Septic Tank Redo

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by dw85745, Jul 30, 2013.

  1. dw85745

    dw85745 Member

    Oct 14, 2004
    Whoever installed the septic tank installed it backwards -- outlet higher inlet by about 1 inch.
    I'm planning on having a new outlet holed cored around 1 1/2 to 2 inches below the current.

    Whoever originally plumbed this used 3 inch ABS Schedule 40 both for outlet and inlet piping
    and appeared to use hydraulic cement around the pipe where it enters the tank. They also cemented
    the 3 inch pipe into the D-Box.

    I would like to upgrade the inlet DWV line from 3 inch to 4 inch but only have 2 inches of fall
    over about 45 feet and am concerned with possibly causing more problems if the inlet pipe
    doesn't get set just right as that will have an impact on outlet..

    On the outlet side the new D-Box, which I'm replacing has built in seals which take a 4 inch thin wall DWV.


    1) Should I upgrade to 4 inch on the inlet side or leave well enough alone.
    2) Would it be better to also keep the outlet side at 3 inch (sch40) instead of going to the thin wall pipe
    because that is what the D-Box will accept?

    This is the outlet end looking from tank to old D-Box

    'This looking at outlet tank end. New D-Box is pictured in foreground.
    Last edited: Jul 30, 2013
  2. leejosepho

    leejosepho DIY scratch-pad engineer

    Nov 23, 2006
    disabled-retired industrial fabricator
    200 miles south of Little Rock
    Are you measuring that from the top of the tank or did you use a transit or a water level to compare one end to the other?

    If that line has been working just fine, I would leave it alone.

    The larger line could help lower the outlet just a bit, but I would not let the D-Box dictate thin-wall pipe if you want Sch40.
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  4. dw85745

    dw85745 Member

    Oct 14, 2004
    Thanks for your input.

    I used a transit, but to confirm, I filled the tank to the inlet invert then measured down from the outlet invert to the water.

    My preference. Now I think of if -- by upgrading to 4 inch (pipe from residence is 3 inch), that will drop me 1 inch lower when the coupling is centered on the 3 inch which gives me even less fall as currently installed -- plus -- would also have to lower outlet pipe about another inch.

    Since the hole is being re-cored, it will be a function of pipe placement. I have two options.
    1) Place bottom of pipe on bottom of cored hole in tank (here pipe size doesn't matter.
    2) Use a link seal which will center the pipe. In this case 4 inch may be lower depending on where hole is cored.

    Right now leaning toward using 3 inch from tank and then upsize to 4 inch thin wall for the D-Box seal.

    Not a fan of the thin wall, but not sure whether its use (thin wall) is standard for residential septic systems?
    In AZ all the thin wall I've found is PVC.

    Can find both 3 and 4 inch (sch 40) in both ABS and PVC (ABS more common) but from what I've seen ABS tends to warp
    more (hard finding a straight piece from supplier).

    When I have the tank cored on the outlet side, I'm also leaning toward using link seals instead of directly cementing the pipe to the tank.
    Only question is whether link seals will stop root intrusion better / worse than cement???
    Last edited: Jul 31, 2013
  5. hj

    hj Moderator & Master Plumber Staff Member

    Aug 31, 2004
    Cave Creek, Arizona
    quote; Are you measuring that from the top of the tank or did you use a transit or a water level to compare one end to the other?

    IF the tank is level, measuring from the top of the tank down would be a precise measurement.
  6. dw85745

    dw85745 Member

    Oct 14, 2004
    I can't speak for other lids, but this one varies. IMHO best is just use tank itself as water level.

    ====== QUESTION ===================

    Went ahead and cored a 5 inch hole for the link seal (which uses 3 inch sch40).
    Opted for a 5 inch hole as I can insert a 4 inch thin wall directly into the hole and hydraulic cement around it.
    Why the industry is using thin wall for their lines (e.g. All D-Box seals and new tank seals just accommodate thin wall)
    is beyond me. Any time you need to dig, good chance putting a pick or shovel through this thin pipe.

    Is there a better (more preferred way) to affix pipe to the tank -- rather than directly cementing pipe to an old tank --using this thin wall pipe??????
    Last edited: Aug 2, 2013
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