A cruise ship bathroom functions through a sophisticated high-velocity vacuum system rather than the gravity-fed system found in most homes. When you press the flush button, a valve opens and the massive pressure difference "sucks" the waste through the pipes at high speed using very little water—usually only about 1.5 liters per flush compared to 6–10 liters on land. This waste is then sent to an onboard Advanced Wastewater Treatment System (AWTS). Here, the "black water" from toilets is filtered, aerated with bacteria to break down organic matter, and then sterilized using UV light. By the time the water is released into the ocean (while the ship is moving and far from shore), it is often cleaner than the surrounding sea water. In the cabin itself, bathrooms are typically pre-fabricated "pods" made of fiberglass to save weight and space. Because the vacuum system is so powerful and delicate, you should never flush anything other than human waste and the provided toilet paper; items like wipes or feminine products can cause a system-wide blockage that affects dozens of cabins simultaneously.