How Does a Septic System Work?

  • Learn that all septic systems perform the same essential function: process wastewater and return it to the water table.
  • Discover that maximizing the time that bacteria can breakdown organic material is critical.
  • Learn that hydraulic loading greatly affects the ability of the septic system to perform.
  • Discover that Aero-Stream can extend the life of your septic system indefinitely by executing a controlled aerobic septic system conversion for under $1500!

Septic System Introduction

Although there are many design variations of a septic system, they all perform the same basic function: to process household black and gray wastewater into a cleaner effluent that can be evenly distributed beneath the ground and blended with the groundwater without creating a health hazard.

There are passive and active systems. The overwhelming majority of residential systems are designed as passive systems. The basic components of a passive septic system are an inlet pipe which carries the household wastewater from the home to the septic tank, the septic tank, and an absorption component, most commonly a gravity drainfield.

Diagram of a functional septic system
Diagram of a functional septic system

Septic System Operation

When a toilet is flushed in a house, the wastewater flows from the house through the inlet pipe into the septic tank. An equal amount of effluent is displaced in the septic tank and flows out of the septic tank to the drainfield and eventually disperses into the ground.


Pumping a Septic Tank Through a Riser

Inside the septic tank is a large population of living microorganisms called anaerobic bacteria. The quantity of anaerobic bacteria is dependent on the solid organic material in the wastewater for a food source. Therefore, when the loading into the septic tank is low, the anaerobic bacteria count declines and when water usage peaks, the anaerobic bacteria count increases. An example would be when a family takes a vacation and the house is vacated. Water usage is nonexistent, so the anaerobic bacteria count decreases.

The septic tank is designed to always maintain minimum water level to allow the anaerobic bacteria to remain in existence at all times. At times of peak water usage such as a clothes washer or dishwasher emptying or when additional people are occupying the home, such as a family gathering, the loading of wastewater into the septic tank is high and the anaerobic bacteria count increases.

Because the anaerobic bacteria are a slow acting bacteria, maximizing the length of time the wastewater is in contact with the anaerobic bacteria in the septic tank is critical. This is referred to as “holding time” and is defined as the amount of time it takes for the wastewater to flow from the inlet of the tank to the outlet.

In general, the septic tank is designed to hold the wastewater for approximately 2 days. During this holding period, the anaerobic bacteria breakdown the organic material in the wastewater, reducing its strength by 40%, on average. As water usage varies, the volume of flow through the septic tank increases or decreases proportionately, which in turn increases or decreases the holding time and the amount of processing in the tank.

Once the wastewater reaches the tank outlet, the semi-processed effluent (still containing on the average 60% of its original organic material) flows out to the drainfield carrying anaerobic bacteria with it. In the drainfield, the anaerobic bacteria partially completes the treatment of the effluent, removing  more of the remaining organic material prior to being absorbed into the groundwater.

The problem with this design, as will be described in detail in later sections of the manual, is that the anaerobic bacteria do not break down enough of the organic material in the tank or in the drainfield, resulting in dire consequences for the service life of the system.

The Septic System Owners Manual

Nobody plans for the expense of having septic tank problems. Whether your septic system is new or failing, this manual is a must read for any homeowner.Understand the causes and discover the solutions to your septic system and septic tank problems.