How long do septic systems last?

Search for "septic system life span" on the Internet, and you will find wildly different estimates, some as low as 25 years. How long to septic systems actually last in West Acton? Pio Lombardo of Lombardo Associates, one of the authors of Acton's Wastewater Management Plan (CWRMP), states that maintained septic systems last 60-80 years:

Do West Acton septic replacement rates support Lombardo's assertion? In a survey of septic replacement records at the Acton Health Department, 14.4% of one- and two-family homes required major septic repair over a ten-year period. See the following Citizen's Concerns document submitted by Mike Geis to the Sewer Committee:

If we assume random installation dates and 14.4/10 = 1.44% fail every year, then the expected lifespan can be estimated: 100/1.44 = 70 years.

However, note that septic system life span doesn't quite follow a Gaussian distribution, as can be seen here (top of page 5):

In the above analysis of Spencer-Tuttle, it was found that a few systems fail relatively quickly, while most systems have a much, much longer life. The reason? Putting oil and grease down the drain can shorten a system's life dramatically. Using a garbage disposal and not pumping regularly can also shorten system life. Finally, in very rare cases, septic systems may fail due to incorrect installation (for example, insufficient grading).

However, the vast majority of septic systems are correctly installed and sufficiently well maintained to avoid premature failures. Actual septic replacements among one- and two-family homes in West Acton over a ten-year period confirm Lombardo's estimate of 60-80 years.