Your system was designed to process waste using a capacity based on the number of bedrooms in the home. Many guests at one time for several days can easily overwhelm your system causing a backup. When a drain field system backs up, solid matter can be pushed into the drain field. If this happens too many times, the drain field pipes will become clogged and will need to be replaced. An aerobic system backup can cause damage to system parts and should be examined by a wastewater system professional.