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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.

Table 3.

Septic Tank Pumping Frequency
Function of Tank Size and Household Population
(assuming no garage grinders)

Tank Size (gal)al)
2 people
3 people
4 people
5 people
900 gal
5.2 yrs
3.3
2.3
1.7
1000
5.9
3.7
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
15%
COD
200 to 327 mg/L
60 to 70 %
BOD 5
120 to 140 mg/L
40 to 50%
Suspended Solidsolids
39 to 155 mg/L
40 to 80 %
T. Nitrogen
36 to 45 mg/L
0 to 50 %
Oil and Greasese
20 to 25 mg/L
70 to 80%

Source: US EPA, Design Manual: Onsite Wastewater
Treatment and Disposal Systems, October 1980