Of particular interest to those living with systems that are 20 or more years old and signs of problems.
Sewage backup into the home is one possible sign of a failing system. However, backup can also be simply the result of a blockage somewhere between the house and the septic tank (this is relatively easy to fix).
Another possible sign of failure is a smell of sewage outside the house. If this smell is more noticeable after a lot of water has been put into the system - multiple showers or several loads of laundry (if the laundry waste discharges into the septic system), for example - this may be an indication that the leach field is failing. The smell may also be accompanied by a "spongy" feeling in some areas of the leach field, near the distribution box, or near the septic tank.
The "spongy" feel may be caused by water and waste being pushed to or near ground level. If ponding water is also seen, this is called "breakthrough" and is an almost positive indication of failure of one or more parts of the system. This smell, however, can also originate at the plumbing vent. In either case, further investigation is warranted.
Dye Testing: If you see such signs, a dye test may confirm your suspicions. For this test, a special strong dye is put in the system - usually by flushing it down the toilet. A significant amount of water is then washed into the system.
If there is "breakthrough", the dye will become visible on the ground surface. If the dye is seen on the surface, this would be a very strong indication that the system has failed. Your Home Inspector, a licensed professional engineer, or a septic system contractor can usually determine if there is a problem visually without further exploratory digging.
One of the more interesting new developments is the use of "tertiary" septic systems where you have a mini processing plant. The result is water that is up 98% pure coming out of the end pipe. In the States, this "effluent" is sometimes allowed to be drained directly into nearby drainage ditches. That is not allowed in Canada because it would create obvious problems in our colder climate. These systems are often the only choice should you not have sufficient land for a raised bed system. The required leaching bed is significantly smaller.