Hi Miriam,
Short term, the sewer system will still work, long term, things change.
As we all know "stuff" flows downhill.
In the mountains, people live up high, and the sewer treatment facilities are down low.
Where the ground is flatter, there are huge networks of pipes called interceptors that collect waste from the sewer mains (what most homes empty into).
They use gravity/are pitched downhill, but at the point the gravity system won't work anymore because you can only go downhill so far, there are pumping stations.
These pumps lift the sewage from depths of 25' and more back to just a few feet below the surface, where gravity takes over again.
The pumping stations have backup generators, in case of power loss, but as in the case of a major disaster, earthquake/flood, etc, the system stops working.
Once the pipes are full, the toilets won't flush as there is no where for it to go.
Septic systems and pit toilets will be the only systems that work, unless there is a flood, and then the septic system or pit is overcome with water, and won't work either.
High ground is a good place to be!
Rob