Fozen sewer pipe

I sometimes go to a temple in Worcester Mass. They've owned the building for a few years and they said that often times the bathrooms can't be used in the colder parts of the winter because the sewer pipe freezes. I told them I'd post a question on the internet about this to see if anyone has any suggestions. I wounder if you could dig up part of the pipe and see how deep it was and then wrap a long heating coil around it and bury it. You would then activate the heating coil in the winter. Would that be a bad idea to be running an electric heating coil under ground ?

My guess is the only other thing you could do is pile more dirt on it. How deep does it need to go ?

Could it be affective to connect the toilet with the hot water instead of the cold and flush it often ?

Thanks