Septic Design - Flows

Question:  Is there any wonder why folks seem to be having so many problems with OSSFs (septic systems) in Texas?

Answer:  Beginning with the new rules that came out in 1997, design flows are on the order of 1/2 what they used to be - even though homeowners aren't universally generating 1/2 as much sewage every day.

The design numbers in these new rules are now mostly average flows, and as such do not provide any reserve for the occasional higher, or peak flows. These higher, or peak flows, often exceed the design values, causing the the system to not be able to perform at the levels expected of these systems.  

For systems to operate correctly, using only these average flows, requires the use of flow equalization.  With flow equalization preceding the OSSF, the peak flows will be moderated, so the balance of the system can be designed for these much lower flows.

Without the use of flow equalization, however, the OSSF must be designed for the higher flows which accommodate peak flows that naturally occur on a daily, weekly, and monthly basis. Otherwise, the OSSF will dramatically under perform.

Number of Bedrooms

Harris County - Example

Minimum Gallons Per Day

30TAC285, Table III

Minimum Gallons Per Day

2

300

180

3

450

240

4

600

300

5

750

360

6

900

420

7

1050

480

8

1200

540

9

1350

600

10

1500

660





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