A small septic system for a two- or three-bedroom home can handle about five loads of laundry per day. This does not include any other water being used and is based on using an old washer with a 1,000-gallon tank. The washer type is a major factor because older washers can use up to 40 gallons per wash. New energy-efficient washers usually use 12 to 15 gallons of water.