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.

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 ...

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...

Aerobic System Pumping Aerobic Treatment Units (ATU) generally require more frequent pumping than do septic tanks of comparable size. There are 2 reasons: The...

News Articles

Septic Infiltrator Infiltrator Systems Inc. is a world leader in the manufacture of plastic drainage chambers for on-site septic and...
Septic Tank Cost You are responsible for maintaining your system. You may wish to hire a professional to make sure your...
Septic System Product A few communities around the country have established management programs that involve having a third party professional look...
Septic Systems Installation Guidelines for designing proper performing OSSFs:1.Flow equalization for all ATU's2.Pressurized time dosing to field for systems with primary...
Cleaners Septic System DETERGENTS: Low sudsing detergents should be used. If powdered detergents are used, only theconcentrated forms are recommended. Filler...