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To size a septic tank? Correct term?

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kami4444
kami4444 Member Posts: 33
Which answer is right?
a) type of building and occupants or
b) expected sewage value per daily

I put (a) because its # of bedrooms. What is the correct 'term' to be used. A or B?

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  • Zman
    Zman Member Posts: 7,579
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    It really could be either. It really depends on the jurisdiction.
    The bottom line is to get to the expected sewage value so you can design the system. Type of building and occupants is one way to estimate that.

    What kind of test are you taking?
    It sounds like a very general test of plumbing and heating systems written by someone with some very general and limited knowledge.
    Best of Luck...
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Jamie Hall
    Jamie Hall Member Posts: 23,454
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    Of course the classic answer is "it depends". However, in the jurisdictions in which I have worked, the sizing is based on the number of bedrooms. For residential structures. For other uses, sometimes the code will allow a non-engineer to design a septic system -- including tank size -- but very often not, with very good reason.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Zman
    Zman Member Posts: 7,579
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    Is the test set up in way that you can explain the answers?
    It would be better if you could show your work and the assumptions you made.
    To be perfectly honest, the right thing to do is explain why each question cannot be answered with the information supplied.
    I don't see much value in learning this way.
    None of this is something you can just memorize and then understand.
    Must be Masters in Architecture...

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    Gordy
  • Zman
    Zman Member Posts: 7,579
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    The system is designed to be able to treat a specific flow rate.
    Many jurisdictions simplify this by using bedrooms and some assumptions about flow rate per bedroom.
    I assure you, the engineer designing the system is not calculating how many bedrooms per hour the ground will absorb.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Jamie Hall
    Jamie Hall Member Posts: 23,454
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    kami4444 said:

    So it's expected sewage value then.. hmph... I knew it

    Having designed and installed a number of septic systems, for both residential and various commercial and industrial applications, allow me to say this.

    Most jurisdictions use the number of bedrooms for residential applications to size the septic tank. Is it usually oversized, strictly speaking? Yes. Which is a good thing, as septic tanks are peculiarly sensitive to flow overloading, and one weekend house party can ruin the leaching field if the tank is even slightly undersized. Seen it happen. For industrial and commercial loadings, it is necessary to take into account peak flow, mean daily flow, BOD, COD and total solids loadings to properly design the tank. It is perhaps worth noting that the tank rarely fails (although it may need pumping more often than it should); it causes the leach field to fail if not properly sized and designed.

    The leaching field is, of course, a totally different problem. Again, most jurisdictions use pretty simple relationships based on the rate of water absorption of the soils; sometimes a table, sometimes a formula. Most of the formulae and tables are pretty good for more or less average conditions. They tend to fail, sometimes rather spectacularly, when soil conditions are significantly worse -- or better! -- than average. Most leach field designs are pretty conventional; unfortunately, they are also serious polluters of groundwater. It is possible to design systems with essentially zero discharge to groundwater and, somewhat more easily, essentially zero pollution loading to groundwater -- I've done a few -- but the regulatory agencies in many jurisdictions don't like them, perhaps with good reason as if not correctly designed and installed they too fail rather spectacularly.

    If you don't really understand how the system as a whole works, and don't understand your loadings, design it by the book. Most times it will work, in the sense that you won't be walking around the place in lamentable puddles. Also most times you will be contaminating the groundwater -- but, out of sight, out of mind, right?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England