ᐅ Combining Wastewater Pipes URGENT HELP

Hello Forum,
I urgently need help. We are extending our existing single-family house to convert it into a two-family house. For this, we have to relocate the sewage pipes located on our property. Our pipe is currently a DN 160 pipe, and the neighbor’s pipe is the same size (also a two-family house). Now, since both pipes need to be relocated, we thought about combining them into one.
What size pipe would be needed in that case?

My initial guess would be a DN 250 pipe.
What do you think?

I believe a DN160 should be completely adequate. Increasing the size will inevitably lead to a significant rise in the amount of water needed to transport the waste. A cross-section that is too large is just as problematic as one that is too small.

A friend of mine built a six-family house with about 700 sqm (7,535 sq ft) last year. The connection was a 160mm (6 inch) pipe.

Hello,

Two houses with two apartments each sharing a lateral sewer pipe? Technically not a problem, but if a clog occurs... which house will be responsible for the costs?
I would definitely discuss the shared wastewater pipe with the municipality’s wastewater department. Will one of the two lateral pipes connect to the municipal sewer, or is a completely new pipe going to be installed? Is there a so-called separated system in the municipality or street (rainwater and sewage are drained separately)? I believe you will need some form of approval from the municipality (check local drainage regulations/planning permission).

If this is allowed, a manhole should definitely be installed at the junction point of the two pipes.

Good luck

I would not connect the two houses to a shared sewer line. As mentioned earlier, who would ultimately be responsible for clearing any blockage? Imagine if the neighbor caused the problem but cannot afford the cleaning costs—then you would be stuck with the expenses, even though it wasn’t your fault. And if a manhole is installed, on whose property should it be placed? In the end, someone will be at a disadvantage. Additionally, one of the connections to the public sewer system would have to be sealed off.

There are clear regulations from the municipality or relevant standards regarding the pipe diameters. This is not your responsibility; the municipality must determine it. Usually, a diameter of 250mm (10 inches) is too large because sometimes the street sewers only have 200mm (8 inches) pipes (maximum 300mm (12 inches)). In principle, combining a sewer line (if the municipality or city approves) is not a problem. Sewers rarely clog quickly, and a simple cleaning does not cost so much that it would be a serious issue. You already save the money for one or two cleanings by combining the lines, so that would not be a reason against it for me. You should definitely install an inspection chamber, and it will likely be required. Of course, it must be placed on the property, but if you position it in a reasonable location, it won’t really limit the use of your property. For those who do not want to see it at all, it can be placed in the driveway and paved over (though this makes accessing it more complicated later).

Keep in mind that almost all municipalities or cities require an inspection of the sewer line. So, only cover the trench with sand and soil again after an official employee from the public authority has reviewed it.