4.1 Septic Tank Treatment Efficiency

The following information provides guidance on the effectiveness of septic tank treatment of domestic wastewater. Table 3 provides the minimum frequency for septic tank pumping as a function of the household population. If garage grinders are used divide the recommended pumping frequency by a factor of approximately two. Therefore, a 3 person family with a 1000 gallon septic tank with no garbage grinder should pump the tank at a frequency of at least 3.7 years. If a garbage grinder is used the pumping frequency should be every two years. Table 4 is a summary of septic tank effluent quality data and approximate percent reductions assuming domestic wastewater.

    Table 3
    Septic Tank Pumping Frequency
    Function of Tank Size and Household Population
    (assuming no garage grinders)
    Tank Size (gal)
    2 people
    3 people
    4 people
    5 people
    900 gal
    5.2 yrs
    3.3
    2.3
    1.7
    1000
    5.9
    5.9
    2.6
    2
    1250
    7.5
    4.8
    3.4
    2.6
    1500
    9.1
    5.9
    4.2
    3.3
    1750
    10.7
    6.9
    5
    3.9
    2000
    12.4
    8
    5.9
    4
    Note: Assuming normal use with no garbage grinders
    Source: The Pennsylvania State University Fact Sheet SW 161
    Table 4
    Summary of Septic Tank Effluent Quality
    Data and Approximate Percent Reduction
    Parameter Mean Effluent Approx. Reduction
    Parameter
    Effluent Concentration
    Percent Reduction
    T. Phosphate
    20 mg P/L
    20 mg P/L
    COD
    200 to 327
    60 to 70 %
    BOD 5
    120 to 140
    40 to 50%
    Suspended Solids
    39 to 155 m
    40 to 80 %
    T. Nitrogen
    36 to 45 mg/
    0 to 50 %
    Oil and Grease
    20 to 25 mg/
    70 to 80%
    Source: US EPA, Design Manual: Onsite Wastewater
    Treatment and Disposal Systems, October 1980
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