Bowls of Water Closets (WC) are now a standard design all over the world. Each one has a health safety feature most people are not aware of; residual water in the bowl that blocks foul gases from the septic tank.
The septic tank, usually situated outside, is a buried chamber where wastes from the WC go to decompose and later seep or soak into the soil. But the decomposition process produces poisonous gases which are allowed to vent into the open air outside the chamber. But the piping of the bowl to the chamber is the same thus creating a problem.
Therefore, the bowl is designed to retain water after flushing in such a way that foul gases returning from the chamber through the pipe would meet the small water obstacle which when full would prevent the gases from entering the toilet house.
So the safety requirement is that after every flushing, the bowl automatically retains the required level of water in the bowl by itself. But if for any reason external flushing aid of water is employed, then the water level can only be maintained if the pouring is done very gradually after the sludge has been forcefully dislodged.