A large part of the problem is because of "a combination of raw sewage and stormwater". That is what's called a "combined sewer".[1] It's an insane thing to have in areas like Seattle and Portland where it rains a lot. It means you're periodically dealing with massive amounts of water.
I live in a newer community near Portland and our stormwater drains are independent of our sanitary sewers. Thankfully.
Combined sewers are generally syptom of aged infrastructure. For many years standard practice has been to separate storm and sewer systems -- in part because the cost of treating sewage is roughly proportional to the volume of water passing through the system.
Going further, stormwater systems are also a sign of obsolescent approaches to infrastructure. Contemporary practice and progressive land use regulations require development generated additional runoff to be mitigated on-site to maintain pre-development flow rates at a development site's edges.
Stormwater systems are superior. On-site mitigation supports mosquitoes and lets automotive fluids seep into the soil. On-site is only preferred due to cost, particularly because of who is paying.
I live in a newer community near Portland and our stormwater drains are independent of our sanitary sewers. Thankfully.
[1] https://en.wikipedia.org/wiki/Combined_sewer