Why is there water present in the toilet after flushing?
When we do our business in the toilet and flush it, we always see that there is water present in it. You must be wondering how do this water is stuck there, well it has a purpose for it. When you turn the flush handle, water from the tank goes and flushes the excreta and little bit water is remaining at the bottom of the toilet. There is a U seal attached to the toilet and pipeline is attached to the U seal. Not only in toilets, this U seal present in wash basins too and this is provided for the water to get stuck in it.
When sewage from the pipeline goes to the sewage tanks, the sewage in the tanks is going to decompose and will produce a foul smell and produce methane gas. As gas is always lighter it will flow upward through the pipeline and strike the water present in the U seal. Due to this U seal and the water present inside, it doesn’t allow the gases from the tank to pass through it and go to the bathroom.
Suppose if the seal is not there, all your excreta is directly going to the tanks but one day methane gas is produced and it goes into your toilet and your toilet is in a locked condition and the moment you enter the toilet you may die in few seconds. This is why this seal is given so that it won’t allow the gases to pass through it.
One important thing to remember is that when water is present in the U seal, there is a pressure head present a due to the shape of the seal (refer to the image given above) which is “ρgh”. The gas coming from the tank which is taken as “P” should have pressure at least equal to the pressure head present in the U seal and if the pressure “P” of gas is greater than the pressure “ρgh” then the gas will displace the water and come out which can be injurious to health. The U seal is always designed in a particular way depending on how much pressure of air from the tank can come through the pipeline and can it displace the pressure head present in the seal.
One more important thing to remember is suppose you have gone out for three months and you have not used this toilet, so the water in the U seal starts getting evaporated and slowly over time it will get completely evaporated, now there is no water seal present. When you return back home and open the toilet room you might get a strong foul smell because due to the absence of water seal the dirty gas and methane gas can come up to the toilet room. So the right thing to do now is immediately flush the toilet which will again create the seal and no more gas can come up and then you can ventilate your toilet.
Note: In ships, generally this happens in case of Hospital toilets which are not used for months, causing U-seal to evaporate causing the smell from STP to reach into hospital.
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