Why does removal of ventilation of a sewage pipe lead to backflow from the toilet?

Question

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

Answer 1

Simply if you remove the ventilation then you create a closed tube where the « piston » ie waste is trying to create a vacuum behind it, that is, untill it can draw past the liquid in the u-bend...

Answer 2

The size and slope of sewer plumbing has been optimized for gravity flow, meaning top part of pipe shall stays dry.

When the vent pipe is not helping to break the back stream vacuum, the flow may stagnate and either leave deposits clogging the sewer pipe or backing up.

Answer 3

To go a bit deeper - you can have a situation where there is no ventilation pipe and the sewer still works, but you will be lucky. In this case, having two ventilation pipes allowed the first building to work when one was removed - the remaining pipe allowed the building to continue to ventilate, but it worked twice as hard. The other situation is when the sewer line is under vacuum, which can happen if you live on a hilly area and the sewage line runs down stream. If you live in the valley, the opposite occurs, and the manway covers can have so much pressure they come off.

In many situations, if you have nuisance odors from the ventilation pipes, consider an odor control system. Activated carbon systems are especially well suited for this application.