<human>: What are the most common types of septic tanks and how do they work?
<bot>: The most common types of septic tanks are gravity-fed, pressure-fed, and vacuum-composting systems. Gravity-fed systems rely on the force of gravity to move the wastewater into the tank. Pressure-fed systems use a pump to move the water into the tank. Vacuum-composting systems use air suction to pull the solids out of the tank. All three types of septic tanks have similar functions in that they remove solids from wastewater before it is discharged into the environment.