Although aerobic treatment plants, such as a Jet system, are typically sized by the gallons per day (GPD) of wastewater that they treat, it is useful to know how many gallons a tank holds when scheduling or paying for a pump out. The basic Jet system holds 1,225 gallons divided into three separate compartments a 500 gallon pretreatment compartment, a 600 gallon treatment compartment, and a 125 gallon settling compartment. Larger jet systems may have an additional 1,000 gallon pretreatment tank before the Jet treatment plant.