I am installing 3/4 hp sewage pump in a downstairs addition because the downstairs bath is lower than the city main. It is installed to code complete with 2" vertical check and all.

I was getting ready to make the final hook-up to my main when it occured to me that if I ever had a plug somewhere down my mainline and effluent couldn't flow to the city main, it would continue to pump and probably come out of the upstairs shower first, then the toilet, etc.

Any ideas save installing a back-flow check at every discharge point? There is a shower, a tub 3 lavs and two toilets upstairs.