The septic tank is full of anaerobic bacteria that consume (albeit slowly) the sewage and get carried out into the leaching bed. Here the dead anaerobic bacteria and remaining sewage collect and wait for aerobic bacteria to finish the decomposition process to water for proper disposal.
For a system to operate properly it requires oxygen to penetrate the soil and feed the bacteria, which is the reason the Ontario Building Code OBC) states that “A leaching bed shall not be covered with any material having a hydraulic conductivity less then 0.01 m/day.” Ref 8.7.2.1.(2)
(The photo is of a failing, 20 year old septic system with 3 feet of cover, and even though the home owners pumped the tank regularly and took care of the system, it still failed because the bacteria couldn't breath).
So if a system has been designed according to code, why did it fail? It could have failed due to one of the following reasons:
- The soil became compacted over time from lawn rolling, lawn tractors, heavy rains, vehicles.
- Increased water use caused the leaching bed to become saturated and the aerobic bacteria died as a result of less oxygen being available in water. (water can only hold about 10ppm, air can hold >200,000ppm).
- The tank was pumped out infrequently which resulted in higher amounts of untreated sewage entering the bed, and again, the aerobic bacteria couldn’t keep up due to the limited amounts of oxygen penetrating the soil.
- Harsh chemicals were used which changed the pH of the tank and leaching bed causing the bacteria to die and accumulate even faster.
