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

Featured Articles

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.

Aerobic Treatment Unit First, please understand that the State has clearly stated it is the owner's responsibility to operate the aerobic treatment unit in compliance with all regulations...

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...

Why Stay On A Septic System? Top 5 Reasons for Preserving Onsite as a Viable, Even Preferred, Sewage Management Option: Better protection of public health is possible ...

News Articles

Septic Tank Design Value Pre 285 ...
Septic Smell Gases which can accumulate in the septic tank or any other anaerobic environment include; hydrogen sulfide (H2S),...
Mound Septic System The drainage area in a mound system is more susceptible to compaction than a conventional drain field because...
Septic Mound Mound Septic Systems are a wastewater absorption trench system which has been constructed using suitable soil which has...
Why Do You Have An Aerobic System? Often its simply because it makes the designers, installers, builders, and manufacturers of these systems more money, frankly....