500 gpd atu with 100' er and chlorinator

If your permits reads "500 gpd atu" you will need a 500 gallon per day treatment unit. Your system will hold more around 800 gallons,It will be designed to allow 500 gallon of influent(sewer) to be introduced to system a day. gpd is for gallons per day,ATU is aeration treatment unit. ER is effluent reduction.