Complex sentence: A proper sump pump, backup sump and/or battery backup sump pump should be installed in a large sump pit with an airtight lid for safety and to keep humidity from seeping through to the basement environment, where it can promote mold growth.
Split into simpler sentences: A proper sump pump, backup sump and/or battery backup sump pump should be installed in a large sump pit with an airtight lid, for safety. A sump system also works to keep humidity from seeping through to the basement environment, where (humidity) can promote mold growth.