Cruise ships are sometimes carrying 10,000 and more people on board which amounts to the population of a small town. When they are set for a sail it will be usually 10 days to one month for a commercial ship to touch the land. Meanwhile, they have to take care of food, washing of clothes and disposing of human excreta for that period.
Not just human waste, there can be leftover food, bottles, cans, cardboard and everything which has to be handled on the land, has to be handled on the ship too.
No. Ships have a more scientific way of disposing of the waste, more than on the land. They have multi-stage treatment systems that meet the requirements of all international regulations.
They have the water filtration systems using reverse osmosis to prepare the fresh and drinking water, to handle the human waste and disposal of soap water etc., After treating, some of the water is let into the sea and some wastage is stored in the aerobic bacteria filled tanks to be disposed of after reaching the shores.