Cruise ships are only permitted to release human wastewater that has been heavily treated. Solid waste is also treated and dehydrated but not released into any open body of water. Instead, it is disposed of through standard, onshore waste disposal methods once the cruise ship is in port.

Modern cruise ships use sophisticated Advanced Wastewater Treatment Systems (AWTS) that often exceed the standards of shoreside municipal facilities. When you flush, the waste is pulled via vacuum suction into a bioreactor where aerobic bacteria break down the organic matter. The liquid then passes through ultra-fine membrane filters and settlement chambers to remove solids. Finally, the water is disinfected using UV light to kill any remaining pathogens, leaving it nearly pure. This treated "effluent" can be discharged into the ocean only when the ship is moving and at a specific distance from land (usually 12 nautical miles), though many modern ships produce water clean enough to be discharged in port if permitted. The remaining solid "biosolids" are either dried and incinerated on board or stored to be offloaded at a specialized port facility for use as fertilizer.