Flush toilets are a type of plumbing fixture and usually incorporate an "S", "U", "J", or "P" shaped bend called a trap that causes water to collect in the toilet bowl to hold the waste and act as a seal against noxious sewer gases. Most flush toilets are connected to a sewerage system that conveys waste to a sewage treatment plant; where this is not available, a septic tank or composting system may be used. When a toilet is flushed, the wastewater flows into a septic tank, or is conveyed to a treatment plant.