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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.
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|
Note: Assuming normal use with no garbage grinders
Source: The Pennsylvania State University Fact Sheet SW 161.
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||15 %|
|COD||200 to 327 mg/L||60 to 70 %|
|BOD 5||120 to 140 mg/L||40 to 50%|
|Suspended Solids||39 to 155 mg/L||40 to 80 %|
|T. Nitrogen||36 to 45 mg/L||0 to 50 %|
|Oil and Grease||20 to 25 mg/L||70 to 80%|
Source: US EPA, Design Manual: Onsite Wastewater Treatment and Disposal Systems, October 1980
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