We are starting to have the occasional warmer weather right now (Alberta, Canada), and every time it warms up our exterior discharge line for the sump pump smells like sewage. Once it gets colder again, the smell goes away. There isn't usually a smell inside, just the line outside. The pipe goes into the ground to tie into the house's drainage system. We also have a backwater valve close to that area (in the basement), in case that's important. Any ideas as to what might be wrong? Any easy fixes we should try before hiring a plumber?