What is the Average Life Span of a Septic System?
he average life span of a typical trench type drainfield is about 9-10 years. Bed type drainfields
and mound systems typically don’t last quite as long as trench systems. There are systems which
defy these statistics and fail much sooner or last much longer. Systems which last much longer
typically were installed in good sandy, gravelly soils, were used moderately (small family), and
were maintained (septic tank pumped at least every 2 years). Systems which failed
"prematurely" were typically installed in poorer soils (silts & clays), used more heavily (larger
family), and/or were not maintained.
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What Causes Drainfields to Fail?
| BIOMAT - It is important to bear in mind that virtually all septic systems (except those with back-up
drainfields and an alternator valve) will fail. The normal cause of failure is due to the buildup of
a thick, pasty, black sludge, called a biomat. The biomat is a biological growth that develops in the
drainfield. The microbes in the drainfield consume the soluble organic material in the
wastewater that comes from the septic tank and convert it to cell mass, the same way we convert
food energy into fat and muscle. Over time this mass of bacteria builds up and plugs the interface
between the drainrock in the field and the surrounding soil, preventing wastewater from
percolating into the ground. The result: a drainfield that fills with water and backs-up into the house and/or
overflows onto the ground. It is important to note that the cold ground temperatures, and the deep
depths at which we install drainfields in Alaska (so as prevent freezing), which are oxygen
deficient, hastens the build-up of the biomat, explaining why drainfields in colder climates don’t
seem to last as long.
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Other Factors
n addition, to old age, there are a number of reasons why systems fail. They include, but are not
limited to, the following:
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- Undersized System: The design of a septic system is based upon a very limited amount
of field testing. It is possible that the actual soils on which your system was installed are not
as porous as the percolation test indicated. Consequently the drainfield was sized too small.
- Poor Construction Practices: If the contractor drove equipment over the surface of the
drainfield (particularly with wet silty/clayey soils), or installed silty (unclean) drainrock, the
performance of the drainfield may have been significantly compromised.
- You may have groundwater problems that were not identified during the design of your septic
system. The groundwater monitoring done by the engineer may not have accurately
identified the seasonal high groundwater table on your property. It is possible that during
spring break-up and significant rainfall events that the groundwater rises to such an extent
that the hydraulics of your septic system are significantly diminished.
- If the septic tank was not pumped frequently enough, or not at all, it is likely that the solids
collected in the septic tank to such an extent that some of them began to wash through the
tank and into the drainfield. In some cases it is possible to jet clean the "muck" out of the
drainfield and retest it successfully. You should consult with your engineer regarding this
option.
- Do you have any additional sources of water that may be hydraulically overloading the
drainfield? Look for the following:
- Do you have leaking toilets (typically the "flapper" in the tank) or other fixtures?
- Is there a sump pump hooked up to the septic system?
- Do you have a water softener that is backwashing excessively?
- Did you just drain your 1000 gallon hot tub into the septic system?
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| If any of these conditions exist, they may be causing your septic system to be hydraulically
overloaded. In some cases, if these sources of water are eliminated, and the system given
several days to recover, it may be possible to retest the system successfully.
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- Excessive usage by residents: As mentioned previously, the average water use for a family is
typically 200-250 gallons per day. If your family has been indiscriminate about water usage,
this may have hastened the failure of the drainfield.
- Did you use biological additives in your septic system? The EPA, ADEC, and the MOA all
discourage the use of septic system additives (enzymes, yeasts, ect). There is no data proving
that they do anything to enhance the performance of the system. In fact there is evidence that
some additives contribute to drainfield failure.
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My septic system is failed, so what are my options?
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hat defines a failed septic system? Most homeowners define a failed septic system as one that
either periodically or continually backs-up into the house or overflows onto the ground in their
yard. In short, the system is a continual or occasional nuisance. If neither of these events occur,
most homeowners assume they have a good septic system. Although a system may be functional
for the occupants, it may or may not meet the Municipality of Anchorage's (MOA) legal definition of a functioning septic
system. The Septic Adequacy Test page will provide specific information on how the test is performed. The MOA defines a functioning drainfield as one that is capable of absorbing (not just
accepting) 150 gallons per day per bedroom. This corresponds to the following:
ost 3 or 4 bedroom houses only have about 4-5 occupants in them and a total daily water usage
of 200 to 250 gallons per day, which is significantly less than the absorption requirements
established by the MOA. As a result, you may have a septic system that functions fine for your
family, but is unable to meet the absorption requirements established by the MOA.
The septic system adequacy test is very demanding. Statistically, about one out of every three
systems is in a technical state of failure. Although that may not sound too optimistic, it is a fact.
