 as their power source.
Bowl siphons are part of flush toilets. Although called a 'siphon' they are not actually a siphon. The name refers to the inverted 'U' of the plumbing attached to the toilet bowl. Water poured into the bowl during the flush increases the total volume of the water from its resting volume and so the water level rises above the lower part of the plumbing and the excess simply drains away until the resting volume returns.
Some toilets also use the siphon principle to obtain the actual flush from the cistern. The flush is triggered by a lever or handle that operates a simple diaphragm-like piston pump that lifts enough water to the crest of the siphon to start the flow of water which then completely empties the contents of the cistern into the toilet bowl. The advantage of this system was that no water would leak from the cistern excepting when flushed. These were mandatory in the UK until 2011.
Early urinals incorporated a siphon in the cistern which would flush automatically on a regular cycle because there was a constant trickle of clean water being fed to the cistern by a slightly open valve.
An inverted siphon is not a siphon but a term applied to pipes that must dip below an obstruction to form a "U" shaped flow path.
Large inverted siphons are used to convey water being carried in canals or flumes across valleys, for irrigation or gold mining. The Romans used inverted siphons of multiple lead pipes to cross valleys that were too big to construct an aqueduct.
Inverted siphons are commonly called traps for their function in preventing smelly sewer gases from coming back out of drains and sometimes making dense objects like rings and electronic components retrievable after falling into a drain. Liquid flowing in one end simply forces liquid up and out the other end, but solids like sand will accumulate. This is especially important in sewage systems or culverts which must be routed under rivers or other deep obstructions where the better term is "depressed sewer".
Back siphonage is a plumbing term applied to the reversal of normal water flow in a plumbing system due to sharply reduced or negative pressure on the water supply side, such as high demand on water supply by fire-fighting; it is not an actual siphon as it is suction. Back siphonage is rare as it depends on submerged inlets at the outlet (home) end and these are uncommon. Back siphonage is not to be confused with backflow; which is the reversed flow of water from the outlet end to the supply end caused by pressure occurring at the outlet end.
Building codes often contain specific sections on back siphonage and especially for external faucets (See the sample building code quote, below). Backflow prevention devices such as anti-siphon valves are required in such designs. The reason is that external faucets may be attached to hoses which may be immersed in an external body of water, such as a garden pond, swimming pool, aquarium or washing machine. In these situations the flow is not actually a siphon but suction due to reduced pressure on the water supply side. Should the pressure within the water supply system fall, the external water may be returned by back pressure into the drinking water system through the faucet. Another possible contamination point is the water intake in the toilet tank. An anti-siphon valve is also required here to prevent pressure drops in the water supply line from suctioning water out of the toilet tank (which may contain additives such as "toilet blue") and contaminating the water system. Anti-siphon valves function as a one-direction check valve.
Anti-siphon valves are also used medically. Hydrocephalus, or excess fluid in the brain, may be treated with a shunt which drains cerebrospinal fluid from the brain. All shunts have a valve to relieve excess pressure in the brain. The shunt may lead into the abdominal cavity such that the shunt outlet is significantly lower than the shunt intake when the patient is standing. Thus a siphon effect may take place and instead of simply relieving excess pressure, the shunt m