Septic Tank Design
Value |
Pre 285 |
Per 285 |
Daily flow |
1080 gpd |
420 gpd |
Area of field |
5191 sf |
2100 sf |
Pretreatment |
2250 gallon |
475 gallon |
Treatment Capacity |
0 gpd |
500 gpd |
Pump Tank |
1000 gallon |
750 gallon |
Observed life span |
15 + years |
3 – 8 years |
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
Previous Article : Septic Tank System - - Next Article : Septic Cleaner
-
Providing Quality Service...Guaranteed In the Greater Austin Area of Travis, Hays, Williamson, Burnet, & Bastrop Counties
, Hudson Bend, Volente, Westlake Hills, Buda, Kyle, San Marcos, Florence, Brushy Creek, Leander, Misc Locations, Bell County.
Call (512) 338-1804 for fast, dependable service.
-
Unhappy With Your Septic System? Why do so many customers end up dissatisfied with their septic systems? We feel it starts with nobody asking or even considering the owner's opinion before...
-
Septic Tank Additives There are many companies promoting their septic tank additives, saying they can solve all of your septic woes. For over 15 years, these claims have...
-
Seller Says "Too Bad" Seller Says “Too Bad” After Homebuyers Find Rotting Septic Tank Under Plywood (From Pumper Magazine, October 23, 2018) A news organization in Cortland Manor,...
-
Alternative Septic System Regarding your Home Aerobic Treatment Unit: The components in your home plant should be serviced at least...
-
Septic Systems Thank goodness that the alternative of onsite treatment and disposal exists. Thank goodness we are here to...
-
Septic System Construction For many years, designers of high strength systems, e.g., for restaurants, RV parks, church halls, etc., due to...
-
Septic Tank Pumping Service More frequent pumping is needed if a garbage disposal is used. Generally the indicated frequency will vary with...
-
Septic Systems Maintenance Why Maintain Your System?There are three reasons why septic system maintenance is important to you and your community....