I have a 1500 gallon septic tank. The tank is 10 years old and has not
been maintained on a regular basis. Recently I dug up the second
access side of the tank and drained both sides completely. I allowed
the tank to refill then added 3 bags of costic soda. The tank appeared
to be working but we keept the water load light. Now, 2 months later i
added the washing machine and returned to full use. Immediatly the
tank filled up and is over flowing the tank. Do you have any ideas as
to how i can try to fix the problem before haaving to dig a new leach
line?

Hello.  I'm sorry you are having these problems.  As described below,
there is a series of tests that I'd like you to perform, and then let
me know via Request for Clarification how they go.

But at this stage I'm afraid that the problem is more likely with the
distribution box and what lies beyond, than with the tank itself.

First some basics:
"A septic system is composed of the following three parts:  

1) Septic Tank: Composed of either plastic, fiberglass, concrete or
steel, the tank's job is to hold and keep solid waste from flowing to
the rest of the system. The tank is the area were bacteria action
decomposes the sewage into liquid and sludge.The sludge settles to the
bottom of the tank, and the liquid moves to the leaching bed.
2) Distribution Box: series of pipes that carries the liquid to the
Leaching Bed.
3) Leaching (Absorption) Field: this is where the liquid seeps out of
perforated pipes and disperses into the surrounding soil. "
Septic Tank Basics
http://home.earthlink.net/~dcheck/septic.htm

You can pretty much diagnose the nature of the problem from where the
problem is seen:
"Sewage backup at the toilets or fixtures is one sign of a failing
system. Backup can also be caused by a blockage somewhere between the
home and the septic tank. Another sign of failure is a sewage smell
outside the house. A failing Leaching field could be the culprit if
the smell is more noticeable when a lot of water is discharged into
the system. This would usually occur after heaving water usage
periods. If sewage or excessive liquid appears at the ground surface
around the field, this indicates systems failure.

If you see any of these signs,  a dye test may confirm the problem.
The test involves flushing a heavy dye into the Septic system, and
running water continuously. Failures are evident by the appearance of
the dye on the ground above the leaching field. The test is relatively
quick and reasonable, it can be performed by a Home Inspector or a
Septic contractor."
Id.

There is a very good and detailed set of trouble-shooting tips that
you can access at
Septic Tank Inspection and Trouble-Shooting
http://www.montana.edu/wwwpb/pubs/mt9403.html

Rather than copy and paste portions of that here, please print out the
entire document for yourself, and run as many of the tests as appear
applicable.

After you have done those tests, please post a "Request for
Clarification" here, telling me what you found, and I'll respond with
the likely diagnosis.

But I'll bet the problem is downstream from the tank, and not the tank
itself.


Search terms used:
"septic tank" backup
"septic tank" backup distribution

Good luck
richard-ga

My closest friends own a septic business.  ASAP, I'll contact him and
see if he can answer this for you.

Im going to assume this septic tank has worked fine in the past. The
1500 gallon septic tank is rated for a 5-6 bedroom house. The first
thing i would do is make sure the outlet from the tank to the leech
line is not clogged. If you have a distribution box and several leech
lines check from the tank to the distribution box. When you drained
the system and let it refill, air is allowed into the tank. With
oxygen in the system, anaerobic bacteria will not do the job they are
supposed to do. While they were "sleeping" solid matter may have been
forced into the leeching lines. So check that first. Second, make sure
your leeching lines are not below the water table. You give no
reference to where you live so take into consideration the rainy
season and the highest water table of the year. The pourous soil under
the leech lines may also be clogged by any solids that were not
decomposed. Also make sure the tank is airtight. As mentioned above,
air in the system makes the system do nothing but hold water. If you
know where your current leech line is walk around by it. If it is a
swamp then your leech lines are working fine but the soil is not
pourous enough to handle the load. Dig up the leech line and 12-16
inches under it and add pourous gravel. If everything above seems to
be fine, either kick up the pipe size of the current leech line or add
a distribution box and add a couple more leech lines to it.

Hi again sm,

Okay, my friend just left after explaining a few things to me. First
off, I'd like to say he basically agreed with what darkdove said. He
also asked me a lot of questions which I had no way of answering. I
told him we would have to look at this problem using only the
information you provided. I said just give me a general idea of what
MIGHT be wrong.

Rather than say 'He said' ad nauseum, I'm just going to tell you what
he told me.

First, because you mentioned digging up the second access and draining
out both sides, he's assuming you had a problem to begin with  but you
haven't said what it was. Because you added caustic soda, the
assumption is that you had clogging somewhere.

Have you heard of bio-matting? Bio-matting is formed when solid
particles which should remain in the large side of the tank don't sink
to the bottom and become part of the sludge layer. If the tank has not
been pumped for awhile and the sludge level rises too high, incoming
water disturbs the top level of the sludge and when the water drains
over to the other side, some of these particles are carried along by
the water. (We're talking about very FINE particles here, as in silt.)

From there the water goes out into the weeping bed, and the particles 
will filter down to the bottom (sand) level where they begin to form a
layer which
gradually seals the sand so the water cannot filter though.
Eventually, the water has nowhere to go but up or out sideways.

What he's thinking is that you unknowingly had this problem before you
did the initial digging and that as long as the incoming water-flow
was restricted, the system could slowly absorb the water. When you
returned to full operation and the washing machine was sending large
amounts at once, the system overloaded.

How to deal with the problem is another matter. He says there are
several things which need to be considered. Oh, yes, and he's talking
about a filter bed, as opposed to other types such as a floating bed.

There are strict regulations about adding to a bed -- technically, he
says you need to check these regulations and use a licensed installer
and the work needs to be inspected upon completion.

What some people have done is to hire a backhoe and remove a foot or
two of the soil all the way down and out from the existing bed and
replace it with new sand or gravel -- in effect, removing the
bio-matting.

I learned more about septic tanks this morning than I ever thought
possible!! That's probably the only thing that I can tell you could be
the problem. Without knowing what type of tank, how often it's been
pumped, how many use it, the type of bed, the type of soil surrounding
the bed, whether it's raised (mounded) or level with the ground, etc.
etc.  he said that was his best educated guess.

Hope it helps,

Regards, 
revbrenda1st