A septic tank works by the bacteria digesting the solids. Is is normal that a shot of bacteria is placed in the tank to reactivate the function of the tank. A tank without bacteria will not function. In the days before additives we used buttermilk and yeast to reactivate a tank that had just been pumped.
How a septic tank works..... link below.....
http://septicpages.com/homeowners/how_does_a_septic_system_work