The nature of the raw sewage is different because of the eating and living habits of the family. The amount of water use is different so that the dilution of sewage is different. Different home cleaning products are used, as are varying amounts of other household chemicals and cleaners. Tank temperatures differ depending on type of water use, depth that the tank is buried, tank insulation, etc. All of these factors affect the bacterial population and the intensity of bacterial action.
Bacteria are always present in sewage, however.
If there is food and the proper temperature, they will grow and multiply. Septic tank bacterial action starts by itself and continues as long as solids are deposited in the tank. An outlet device, baffle or a tee as shown in Figure 1, definitely is needed on every septic tank. The bottom of this device should extend into the liquid a distance equal to 40 percent of the liquid depth. This is the location determined by a number of tests to provide for the clearest effluent to leave the septic tank.