where ships have discharged close to land or in harbours. E.g. In 2002 Holland America was fined $2 million for discharging 20,000 gallons of untreated sewage (by accident) in the harbour at Juneau, Alaska but could have discharged the same raw sewage without penalty only 3 miles from shore provided it was outside the Inside Passage.
This may all sound horrendous but let’s not forget that UK coastal towns dumped raw sewage into the sea for many many years often quite close to the shore. At Bognor Regis the rock pools often had Izal toilet paper floating into water in the 1950s.
The cruise companies will tell you what you would like to hear but the above is what the law allows and you should decide for yourself what the companies are most likely to do.
All modern ships have to have there own environmental control department all wast is treated saved in tanks. and pumped out into road tankers at a port of call,other wast tins/food etc are compacted placed in containers and are of-loaded onto container trucks also. Nothing whatsoever is allowed to go into the sea,it would be against inernational law to do so.
It may be the ideal for no types of waste to be dumped at sea but, although there are strict rules about what can be dumped and where, there is no absolute international ban. Most countries do have specific restrictions but, here again, they rarely amount to a total ban away from land. I read an interesting official notice on my cruise ship which listed the restrictions, e.g. no plastic to be dumped and nothing nasty in shallow water. Neither waste food nor sewage is necessarily harmful to marine life and of course it does soon degrade. No doubt a lot of useful material (such as metals) is collected at ports for recycling, but not everything.
I belive it is all sucked out by a "Poo" lorry at certain ports.
The liquid sewage is vacumed to treatment tanks and processed to provide the water for the flush system in the toilets and other water wash utilities around the ships.