Septic Tank Design

Let's consider one real life example:
Residence in Austin, TX 78734
5,164 sf residence with an Aerobic/drip system
Class II soils placed over fractured rock
Area known for groundwater problems

Determination of Design Flow, Q:
Pre 285:
0 - 1500 sf 2 BR
1500 – 1900 sf 3 BR
800 sf = additional BR
1900 sf = 3 Bedrooms
(5164 – 1900) / 800 = 4.08 additional bedrooms.
Must round up to 5 additional bedrooms.
Number of Bedrooms = 3 + 5 = 8 Bedrooms
Q = (B + 1) x 120 gpd (with a 20% credit for low flow fixtures)
Qpre-285 = (8 + 1) x 120 gpd = 1080 gpd.
Per 285:
Table III, says less than 5500 sf is 6 Bedrooms.
6 Bedrooms, Qper-285 = 420 gpd.

Determination of Field Area:
Pre 285:
No specific guidance on drip, but
Generally accepted Ra < 0.1 gpd /sf
A = Q / Ra = 1080 / 0.1 = 10800 sf
Per 285:
Fractured rock, Ra = 0.2 gpd / sf
A = Q / Ra = 420 / 0.2 = 2100 sf
Note: Area provided = 2600 sf

Sizing of Tanks:
Pre 285:
Q = 1080 gpd
Pretreatment = 1000 gallon single compartment tank
Treatment = 1250 gpd
Pump tank = 1000 gallon single compartment tank, minimum
Per 285:
Q = 420 gpd
Pretreatment = 475 gallon
Treatment = 500 gpd
Pump tank = 750 gallon

Let's consider another real life example:
Residence in Austin, TX 78734
5,164 sf residence with an Aerobic/drip system
House built on fractured rock
Area known for groundwater problems

Determination of Design Flow, Q:
Pre 285:
0 - 1500 sf 2 BR
1500 – 1900 sf 3 BR
800 sf = additional BR
1900 sf = 3 Bedrooms
(5164 – 1900) / 800 = 4.08 additional bedrooms.
Must round up to 5 additional bedrooms.
Number of Bedrooms = 3 + 5 = 8 Bedrooms
Q = (B + 1) x 120 gpd (with a 20% credit for low flow fixtures)
Qpre-285 = (8 + 1) x 120 gpd = 1080 gpd.
Per 285:
Table III, says less than 5500 sf is 6 Bedrooms.
6 Bedrooms, Qper-285 = 420 gpd.

Determination of Field Area:
Pre 285:
LPD, ET sizing due to the lack of soil.
A = 31000 (1+ 8) / (70 – 16.25) = 5191sf (in two alternating fields)
Per 285:
LPD, Fractured rock, Ra = 0.2 gpd / sf
Theoretically, A = Q / Ra = 420 / 0.2 = 2100 sf
Realistically, A = 2100*5/3 = 3500 sf.

Sizing of Tanks:
Pre 285:
Q = 1080 gpd
Pretreatment
1250 + 4(250) = 2250 gallon dual compartment tank
Pump tank = 1000 gallon single compartment tank, minimum
Per 285:
Q = 420 gpd
Pretreatment = 475 gallon
Treatment = 500 gpd
Pump tank = 750 gallon