A building where three toilets (toilet + urinal each) are connected to the same sewage pipe under the building. The pipe is DN 100 and should have (according to old plans) a gradient of 1:50. Two floor drains are also connected to this pipe. It empties after a few meters into a DN125 sewage pipe that collects the water from a dozen showers. This empties after 6m or so in a larger pipe outside the building.
The branch of the network near the toilets hade a ventilation pipe straight up through the roof, that was errouneously removed during recent refurbishments.
Since then, at certain shift changes when the toilets and showers where heavily in use, the floor drain overflowed and sewage from the toilets flooded the area. The whole network was checked with cameras. There's no blockage, only a slight sag near the toilets. It was observed that when you flush a toilet, the urinal in the same stall by will empty partially.
In the same building, in a nother branch of the sewaer pipe network, the ventilation pipes where removed also. Here, we have a lot of pipe upstream of the toilet and so far no problems occured.
Now, our assumption is that due to the removal of the ventilation, we have this backflow problem. I want to check if my reasoning here is correct & if we are not solving the wrong problem by installing a new ventilation:
When flushing the toilet, I have a plug of water in the pipe and an area of underpressure behind until air can pass over this plug.
When the, downstream of the toilets, the pipe is relativly full because the showers are in use, the water from the toilets flows off slower and the time till equalization is longer
Hence backflow and overflowing floor drain