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Thread: Septic system sizing = GPD needed

  1. #1

    Default Septic system sizing = GPD needed

    I知 working on building my shop / office for my farm. I知 about to go apply for a permit on the septic system but I need a design. The building has 2 full bathrooms, a sink for a food fixing area (fridge and microwave), and 2 utility sinks. The floor drains, utility sinks, and showers are on a gray water system.

    Code calls for 2 tanks and an aerator (sp) in the second tank for houses. I can skip the aerator for this because this building will not see continues use. We do have to get a pump done every 5 years by code.

    I知 thinking I should go with 1000 primary and a 500 secondary tank witch would be smallest by code I could use. I知 planning on using the half moon plastic leaching field system also. Were a family of 5 and can see us spending time around the shop in the summer a few days a week so I知 sure it will be used for the bathrooms and for cleaning up after being out side all day long. I can also see family gatherings a few times a year.

    I知 thinking 3.5 gallons to flush and wash your hands. If every one went 4 times in 8 hours would give us 20 rounds on a normal day. So 70 gallons a day for normal times but for family get-togethers I would say 57 rounds in the bathrooms witch would put us at 199.5 gallons per day, but this would only be a few times a year.

    Should I use 100, 150, or 200 as my number for sizing this systems leaching field? I do not want to have issues with this down the road. I want to do it right the first time.

    Is there a better way to come up with this number (they want to see how I got the number)?

    Any other suggestions that everyone over looks?

  2. #2
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona

    Default Size

    Septic systems are seldom sized by guesswork. They are sized by usage AND a factor called seepage. The ONLY way to get a handle on seepage is with a percolation test, usually administered by a certified septic system contractor or plumber. That is the number they will want to see when they evaluate your seepage field to see if it is the proper size.

  3. #3
    Master Plumber nhmaster's Avatar
    Join Date
    Jul 2008
    S. Maine


    You will need a plan that has been drawn and certified by an engineer. Your local building department will not accept anything without an engineers stamp that has been accepted by the state first.

  4. #4
    In the Trades jnaas2's Avatar
    Join Date
    Mar 2009
    Evansville In


    It depends upon where you are, The county I live in requires a perc test to see how much field bed you will need depending on the number of bedrooms in the house.When you go to get the permit the building inspector doesnt even look at the perc test but says FOR A 2 BEDROOM HOME 500 LINEAL FEET OF FIELD BEDS ARE NEEDED, thats 5 100 foot runs. In reality most field beds are figured on the bedrooms and the amount of waterflow per day and the perc rate of the soil determines the square feet of field bed needed

  5. #5
    DIY scratch-pad engineer leejosepho's Avatar
    Join Date
    Nov 2006
    200 miles south of Little Rock


    Same here: The size of the tank(s) depend(s) upon the number of bedrooms, then the size of the drain field additionally depends upon percolation testing to see what drainage your soil can actually handle.

    The plastic chamber system as a drain field is definitely better than the old-style, rock-filled trenches, and sometimes only half as much is needed.

    Do not make the mistake of installing absolute minimums now only to later have trouble when usage changes or when trying to sell the property.

  6. #6


    however, it is very important to use properseptic tank cleaning mechanisms to improve you garden health


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