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How do water efficient fixtures affect the performance of a septic tank?

Michael Haluschak | Posted in General Questions on

We recently worked on a job where we replaced all of the old fixtures with ultra high efficiency toiles (UHET), low flow faucets and shower heads and a very low water use washing machine.

Afterwards, we got to talking about how this would affect the septic tank that services the house. The tank was sized years ago (based on number of bedrooms) considering the fixtures of the day. I don’t know that much about septic systems, but I do know they operate on the principal of a certain amount of flow-through, and I wonder how it will function with far less water flowing through it.

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  1. Daniel Ernst | | #1


    You're right. Septic tanks and leaching fields are typically sized by the number of bedrooms in a house.

    Although it varies somewhat from state to state, most regulations assume ~ 75 gallons of water per day (GPD), per person. Usually it's broken down something like this:

    * Shower - 20 Gallons
    * Toilet - 20 Gallons
    * Laundry - 15 Gallons
    * Kitchen - 15 Gallons
    * Bathroom Lavatory - 5 Gallons

    The regulations also assume two people per bedroom.

    Tanks are sized based on a minimum two-day detention time. This in turn is based on the total daily flow. So a three bedroom house would require a tank that holds at least 900 gallons of sewage:

    (3) Bedrooms x (2) Persons / Bedroom x (75) Gallons / Day / Person = 450 Gallons / Day

    450 GPD x 2 Days = 900 Gallons

    I believe many of the GPD assumptions are based on the older toilets, which used between 3.5 - 5.0 gallons per flush. The calculations also include a safety factor.

    In your case, less flow = greater detention time. That's not a bad thing. It allows the tank bacteria more time to work their magic on the nutrients. The effluent going out of the tank and into the leaching field will probably have a lower BOD (biochemical oxygen demand) and TSS (total suspended solids). In simple terms, the effluent will be weaker and clearer, with less potential to pollute the local ground water.

  2. Roger Anthony | | #2

    There is a side issue on this. The transport of solids from the toilet to the septic tank, relies on the solids being suspended for the length of their travel in a bubble of water. If the pipe is at a too shallow an angle or too steep an angle, or if the pipe has a dip, this may lead to pipe blockage as the solids get left behind.

  3. Daniel Ernst | | #3

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