When sewer lines become obstructed, whereby preventing wastewater from flowing through drainage pipes, blockage occurs. The thick, black water that forms as sewage is a potential hazard, as it contains contaminants and viruses that can present a risk of severe illness if exposed to humans or animals. If you have a sewage backup in your basement, you will need to fix it quickly. However, knowing what causes sewer backups can help you prevent a hazardous situation in your basement or home.