A backwater valve is simply a device in the waste pipe that prevents sewage from flowing back into the building. When the city sewer in the street backs up downstream of your connection, there is a potential for the upstream flows to find their way into your building and work back uphill to your fixtures. In these instances, the effluent flows out of the lowest opening it can find, typically a basement floor drain or similar fixture.
Plumbing codes require backwater valves when a fixture is installed on a floor that is below the next upstream manhole. The obvious example is the basement floor drain. Less obvious is a first floor fixture in a lot at the bottom of a hill.
In this case, the next upstream manhole is often higher than the first floor slab. A sewer clog in the street downstream of the lot can collect everything from the top of the hill until it comes flowing out of the unprotected first floor fixtures.
The next upstream manhole is key, because it is the source of relief. When the street backs up to the next upstream manhole, the sewage will lift the cover and the flow will cascade down the street. It’s not a pretty site and certainly bad for the ocean to have that in our storm drain, but it’s better than having it in your living room.
As much as the codes require backwater valves for lower fixtures, they prohibit backwater valves for higher fixtures. Second story fixtures should not flow through a backwater valve since a valve failure resulting in a clog will divert the second story flows out the first floor fixtures you are trying to protect.
Some codes state that backwater valves are required when the flood level rim of the fixture is installed below the next upstream manhole. Taken to the next level, if a first floor slab is six inches below the next upstream manhole, which fixtures should be protected?
Certainly the floor drains, mop sinks and showers should be protected, but what about the toilets, lavatories and sinks? Since they sit 15″, 30″ and even 36″ above the floor, they should be plumbed separately around the backwater valve. This results in additional piping and invert challenges where the two lines may cross. Nevertheless, they should be separated.
The first design task is then to separate and route the flows from fixtures that need protection and those that don’t. When doing so, avoid crossing lines where it may lead to lower inverts. If you must cross, route the unprotected line lower than the line going to the backwater valve. You will need the additional height for the proper operation of the valve. Locate the valve where it can be accessed. This too is a code requirement.
Consider that it may need frequent access for maintenance and this will be a messy, foul smelling task. Check the inverts in and out of the valve. Additional design requirements will depend on the type of valve selected.