Many people fill in old septic tanks when they switch to a public sewer system for safety reasons. Before filling a septic tank, make sure laws in your state allow it to be filled instead of having to remove it. Filling an unused septic tank will prevent it from collapsing, creating a sinkhole in your yard. Have a professional sewer company pump out the tank before filling it.
Ask your local health department to see whether you need a permit to fill the septic tank. If so, obtain a permit.
Pump out any water in the septic tank with a water pump. Any water in the septic tank will be from rain or groundwater.
Remove the lid and destroy it. Use a jackhammer to break up the concrete lid for easier disposal. Jackhammers can be rented at most hardware stores.
Drill holes in all of the side walls and bottom of the septic tank. This will allow drainage for all future rain and groundwater. If you do not drill holes and water collects in the septic tank, it can possibly float up out of the ground.
Fill the septic tank with dirt or gravel. This will prevent the ground above the tank from collapsing.