Best place to put a check valve when there are 2 lines in the house

Question

I had some bad sewage backup during the flash flood surge from Ida here in New York City. I'd like to prevent ever having sewage backup and am looking to put in a check valve. There are a couple of options to put the check valve and not sure which place is optimal to prevent sewage backup and flooding in the basement from the floor drains.

I have a storm line and a house sewer line that meet and form to a single combined city sewer connection point at the very front of the house in the basement under the concrete. I have the two house traps accessible in the cutout on the concrete floor, but there is not a lot of room. Digging up the concrete floor is the majority of the work/labor in this endeavor.

Option 1.

Place a combined check valve in front of both house traps where it combines to the single point of connection to the city sewer. Since there is not a lot of room in front of the house traps, this would mean moving the existing house traps further back away from the wall and digging up concrete in front and behind the existing traps. I could still have flooding from storm runoff from my roof and my backyard - so I would need to install floor check valves in my floor drains in the basement. This would be the most laborious option as it would require moving house drains and digging up a large amount of concrete.

Option 2

Place a check valve behind the existing house sewer trap and leave the storm drain side alone. This means that if there is sewer backflow, it would push into my storm drain and potentially come out my backyard drain. This means that most likely the sewage backup would not come up into my backyard unless everyone in the neighborhood also installed check valves and the sewage rises above the basement. This option requires the least amount of work and uses the existing house traps, with only a smaller concrete excavation for the single check valve.

Is option #1 overkill? I assume that the floor drains don't connect to the storm drain because of my blueprints.

Answer 1

Not a full answer, but some comments and alternative suggestion:

You should confirm with your local code if you are allowed to install a backwater valve (drainage check valve) on the entire system(s). Some codes, or even the local authority having jurisdiction, may only allow backwater valves on the portion of a drainage system below the municipality's system.

Regarding the use of backwater valves, in particular with sanitary drainage, they themselves can cause damage when they fail. They increase the chance of blockage that overtime will cause a backup of your own sewage that often will not be know until it overflows out of the lowest fixture. Additionally if they become partially blocked, or the swing check becomes seized, they may not fully close in a back-surge situation.

Some locations allow the installation of a manually operated gate valve. In this case, the gate of the valve would be fully out of the way of the sewage when open. When there is signs of a back-surge, you would manually close the valve until the issues has been resolved.